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The On-line Mud Logging Handbook Alun Whittaker

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The On-lineThe On-lineMud Logging Mud Logging HandbookHandbookby Alun Whittakerby Alun Whittaker

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Hydrocarbon EvaluationHydrocarbon Evaluation- Oil, Gas, & - Oil, Gas, & PrecursorsPrecursors

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Hydrocarbon Evaluation – Oil, Gas & Precursor

Mud logging was born as a well-site extension of petroleum laboratory analysis. It is logical that modern mud logging should include, where cost justifies, well-site application of other, newer laboratory geo-chemical tests and analyses. Recent advances in solid state electronics have finessed the Mud Logging Golden Rules by allowing the development of analytical hardware that is smaller, more rugged, easier, and less time consuming to operate – also much cheaper to buy and use. Techniques in use include:

✔ Enhance hydrocarbon gas analyses

✔ Cuttings testing for oil, gas and chemical signs

✔ Total Organic Carbon in kerogen, pyro-analysis, and other source bed geo-chemical tools

✔ Oil fluorescence, contaminants and quantitative fluorimetry



Table of ContentsGeo-chemistry in Work Gloves.......................................................................................................................................................................9

Hydrocarbon Gas Analysis...........................................................................................................................................................................10

Cuttings Evaluation.......................................................................................................................................................................................14Early Signs..................................................................................................................................................................................................14

Petroleum Odors....................................................................................................................................................................................14Oil Pops..................................................................................................................................................................................................15Rainbow and Sheen...............................................................................................................................................................................15

Detailed Examination..................................................................................................................................................................................15

Oil Fluorescence............................................................................................................................................................................................17Mineral Fluorescence..................................................................................................................................................................................20Petroleum Product Fluorescence................................................................................................................................................................21

Mud Additives.........................................................................................................................................................................................21Oil-base & Additives...............................................................................................................................................................................21Pipe Dope...............................................................................................................................................................................................21

Solvent Cut Test..........................................................................................................................................................................................22Cut Speed..............................................................................................................................................................................................23Cut Nature..............................................................................................................................................................................................23Cut Colors..............................................................................................................................................................................................23

Sample Examination Procedure...................................................................................................................................................................26Mud Oil Show Tests....................................................................................................................................................................................28Unwashed Cuttings Oil Show Tests............................................................................................................................................................29

Automated Fluorimetry .................................................................................................................................................................................31

Refractometry................................................................................................................................................................................................39

Wellsite Geo-chemistry.................................................................................................................................................................................42Total Organic Carbon..................................................................................................................................................................................44Pyro-analysis..............................................................................................................................................................................................47



Temperature Programmed Chromatography...............................................................................................................................................67Pyro-chromatography..................................................................................................................................................................................69

Worst Case Scenario.....................................................................................................................................................................................71

The Swan Song..............................................................................................................................................................................................73

And Next.........................................................................................................................................................................................................73



Didn't find what you needed here? Sorry.

Why not go back to the Chapter Summaries, and fine a better place to start, or use the Index to search for the subject you need.

List of Figures & TablesFigure 1: A chromatogram containing sufficient components in characteristic proportions may be as unique as a finger print in correlating migrated or accumulated hydrocarbons. These chromatograms are from known, pure samples of producing zones #1, #2, and from a real-world (somewhat contaminated and degraded) oil-based drilling fluid.............................................................................................................11

Figure 2: These chromatograms are from known, a pure sample of producing zones #1, from the real-world, oil-based drilling fluid, and from two different composition mixtures of the two..................................................................................................................................................12

Figure 3: These chromatograms are from known, a pure sample of producing zones #2, from the real-world, oil-based drilling fluid, and from two different composition mixtures of the two..................................................................................................................................................13

Figure 4: Consisting of homologous series or families of hydrocarbons ranging in molecular weight, physical and chemical properties of a crude oil is a reflection of it’s chemical composition. Density, viscosity, natural and fluorescence colors of a crude oil, and it’s associated gas reflect the distribution of molecular weight homologs of compounds in the mixture. .......................................................................................19

Figure 5: Fluorescent minerals commonly found in well cuttings.....................................................................................................................20

Figure 6: Fluorescence and cut testing for oil must be performed in a rigorous and systematic manner in order to avoid erroneous interpretations, missed shows or false alarms. ...............................................................................................................................................24

Figure 7: Evaluation of mud and cuttings for signs of oil or residuum should be performed routinely, Approximately once each hour in all formations. In potential or actual reservoir rocks, the smallest possible sample interval should be used. .......................................................27

Figure 8: Quantitative Fluorescence intensity improves correlation of show quality between zones. On a computer-generated mud log, both intensity, and color-coded approximation of fluorescent color helps illustrate both the oil show quality and type.............................................32

Figure 9: A fluoro-spectrometer can detect quantitative fluorescence at a range of excitation wave lengths...................................................33

Figure 10: Quantitative fluorescence using a scanning spectrometric ............................................................................................................34



Figure 11: Quantitative fluorescence using a synchronous scanning spectrometry plotted as two-dimensional contour plot. Note that the colors used represent, gradations of fluorescent intensity, and not fluorescence color (as shown in Figure 8). When setting up a computer plotter to create a contour plot of this this type, you should take care to select colors or textures from the device's palette that cannot be misunderstood. ...............................................................................................................................................................................................36

Figure 12: Well-to-well correlation using a QFT fluorescence log....................................................................................................................37

Figure 13: This Atago hand refractometer is made in Japan and is (not surprisingly) compact, reliable and reasonably priced. Need I say more?..............................................................................................................................................................................................................40

Figure 14: Refractometry was developed for use in petro-chemistry and food technology to determine oil purity from a measurement of Refractive Index. This empirical relationship between Refractive Index and API Gravity of crude oil gives reasonable accuracy even when the oil base type is unknown. ..........................................................................................................................................................................41

Figure 15: The LECO® CR-12 Total Organic Carbon (TOC) analyzer provides the basic geo-chemical measurement used to indicate source bed quality and against which to normalize all other geo-chemical measurements. (Illustration courtesy of EXLOG, Inc.).............................43

Figure 16: Total Organic Carbon content and source bed quality for argillites (claystones and shales) and carbonate rocks (limestones, dolomites and calcareous mudstones). ..........................................................................................................................................................45

Figure 17: Total Organic Carbon provides a reliable quantitative estimate of the richness of organically-derived carbon. Unfortunately it cannot indicate the type or present diagenetic state of the organic material without additional geo-chemical or mud logging data. ...............46

Figure 18: The Rock-Eval pyro-analyzer heats a cuttings sample in a helium atmosphere, detects free hydrocarbons, kerogen abundance, type and maturity. ...........................................................................................................................................................................................48

Figure 19: Results from the Rock-Eval pyro-analyzer indicate the presence of free hydrocarbons, and the abundance, type and maturity of kerogen from the parameters S1, S2, S3 and Tmax. ......................................................................................................................................50

Figure 20: The S2 peak alone is a measure of potential hydrocarbon yield of the whole rock.........................................................................52

Figure 21: The S2 and S3 peaks together with the TOC value are a measure of kerogen type and maturity..................................................53

Figure 22: The Van Krevelen diagram relates Hydrogen Index (S2/TOC) and Oxygen Index (S3/TOC) with source type and maturity...........54

Figure 23: The relative richness of Hydrogen (reflected in the S2 peak) and Oxygen (reflected in the S3 peak) in the kerogen is diagnostic of the type and yield of hydrocarbons to be expected from it...............................................................................................................................55

Figure 24: From correlation with the IFP database, Tmax can be used to indicate maturity of the source bed ...............................................56



Figure 25: The Well-site geo-chemistry Log displays Rock Eval® pyro-Analysis, TOC or associated mud log and geo-chemical data and may be used to monitor source rock and reservoir occurrence, type and maturity. ................................................................................................57

Figure 26: The Rock-Eval source bed evaluation may be modified by the presence of other non-source materials, such as catalytic, high CEC smectite clays that can enhance reaction in the Rock-Eval oven, increasing hydrocarbon yield, or achieving peak hydrocarbon evolution at a lower Tmax. ..............................................................................................................................................................................58

Figure 27: The Rock-Eval source bed evaluation may be modified by the presence of other non-source materials, such as unstable Carbonates. These may initiate decomposition of stable carbonates at lower temperatures, causing a falsely higher and wider S3 peak, and a pessimistic Oxygen Index.............................................................................................................................................................................59

Figure 28: The Rock-Eval source bed evaluation may be modified by the presence of other non-source materials, such as dense, involatile bitumens resulting in widened, and partially merged S1 and S2 peaks. .........................................................................................................59

Figure 29:The LECO CR-12 measures only total organic carbon (TOC) in the source rock............................................................................60

Figure 30: The Rock-Eval I and II differ only in electronics and processor design. The measurements are essentially the same: S1, S2, S3 peaks and temperature Tmax..........................................................................................................................................................................61

Figure 31: The Oil Show Analyzer, or Rock-Eval III, measures Carbon Dioxide for total organic carbon (TOC) content, and not for kerogen Oxygen Index..................................................................................................................................................................................................61

Figure 32: The Thermolytic Hydrocarbon Analyzer is a simplified pyro-analyzer, indicative of what may become the next generation mud logging analyzer...............................................................................................................................................................................................62

Figure 33: The Pyrologger is a another candidate to be the next generation mud logging analyzer...............................................................62

Figure 34: Measured and computed parameters available from the programmable Rock-Eval 6....................................................................64

Figure 34 (continued): Measured and computed parameters available from the programmable Rock-Eval 6.................................................65

Figure 34 (continued): Measured and computed parameters available from the programmable Rock-Eval 6.................................................66

Figure 35: Temperature-programmed chromatography can provided extended hydrocarbon analyses allowing recognition of reservoir character and productivity, for example, petroleum condensate or wet gas....................................................................................................67

Figure 36: Temperature-programmed chromatography of a high gravity (light and volatile) oil. ......................................................................68

Figure 37: Temperature-programmed chromatography of a low gravity (dense and heavy) oil........................................................................68

Figure 38: Temperature-programmed chromatography of a very dense, immovable residual oil.....................................................................68



Figure 39: A pyro-chromatogram of the S1 peak from a pyrolized source bed sample. .................................................................................70

Figure 40: A fragmentogram of the S2 peak from a pyrolized source bed sample. .........................................................................................70



Hydrocarbon Evaluation – Oil, Gas & Precursors

Geo-chemistry in Work Gloves

Mud logging involves the analysis of organic and petroleum material from drilling fluids and rocks. So does petroleum geo-chemistry! Most mud loggers don’t have Ph.D.’s, but some of the best ones do. Many petroleum geo-chemists do have doctorates, although some very good ones don’t.

The dividing line between mud logging and petroleum geo-chemistry is an arbitrary and, to my mind, a quite unnecessary one. It allows some narrow-minded geo-chemists to sustain an absurd snobbery. It also allows some inferior mud loggers to avoid applying rigorous scientific standards to their work. The only real difference is that:

✔ Mud logging measurements are commonly targeted at tactical, real time problems involving real hydrocarbons, usually in existing reservoirs, while

✔ Petroleum geo-chemistry measurements are often used to address strategic, historical problems involving source beds and migration pathways.

In reality, as we has already begun to see, both kinds of data can contribute to either kind of problem. So we are left with the only real difference being that geo-chemical analytical instruments are more expensive, and less suited to use at the well site, and even that difference, as we’ll see in this chapter, is starting to go away.

Both mud logging and petroleum geo-chemistry involve the extraction, separation, and identification of hydrocarbon components using organic chemical methods. In reality, any laboratory geo-chemical procedure that may be applied in real-time at the well site can be added to the suite of mud logging services, if it complies with mud logging golden rule number one — it can be carried out without placing too great an extra strain on the mud loggers’ time and attention. In recent years, improvement and simplification of sample processing procedures and the development of reliable, robust, semi-automated analyzers have allowed a number of procedures to make this transition.

The first of these was gas chromatography which is used to separate the individual hydrocarbons for more precise analysis. The technique, first introduced in the fifties, is now standard in virtually all modern mud logging units.


In Datalog’s GC-TRACER (see In Datalog’s GC-TRACER (see Chapter 5Chapter 5), the chromatograph has even), the chromatograph has even gone beyond the mud logging unit, to the mud flow line itself!gone beyond the mud logging unit, to the mud flow line itself!


Techniques that have more recently made the transition to well-site application include pyro-analysis, in which petroleum oils and precursors are thermally decomposed in an inert atmosphere. Analysis of the gaseous products of this thermal cracking yields information on the richness, maturity and eventual products of organic source material in the geological section.

More recently, optical techniques, such as refractometry and fluorimetry now make it possible to determine something of the physical properties of oil from the small amounts recovered in cuttings while drilling.

The newest addition to well-site geo-chemical services involves the combination of pyrolysis techniques with chromatography. Controlled progressive heating cracks and volatilizes oil and source material which is then separated in a heated chromatograph column. A technique that has been long used in the laboratory, this type of instrumentation has now been made in a compact configuration suitable to the logging unit and automated to allow rapid analyses to be performed.

The selection of services in a drilling operation should not be made on the basis of whether mud logging or well-site geo-chemistry is required. Instead, the decision should be made on the basis of:

✔ What information is required,

✔ When is it required, and

✔ Can this schedule be met more reliably and economically:

✔ In the well-site mud logging unit, or

✔ At an off-site laboratory.

Hydrocarbon Gas AnalysisThe simplest form of geo-chemical analysis is nothing more than a reversion to the earliest of mud logging techniques: container head-space analysis. This type of analysis is not normally performed on every sample caught. Preference is given to samples caught from zones of particular interest, and samples may be allowed to back-up for analysis when time is available.

Instead of using the gas trap, a standardized gas sample is withdrawn from a blender, or a sealed sample container, at a controlled pressure and temperature. To do this, canned samples of mud and cuttings can be carefully punctured and then re-sealed with a rubber septum. A syringe is then used to extract a measured volume of gas to be injected into a chromatograph.


Samples may also be extracted from the canned cuttings and mud, usingSamples may also be extracted from the canned cuttings and mud, using a mud (steam or vacuum) still — but this is a a mud (steam or vacuum) still — but this is a complication with littlecomplication with little benefit. benefit.


The chromatograph is similar to the type used in regular mud logging except that is has a heated, manual sample injection port, to keep the sample volatile, and not condense before getting into the chromatograph column. The column material, temperature and carrier gas flow rate are also optimized for the separation and detection of heavier volatile hydrocarbons. Not being bound by real-time logging priorities, the analysis may be allowed to run longer in order to identify alkanes as heavy as Duo-decane (C12H26) with good peak separation and baseline resolution.

This type of chromatography is used as a supplement to, not a replacement for conventional mud logging chromatography. It’s improvements in analytical accuracy are gained only at the expense of loss of timeliness (more time is needed to process each sample) and loss of vertical resolution in the log (less samples can be processed in the time available at the well site, although some may be held back for later processing to fill in detail as required). It is in fact a very useful tool for correlation of hydrocarbons between different reservoirs, recognition of migration pathways or discrimination or formation fluids masked by oil-based drilling muds.

Figure 1: A chromatogram containing sufficient components in characteristic proportions may be as unique as a finger print in correlating migrated or accumulated hydrocarbons. These chromatograms are from known, pure samples of

producing zones #1, #2, and from a real-world (somewhat contaminated and degraded) oil-based drilling fluid.

Although the alkanes are an homologous series, and commonly occur together, any mixture will contain a unique selection and distribution of compounds. A chromatogram with enough components and good enough resolution can be used to recognize characteristic combinations



and proportions of compounds. This is illustrated in Figure 1, Figure 2, and Figure 3, in which chromatograms from two different, known petroleum reservoirs, and from a sample of the base oil used to build an oil-based drilling fluid can be used to identify a newly discovered reservoir, penetrated using this oil-based mud. Electronically subtracting the drilling mud chromatogram from that of the mixture leaves behind a chromatogram containing the same components in the same proportions as in the reservoir. We are able to remove the effect of the mud contamination, and demonstrate a relationship between a newly-drilled reservoir and a known previously-drilled reservoir. This type of mathematical treatment may also be applied to the routine chromatogram but the greater the number of components, the greater will be the reliability and discriminatory power of the technique.

Figure 2: These chromatograms are from known, a pure sample of producing zones #1, from the real-world, oil-based drilling fluid, and from two different composition mixtures of the two.



Electronically subtracting a multiple of the drilling mud chromatogram leaves behind a chromatogram containing the same components in the same proportions as in Reservoir #1. We are able to remove the mud contamination and demonstrate a relationship between the present and one of the older reservoirs.

Figure 3: These chromatograms are from known, a pure sample of producing zones #2, from the real-world, oil-based drilling fluid, and from two different composition mixtures of the two.



Cuttings EvaluationIn Chapter 7, we reviewed the collection, preparation and inspection of cuttings for lithological and mineralogical evaluation.

There are a number of additional procedures in standard, descriptive mud logging for the evaluation of liquid and solid hydrocarbons: oils, tars, and waxes in cuttings.

Before moving on to geo-chemical procedures carried out at the well site or in the laboratory, on unwashed cuttings, we’ll look at the additional steps in routine mud logging sample examination for evaluating an oil bearing sample. The most important of these is the recognition, and use of oil fluorescence.

Early SignsUse your eyes, your noise and anything else available. The first indications of the presence of oil in a well-site sample begin in the sample catching and preparation stages.

Petroleum OdorsAt the shale shaker, there are usually prevailing smells of drilling mud and exhaust from motors around the rig, Nevertheless, it is possible to detect the cleaner, sweeter odor of crude oil carried in cuttings on the shaker screens.


It is common for reference books to describe crude oil as having a paint-like odor. I have smelled many types of paint.It is common for reference books to describe crude oil as having a paint-like odor. I have smelled many types of paint. They all have different odors and none of them, in my experience, has a smell that is anything like that of crude oil. IThey all have different odors and none of them, in my experience, has a smell that is anything like that of crude oil. I can also say that crude oil does not have the acrid or bitter odor commonly associated with refined oil or gasoline.can also say that crude oil does not have the acrid or bitter odor commonly associated with refined oil or gasoline. Commonly containing more aromatics (literally and descriptively) and more volatiles than refined products, crude oilCommonly containing more aromatics (literally and descriptively) and more volatiles than refined products, crude oil can best be described as having a sweeter, more pleasant, perhaps even slightly fruity odor. can best be described as having a sweeter, more pleasant, perhaps even slightly fruity odor.

Remember that, before mud logging, well-site geologists were nicknamed Remember that, before mud logging, well-site geologists were nicknamed Shale Sniffer sShale Sniffer s . At other times, they were even called. At other times, they were even called Shale Eater sShale Eater s , because it was claimed that even the slightest oil trace could be tasted by placing a cutting on the tongue., because it was claimed that even the slightest oil trace could be tasted by placing a cutting on the tongue. Another trick was to grind a cutting between the teeth — a sensation of grittiness could help discriminate a siltstoneAnother trick was to grind a cutting between the teeth — a sensation of grittiness could help discriminate a siltstone (containing fine clastic material) from claystone (with pure clay minerals). While I don’t recommend either of these as a(containing fine clastic material) from claystone (with pure clay minerals). While I don’t recommend either of these as a standard practice, I can testify that they both work. standard practice, I can testify that they both work.


Petroleum odors may also be detected on washed cuttings, particularly as they are warmed in the sample dryer, or by the heat of the microscope illuminator.

Oil PopsWhen catching a pan full of cuttings for bagging unwashed samples, you may notice small droplets or globules of oil popping out of the coating of drilling fluid on the cuttings. These may become even more visible when the mud is diluted while you are washing the cuttings.

Rainbow and SheenA sheen or rainbow-like luster may first be identified on the mud coated cuttings on the shale shaker, or even on the mud itself in the possum belly ditch, and running off below the shale shaker. The sheen may become more visible on the water running off while washing, or on the surface of wet cuttings in the sieve. The sheen can be more apparent in bright outdoor sun light, than in the more uniform, less intense, fluorescent lights inside the mud logging unit, or shale shaker house.

Always check the mouth of blender jar, after running the blender test both for petroleum odors and for a rainbow sheen on the water surface.

The mud logger should be aware of, and looking for all of these signs all of the time. When using oil-based drilling fluid or additives, some of these signs may be observed in every sample.

If so, that should be noted and described on the mud log worksheet:

✔ To prevent later, off-site observers of samples from drawing false conclusions, and

✔ To establish a baseline oil trace description, against which future genuine oil shows can be judged.

Detailed ExaminationIn addition to looking for first signs, there is a series of detailed examinations and oil tests that must be performed on every sample when conditions indicate the possibility of an oil zone (or potential oil zone) having been penetrated, such as:

✔ A drilling break,

✔ Early signs, such as petroleum odor, oil popping, or sheen,


If we see clues that oil may present, then we must search diligently to confirm that presence. If we see clues that oil may present, then we must search diligently to confirm that presence.

Conversely, when we drill into a potential reservoir but see no obvious first signs of theConversely, when we drill into a potential reservoir but see no obvious first signs of the presence of oil, then we much search equally diligently to confirm, and explain that absence.presence of oil, then we much search equally diligently to confirm, and explain that absence.


✔ A gas show consisting of an increase in total combustible hydrocarbons,

✔ A gas show with the first appearance of heavier alkanes (Propane, Butanes, and Pentanes) in the hydrocarbon chromatogram, or an increase in their concentration, relative to that of the lighter alkanes (Methane and Ethane),

✔ A change in cuttings lithology with the appearance of potential reservoir rock types, such as fine, unconsolidated sandstone, or porous limestone, or

✔ Identification of the top (depth or lithological boundary) of a formation which the well prognosis (or mud logging instructions) indicates to be a zone of interest.

In this zone, samples for oil testing should be caught as often as possible (no less often than every 15 minutes) and, if necessary, labeled by depth, and then set aside to back up for later examination. Continue to catch, process and package other sets (unwashed, rinsed, washed and dried, and so on) on their normal schedule.

For each oil evaluation, the following samples are required:

✔ Fresh drilling mud

✔ Fresh (unwashed) cuttings

✔ Washed and sieved cuttings

✔ Blender residue

✔ Diluted mud

✔ Crushed cuttings

Each of these samples must be examined while fresh — old, dried, cuttings samples are of little value in oil evaluation. If, after the fact, your inspection of washed and dried samples does not agree with fresh, well-site observations reported on the mud log, it is probably better to trust the fresh observations over your own!


The alkanes that constitute the majority of hydrocarbons in a petroleumThe alkanes that constitute the majority of hydrocarbons in a petroleum reservoir are an homologous series, varying progressively and predictablyreservoir are an homologous series, varying progressively and predictably in physical and chemical properties from the lightest (Methane) to thein physical and chemical properties from the lightest (Methane) to the heaviest (petroleum waxes and tars). The composition of a reservoir fluidsheaviest (petroleum waxes and tars). The composition of a reservoir fluids is similarly progressive from lightest to heaviest components. If heavyis similarly progressive from lightest to heaviest components. If heavy alkanes (oils) become present in the reservoir, then medium alkanesalkanes (oils) become present in the reservoir, then medium alkanes (heavier gases) will also be added to the mixture. This is discussed in more(heavier gases) will also be added to the mixture. This is discussed in more quantitative detail in quantitative detail in Chapter 13Chapter 13..


Each of them should be examined:

✔ With the naked eye and the microscope,

✔ In visible and ultraviolet light, and

✔ While wet, dry and treated with an organic solvent, or hot water.

Any visible traces of oil, tar or residuum on the cuttings, or in the water must be reported on the mud log worksheet. All samples, regardless of whether visible staining is observed, must be processed through the following fluorescence, solvent cut and examination procedures, and the results fully documented. In a potential reservoir, is as important to report negative results, as positive ones.

Oil FluorescenceLuminescence is a common property in nature. Molecules of many compounds will absorb electromagnetic radiation (infrared, visible, ultraviolet light, and so on), at one or more characteristic excitation wavelengths and, as a result, some of their bond electrons absorb energy, and move to a higher energy level orbital. The molecule is less stable in this excited state, and so the electrons soon fall back to their original energy level with the re-emission of electromagnetic energy. This emission is commonly not at the original excitation wavelengths, but at a different, but equally characteristic, longer emission wavelength (see Whittaker, 1985).

In some cases, the molecule in an excited state is meta-stable, and so the emission of energy is slow and may continue for some time after the excitation energy source is removed. Compounds that behave in this manner are said to be phosphorescent. A phosphorescent compound will, after excitation, continue to emit energy for some time afterwards.

In other compounds, the excited state is much less stable. Emission begins rapidly after excitation, and continues only so long as the excitation source is active. These are called fluorescent compounds.

There are numerous fluorescent compounds commonly found in crude petroleum with a wide range of characteristic pairs of excitation and emission wavelengths. Unfortunately, there are other, equally common, naturally occurring fluorescent minerals to be found in cuttings samples (see below).


A system is said to be meta-stable when it exists in a state of pseudo-A system is said to be meta-stable when it exists in a state of pseudo-equilibrium, such that it has a free energy higher than that of the trueequilibrium, such that it has a free energy higher than that of the true equilibrium state. A metastable system will not spontaneously nor rapidlyequilibrium state. A metastable system will not spontaneously nor rapidly change. Instead, it will do so slowly or intermittently over a period ofchange. Instead, it will do so slowly or intermittently over a period of time, or under the influence of external stimulants. For example, aftertime, or under the influence of external stimulants. For example, after excitation by daylight, the luminous hands of an alarm clock willexcitation by daylight, the luminous hands of an alarm clock will continue to emit light all night, but if shut away in a dark cupboard, theycontinue to emit light all night, but if shut away in a dark cupboard, they will eventually darken and remain dark until, one again, brought outwill eventually darken and remain dark until, one again, brought out into the light.into the light.


The most useful fluorescent crude oil components to mud logging, are a number of poly-cyclic (multi-ringed) aromatic, and alicyclic compounds that are common in almost all petroleum oils and which, when excited by shorter wavelength ultra-violet light (200 nanometers or less), strongly fluoresce in the visible spectrum.

The wavelength of the fluorescence to which a compound is susceptible is closely related to the molecular weights of the fluorescing compounds (see Jorden & Campbell, 1984, and Lynch, 1962). Once again, the familial nature of hydrocarbon compositions, is reflected in the range and distribution of molecular weights in the oil and the overall density (or gravity) of the mixture. Within bounds, controlled by extremes of base chemistry:

✔ Low gravity (or denser) oils fluoresce at longer, infrared, red and orange, visible wavelengths, while

✔ Lighter, high gravity oils will fluoresce at shorter, yellow or blue, visible wavelengths (See Figure 4).

The ultra-violet (black light) inspection box is ubiquitous in mud logging units.

The usual configuration is a light-tight, black-lined box that can be illuminated internally with either natural white, or ultraviolet light. The box usually has light-tight flexible doors at each side so that a large object, such as a core can be passed through the light box. They also allow the mud logger to manipulate samples, test tools, and chemicals inside the box.

An alternative consists of a conventional binocular microscope with an enclosed stage and an illuminator capable of supplying both white and ultra-violet light.

Another useful option is a battery-powered, portable UV light, that can be taken from the unit to the core processing area or even the shale shaker to examine fluorescence.

All of these designs have advantages and probably all three should be carried in a well equipped mud logging unit. It is essential to have a means of illuminating and magnifying oil stains within the pore spaces and surfaces of a single cuttings. On the other hand, an enclosure large enough to accommodate something as large as a piece of core is also needed. Finally, for inspecting whole core, or large volumes of mud and cuttings at the shale shaker, a portable unit can be very useful.


In 1969, a mud logger's ultraviolet inspection box achieved the dubiousIn 1969, a mud logger's ultraviolet inspection box achieved the dubious honor of being the only mud logging instrument to appear in a Playboyhonor of being the only mud logging instrument to appear in a Playboy centerfold! No, I’m sorry, I don’t remember which month (Try centerfold! No, I’m sorry, I don’t remember which month (Try Googl-ingGoogl-ing that one).that one).

Talking of movies, hand-held, ultra-violet illuminators have become easier,Talking of movies, hand-held, ultra-violet illuminators have become easier, and cheaper to get hold of, ever since they have become so common on TVand cheaper to get hold of, ever since they have become so common on TV crime shows, used along with crime shows, used along with lum ino llum ino l , to highlight tell-tale traces of blood, to highlight tell-tale traces of blood and ... other stuff... at crime scenes. Why would watching something likeand ... other stuff... at crime scenes. Why would watching something like that inspire someone want to buy one a Radio Shack? Who knows?that inspire someone want to buy one a Radio Shack? Who knows?


Figure 4: Consisting of homologous series or families of hydrocarbons ranging in molecular weight, physical and chemical properties of a crude oil is a reflection of it’s chemical composition. Density, viscosity, natural and

fluorescence colors of a crude oil, and it’s associated gas reflect the distribution of molecular weight homologs of compounds in the mixture.



Mineral FluorescenceUnfortunately, crude oil is not the only material found in well cuttings that fluoresces under ultraviolet light. The most common source of fluorescence in cuttings are rocks and minerals. Fortunately, although widespread, the fluorescence colors are subdued and the intensity low. Common examples are shown in Figure 5.

Mineral or rock type Emission fluorescence color

Dolomite and Magnesian Limestone Yellow, yellowish-brown to dark brown

Aragonite and Calcareous mudstones Yellow-white to pale brown

Chalky Limestone Purple

Foliated “Paper” Shale Tan to grayish-brown

Anhydrite Blue to mid-gray

Pyrite Mustard yellow to greenish-brown

Figure 5: Fluorescent minerals commonly found in well cuttings.

Fluorescence of these colors and intensity are unlikely to be mistaken for a true oil show but the background of mineral fluorescence behind an oil show may modify the color and intensity, and so complicate comparative estimates. It is important to check all samples under ultraviolet light regardless of whether oil is suspected. Specimens with mineral fluorescence should be labeled and set aside for later reference and comparison with oil stained samples.



Petroleum Product Fluorescence

Mud AdditivesSome mud additives may also exhibit traces of fluorescence. I hope you took my advice (in Chapter 7) and remembered to keep samples of all mud additives, dry, water and oil wet? These should now be close to hand, while you are inspecting cuttings under the microscope, in natural and ultraviolet light. With these available for reference, few mistakes should occur.

Oil-base & AdditivesOil and oil-based mud additives can be more of a problem. Mineral oil and clean diesel oil have no fluorescence or, at worst, a dull, dark brown fluorescence. Combining this observation with a genuine hydrocarbon gas show that goes along with oil additions (see Slug Gas in Chapter 6) should allow easy recognition.

Unfortunately, the mutual solubility of oils can complicate the situation. Formation crude oil components will dissolve into oil-based additives and oil-based muds adding their brighter fluorescence. Unlike dissolved gas, these oils do not gradually evaporate from the mud. The extra fluorescent component will be carried in the mud throughout the well. Even worse, with a re-used, expensive oil-based muds, the fluorescence may be carried from well to well, adding fluorescence color and intensity each time an oil-bearing zone is penetrated.

When a routine sample is caught, the mud and unwashed cuttings should be briefly inspected under ultra-violet light. This will allow you to recognize the background fluorescence and, hopefully, to prevent a later false alarm. However, you must remember that it is not the oil-based mud that is fluorescing, but the true crude oil components carried in it. Even the best mud logging technique has its limits. It may be impossible to determine the nature of a newly penetrated potential reservoir if the mud already contains amounts of similar oil and gas from previously drilled intervals. Even comparative techniques may not work if the mud contains a broad spectrum of compounds from a range of previous reservoirs see Figures 2 and 3). The true formation oil and gas show can be swamped with other oils and gases.

Pipe DopeA final source of fluorescence contamination is pipe dope, the heavy, metalized grease used to lubricate and seal the threaded tool-joints of drill pipe and drill collars:

✔ Pipe dope has a very bright gold, white or blueish-white fluorescence apparently indicative of the lightest, highest gravity oils or condensate.

✔ Fortunately, such oils, in natural light are usually transparent, gold in color and readily evaporate under the heat of the illuminator.



✔ The heavy, viscous appearance and blue-black, or brown metallic color of pipe dope in natural light are obvious confirmation of the true nature of this show.

Samples of pipe dope, like anything else likely to find its way into the mud, should be kept in the mud logging unit, dry, and in mixtures with water and oil. Be sure to get samples of all types: often the connections of drill pipe, drill collars, casing collars, and other drill-string components require different types of pipe dope or EP (Extreme Pressure) greases.

Mistakes can be made, but they can be minimized if an evaluation is made based upon all data available in the mud logging unit.

Consider a reservoir containing heavy, low gravity, crude oil:

✔ The oil is dense, dark brown in color and has a low intensity, dark red-brown fluorescence (actually, it will have bright infra-red-to-red fluorescence but most of this is not in the visible range).

✔ The addition of pipe dope contamination to the cuttings from this formation may be disastrous. A little more heavy, brown material may be seen in natural light, but the pipe dope’s bright white fluorescence will mask the real crude oil’s duller colors.

✔ Based on visual inspection, the zone may be passed over, dismissed as a false alarm, but...

✔ If this is a true oil reservoir, then the oil will contain some dissolved gas, including the heavier alkanes, propane, butanes and so on.

✔ Drilling the zone will produce a gas show of increased magnitude and increased richness. This would not be expected from a small slick of pipe dope.

✔ The increased porosity in the zone may also be reflected in drilling break — also not to be expected from man-made additives.

These indicators should justify careful, systematic sample evaluation and lead you to a correct and complete evaluation.

Solvent Cut TestOil solubility is not always a drawback in reservoir evaluation. The solvent cut test is a useful companion to the cuttings blender test in determining oil mobility, a combination of the effects of oil viscosity, gas-oil ratio and permeability. It is also an aid in extracting the oil from the cuttings and giving a clear view, removed from the colored mineral background.

Several different solvents are used for this test:

✔ Chloroform, or Trichloro-methane

✔ Carbon Tetrachloride, Tetrachloro-methane, or Perchloro-methane

✔ Ethylene Dichloride, Sym-dichloro-ethane, 1,2-Dichloro-ethane (also known as Dutch Oil)



✔ Methylene Chloride, Methylene Dichloride, or Dichloro-methane

✔ 1,1,1-Trichloro-ethane, Methyl Chloroform, or Chlorothene

✔ 1,1,2-Trichloro-ethane, Vinyl Trichloride, or Beta-trichloroethane

✔ Trichloro-ethylene, Ethylene Trichloride, Triclene, or TCE, and sometimes the slang names: tri, or trike

You may also come across different trade names, blends and variants of these. Each has slightly different solvent properties and all of them are, to some degree, toxic. When working with solvents always work with small quantities in a well-ventilated area. Keep containers closed, do not smoke and, leave the work area and carefully wash your hands before eating or drinking anything. Dispose of used solvents carefully, following all requirements provided by the manufacturer, the rig safety rules, and the environmental laws of the region in which you are working.

These are organic solvents and that includes you!

In the solvent cut test, selected rinsed cuttings are placed in the depressions of a white porcelain spot plate, and a solvent is added. Observing the cuttings in natural and ultraviolet light, with the naked eye and under the microscope, the following characteristics can be observed:

Cut SpeedSolution of oil in the solvent can take place instantly, rapidly, slowly, or not at all. This is an indication of both the solubility of the oil and the permeability of the cutting — the ease with which solvent can flow into the pore space, dissolve the oil, and carry it out of the cutting.

Report you observations of cut speed as seen in both natural light (and the color of the cut), and ultraviolet light (and fluorescent color).

Cut NatureColoration of the solvent with dissolved oil may occur in a uniform manner, or with streams of color spreading unevenly out from the cutting. A streaming cut also indicates low oil mobility, either due to unconnected pores, or a discontinuous (or immovable) oil phase.

Cut ColorsThe color of the oil dissolved in the solvent is observed in natural light, and ultraviolet light. After solution, the solvent rapidly evaporates under the heat of the illuminator leaving a residue of oil around the cutting on the spot plate (see Figure 6). The true color of the oil can then be observed.



Figure 6: Fluorescence and cut testing for oil must be performed in a rigorous and systematic manner in order to avoid erroneous interpretations, missed shows or false alarms.

Natural and fluorescent colors may be seen more reliably when the oil is removed from the background color or fluorescence of the cutting onto the clean white porcelain surface of the cut dish.



The intensity and opacity of color, especially of the residue after the solvent has evaporated, is an indicator of the oil density and of the quantity of oil originally contained in the cutting

Remember, that oil-based muds, additives and pipe dope all dissolve in the solvent and add natural or fluorescent coloration to it. There is yet another source of fluorescent contamination: the solvent itself! Although bulk supplies of solvent are commonly stored in metal or glass containers, it is quite common for the unit’s laboratory bench top supply to be kept in a small plastic squeeze or dropping bottle.

Now these bottles, and the material of which they are molded, are commonly described as being inert and non-fluorescent. This is probably true for most applications and short periods. However, I have regularly observed that, after a few weeks of use, both the bottle and its contained solvent will adopt a low level of pale yellow-white fluorescence. This is not enough to cause a false oil show but enough to modify the appearance of a genuine oil show.

✔ In the logging unit, bulk containers for organic solvent should be made of glass or metal and clearly labeled as being:

✔ FOR OIL TESTS or

✔ FOR DE-GREASING PURPOSES ONLY

✔ At the beginning of each well, the bench top dropping-bottle must be thrown out and replaced.

✔ Once each week the bench top container should be emptied into the de-greasing fluid container and refilled from the clean solvent supply.

✔ Before performing the cut test, the solvent and its bottle should first be checked under ultraviolet light for fluorescence.

One final reminder: never use a plastic spot plate for a solvent cut test or any kind of oil evaluation. I don’t care what the guarantee says.

✔ It may dissolve in the solvent.

✔ It may fluoresce.

✔ It may become permanently stained and discolored.

✔ Don’t risk it.


This is assuming the mud logger is using laboratory quality, porcelain, spotThis is assuming the mud logger is using laboratory quality, porcelain, spot plates — not cheap plastic ones,plates — not cheap plastic ones,

And the mud logging unit has a plentiful supply of spot plates on hand —And the mud logging unit has a plentiful supply of spot plates on hand — enough so that every test can be performed on a fresh, clean spot plate. Spotenough so that every test can be performed on a fresh, clean spot plate. Spot plates should be rigorously cleaned between uses to remove all staining and anyplates should be rigorously cleaned between uses to remove all staining and any remnants of fluorescence, and they should be replaced when they start toremnants of fluorescence, and they should be replaced when they start to become discolored.become discolored.


Sample Examination ProcedureThe first sign of a entry into a potential oil reservoir is commonly a drilling break. This is an increase in rate of penetration in response to the low strength of a porous formation.

If the mud logger has a drilling data acquisition system (see Chapter 9), or if a cooperative driller is available, he may also become aware that the rotary torque has increased, and become more irregular. This might suggest a worn bit with damaged bearings but, hopefully, in this instance it is responding to inhomogeneities in a granular or fractured formation: another indication of good porosity.

On explorations wells, the drilling program will often dictate that, after two or five meters of such behavior, drilling should be halted, and circulation continued until samples are recovered and evaluated. This is called circulating bottoms up.

Whether circulating or drilling ahead, the mud loggers should now be getting ready for their most hectic work period. The laboratory area should be cleaned up. Sample containers, sample examination tools, chemicals and reference materials laid out. The mud log is brought up to date and then put away.

For a while, there will be time for sample evaluation only… We hope.After the lag time has gone by, the first gas shows from the formation will arrive and an increase of total hydrocarbons confirms that a hydrocarbon zone has been penetrated.

✔ The proportions of the alkanes in the chromatogram will be a clue to the nature of formation hydrocarbons: gas or oil. If there is little or no change, then the drilling break may indicate a water-bearing zone, or just a lithological change.

✔ Samples of well cuttings must be caught in order to evaluate the reservoir lithology, porosity, permeability and fluid saturations.

✔ From a time, ten minutes before the expected first arrival of cuttings from the formation, until uniform sample characteristics are established, samples of mud and cuttings for evaluation should caught at the smallest possible interval.

✔ If it is not possible to complete all of the tests and procedures for each sample at this rate, then portions of mud, unwashed or rinsed samples should be labeled, sealed if necessary, and set aside for evaluation later.


A water-bearing reservoir zone requires asA water-bearing reservoir zone requires as intensive sampling and evaluation as an oil, or gasintensive sampling and evaluation as an oil, or gas filled one. On this well, we may have penetratedfilled one. On this well, we may have penetrated the reservoir below the oil-water contact.the reservoir below the oil-water contact. Evaluation of the zone can still tell us a great dealEvaluation of the zone can still tell us a great deal about it’s porosity, permeability and reservoirabout it’s porosity, permeability and reservoir potential. Evaluation of remnant traces of oil, maypotential. Evaluation of remnant traces of oil, may tell us something about the oil source, maturitytell us something about the oil source, maturity and migration history. All of this is needed to planand migration history. All of this is needed to plan the next well.the next well.


Figure 7: Evaluation of mud and cuttings for signs of oil or residuum should be performed routinely, Approximately once each hour in all formations. In potential or actual reservoir rocks, the smallest possible sample interval should be used.



Mud Oil Show TestsThe mud sample is divided into four portions (see Figure 7):

✔ Portion #1 is immediately poured into a shallow dish and placed under the ultraviolet light to observe sign of fluorescence in the mud itself or droplets of immiscible, light oil popping to the surface of the denser mud.

If at first, nothing is seen, the sample a left to stand and observed periodically over several minutes.

✔ Portion #2 is mixed with an equal quantity of water to decrease its viscosity, stirred and then treated in the same manner as Portion #1.

✔ Portion #3 is used for the mud blender gas analysis.

After analysis, lift the blender gas carefully and check for a petroleum odor.

✔ Contrary to a common misconception, light alkane gases have no detectable odor.

✔ Oil-like odors are a definite indication of the presence of oil.

✔ Touch the surface of the water with a dry filter paper.

✔ This will pick up any oil droplets which may then be seen under ultraviolet light.

✔ After these tests, proceed to the normal sample processing procedure as discussed in Chapter 7.

✔ Portion #4 is filtered with the mud filter press and a coarse grade of filter paper (this does not yield a valid water loss measurement but produces a rapid sample of mud filtrate).

✔ The salinity is determined with a electrical resistivity meter, or silver nitrate titration.

✔ A pH meter, pH test paper or titration is used to determine the acidity (pH).

Changes in mud salinity within the reservoir may indicate a high water saturation or even identify the oil water contact. Decrease in mud pH may indicate the presence of sour gas — Carbon Dioxide or Hydrogen Sulfide — in the reservoir.



Unwashed Cuttings Oil Show TestsThe sample of unwashed cuttings is treated in the normal manner (see Chapter 7) except that two small portions are taken aside and checked for fluorescence in the same manner as the first two drilling mud samples (above):

✔ Portion #1 is immediately poured into a shallow dish and placed under the ultraviolet light to observe sign of fluorescence in the mud itself or droplets of immiscible, light oil popping to the surface of the denser mud coating the cuttings.

If at first, nothing is seen, the sample a left to stand and observed periodically over several minutes.

✔ Portion #2 is mixed with an equal quantity of water to decrease its viscosity of the mud coating, thoroughly mixed and then treated in the same manner as Portion #1.

After the cuttings blender test, the jar and water are inspected for oil signs just like the mud blender test (above).

After routine cuttings sample processing (see Chapter 7), the lithology evaluation sample is divided into two more portions (#3 and #4). Each consists of a representative sample of cuttings, spread thinly on a sample tray (see Figure 7).

✔ Portion #3 is placed under the microscope and examined in the normal manner for lithological evaluation.

An estimate is made of the proportion of apparently oil stained material, and visible oil staining is described:

✔ Appearance: oily, waxy, dry residue

✔ Distribution: even, spotty, patchy, streaks


This probably belongs, not here, but in a sample examination manual but I want to bring it up. Far too often, mud loggersThis probably belongs, not here, but in a sample examination manual but I want to bring it up. Far too often, mud loggers and geologists pile cuttings into a sample tray, a half centimeter or more deep. This may be a good way to load a drying trayand geologists pile cuttings into a sample tray, a half centimeter or more deep. This may be a good way to load a drying tray for making washed-and-dried sample. It is no way to perform geological, or oil evaluation. for making washed-and-dried sample. It is no way to perform geological, or oil evaluation.

Under the microscope, this tray of cuttings will present an irregular top surface, part above and part below of the range ofUnder the microscope, this tray of cuttings will present an irregular top surface, part above and part below of the range of focus. When you try to manipulate or pick up a cutting with a probe or tweezers, most likely you will push it down into thefocus. When you try to manipulate or pick up a cutting with a probe or tweezers, most likely you will push it down into the mass of other cuttings and out of focus. Cuttings should spread across the surface of a clean sample tray or filter paper, nomass of other cuttings and out of focus. Cuttings should spread across the surface of a clean sample tray or filter paper, no more than one cutting deep.more than one cutting deep.


✔ Hue and intensity of color on wet and dry cuttings.

Take a representative number of apparently oil stained cutting from the sample tray and place them in the depressions of a white porcelain spot plate, with no more than one cutting in each depression. Add solvent to half of the depressions on the spot plate to perform a cut test. Solvent cut is described:

✔ Rate,

✔ Nature,

✔ Color and intensity of solvent cut and oil residue (after evaporation).

✔ Portion #4 is inspected under ultraviolet light. An estimate is made of the proportion of fluorescent cuttings and fluorescence is described:

✔ Distribution: even, spotty, patchy, streaks),

✔ Hue and intensity of color fluorescence.

Take a representative number of fluorescent cuttings from the sample tray and place them in the depressions of a second spot plate. Add solvent to half of the depressions on the spot plate to perform another cut test. Solvent cut is described:

✔ Rate,

✔ Nature,

✔ Color and intensity of fluorescence.

You have now positively identified and placed on spot plates:

✔ Spot Plate #1: Cuttings with an oily appearance under white light, and

✔ Spot Plate #2: Cuttings with fluorescence under ultraviolet light

✔ Spot Plate #3: Cuttings that with a solvent cut visible under natural light.

✔ Spot Plate #4: Cuttings that with a solvent cut visible under ultraviolet light.



Next, swap the spot plates, so that:

✔ The cuttings described as oil stained can be evaluated for fluorescence and cut.

✔ The fluorescent cuttings can be evaluated for visible oil staining and cut.

✔ The color and behavior of the cut can be observed in natural light, ultraviolet light, and under the microscope.

By performing this double blind test you can be certain that the cuttings contained a true, fluorescent oil stain, wax, or tar residue. You have eliminated the risk of false alarms such as seeing (non-fluorescent) diesel oil stain on some cuttings, and falsely matching this with mineral fluorescence on others, and a pipe dope cut on yet others.

I have regularly seen mud loggers and well-site geologists take spoons-ful of cuttings in a dish, add solvent, swirl and look at them under ultraviolet light. This can make a quite impressive light show, but it is just a show. It is also a waste of valuable time and valuable sample. If the sample shows all possible signs of being oil-bearing but has no solvent cut, it may be worth trying to force a cut by crushing the sample, adding a little dilute acid, hot water, or trying the test on a warm, rinsed and dried sample.

This sometimes works on carbonates with extremely low effective porosity and permeability could be improved in producing wells by fracture or acid stimulation. If the cut test is successful under these conditions then describe the results in the normal way. Of course, when reporting the results on the log or core report, the added steps needed to induce the solvent cut should be explained.

Automated FluorimetryOil fluorescence evaluation is a simple, reliable method of locating and characterizing oils. Unfortunately, when many different fluorescent compounds are already present the mud, the addition of small amounts of one new component may be missed. Fluorimetry upgrades fluorescence analysis with automation and an extra dimension of measurement.

You will remember that, after excitation, a fluorescent compound may emit radiation at one or more characteristic longer wavelength. Thus, it should be possible to characterize fluorescence by both the wavelength (or color), and the intensity (or brightness), of the emitted light.

If our ultraviolet inspection system used a photocell instead of our eyes then, it would be possible to measure the intensity of fluorescence


If you don’t remember, please go back to the If you don’t remember, please go back to the beginningbeginning , and I’ll wait for you here..., and I’ll wait for you here...

... as I keep telling you, once you get the basics (no matter how irrelevant they may seem at the... as I keep telling you, once you get the basics (no matter how irrelevant they may seem at the time), you will be able to figure out any new situation or technology that you meet up with later.time), you will be able to figure out any new situation or technology that you meet up with later.


and detect an increase in intensity when a oil-bearing zone is first encountered (see Figure 8).

Figure 8: Quantitative Fluorescence intensity improves correlation of show quality between zones. On a computer-generated mud log, both intensity, and color-coded approximation of fluorescent color helps illustrate both the oil show

quality and type.

In order to improve reproducibility, two additional steps are required:

✔ Establishing a baseline intensity (the fluorescence intensity of mud filtrate, cuttings minerals, and contaminants prior to addition of the oil stain to be evaluated), and

✔ Preparing a standard sized, and standardly processed sample.



Oil Fluorescence can be further automated using scanning spectroscopy (see Law, 1981, and Figure 9). Using a spectrometer, a sample of flow-line mud (or unwashed -- easier to standardize than selecting from washed cuttings) is excited at a range of excitation wavelengths, and the intensity of emission fluorescence can be measured at each of its separate excitation wavelengths.

Figure 9: A fluoro-spectrometer can detect quantitative fluorescence at a range of excitation wave lengths


The question arises -- are we planning to measure the intensity of fluorescence from a standard volume of:The question arises -- are we planning to measure the intensity of fluorescence from a standard volume of:

Cuttings selected at randomCuttings selected at random

Selected fluorescent cuttingsSelected fluorescent cuttings

Selected (apparently) oil-stained cuttings Selected (apparently) oil-stained cuttings

A combination of the second and third options (oil-stained AND fluorescent) would be more valid and reproducible, but it wouldA combination of the second and third options (oil-stained AND fluorescent) would be more valid and reproducible, but it would it require quite an effort in cuttings picking to produce a large enough sample for measurement.it require quite an effort in cuttings picking to produce a large enough sample for measurement.


Oil-based components and contamination in the drilling fluid may contain some similar or related hydrocarbons to those in the reservoir crude oil. Other components will be unique to each of them. Often, those unique components will respond to different excitation wavelengths and emit at different emission wavelengths. A spectrogram of emission fluorescence intensity over a range of excitation wavelengths shows differences in fluorescence intensity between an oil show sample and the established background. Subtracting the base-oil signature it is possible to recognize the presence and relative amount of a genuine new oil show (see Figure 10).

Figure 10: Quantitative fluorescence using a scanning spectrometric



For example, in Figure 10, you can see that:

✔ At the excitation wavelengths labeled A, emission fluorescence is present in both background and oil show samples with a similar intensity. Obviously, the components excited at these wavelengths are present in similar amounts in both. They are most likely components of the drilling fluid or contamination from previously drilled zones.

✔ At the excitation wavelength labeled B, emission fluorescence is present in both background and oil show samples, but there is greater intensity in the oil show sample. The components excited at this wavelength are likely present in both samples, but more has been added to the oil show sample. These components are most likely formation hydrocarbons, with trace amounts carried as contaminants in the background sample, with increased amounts added from the drilled reservoir being seen in the oil show sample.

✔ At the excitation wavelengths labeled C, emission fluorescence is present in only the oil show sample. The components excited at this wavelength are most likely unique components of formation hydrocarbons, not seen before, and introduced by drilled cuttings contributing to the oil show sample.

Using water-based muds, the base-line (or background) values of fluorescence can be reliably obtained from circulating fluids, stabilized by one or two full circulations (much like establishing the circulating background for total hydrocarbon gas detection).

However, it has been found, when using oil-based mud, that a base-line (or background) value of fluorescence intensity can only be reliably achieved by continuous sampling the mud as it enters and exits the hole. Knowing the down-time and lag time it is possible to determine a continuous differential intensity. This is the increase in fluorescence intensity as the mud passes through the bore hole, and carries the cuttings from each newly drilled interval.

Sample standardization (and a reduction in the necessary sample processing effort) can be achieved by taking a fixed volume of mud or unwashed cuttings and agitating it with an equal volume of a clean organic solvent such as Hexane. This can be done by hand but even better standardization is achieved with mechanical grinding and agitation.

The method is a contribution to formation evaluation but in oil-based muds, and when drilling extensive or multiple pay zones, the overall intensity and variability of the background may be so great that smaller variations due to changes in formation hydrocarbons may be difficult to isolate.

Another possible improvement through automation involves performing synchronous scan fluorescence spectrometry (see Bather, 1984 , and Connell, Coates, & Frost, 1986). In this method, the excitation wavelength is progressively changed and a series of fluorescence emission spectra are generated. When stacked together, these can be used to create a three-dimensional spectrogram (see Figure 11) in which fluorescence intensity peaks are defined by excitation wavelength and emission wavelength.

Just as in single scanning spectroscopy, it is possible to recognize the presence and relative amount of a genuine new oil show from the location and height (either in a 3-D graphic display or, in two dimensions, as a contour plot) of peaks. Unfortunately, even with synchronous scanning spectroscopy, practical experience has failed to achieve complete reliability when faced with the problem of contamination from



saturated, oil-based mud. Often the mud, contaminated with oil from many different sources, produces such intense fluorescence from so many wavelengths that it swamps the entire fluorescence spectrum and prevents any unique peak from being recognized.

Figure 11: Quantitative fluorescence using a synchronous scanning spectrometry plotted as two-dimensional contour plot. Note that the colors used represent, gradations of fluorescent intensity, and not fluorescence color (as shown in Figure 8). When setting up a computer plotter to create a contour plot of this this type, you should take care to select

colors or textures from the device's palette that cannot be misunderstood.



Fluorescence spectrometry has a great potential as a well-site analytical tool and it could become an ideal complement to chromatography. Some recent work by Texaco (now Chevron-Texaco) has made an important contribution by developing a systematized methodology for sample processing, analysis, and data presentation (see Delaune, 1992) and Delaune, Spilker, & Wright, 1999) . These methodologies have been patented under the names Quantitative Fluorescence Technique (QFTTM and QFT2TM).

✔ The original process, called QFT, provides a plot of fluorescence intensity against depth, similar to that illustrated in Figure 8. Using an empirical calibration against a large database of samples from oil producing intervals, the QFT method attempts to determine a quantitative oil concentration function.

✔ QFT2 (see Dick, 2005) extends the basic process using two simultaneous excitation wavelengths, a computer program to calculate the ratio of the emission fluorescence intensities (see Figure 12). From that, it is also claimed that it is also possible to compute empirical estimates of oil percentage (by weight), oil density (as API Gravity), and the volume of oil-filled porosity.

Figure 12: Well-to-well correlation using a QFT fluorescence log

The Texaco QFT, and the QFT2 techniques have been licensed and are now offered by a number of mud logging contractors. As solid state, analytical instruments becomes more rugged, more capable, and cheaper, we should expect to see more efforts along the lines proposed by Texaco.



Of course, the methodology is entirely empirical, but if your conclusions are to stand only upon an established record of previous results, then probably those methods developed by huge, international exploration companies (with the benefit of their huge, international databases) should be most worthy of trust.

Quantitative fluorimetry does break mud logging golden rule number two, by requiring a day rate instrument to be kept (and paid for) on site for the whole well, although it is to be used only for short periods. On the other hand, it meets the approval of golden rule number three, in that it requires little specialized additional skill or time to run.

Any fluorescence measurement should be performed on very fresh samples. Analysis at some later time and place is unlikely to be so useful, nor so valuable. Even with carefully sealed samples, sample fluorescence will be depleted and, after the well has been completed, any real-time decisions to which a mud logging measurement could have contributed will have already been made, or will be no longer relevant; better, alternative data now being available.

Even without the somewhat extravagant claims made for QFT, quantitative fluorimetry, particularly when enhanced with some element of spectroscopy as discussed above, will eventually become an important tool in evaluating difficult drilling problems and situations.

These include:

✔ Drilling with oil-based drilling fluids,

✔ Evaluating multiple or repeated oil zones, and

✔ Correlation and steering in boundary detection on horizontally drilled wells.


OObviously, the success of the method as with others (see bviously, the success of the method as with others (see Chapter 13Chapter 13) can only be) can only be maintained if new data is gathered to both reinforce the general model, and to develop andmaintained if new data is gathered to both reinforce the general model, and to develop and refine more precise region-specific, or reservoir-type-specific correlations.refine more precise region-specific, or reservoir-type-specific correlations.


RefractometryUntil fluorescence spectrometers have been reduced enough to fit every pocket — in both size and price — there is another quantitative optical technique that is already able to fit that specification: refractometry.

Refractometry, has been used for many years in industrial petro-chemistry. Precise correlations has been developed relating oil gravity and refractive index for various oil base types and refined products. Where the base type of a crude oil is unknown, a simple empirical, generalized relationship can be used with acceptable accuracy (see Figure 14). In the past, refractometers have rarely been used at the well site because of the high price and delicacy of the traditional Abbé refractometer.


It is unfortunate that the introduction of any new or enhanced mud logging service is, more often than not, accompanied by overblownIt is unfortunate that the introduction of any new or enhanced mud logging service is, more often than not, accompanied by overblown claims. It never seems to be enough to demonstrate an improvement in the real time mud log data. Claims to provide data comparableclaims. It never seems to be enough to demonstrate an improvement in the real time mud log data. Claims to provide data comparable with, or better than that from wire-line logs, core analysis, or well tests always seem to be piled on. with, or better than that from wire-line logs, core analysis, or well tests always seem to be piled on.

In my opinion, this over-claiming is stimulated by the mud logging contractors failure to sell (and their customers to appreciate) the real-In my opinion, this over-claiming is stimulated by the mud logging contractors failure to sell (and their customers to appreciate) the real-time importance of mud log data (see time importance of mud log data (see Chapter 1Chapter 1). Instead of marketing timely data, that allows improved and accelerated real-time). Instead of marketing timely data, that allows improved and accelerated real-time decision making, the mud logging contractors try to promise data of a quality surpassing that available later from other services. Sincedecision making, the mud logging contractors try to promise data of a quality surpassing that available later from other services. Since these other services will continue to be routinely performed, and so these over-ambitious claims cannot offer any cost benefit. No matterthese other services will continue to be routinely performed, and so these over-ambitious claims cannot offer any cost benefit. No matter what the salesman has to offer, it is ludicrous to expect the some new mud logging technique will result in running less wire-line logs,what the salesman has to offer, it is ludicrous to expect the some new mud logging technique will result in running less wire-line logs, cutting less cores, or testing fewer intervals. The explorationist cares about having the most data at the end of the well, no matter how orcutting less cores, or testing fewer intervals. The explorationist cares about having the most data at the end of the well, no matter how or when it was gathered.when it was gathered.

Drilling foremen and engineers, on the other hand, are in the business of “makin' hole”. They want all the information then can get, asDrilling foremen and engineers, on the other hand, are in the business of “makin' hole”. They want all the information then can get, as soon as they can get it. It is surprising that when, during the soon as they can get it. It is surprising that when, during the 70s and 80s, there was major growth in the cost and complexity of mud70s and 80s, there was major growth in the cost and complexity of mud logging services, it mostly came from the addition of drilling data acquisition, optimization, and pressure evaluation services. Commonlylogging services, it mostly came from the addition of drilling data acquisition, optimization, and pressure evaluation services. Commonly it seemed that the Drilling Department was better equipped than the Exploration Department to make use of real-time data, both init seemed that the Drilling Department was better equipped than the Exploration Department to make use of real-time data, both in time saved, and in decisions made sooner which, for them, also meant better.time saved, and in decisions made sooner which, for them, also meant better.


Figure 13: This Atago hand refractometer is made in Japan and is (not surprisingly) compact, reliable and reasonably priced. Need I say more? This particular one I donated to the Stanford University Petroleum Engineering Department

(OK, it's not a whole building, but then I'm not Bill Gates!)

A portable refractometer, similar to the Atago device shown in Figure 13, provides a simple, speedy alternative for measuring refractive index. An added advantage of the technique is that it requires only a small volume of oil. A sample of well cuttings can provide sufficient oil to cover the small optical cell in the refractometer (see Kinghorn, 1983). Alternatively, you can use the smear of oil left behind in the cut dish after the solvent cut test, and before complete evaporation of the solvent.

A hand refractometer is, obviously less accurate than the laboratory Abbé refractometer but it is simpler, easier to operate and requires no more sample than can be squeezed from a few grams of crushed cuttings. By using only cuttings samples, it is possible, with minimal effort to avoid the worst of oil-mud contamination problems and detect any true crude oil, if present, by its difference in refractive index and gravity from that of the refined oil-base and its contaminants.



Figure 14: Refractometry was developed for use in petro-chemistry and food technology to determine oil purity from a measurement of Refractive Index. This empirical relationship between Refractive Index and API Gravity of crude oil

gives reasonable accuracy even when the oil base type is unknown.



Well-site Geo-chemistryMud logging has always been a kind of well-site geo-chemistry. In recent years, improved instrument construction technology has allowed more types of geo-chemistry to become part of mud logging. More could follow but, in practice there are limitations.

Firstly, the petroleum exploration industry defines geo-chemistry as the organic chemistry of petroleum precursors — it is the science of the source bed, not of the reservoir. The chemistry of petroleum itself is peripheral; minerals and pore waters are quite out of the picture. Usually, these fall within the realm of the petro-chemists and petro-physicists respectively. That’s okay, there’s plenty of problems to go around.

Next, there are practical and cost limitations to performing geo-chemistry at the well site. Much geo-chemical sample processing and analysis equipment is large, delicate, expensive and slow. It does not lend itself to operation in real-time operations and to the crowded, non-sterile conditions of the mud logging unit. Much of it cannot survive without skilled technicians to keep it in-tune, and analytical chemists to follow detailed sample preparation procedures. Finally, the data obtained from this kind of oilfield geo-chemistry is often required only from limited sections of the well and mostly for strategic, rather that real time decision making. The data may not be used until late in the exploration or development program. In most circumstances, it would be wastefully expensive to install this type of equipment at the rig site for an entire well, or every well.

On the other hand, the limitations of remote and international operations (for comparison, see Core Analysis in Chapter 7) may make it easier to have the equipment available at or near the well site, rather that to move the samples to the equipment on another continent. This does not necessarily mean that the geo-chemical analyzers are literally installed in the mud logging unit. On many large, offshore, arctic and jungle camp locations, comparatively fully equipped geo-chemistry laboratories have been installed, complete with the most delicate instrumentation and the specialized personnel to maintain it. Where this is not possible, some geo-chemical or mud logging contractors can provide a field geo-chemistry unit for location at remote shore base, or operations centers.

These field geo-chemistry laboratories are self-contained and portable, like a mud logging unit, although Rafael Gurvis, one of the pioneers of field geo-chemistry, always referred to them as not portable but transportable (see Gurvis, Whittaker, & Aremburu, 1982). By this, he meant that the portable buildings could be moved from place to place but that they required considerably more effort in setting-up, calibrating, and standardizing of the equipment than a regular mud logging unit would need.

An alternative to this are locally established laboratories of major geo-chemistry contractors, local companies and joint ventures. These are often better equipped though they may be less well staffed and standardized than the field laboratories operated by a well organized multi-national service company. Finally there are the laboratories operated by the major geo-chemical service companies and by oil companies


There may be truth in this, but it always brought into my mind an image of a slick salesmanThere may be truth in this, but it always brought into my mind an image of a slick salesman who hyped his product by explaining that it was not a who hyped his product by explaining that it was not a por tab lepor tab le house trailer, but a house trailer, but a transpor tab letranspor tab le mobile home. mobile home.


themselves in oilfield centers, like the Los Angeles, London, Paris, San Francisco, Calgary, Zhanjiang, Houston and so on. These are obviously the ideal location for analyses to be performed, with standardization of procedures and personnel. Unfortunately, as we’ve seen, it is not always possible to get the samples to such a location in a time and condition acceptable for analysis.

In the following discussion, I will limit myself to those analyses which have been offered and provided as well-site services in mud logging units on exploration wells. You must decide which of them fulfill the cost, practicality and timeliness parameters for your own operation.

Again, with improved solid state components and processor power, this list will continue to grow and change, both by improved and low-priced instrumentation, and more productive utilization of geo-chemical data while drilling.

Figure 15: The LECO® CR-12 Total Organic Carbon (TOC) analyzer provides the basic geo-chemical measurement used to indicate source bed quality and against which to normalize all other geo-chemical measurements. (Illustration

courtesy of EXLOG, Inc.)



Total Organic CarbonThe earliest and, you have my personal assurance, heaviest geo-chemical analyzer to make the move from laboratory to well site was the Total Organic Carbon (TOC) analyzer. There are many versions of this but, in the oilfield, the LECO® CR-12 (see Figure 15) is probably the standard instrument.

Sample preparation for TOC analysis:is very time and labor intensive, so it is usual to process a large number of samples and analyze them as a batch. If the analysis is being performed in a mud logging unit, this is best done during trips or other down time when the mud logging work load, and other unit activity is reduced.

Sample preparation is as follows:

✔ Approximately 20 grams (about ¾ ounce) of sample must be selected from 170-mesh sieved cuttings carefully inspected for uniformity and the absence of mud contaminants and organic debris, particularly rubber and plastic.

✔ The cuttings are air dried for about two hours and then ground in a mortar and pestle, slowly and carefully, to avoid frictional heating.

✔ After grinding, half the sample is set aside for re-testing and other analyses (see below).

✔ From the other half of the sample, one gram of sample is weighed and then digested with warm, dilute (10%) Hydrochloric acid until all carbonate material has fully reacted and Carbon Dioxide evolution is complete (see Waples, 1985).

✔ The sample is then re-washed, air dried at low temperature and carefully transferred into an unglazed ceramic crucible.

Analysis itself is much simpler and quicker than sample preparation.

✔ The sample is enclosed in a induction furnace and swept with a supply of pure oxygen.

✔ As temperature increases, complete combustion of all remaining free carbon and carbon compounds in the sample produces Carbon Dioxide which is detected by a thermal conductivity detector (TCD).

✔ Using pre-mixed standards of known carbon content, the detector can be calibrated in terms of percentage carbon content by weight.

With all carbonate material content removed from the sample, the TOC analyzer will reliably indicate only the organic carbon content of the rock. TOC is a useful basic geo-chemical tool in obtaining a quantitative estimate of the organic-richness and the petroleum producing capability of clay or lime mudstone source beds. Figure 16 is a table illustrating the general relationship between source bed quality and TOC for shale and carbonate source beds.



Total organic carbon percentage (by-weight): Source bed quality

Claystones & Shales Carbonates

Less than 0.10 Less than 0.01 Insignificant

0.1 to 0.25 0.01 to 0.10 Poor

0.25 to 0.50 0.10 to 0.25 Fair

0.50 to 1.00 0.25 to 0.50 Moderate

1.00 to 2.00 0.50 to 1.00 Good

Greater than 2.00 Greater than 1.00 Excellent

Figure 16: Total Organic Carbon content and source bed quality for argillites (claystones and shales) and carbonate rocks (limestones, dolomites and calcareous mudstones).

Unfortunately, these values are not entirely decisive because Total Organic Carbon alone is not capable of discriminating between:

✔ Fresh organic debris,

✔ Kerogen of various types, or diagenetic levels,

✔ Hydrocarbon oils, tars, or

✔ Coal, or metamorphic graphite

Gases and the lighter, more volatile oils, of course, are not a factor, being lost from the sample during the grinding and air drying stages of preparation. So TOC alone has little diagnostic value as a real-time tool operating at the well site while drilling. In the laboratory, TOC data is combined with other analyses so that the amount and type of organic material can be determined. They are plotted together on a separate geo-chemical log.



At the well site, in the absence of other geo-chemical analyses, mud log hydrocarbon and lithology data may be added to TOC data to produce a well-site source bed evaluation log (see Figure 17).

Figure 17: Total Organic Carbon provides a reliable quantitative estimate of the richness of organically-derived carbon. Unfortunately it cannot indicate the type or present diagenetic state of the organic material without additional geo-

chemical or mud logging data.

Using this combination of data, TOC becomes a useful part of the mud log suite, enhancing formation evaluation and improving the qualitative and quantitative interpretation of both source beds and reservoirs. To put it another way, even though the geo-chemical analyses are being performed for other audiences and on other budgets, they still can be used to yield useful information for the explorationists



working at the well site.

Pyro-analysisIn the study of hydrocarbon detection we have looked at both normal and catalytic combustion. Despite occurring at different gas-air compositions and rates, both of these involve the thermal decomposition of a hydrogen and carbon-containing compounds in air, and the combination of the products of decomposition with of oxygen to form Carbon Dioxide and water. They are both thermal, oxidation processes. The LECO CR-12 total organic carbon analyzer also operates by a thermal oxidation process.

Pyrolysis is a very different form of thermal decomposition. In pyrolysis, a compound is heated in an enclosed space, to a temperature usually much higher than that involved in combustion. The enclosed space contains no oxygen, so that thermal decomposition is not accompanied by oxidation. The hydrocarbon compound is simply broken down, or cracked, into simpler and lighter carbon and hydrogen containing fragments. The only oxygen involved is that originally contained in the source material, and liberated by the cracking process.

At less extreme temperatures, and over much. Much longer periods of time, this is very much the same process by which organic debris in sediments matures to kerogen and, eventually, to oil and gas.

A Pyro-analyzer provides sufficient controlled heating to emulate those maturation or cracking processes, and then the fragmentary products of pyrolysis are swept from the chamber by a stream of an un-reactive, inert gas to various types of gas detectors.

For several years, accelerated pyrolysis, at higher temperatures has been a popular laboratory technique for the study of petroleum maturation processes, and their intermediates and product yields (see Barker, 1974).

In the late 1970s, work performed at the Institut Francais du Petrolé (IFP, the French Petroleum Institute) and the PETROFINA research center in Belgium led to the development of an analytical instrument that used pyrolysis to make measurements indicative of source rock quantity and quality in a single analysis (see Espitalie et al, 1977).

Various versions of the IFP-PETROFINA device known as the Rock-Eval® were manufactured in France, and the USA by Geocom (also known as Geomechanique) and DELSI (also known as Girdel, and Delsi-Nermag) The latest version (that I know of) is the Rock-Eval 6, manufactured by Vinci Technologies in France.

The Rock-Eval pyro-analyzer works by heating a formation sample in an inert helium atmosphere.

As temperature rises:

✔ First, free hydrocarbons are liberated first from the sample, then

✔ Then, source material is cracked to liberate free Hydrogen which rapidly combines with Carbon and produces light alkanes in amounts indicative of the hydrogen richness of the source rock,



✔ Any oxygen within the source rock combines with carbon and is liberated as Carbon Dioxide, and

✔ Finally, after all of the Hydrogen and Oxygen have combined with carbon, and been removed as alkanes and Carbon Dioxide, any surplus carbon in the source material remains as an elemental carbon residue.

Figure 18: The Rock-Eval pyro-analyzer heats a cuttings sample in a helium atmosphere, detects free hydrocarbons, kerogen abundance, type and maturity.



The relative amounts of gases produced, and the temperature of peak evolution are compared with a database of results from a large number of standard rock samples prepared by the IFP. From this comparison, it is possible to make semi-quantitative deductions about the organic richness and maturity of source rocks.

The original Rock-Eval does not detect the total carbon in the sample, only that which combines with hydrogen or oxygen to escape as gases. It is very convenient to run both TOC and Rock-Eval devices together and process sample batches in parallel to give measures of the total Carbon, Hydrogen and Oxygen content and, as we'll see, the diagnostically significant ratios of those elements.

Sample preparation for the Rock-Eval is similar to that used for TOC analysis except that acid washing is not necessary because the Rock-Eval does not heat the sample to high enough temperatures for carbonate-cracking to occur.

Although the pyrolytic heating and analysis program is more complex that the TOC analysis, it is controlled in the Rock-Eval instrument by a microprocessor controller for each of the stages of heating and gas analysis. The device can even be loaded with several pre-weighed samples and left to run unattended for up to eight hours.

The basic Rock-Eval program works like this:

✔ The sample is loaded into the Rock-Eval oven, and it is first heated up to a constant oven temperature of 300°C as the system is purged with inert helium.

✔ The helium flow is maintained as oven temperature is programmed to rise at 25°C per minute, up to a maximum temperature of 550°C (see Figure 19).

During this heating program:

✔ Free hydrocarbons in the sample (mainly oil with a little remaining gas) is driven off, carried along with the helium flow, and detected by the FID (flame ionization) detector (see Step 1 in Figure 18).

This is displayed as the S1 Peak (in Figure 19).

✔ At higher temperatures, kerogen in the sample is cracked to yield more hydrocarbons which is also detected at the flame ionization detector (FID) (see Step 2 in Figure 18).


✔ Carbon Dioxide, derived from the cracking of oxygen-rich kerogen is trapped in a chemical trap (see Step 3 in Figure 18) to be released later by heating and detected at a thermal conductivity detector (see Step 4 in Figure 18).




Based upon calibration with IFP-prepared and certified organic standards and pure compounds, the gas peaks are directly reported as milligrams per gram of rock.

Figure 19: Results from the Rock-Eval pyro-analyzer indicate the presence of free hydrocarbons, and the abundance, type and maturity of kerogen from the parameters S1, S2, S3 and Tmax.



✔ With increasing kerogen maturity, the Rock-Eval needs to add more and more heat to the sample before it begins to crack and liberate the S2 hydrocarbons. The oven temperature in degrees Centigrade, Tmax , at which the maximum of the S2 Peak occurs (see Step 5 in Figure 18) is recorded and reported as a measure of kerogen maturity.

This is displayed as Tmax (in Figure 19).

The S1 peak has similar significance to the cuttings blender test, reflecting the free hydrocarbons retained in the cuttings. It is more reproducible, however, because it uses a precisely weighed and uniformly processed sample. Because of the time delay in storage and processing involved, the S1 result is unlikely to include any of the very lightest hydrocarbon gases.

The S1 peak increases with the total amount of the kerogen in the rock but it also reflects the maturity and hydrocarbon yield of the kerogen.

From the S1 and S2 peaks together, a hydrocarbon production index, expressed as a fraction of the total hydrocarbon productivity of the sample, may also be calculated. This is a measure of how much hydrocarbon has already been generated (S1), compared to the total amount of hydrocarbons that could be generated (S1 and S2) from the organic content of the rock:

..................... Equation 1

✔ Immature source material will have a very low value of production index, because hydrocarbon generation has not yet begun (S1 is very low).

✔ With increasing maturity, the production index increases (as S1 gets higher), but

✔ It may decline again in post-mature source beds where the produced oil and gas have long ago all been generated. Most of them will have already migrated away, and are no more are being replaced with newly generated hydrocarbons.

✔ The very highest values of production index, perhaps as high as 1.00 (because S2 is zero) occur in reservoirs that have little or no source material but a large accumulation of free hydrocarbons from other sources.



The S2 peak alone can provide a guide to the total hydrocarbon yield to be expected from the source bed (see Figure 20). However, this figure combines both the amount of organic matter in the rock and the productivity of that matter.

S2 Peak

(mg /g of Organic Carbon)Source Bed Quality

Less than 200 Gas Source

200 to 300 Oil & Gas Source

Greater than 300 Oil Source

Figure 20: The S2 peak alone is a measure of potential hydrocarbon yield of the whole rock.

Alternatively, if TOC data are also available, a true Hydrogen Index may be calculated, and this is independent of the amount of organic material in the rock, responding only to the kerogen type and richness.

..................... Equation 2

A similar Oxygen Index may be calculated from the S3 peak:

..................... Equation 3



Figure 21 shows the relationship between these quantities, source bed quality and kerogen type.

Hydrogen IndexS2/TOC

(mg/g of Organic Carbon)

Oxygen IndexS3/TOC

(mg/g of Organic Carbon)

Source Bed Type

Less than 200 Greater than 100 Gas Source

200 to 300 50 to 100 Oil & Gas Source

Greater than 300 Less than 50 Oil Source

Figure 21: The S2 and S3 peaks together with the TOC value are a measure of kerogen type and maturity

Peaks S2 and S3 are indicative of the amount of kerogen present in the rock and its hydrogen and oxygen content respectively. The relative richness of hydrogen and oxygen in the kerogen indicates the type and yield of hydrocarbons to be expected from it (see Figure 21), and can be illustrated in a Van Krevelen diagram (see Figure 22).

The ratio of S2 to S3 is a measure of the slope of the lines representing kerogen types in the Van Krevelen diagram and can give a general indication of the kerogen type and likely hydrocarbon production (see Figure 23).



Figure 22: The Van Krevelen diagram relates Hydrogen Index (S2/TOC) and Oxygen Index (S3/TOC) with source type and maturity



Figure 23: The relative richness of Hydrogen (reflected in the S2 peak) and Oxygen (reflected in the S3 peak) in the kerogen is diagnostic of the type and yield of hydrocarbons to be expected from it



Because of the difference in time scale, the oven temperature at maximum hydrocarbon evolution, Tmax , has no meaningful relationship to the actual formation maturation temperature. However, by correlation with the IFP database of standard samples, an equivalent Tmax Oil Window can be defined (see Figure 24).

Tmax (oC) Productivity “Window”

Less than 435 Gas Source

435 to 470 Oil & Gas Source

470 to 500 Oil Window

Greater than 500 Post-mature

Figure 24: From correlation with the IFP database, Tmax can be used to indicate maturity of the source bed

The Rock-Eval data, with TOC and associated mud log data are plotted on the special well-site geo-chemical log format (see Figure 25).

The process of thermal decomposition is not a series of predictable, stoichiometric chemical reactions. The analysis is never completely reproducible and, in fact, calibration is performed by adjusting instrument parameters until correct results are obtained matching the IFP standard source rock samples. Two or more calibration standard samples are included with every batch of drilled samples and, if two machines are being used to process the samples (one at the well site, and another at a base laboratory) inter-calibration samples must also be run on a daily basis (see Clemenz, Demaison, and Daly, 1979).

Some objections have been lodged against the validity and reproducibility of Rock-Eval for a number of reasons, such as:

✔ Thermal decomposition is catalyzed by clay minerals in the rock matrix. An abundance of high CEC clays will enhance reaction in the Rock-Eval oven, increasing hydrocarbon yield, or achieving peak hydrocarbon evolution at a lower Tmax (see Figure 26).



✔ Some less common carbonates decompose and yield Carbon Dioxide below 550°C. Presence of these carbonates in trace quantities with calcite and dolomite may initiate even these more stable carbonates to partially decompose. The evolution of Carbon Dioxide from carbonates will cause a broadened or falsely high S3 peak, and a pessimistic Oxygen Index, S3 /TOC (see Figure 27).

✔ Some bitumens, tars and heavy petroleum residues, may have a low volatility and require higher temperatures and longer cracking before they to are evolved and detected. Instead of forming part of the S1 peak, they will merge with the kerogen-derived hydrocarbon, producing a broadened S1 or S2 peak. Alternatively this may also result in a mis-identified S2 peak and pessimistic Hydrogen Index, S2 /TOC, and kerogen indication, and perhaps even a falsely low Tmax maturity indication (see Figure 28).

Figure 25: The well-site geo-chemistry log displays Rock Eval® pyro-Analysis, TOC or associated mud log and geo-chemical data and may be used to monitor source rock and reservoir occurrence, type and maturity.



Figure 26: The Rock-Eval source bed evaluation may be modified by the presence of other non-source materials, such as catalytic,

high CEC smectite clays that can enhance reaction in the Rock-Eval oven, increasing

hydrocarbon yield, or achieving peak hydrocarbon evolution at a lower Tmax.


The VR plotted in the same track as TThe VR plotted in the same track as Tmaxmax in in Figure 25Figure 25 refers to Vitrinite Reflectance. This method, first developed in coal exploration, has refers to Vitrinite Reflectance. This method, first developed in coal exploration, has become the most important method for determining source bed maturity and history, by its excellent correlation with the maximumbecome the most important method for determining source bed maturity and history, by its excellent correlation with the maximum formation temperature experienced by kerogen. Vitrinite Reflectance measurements require the use of a photometer-equippedformation temperature experienced by kerogen. Vitrinite Reflectance measurements require the use of a photometer-equipped petrographic microscope, and is not well-suited to well-site operations (particularly dusty, and vibrating environments – like mud loggingpetrographic microscope, and is not well-suited to well-site operations (particularly dusty, and vibrating environments – like mud logging units!) units!)

The TThe Tmaxmax oil window of 435-to-500 oil window of 435-to-500 ooC correlates well with Vitrinite Reflectance values of 0.5-to-1.1%C correlates well with Vitrinite Reflectance values of 0.5-to-1.1%. .


Figure 27: The Rock-Eval source bed evaluation may be modified by the presence of other non-source

materials, such as unstable Carbonates. These may initiate decomposition of stable carbonates at lower temperatures, causing a falsely higher and wider S3

peak, and a pessimistic Oxygen Index

Figure 28: The Rock-Eval source bed evaluation may be modified by the presence of other non-source materials, such

as dense, involatile bitumens resulting in widened, and partially merged S1 and S2 peaks.



These factors, could cause serious concern if Rock-Eval results were to be taken alone as having absolute meaning in formation evaluation. In reality, like all mud log hydrocarbon results, Rock-Eval data has only comparative value. Single values can never be used alone. The trend of data through a whole section is used in combination with all other measurements and observations, to make well-to-well or zone-to-zone correlations. Used in this manner, possible errors can be acceptable (though sometimes troublesome) in a well-site screening tool.

There are other, more practical objections to using the Rock-Eval and TOC analyzer at the well site. The machines are very large, very expensive and produce great amounts of waste heat. The complicated sample preparation procedure is time consuming and more suitable for large batch processing than for the sample-by-sample, real-time schedule of mud logging.

Several alternative devices have been introduced that attempt to simplify or reduce the cost of well-site geo-chemistry. These include:

✔ Oil Show Analyzer® (OSA), or Rock-Eval III from DELSI (Figure 31),

✔ Thermolytic Hydrocarbon Analyzer® (THA) developed by LECO with Exploration Logging (EXLOG) (Figure 32),

✔ Pyrologger® from Geoservice (Figure 33), and

✔ Rock-Eval 6 from Vinci Technologies (Figure 34).

The differences between these devices and the original TOC (Figure 29) and Rock-Eval (Figure 30) are shown below.

Figure 29:The LECO CR-12 measures only total organic carbon (TOC) in the source rock.



Figure 30: The Rock-Eval I and II differ only in electronics and processor design. The measurements are essentially the same: S1, S2, S3 peaks and temperature Tmax.

Figure 31: The Oil Show Analyzer, or Rock-Eval III, measures Carbon Dioxide for total organic carbon (TOC) content, and not for kerogen Oxygen Index.



Figure 32: The Thermolytic Hydrocarbon Analyzer is a simplified pyro-analyzer, indicative of what may become the next generation mud logging analyzer.

Figure 33: The Pyrologger is a another candidate to be the next generation mud logging analyzer.



The newer devices have numerous benefits and drawbacks. However, in summary:

✔ The Oil Show Analyzer (and the Rock-Eval III, see Figure 31) attempt to combine the functions of the Rock-Eval and Total Organic Carbon analyzer, but in doing so they pay the price of losing the original’s S3 Oxygen Index peak.

The OSA also has a lower temperature stage to the temperature program allowing the S1, free hydrocarbon, peak to be subdivided into an S0, free gas peak and an S1’, free oil peak.

Unfortunately, this bonus is probably lost because the time-consuming sample grinding, acidification (necessary to eliminate carbonate minerals contamination of the TOC peak), and air drying steps. It is likely that, during these processes, evaporative loss of free gas from the sample will be completed long before the it reaches the pyrolysis oven.

✔ The Thermolytic Hydrocarbon Analyzer (THA, see Figure 32) and Pyrologger (see Figure 33) are simpler devices that attempt to reduce the cost and complexity of the devices while speeding up the analysis. Neither device is equipped for automatic sample handling and part of the simplification for both includes an easier, quicker and partly automated sample processing, using a fixed volume (rather than mass) of pressure dried (rather than air dried) sample.

Like the OSA, the THA adds a lower temperature stage to the temperature program producing S0, free gas, peak and S1’, free oil, peaks. Unlike the OSA, the less extreme and quicker sample processing allow these peaks to be more valid, and useful in source and reservoir evaluation.

✔ The Rock-Eval 6 (see Behar, 2001, and Figure 34) apparatus is the latest version of the Rock-Eval product line, developed in 1996 by Vinci Technologies. It is more complex, automated, and extensively programmable device best suited to a standalone field laboratory than a mud logging unit.

By varying it’s program, the Rock-Eval 6 can determine any of the parameters available to all of the other pyro-analysis devices, and more.

Sample processing and quality is improved because acidification is not required. The Rock-Eval 6 temperature programming allows Carbon Dioxide and Carbon Monoxide from the pyrolysis of organic and mineral sources to be separated and independently analyzed using an infrared (IR) gas detector. It can also measure Carbon Dioxide and monoxide created by oxidation in the high temperature (TOC) phase of the analysis. The Rock-Eval output parameters are shown in Figure 34.

Remembering the non-stoichiometric nature of sample pyrolysis, it is not surprising that analytical results from any of these pyro-analyzer are rarely in exact agreement with each other. As mentioned before, if two or more devices (of the same or different types) are being used, then inter-calibration samples must run regularly and machines adjusted to give compatible results.



Parameter Unit Source

S1 mg HC/g rock Free hydrocarbons in helium gas stream, measured at FID (flame ionization) detector

S2 mg HC/g rock Hydrocarbons from pyrolysis of organic source material in helium gas stream, measured at FID (flame ionization) detector

TpS2 °C Measured sample temperature at maximum of S2 peak.

S3 mg CO2/g rock Carbon Dioxide from pyrolysis of organic source material in helium gas stream, measured at IR (infra-red) detector

S3' mg CO2/g rock Carbon Dioxide from decomposition of carbonate minerals in helium gas

stream, measured at IR (infra-red) detector

TpS3’ °C Measured sample temperature at maximum of S3‘ peak.

S3CO mg CO/g rock Carbon Monoxide from pyrolysis of organic source material in helium gas stream, measured at IR (infra-red) detector

TpS3CO °C Measured sample temperature at maximum of S3CO peak.

S3’CO mg CO/g rockTotal Carbon Monoxide from pyrolysis of both organic source material and decomposition of carbonate minerals in helium gas stream, measured at IR (infra-red) detector

Figure 34: Measured and computed parameters available from the programmable Rock-Eval 6.




S4CO2 mg CO2/g rock Carbon Dioxide from oxidation of organic source material in oxygen gas stream, measured at IR (infra-red) detector

S5 mg CO2/g rock Carbon Dioxide from decomposition of carbonate minerals material in oxygen gas stream, measured at IR (infra-red) detector

TpS5 °C Measured sample temperature at maximum of S5 peak.

S4CO mg CO/g rock Carbon Monoxide from oxidation of organic source material in helium gas stream, measured at IR (infra-red) detector

Tmax °C Computed from TpS2

Production Index (PI)

Pyrolyzable Organic Carbon (PC)

% by weight Computed from S1, S2, S3, S3CO, and S3’CO:

Residual Organic Carbon (RC)

% by weight Computed from S4CO and S4CO2

Total OrganicCarbon (TOC)

% by weight Computed from PC and RC

Figure 34 (continued): Measured and computed parameters available from the programmable Rock-Eval 6.




Hydrogen Index (HI) mg HC/g TOC

Oxygen Index (OI) mg CO2/g TOC

Mineral Carbon (MinC)

% by weight Computed from S3’, S3’CO and S5

Figure 34 (continued): Measured and computed parameters available from the programmable Rock-Eval 6.

Pyro-analyzer technology continues to evolve. As yet there is no consensus on the optimum group of capabilities and acceptable limitations. However, it appears likely that further developments will continue the parallel trends toward more capable laboratory instruments, like the Rock-Eval 6, and simpler, more rugged well-site instruments, like the Pyrologger. Figure 25 demonstrates that pyro-analysis can develop data and information that is compatible with mud logging and enhances the standard measurements in a manner that can truly be called advanced or next generation mud logging. I confidently predict that some form of pyro-analysis will become a standard feature of mud logging.



Temperature Programmed ChromatographyMore sophisticated laboratory chromatographs also use temperature programming. Temperature-programming extends the range of well-site chromatography, allowing the volatilization and separation of many higher alkanes and more complex hydrocarbons. The sample port and columns are heated through a controlled range of temperature during analysis, This is not intended to crack or decompose hydrocarbons but to increase the volatility of heavier components, and improve chromatographic separation of existing lighter components.

Figure 35: Temperature-programmed chromatography can provided extended hydrocarbon analyses allowing recognition of reservoir character and productivity, for example, petroleum condensate or wet gas.

With improved reliability and reduced cost of technology, this type of chromatograph is becoming available for use at the well site. It is now possible to extract oil, gas and bitumens from cuttings samples and to obtain a chromatogram representing the entire fluid composition. As mentioned earlier in this chapter, with more components in the chromatogram it becomes easier to identify fluids from different reservoirs, source beds and oil-mud contaminants (see Figures 35, 36, 37 and 38).



Figure 36: Temperature-programmed chromatography of a high gravity (light and volatile) oil.

Figure 37: Temperature-programmed chromatography of a low gravity (dense and heavy) oil.

Figure 38: Temperature-programmed chromatography of a very dense, immovable residual oil. residual oil



On the other hand, despite improvements:

✔ These temperature-programmed chromatographs are still large, expensive and delicate machines, and

✔ They need skilled and attentive operators, and a good deal of time to prepare and run the specialized samples required.

✔ The output of the devices, although unquestionably informative, is also complex and requires careful and knowledgeable interpretation.

✔ Finally, the data produced from these machines will probably be required only through limited sections of a well or on limited wells of an exploration and development program.

All-in-all, there are few strong arguments for putting this type of equipment at the well site. Only logistic or political difficulties would justify the expense and personnel necessary to dedicate this type of equipment to a single rig.

Pyro-chromatographyA further development in gas analysis has been the coupling of a simple pyrolysis device to a chromatograph. Pyro-gas-chromatography combines a simple pyro-analyzer with a chromatograph.

✔ Free hydrocarbon components that comprise the S1 peak can be separated and displayed as a chromatogram much like that seen in temperature programmed chromatography (see Figure 39).

✔ Kerogen is thermally cracked and the gases from the S2 peak can be separated and displayed in the same way. Because the components in the S2 peak did not exist in the sample formation fluids, but are newly created in the pyro-analyzer, this is referred to as a fragmentogram (see Figure 40).

Examples of pyrolysis-gas-chromatography are shown in Figure 39 and Figure 40. There is, again, no doubt that these data are informative. However, these examples may be misleadingly simple. In practice, the results may be of little diagnostic value at the well site. I doubt that this is the type of data that is required in real-time or that can be reliably interpreted in the time or with the personal skills available while drilling. These are specialized tools more suited to the research laboratory. Before they find a place at the well site, we will need to find better and simpler means of understanding and interpreting them.



Figure 39: A pyro-chromatogram of the S1 peak from a pyrolized source bed sample.

Figure 40: A fragmentogram of the S2 peak from a pyrolized source bed sample.



Worst Case ScenarioOil-based drilling fluids have been used on-and-off throughout the history of rotary drilling. They have always been unpleasant to work with. and the cause of serious difficulties in formation evaluation. Unfortunately (for the geologist) they have always been able to solve difficult and expensive drilling problems caused by chemical reaction between a water-based drilling fluid and exposed formations.

In the late sixties and seventies, we thought that the problem had been solved for us. Increases in exploration offshore coupled with greater environmental concern and regulation almost managed to put and end to oil-based muds. In order to use the muds offshore, and even in some onshore locations, required complicated and expensive clean-up efforts to prevent oil-coated cuttings from polluting the area around the rig. The cost of these activities overwhelmed the savings achieved by using the oil-based mud. Oil-based muds became rare. In many cases, water-based mud systems and additives were developed to cope with the same problems although, admittedly, not quite so well and usually more expensively.

Since the nineties, there was been a resurgence in the use of oil-based muds with the development of the so-called environmentally safe oil-based mud materials. Usually based upon mineral oils (also known as white oil, liquid paraffin, or de-aromatized kerosene), and modified vegetable, or fish oils, these may contain pure, high molecular weight alkanes or poly-alicyclic compounds. They are low in volatiles and aromatics, and are claimed to be harmless to the environment and inert in formation evaluation.

As we have discussed in previous chapters, these muds have proven to be as much of a problem for mud log interpretation as the older toxic oil muds. There have been many arguments regarding the possibility of cracking of the oils by down-hole temperature and clay catalysis to form lighter, more volatile alkanes. It does appear that it is most often in areas of higher geothermal gradient that use of mineral oil muds is associated with anomalously high total hydrocarbons backgrounds.

However, the real problem with the muds is their affinity for dissolving formation hydrocarbons and their ability to carry high backgrounds of oil and gas from disparate, multiple origins. These backgrounds can be carried over long periods through a well or even from well to well. In some cases, mud logging formation evaluation becomes almost meaningless. If a well is spudded with a drilling fluid system already containing all of the oils and gases anticipated for the well, log interpretation becomes, at best, dubious, regardless of the quality of equipment and personnel.

Throughout this period the mud chemical manufacturers continued to congratulate themselves on the perfection of their products while the mud logging contractors were bombarded with demands for a solution to the problem. There has been experimentation with several techniques attempting to find means of identifying an oil or gas show in the presence of this massive and broad-based contamination. Some achieved partial success but at unacceptably high cost. None of them have had any more degree of success than the good logging procedure and logical data evaluation discussed in previous chapters.



If things were not bad enough, a significant stimulant to the resurgence of oil-based mud has been the increased incidence of very high angle drilling. In these steeply angled, and even horizontal wells, the problems associated with drill string friction and sticking in the bore hole are increased by orders of magnitude. The much greater lubricity of oil-based drilling fluids is a great benefit in reducing drill string wear, damage and sticking in the bore hole. This good news for the driller translates into the worst possible news for the mud logger. Recognition of formation tops from drilling breaks, cutting lithology and gas shows all become more difficult when drilling horizontally, or at a high angle through the section:

✔ ROP becomes less responsive to changes in formation drillability when much of the drill-string weight is resting on stabilizers and rotary reamers on sections of the bore-hole wall.

✔ Understanding the mixing of cuttings and gas in the drilling fluid’s laminar or turbulent flow becomes even more difficult when for much of the bore hole, mud flow is horizontal so that annular velocity profiles and cuttings settling forces are acting vertically across the bore hole.

✔ Increased friction of the drill collars, stabilizers and rotary reamers adds to increased damage to the bore-hole wall and contamination of the mud stream with cavings and other debris. All this material is subject to frictional grinding between the drill string and bore-hole wall making the cuttings more difficult to use, and cavings are ground smaller, and less easy to discriminate from cuttings.

✔ When these problems are compounded by the use of an oil-based drilling fluid carrying a high and regularly recycling background of oil a gas, you have the worst possible case for mud logging.

It is unlikely that preventing mud logging difficulties is going to be a major consideration in the future use of either horizontal drilling or oil-based drilling fluids. So, it is up to us to find better ways of doing mud logging in those circumstances. In this chapter we have looked it some tools, such as fluorimetry, refractometry and pyro-analysis, that can make a small, though possibly important improvement to routine formation evaluation. Perhaps, one or more of these techniques can provide the extra component that solves our problem with horizontal drilling and oil-based drilling fluids.



The Swan SongWhen I told people that I was writing this book, the standard reaction was:

“What are you going to say about oil-based mud and horizontal drilling?”. Well. what I have to say is this, there are two easy answers to formation evaluation in oil-based mud:

✔ Easy Answer #1: “Don’t use the stuff.”

✔ Easy Answer #2: “If you must, then plan on coring all potential pay zones.”

That may sound unrealistic and academic but it is also true. If the oil-based mud is saving so much money, then why not spend a little of it curing the disaster it wrecks on formation evaluation?

If you must add something to the mud logging unit, try the Atago refractometer. From practical experience, I would claim the refractometer to be at least as reliable as synchronous scanning fluorimetry in identifying oil shows. This is still not very good: about 40 to 60 percent. On the other hand it costs only about one hundredth the price of a fluorimeter!

And NextIn the next chapter, we are going to look at drilling monitoring sensors and systems that have proliferated in mud logging units over the last few years. The primary purpose of these systems is to improve drilling economy and safety, and the mud logger needs to know how to communicate the sensor measurements to the drillers and engineers who need them.

We may also learn a little bit about how these data can also provide useful information and interpretive assistance to the mud logger and geologist.


the on-line mud logging handbook alun whittaker the on-line...

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