MODERN LOGGING PROGRAMS AND INTERPRETATION METHODS

by

R. p. Alger

To be presented at theFormation Evaluation Symposium

November 21-22, 1960, University of Houston

MODERN LOGGING PROGRAMS AND INTERPRETATION METHODS

R. p. Alger

Recent logging developments have provided more dependable data for reservoirevaluation. These new methods, ~bich include both resistivity and porosity meas-uring devices, have nOw been adequately checked against older techniques, core

analysi~and production results. Their responses with respect to varied bore-ho~e and formation conditions are sufficiently understood to permit the properselection of legs for most efficient interpretation.

These devices, including the Induction Log (medium and deep), Laterolog,Proximity Log, Sonic Log, and Gamma-Gamma Density, have largely solved the prob-lems of thin-bed resolution and borehole effects. All are essentially focuseddevices and have improved and predictable response where flushing or invasionis a problem. Their uti1ity is such that any logging program today is likely toinclude one or several of them. When detailed reservoir information is required,where reservoir conditions are complex, and when wildcats or outposts are drilled,the best available logging combination should be used.

The first and most important use of logs is to identify zones for testing orfurther evaluation. Although the discussion to follow presupposes that logs willprovide complete information, it must be remembered that additional data, such assidewall cores, conventional cores, drill-stem and wire-line formation tests, andcuttings, will assist in the final analysis.

The objectives of this paper are twofold: first, to discuss the new logs,briefly indicating where each fits in the several families of logging devices,and, second, to outline essential logging combinations or programs by mud andformation type.

The basic requirements of any logging program should be:

1. Correlation - the program should include curves that may be reliablycorrelated with logs (sometimes old logs) from nearby wells. The SP,Short Normal, and Gamma Ray curves are normally used for this purpose.The Laterolog is often used in salt muds. The Sonic Log can giveunique correlations in carbonate reservoirs, evaporite sequences, orin lignite or coal. l

2. Depth control - the tops and bottoms of beds should be accuratelydefined. Pad devices, such as the Microlog and Microlaterolog, pin-point bed boundaries.

3. Net pay definition - the net thickness of the productive zones (thesand count) is reqUired for recovery computations. The oldest, and.still useful, method involves the SP. The modern combination of theMicrolog2 with a caliper gives maximum efficiency. Net pay may also

be determined by selecting a cut-off value for porosity logs (Sonic,Neutron, or Gamma-Gamma).

4. Location and evaluation of oil or gas shows - the first and mostimportant use of logs is to find zones worthy of further evaluationor testing. After such zones are identified a more quantitativeevaluation, involving both porosity and fluid saturation, can beeffected.

All of the above listed uses of logs, are important; however, this paperdeals primarily with the location and evaluation of oil or gas shows. For thispurpose certain fundamental formation properties must be provided by the loggingprogram. The program must provide values of true formation resistivity, poros-ity, and formation water resistivity.

True Resistivity Determination

The true formation resistivity is required for a quantitative evaluation ofhydrocarbon saturation. Determination of true resistivity is accomplished byone of three logging devices--the conventional Electrical Survey, the InductionLog, or the Laterolog.

The conventional Electrical Survey can give approaches to Rt measurementunder favorable conditions: when the mud resistivity is not much lower than Rt,when borehole size is known and is not very large, when the beds are thick andhave resistivities not sharply different from those of surrounding beds, andwhen invasion is not excessive. These conditions are so rarely encountered innormal practice that the use of focused devices is becoming essential wherereliable quantitative or qualitative interpretation is desired; as a result,there is a steady shift from conventional Electrical Logging to Induction Log-ging in all parts of the country.

Choice of the Induction Log for fresh mud cases and of the Laterolog forsalt mud cases practically resolves all problems due to the borehole and bedthickness. In regard to invasion, the performance of each tool is known for allinvasion ranges, so proper corrections can be applied. The extent of invasionmay be estimated on the basis of the rock type and porosity.4 More accuracy isgained by using a suitable combination of resistivity curves.5 Detailed studiesof these curves show that the invasion diameter can be quite variable, even inthe same zone; the variation in Di in sandstones seems to depend more on permea-bility than on porosity.

The Rt tool selection should be based on the levels of mud and formationresistivities. The Induction Log is always preferred when Rt is less than 5ohm-m. Conversely, the Laterplog is required when Rt exceeds 200 ohm-m. Themud resistivity must always be considered in selecting the appropriateresis-tivity device ,for intermediate ranges of Rt These values refer to what is tobe measured in the reservoir rocks, not in the surrounding impermeable zones.

Porosity Determination (Direct Measurement)Porosity information is essential for both qualitative and quantitative

evaluations. Porosity data are of course necessary for reservoir volume; they

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are equally important as a basis for saturation determinations. Sonic., Neutron,and Gamma-Gamma Density Logs are more or less directly related to porosity:Each has certain advantages and disadvantages.

Borehole

Size

Irregularity

Mud cake

Fluid type

No fluid

Lithology

Sonic

no effect

large at boundary

no effect

no effect

not used

Gamma-Gamma

very small effect

large effect

small (except)weighted mud)very small effect

small effect

Neutron

must be accuratelyknown

large effect

some effect

some effect

large effect

Clean rocks

Gypsum impurity

Shaliness

Oil or GasEffect

need Vm

small effect

large effect, butcorrectable(shaly sands)

need grain density usually no effect

large effect adversely affected

small effect very large effect

In low

In high

Max. RecordingSpeed

no effect(if invaded)

too high(correctable)

5000'/hr.

no effect(if invaded)small effect(correctable)

3000' /hr.

gas may affect

too low for gas

1800'/hr.

When rocks are composed of alternating beds of sandstone (or chert), lime-stone, dolomite, gypsum, or anhydrite, the problem of finding a correct porositycan become very difficult. Sometimes the Gamma Ray can distinguish sandstones,and thus will govern the Vm to be used on Sonic Logs. If matrix changes are nottoo complex, the use of two of the porosity devices can yielg improved porosityand also some lithologic information. Several cases are cited:

.

Sandstone-Limestone-Dolomite Beds. When Neutron deflection is plottedagainst A~ from a Sonic Log, the grouping of the points permits lithologic identi-fication. This is due to the characteristic variations in matrix velocity andpossibly some matrix effects on the Neutron-. For such a plot the Neutron read-ings should be normalized to a single hole size. Actual porosity values should

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be obtained from the Sonic Log, using established ~t-porosity relations for thelithology found. Illustrating this method is Figure 1, which is from theDevonian section of a well in Utah. Core descriptions verified the lithologicidentification obtained from the plot.

Dolomite-Anhydrite or Dolomite-Limestone Mixtures. Actually, the Neutronis theoretically sati~factory for these sequences. In some ca~es, better andmore detailed results are obtained when the Sonic and Gamma-Gamma Logs are com-pared. The difference ~n matrix effects is used to advantage. Figure 2 showsporosity logs from a portion of the Montoya formation of New'Mexico, togetherwith a lithology-porosity chart involving Sonic and Gamma-Gamma data. Anassumption was made that anhydrite usually occurs in dolomite rather than inlimestone, so to this extent the chart is an approximaM:on. The points enteredon the chart are taken from the entire Montoya section and show an interestingtrend.

Porosity Determination (Indirect Measurement)Porosity evaluation is often achieved by use of a resistivity device. The

formation factor (F) is determined, and it is converted to porosity through anempirical formula. When saturation is to be computed, it is the F, rather thanporosity (), which is required; consequently, where F- relationship is indoubt, F determination should be made from an appropr~ate device for the meas-urement of the resistiVity of the flushed zone (Rxo)'( .

Formation factors can be found from two sources: 1) noninvaded water-bearing zones, 2) invaded zones. In both cases the fluid resistivity must beknown. In the invaded case, presence of residual hydrocarbons must be accountedfor. Usually for the invaded cases, one or more of the following devices areused: Microlog, Microlaterolog, Proximity Log, Short Normal. Pad devices shouldbe used because of their superior vertical resolution and shallow penetration.They measure Rxo, the flushed-zone resistiVity, when conditions are correct.

Microlog. To properly measure Rxo, an. invasion diameter (Di) of at least16" is required (adequate invasion should be present if R16":::: 2 Ro in sa;lt-water sands drilled with fresh mud). Accurate values for mud resistivity areneeded for good results. When,porosities are low, results are no longer reliable.

Microlaterolo~~ This device requires slightly deeper invasion than does theMicrolog to measure Rxo. It is reliable in low porosities if the mud cake thick-ness does not exceed 1/4".

proximit~ Log. This is not truly an Rxo tool, since an invasion diameterof at least 0" is needed for the device to read Rxo It is'practically un-affected by moderate to high resi~tivity mud'cake~ Its true place is as anauxiliary resistiVity measurement for invasion study in fresh mud. The Proximity-Sonic combination makes an excellent oil locator.

~

Short Normal (R16"). W~en invasion is moderate, as is frequently the casein medium and low porosit~rocks, the Short Normal will yield an Ri value(resistivity of the invaded zone). From this, porosity is found by the relation = c JRm!Ri, where Rm is tge mud resistivity" and c is a function~of the SP andpresence or absence of oil.

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Source of Data on Rw

Much data on produced formation water has been assembled and is very helpfulto the log analyst. However, Rw can be quite variable in some regions; it- isimportant to check it by $P analysis. Where no SP is available and where waterzones are present, use of a porosity device with an Rt device can provide a goodvalue for Rw. Sonic-Resistivity methods are particularly helpful for this.9

The problem of recognizing or estimating static SP in shaly sand regionscan be solved if both Sonic and Gamma-Gamma Density are available. The latterprovides effective porosity if the sand is not too shaly, while the Sonic poros-ityl is too high by a factor of about (2 - a), where a is the SP shale reductionfactor. In clean sand the Sonic and Gamma-Gamma should yield the same porosityvalue. Table I shows data from a South Texas well. The ratio of apparentporosities is p19tted versus PSP, the trend being extrapolated to a ratio ofunity, where the SSP is read (see Figure 3). Curve A shows a trend for a com-plete range in shaliness. When the SP is greater than 30 mv., the trend seemsto be linear; Curve B is a replot of this portion of the data on an expandedscale Remember that for proper use of the Gamma-Gamma Log, fuud cake thicknessand the extent of hole enlargement must be known, possibly from a microcaliper.

Logging Programs and Interpretation Methods

Choice of logging combinations depends on both the borehole and formationenvironments. The former relates to the type of drilling fluid, the latter tothe type of porosity and to the salinity of the interstitial water.

Tpe selection of the appropriate interpretation methods depends a good dealon the log combination used, so each method will be discussed along with theexample of the logs in each category. In these examples, we will assume thatlocation and evaluation of potential pay zones is the primary objective.1) Fresh Mud

a) High Porosity FormationsThese are sometimes called "soft" formations and typically their

porosity exceeds 25% Shale-free or "clean" formations are easiest bothto log and to interpret. The minimum recommended combination is theInduction-Electrical Survey plus Sonic Log or Microlog.

In the example {Figure 4-A) from California, both the Sonic and theMicrolog are shown- Two methods of interpretation were applied: oneutilizing the Induction Log for Rt and the Sonic for porosity, the otheremploying the ratio method with Rxo obtained from the Microlog and Rtobtained from the Induction Log. Results are tabulated below:

Zone Induction-Sonic Induction-Micr~~ o:L ~

A ~32 28% 30 29%

B 1.00 29% 75 38%

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Sand A is a gas producer; sidewall cores gave = 28~ andpermeability = 27 md.

The Induction-Microlog interpretation yielded good results in thepay zone but led to a high value of porosity in Zone B. In the lattercase the invasion was too shallow to yield a suitable flushed zone forthe Microlog to~'measure. The falsely high value of porosity led tothe computation of a low value of water saturation. In Zone A thevalues of Rxo and Rt were similar; therefore, insufficient invasion didnot constitute a problem.

In moderately shaly sands logged with Induction and Microlog theratio interpretation methodll is quite effective if invasion is suf-ficient. When the shaliness increases so that only a very small PSPexists, this method has poor resolution. Stud~es have shown that aSonic-Resistivity approach will qualitatively locate oil or gas showswhere the Microlog is not effective.

, In a recent paper9 it was shown that Rwa (apparent water resis-tivity) can be computed from Rt and FS' an apparent formation factorwhich is derived from the Sonic Log. The relation is Rwa =Rt/F.s andit is solve4 by Chart 5-B. In clean water sands Rwa = Rw, in shalywater sands Rwa may approach 2 Rw, and in shaly oil sands Rwa shouldexceed 2 Rw Basing interpretation on knowledge of Rw is an uncertainpractice in shaly sand regions. Therefore, a study was undertaken tosee if use of the resistivity and the Llt in shale would help. Whenpure shales are used to compute Rwa by the same method-as is used tofind Rwa in sands, the ratio of these two values qualitatively locatesshows. When Rwa (sand) - 1 the zone is of interest. When a can be

Rwa (shale) ::::>estimated, this ratio can be used to approximate SW' Figure 5-C wasprepared for this purpose. The dashed curve is provisionally used asa critical line, such that points plotted above it may have shows. Anypoint of interest should have some SP. Points falling to the left ofthe ordinate scale indicate limey zones Use of the chart is illus-trated by a South Louisiana well (Fig. 5-A). For this 12,000 ft. wellcLltsh was taken as 100. Commercial gas production was obtained fromthe interval including levels 9 and 10. It appears that level 9 con-tributes most of the production, since it has higher porosity. Levels11, 12, and 13 also probably contain gas or oil.

To. summarize for high porosity formations, the most effective logcombination which will locate potential pay zones consists of theInduction-Electrical Log plus Sonic Log. When invasion is just right,the Microlog may replace the Sonic, but even here very shaly sands maybe ,missed. For clean'or shaly sands the Sonic-Rwa technique should beused for saturation; when Rw is properly known, sw~JRw7Rwa(sd)'Porosity in shaly sand'13 requires determination of a both for tpe Sonicand Micrologmethods. 'The Gamma-G~mma Density seems to give close toeffective porosity in not-too-shaly sands; corrections are requiredonly for the borehole and mud cake.

b) Medium Porosity FormationsThese are the "Mid-Continent" types and exhibit porosities in the

13 to 25~ range In this class we can have moderate to deep invasion,

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so the new deep investigation Induction-Electrical Log9 (6FF40) isrecommended. The Sonic Log is definitely the best source of porosityinformation and does not require correction for compaction or fluideffects. A mud-cake detector such a~ a caliper--or better, a Microlog--is advisable for location of permeable zones and for sand count.

When the 6FF40 log and the Sonic Log are used, the plotting methodgives a fast, visual interpretation.9 When no trend to establish Vmis seen, the control line (equal to Ro) can be quickly drawn fromknowledge of Rw and lithology.

Figures 6-A and 6-B show an application of the plotting method forthe Strawn formation. Here, the 6FF40 Induction device reads Ro insome of the water-bearing zones (points 2, 5, and 7). The Rw valuecomputed from this plot (0.052) is close to the SP-derived value of0.06. The invasion is eVidently not excessive as the Short Normalleads to plots above FRz, the line representing moderately invadedwater zones. The exceptions are points 9 and 10 which contain shows.Point 10 is definitely pay, since the Induction plot falls on a lineequal to 20 Ro and since the Short Normal plot is greater than twiceFRz The water saturation is computed to be 022.

Ratio methods of interpretation, when checked by porosity balance, .are effective in these medium porosity zones The example interpretedabove by the plotting technique was also interpreted by the ratiomethods (see Fig. 7). After balancing, Sw is of the order of 0.21 to0.25 (see Fig. 8).

The lower porosities of this class sometimes invade so deeply thateven the 6FF40 reads too high in water-bearing zones. An example ofthis is taken from a well in Mississippi (Fig. 9) Taking the Inductionvalue as Rt and solVing the Archie Saturation formula leads to Sw = 0.47The ratio method (using the Rocky Mountain Chart) provides Sw = 075Since the sand is actually 100% water-saturated, the ratio method ismore diagnostic. The fact that no residual hydrocarbons are present isconfirmed by the Short Normal and PrOXimity Log reaalng approximatelythe same while the Induction reading is significantly lower.

This example points to the difficulty in log analysis when Rmf /Rw >10and invasion is very deep. For oil zones in this well, interpretationis straightforward, since Ri and Rt values are fairly close.

For maximum refinement in determining Sw and for studying invasioneffects, a suite of focused resistivity 10gs,5 popularly called the"Grand Slam," is available. In terms of increasing penetration, theyinclude a Proximity Log, a Laterolog with shallow investigation, amedium Induction Log (5FF40), and a deep Induction Log (6FF40). TheSonic Log is also vital for porosity control. With the additional resis-tivity curves the effects of invasion may be more precisely determinedand accounted for. This is particularly beneficial when unusual invasionconditions, such as inefficient flushing or annulus, exist.

Figure 10, from the Texas Panhandle, shows the set of resistivitycurves that were obtained on three trip,sinto the borehole. All are

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recorded on a logarithmic scale. A detailed analysis of these logs isbeyond the scope of this paper; however, results were obtained and arepresented below:

Zone

A

B 60"

Rxo!Rt

23

15

100

37

Remarks

Inefficient flushing

Annulus present

If deep invasion is a problem, hydrocarbons can be located by usingan invaded resistivity value plus the Sonic Log.9 Since the filtrateresistivity is known from mud samples, the resistivity from a Micro-laterolog, Proximity Log, or Short Normal will give an apparent forma-tion factor, FR, which is compared to the FS from a Sonic Log. When theratio FR!FS is greater than one, shows of oil should be present. If theSP is very high, ratios less than this may still be significant. TheFR!FS ratio method has been very successful in tight sandstones withsmall sp.13 .

c) Low Porosity FormationsWhen low porosity reservoirs are predominantly of a granular type

and Rw is low, the best logging combination is 6FF40 with Sonic. TheMicrolog gives a clear picture of net pay, whereas the caliper and SPare not as easy to use for this purpose. Interpretation methods arethose found under medium porosity, except the ratio method requiresreplacement of the 16" normal by the Laterolog-8, a new Ri device runwith the 6FF40 Induction Log. Although the Laterolog-8 has slightlyshallower penetration than the Short Normal, it can be used uncorrectedas the Ri value in the Rocky Mountain Method Chart (Fig. 7) with verysatisfactory results.

When secondary porosity is predominant in these reservoirs, thepay section can have very high resistivity. In such cases invasioninto the matrix is usually limited, so the choice of tools shouldtake this into account. The combination Gamma Ray-Neutron-Laterolog-3is recommended for carbonates in this class. The Laterolog will notread true resistivity in the water zones (Ro), but the Rt values inthe pay should be good.

An example of this (Fig. 11) is from the Abo trend in New Mexico.The presence of considerable anhydrite complicates the use of a SonicLog; however, the Neutron does a good job for porosity. The water con-tact is easily seen on the log. For saturation interpretation use theArchie Method, with the Neutron porosity having been converted toformation factor by a high Um" exponent of the order of 2.2.

d) Fresh Formation WatersIn some regions, particularly in mountainous areas, the formation

waters can be fresh. This complicates SP analysis and also producesvery high resistivities in pay zones. A typical example (Fig. 12-A)

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is the Tensleep sandstone of Wyoming. Minimum logs required are Laterolog,Gamma Ray, Sonic, and a permeability indicator such as the Midrolog.Interpretation is by Sonic-Resistivity plotting (Fig. 12-B).

The producing level is at point 12, where the plotting techniqueindicates the Sw = 0.22 and the porosity is approximately 20%. Perfo-rations from 3362-77', after a small acid job and sand frac, yielded630 BOPD, 163 gravity, 11% water. With low gravity oils a very low Swis required, probably less than 35%, to keep the water cut reasonable.Agreement of the Sonic Log with core-analysis porosity is good except fora dolomitic section at level 6. One of the weaknesses in the single-resistivity-curve method is lack of means to recognize, or correct for,deep invasion. This can sometimes be done by using a Microlaterolog,but the possibility of mud cake influence must be recognized. ProximityLogs should be helpful.

In summary, for fresh mud logging the deep Induction is the best Rt toolexcept when formation resistivities become too high. For porosity, the basictool,is the Sonic Log; in some cases the Neutron or the Gamma-Gamma may be prefer-able, and in others, two porosity devices are required for good results. Themost universal log analysis method is Sonic-Resistivity plotting or a modifica-tion of it, such as the Rwa fluid comparison method For problems related toinvasion a resistivity ratio method is often safer, especially when followed bya porosity balance or checked with additional resistivity spacings. When thecomplete ,set of logs which comprises the "Grand Slam" is run, the type and depthof invasion, as w~ll as values for. porosity and water saturation, may be obtained.

2) Salt MudsA completely satisfactdry definition of a salt mud cannot be made. In

general, either of two conditions can give some limits: 1) salt concentrationequal to or above sea water; 2) the resistivity of the filtrate is close to orless than formation water resistivity. The former gives borehole effects onsome devices (e.g., 5FF40, Microlog, SP, Short Normal, etc.); the latter tendsto give Rxo~Ro in water zones and Rxo

give the usual SP appearance). The porosity in the pay zone is foundto be between 26 and 33~; the Sw is as low as 0.24 near the top of thesand. Note that the Micrologqualitatively shows the oil/water contact,but its interpretation would be rather difficult.

b) Medium and Low PorosityIn salt muds the Sonic plotting method is highly advantageous since

invasion does not appreciably affect water-zone rerastivities. Figs.l4-A and l4-B show the application of this method. Here Rmf:::: Rw,the ideal situation. Note that the Microlaterolog gives the requiredcontrol for the FRw line, possibly due to better thin-bed matching withthe l-foot Sonic. The pay zone is easily picked; points 7, 8, 9, and10 are included. Production of 113 BOPD was obtained from this zone.For accurate values of Sw the Laterolog values should be cor~ected forinvasion. Using the correction method shown as Chart 4 in the WillistonBasin reference,4 the estimated values of Rt are plotted as XIS onFig. l4-B. Sw should be taken from these new values.

Other popular salt-mud combinations are the Gamma Ray-Neutron +Laterolog or Laterolog + Microlaterolog. An example from Kansas (Fig.15) provides a study of the use of all these devices. Table II givesthe data and results. The interval 4291 to 4326 ft. is Mississippianconglomerate. When the porosity index (PI) from the Microlaterolog,uncorrecte~ for reSidual oil saturation (ROB), is compared with theNeutron porosity (N), the percent ROS can be found. The upper threelevels in Table II show N

are so effective that the Sonic Log becomes the preferred porosity tool. AGamma Ray is required to identify shales. Figure 16 shows logs from a shalysand in California. Levels 3, 4, and 5 show Rwa values well above those fromshale levels, thus indicating pay. This section produces gas. Correctionson porosity for shaliness by using the Gamma Ray could be made, following anempirical study somewhat as is being done for the Delaware Sand.15

4) Empty or Gas-Drilled HolesHere the problems are similar to those of oil-base mud, in that the Induction

is the only Rt tool a~d it is unaffected by the hole or invasion. For porosityonl~radiation tools can be used, and the Gamma-Gamma device has been found tobe best except where the hole caves slightly. Since the caves usually occur inshale, this is not too important; but it does mean that a caliper should

6be run

The temperature log is very helpful in locating the producing interval. lFigure 17-A shows'typical logs for such holes.

Quantitative interpretation in gas reservoirs is accomplished by a combi-nation chart, such as shown as Figure 17-B. A correction for gas is automati-cally made on the Gamma-Gamma by relating iit to percent liquid saturation.Knowledge of lithology and Rw are important and cannot be obtained directly fromlogs. Assuming that Rw = 0.2 and the formation is composed only of silica,levels A and B are plotted. The temperature c~rve confirms the presence ofproducible gas at level A, where Sw = 0.25 and = 13~Permeability

Direct measurement of the very important parameter of permeability cannotbe accomplished with eXisting logging devices. There are, however, manyapproaches to the recognition of the existence of permeability. Under most con-ditions the Microlog-Caliper indicates permeability by detecting the. presenceof filter cake.

The SP curve has long been used to locate permeable zones; however, it iswell to remember that this curve responds to ionic rather than to hydraulicpermeability. Careful analysis of curve shapes is necessary to use the curvefor this purpose.

Whenever curve separation (not due to hole or bed thickness effects) exists.on resistivity logs, it is obvious that invasion has taken place. Such separa-tion thus indicates that at least some hydraulic permeability exists.

Where permeability is rather low, analysis of pressure bUild-up from theFormation Tester,3 can give permeability ranges.

When permeability values are available, they may be,used to better predictproduction on the basis of computed Sw and porosity figures.

If the zone has principally a granular type of porosity, the Sw and shouldbe plotted on a permeability chart lO (Fig. 19). If the permeability shown isreasonable, or close to known values, the zone,shpuld produce clean 9il. 'If thechart value of k is much too low, a transition zone is suggested. If,the chartvalue is much too high (e.g., by a factor of 10), the zone likely contains gas.The interpretation of the log on Figure 4-A may be checked by this technique.

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Summary

The appropriate usage of tools can be recapitulated as follows:

I POROSITYSINGLE TOOL APPLICATIONS

t L11GtSon1.c Neu ron amma-Gamma Micro; Oil Micro atero og Prox1.mi;y og,

Basic Low matrix Empty hole If inva.sion adequate, can beporosity plus used as porosity tools.tool in secondary Shallow sandsmost Hig~ sd. Med-low Med-low types of For some Shaly sandslithology anhydrite- Shaly sd. Salt mud Fresh mud

dolomiteformations

other Uses

Integrated Gas indicatortraveltime

Geophysical(Density)

Permeabilitylocation

To correctLaterolog

Di indi-cation

MULTIPLE TOOL APPLICATIONS

Sonic + Microlaterolog or Proximity Log - - Plotting to find Vm, FR/Fs method,study of flushing

Sonic + Neutron - - - - - - - - - - - - - - Gas location, in sandstone-carbonatebeds

Sonic + Gamma-Gamma - - - - - - - - - - -- - in anhydrite-carbonate mixtures in sandstone-limestone mixturesEstimation of compaction correctionsEstimation of SSP

II. TRUE RESISTIVITY

Deep investigation InductionFresh mud--all formations if Rt< 200 in pay zonesSalt mud--high porosity formation

Standard Induction devicesEmpty hole, oil-base mudHigh porosity in fresh mud

LaterologSalt mud--all formationsFresh mud--high resistivity, Rt > 200

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III . INTERPRETATION TECHNIQUES APPLICATIONSonic-Resistivity plots

Salt mudFresh mud--medium and low porosityOil-base mud

Sonic-Resistivity comparisonRwa Method---high and medium ; shaly sl:!-nds (any hole fluid)FR/Fs--medium and low (if SP< 100 mv.)

Gamma-Gamma Density-InductionGas-filled holes

ResistiVity Ratio MethodsFlushed Zone (Rxo/Rt) Method--shaly sandsRocky Mountain Method + Porosity Balance--medium and low

Often several porosity classes, formation water changes, or diffe+entlithologies are present in the same hole. For such cases care must be taken tochoose the proper logs; sometimes additional logs will be essential. When rankwildcats are to be logged, a complete logging program is especially important toinsure full evaluation of all possible intervals before the well is plugged orcased. Location of noncommercial shows can point the way to an oil or gas field.To do this, a combination of refined logging todls and careful interpretation--theultimate of which is the Grand Slam technique--are vital. Finally, in provenfields the essential logs will produce data on reserves and for geology and willprOVide a basis for correlating all engineering information.

Acknow~edgements

The author acknowledges the able assistance of W. P. Biggs and F. Segesmanin the preparation of this paper. The courtesy of oil companies which releasedlog data for illustration of this paper is appreciated.

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LOGGING SYMBOLS AND ABBREVIATIONS

Symbols

RmfRxoRwRoRzEiR16cRtFFSFRmPIVm.6tRwaSSPPSPaDidSw

Resistivity of mud filtrateResistivity of flushed zoneResistivity of formation waterTrue resistivity of zone when completely filled with formation waterResistivity of a mixture of filtrate and formation waterAverage resistivity of the invaded zoneApparent resistivity from 16n normal, after borehole and bed correctionsTrue formation resistivityFormation resistivity factorAn F derived from Sonic LogAn F derived from a Resistivity LogCementation exponentPorosity: percentage of pore spacePorosity index: computed apparent porosity without residual oil correctionMatrix velocity: acoustic velocity of rock at zero porosityAcoustic travel time in ~sec. per footApparent fluid resistivity from Sonic-Resistivity Logs: Rwa = Rt/FsStatic spontaneous potential in thick shale-free permeable zonesPseudo static SP: Value of SP obtained in thick shaly sandsSP reduction factor: PSp/SSpAverage electrical equivalent diameter of invaded zoneDiameter of the boreholeFormation water saturation, fractional portion of pore space

Abbreviations of Logging Curves

IL Induction LogSN Short NormalLL LaterologML MicrologMLL MicroLaterologPL Proximity LogGR Gamma Ray Log1-1 Gamma-Gamma Density LogN Neutron LogSL Sonic Log

TABLE I

SP deflection compared to apparent porosities from Sonic and Gamma-Gamma Density Logs - South Texas

(See Fig. 3)Sonic Sonic Density Log Density Log Ratio of Apparent

Depth SP 6t Porosity (cps) Porosity Porosities (s.!.,-7)8445 0 79 175 420 45 39

70 8 75 145 450 55 2.63

8532 40 825 203 620 150 13552 45 82 198 650 163 1.21

60 50 81 190 660 170 1.12

95 50 85 22.0 750 205 1.07

99 55 825 203 740 20.0 1.02

8625 36 825 203 635 155 13162 32 86 22.8 630 153 1.49

90 36 825 203 600 145 1.40

8705 47 83 20.8 675 16.7 1.2580 20 81 190 530 110 1.83

8820 7 76 153 430 50 30530 15 75 145 450 55 2.60

53 50 80 183 690 185 1.02

TABLE II

Data and Results for Laterolog-MicroLaterolog and Laterolog-Neutron Interpretation - Edwards County, Kansas

(See Fig. 15)Neutron by Laterolog-3 MicroLaterolog Porosity Residual Oil Sw, Based On

Depth cps Neutron Value Value Index Saturation i Flushed Zone Archie (Neutron)4291-94 1000 8.0 165 70 8.6 (0 .43 53 *

94-96 980 8 5 12.0 65 8.8 (0 50 50 *96-00 1100 55 17 5 10.0 73

REFERENCES

1. Tixier, M P., Alger, R. p. and Doh, C. A.: "Sonic Logging,lI Trans.,AIME (1959) 216, 106.

2. Doll, H G.: liThe Microlog - A New Electrical Logging Method for DetailedDetermination of Permeable Beds," Trans., AIME (1950) 189, 155.

3 Finklea, Ernest E: "Formation Testing on Logging Cable - New Applicationsin Consolidated Formations," API Spring Meeting, Amarillo, April 22, 1959.

4. McVicar, B M, Heath, J. L. and Alger, R. p.: "New Logging Approaches forEvaluations of Carbonate Reservoirs;" First Williston Basin Symposium.

5 Doll, H G, Dumanoir, J. L. and Martin, Maurice: "Suggestions for BetterElectrical Log Combinations and Improved Interpretations," Geophysics,Vol. 25 No.4, Aug. 1960, p. 854.

6. Chambliss, G. F: "Devonian Lithology Identification utilizing PorosityLogs," Four Corners Geological Society ThirdField Conference, Moab, Utah,Oct. 5, 1960.

7. Chombart, L. G: "Well Logs in Carbonate Reservoirs," Geophysics, Vol. 25No.4, Aug. 1960, p. 779

8. Tixier, M. p.: "Porosity Index in Limestone from Electrical Logs," Oiland Gas Journal, Nov. 15 and 22, 1951.

9. Tixier, M P., Alger, R. p. and Tanguy, D R.:"New Developments in Inductionand Sonic Logging," Journal of Petroleum Technology (May, 1960) p. 79

10. Schlumberger Well Surveying Corporation (Editor), Log Interpretation Charts,Houston, August, 1960.

11. Poupon, A., Loy, M E and Tixier, M. p.: "A Contribution to ElectricalLog Interpretations in Shaly Sands," Journal of Petroleum Technology (June,1954 ), p. 27

12. Tixier, M. p.: "Electric-Log Analysis in the Rocky Mountains, 11 Oil and GasJournal, June 23, 1949

13. Burton, R. P.: "New Log Interpretation Techniques for the Gulf Coast," 9thAnnual Convention of GCAGS and the 1959 Fall Regional Meeting of AAPG,Houston, Nov. 11-13, 1959

14. Tixier, M P., McVicar ~ B. M. and Burton, R. P ~: "Progress in Sonic Log Appli-cation," The First Joint Tech. Meeting of the CIM Petroleum and Natural GasDiv. and Rocky Mountain Sections of SPE of AIME, Calgary, Alberta, May 4-6, 1960.

15. Millican, M. L.: "The Sonic Log and the Delaware Sand," Journal of PetroleumTechnology, Vol. 12 (Jan. 1960) p. 71.

16. Kunz, K S. and Tixier, M p.: "Temperature Surveys in Gas Producing Wells,"Journal of Petroleum Technology (July, 1955) p. 111.

Sonic and Neutron Logs used for porosity determination In a bedded lithology.

DEVONIAN POROSITY LOGS

SONIC VS. NEUTRON

421

SONIC

JJ. sec./ft.

...-::..::,

NEUTRON

cts.lsec.1020 1320 162

26

GAMMA RAYi 8200

microgramso Ra - eq/ton 7

I ". - - - '" r 18 I ~ I I ........... "\i: I

SANDSTONE

_7

13001 " I \ I I I I

J-1/ I -21

DOLOMITE 1\ ILIMESTONE10001 I r ,

40 50 I ISONIC - )J. S EC./ F~~ 70

(JIJJen.......

.

en....

(J I2001 I \ I )~ci....

enI

zo~ 11001 I \ I \ I . -j I I=>IJJZ

FIG. 1

Sonic and Gamma-Gamma Density Logs used for porosity determination in a mixed lithology.

MONTOYA DOLOMITE

'?~.~//I~

100 % ANHYDRITE

'S' ;:L-+---t,~;;>?J-I4001

300, I

2001 /;> / - >:

SONIC - ~ ~ COMBINATION500. I I l:::>i

~ ~ LOG

')"~

~,/~,

.:::::>-"'>

-

Ratio of apparent porosities from Sonic and Gamma-Gamma Density Logs compared to SP in shaly sands.

4.0

7050

o

30 40SP

2010

I I I ~ I 1\ I II. I

I \ I I I \0: I I I I I. 5

CURV I 11.4A Q I IIIn I II ~

I I\:I I I 4 I I 11.2

o

I X I\ X I I 0 1\ I I II. 3 (/) S0'671

3.0

1.0

(/)s(/) '1 'if

2.0

FIG. 3

Interpretation in clean sands of high porosity (Cal ifornla).

HIGH POROSITY

POROSITY a SATURATION FROMSONIC a RESISTIVITY LOGSUnconsolidated or Shaly Sands

FIG. 4 - CM (Unconsolidated Sand)

5

30~.-- 10 9 .3I 50 (Woter Sand)20

tj zT~kI l1ig

Clean Gas Sand

Clean Oil Sand

~93Start here when 100 (Pay Sand)11t(shale)

Interpretation in very shaly sands (South Louisiana).

SONIC

.~ Rwa AND FR/Fs COMPARISON METHODS125~t 1/>5 Fs

.75~SP 14~'r22.4~ 1. 16~ 18 I 90"r 10088 ",'/ " 25.3 .Q44 22.5 2509 ,1,..--' I~.l:l 25.4 1---:='0;- .027 =:::-z 23 I 100 1.020

20110-1 .........1 I ......... 1 I 15 10 .5

1205

--

30.4 ~ ~ t::~ sn. 10 ----..-"---"--- 2 .230.2 g 16 1.0140 ~5_-

---

21.7 12.5 .5.1

521.7 12.5 160 L.2

t RESISTIVITY .0527 14 3~t 50

180 140 120 100 .02

o lS:- I 16H 33.4 - ... ~ ~-Rwo - FLUID RES.

1224.6 I JI~I~IA I @ Schlumberger FIG. 5 - B

FIG. 5-A

30202346810RESISTIVITY INDEX

I. tty./ V./~~ V0.6.....

VVVV~ V "~

j....--I-SHAL ELINE ~ V >.....

l4r:: v j.-o'" 0.2le. V - 9x7 ~ l-I-t-"I---I-- a. =0ret-,

-'t ~15...... , ..&:, 19~ _xIO 13 ~~ ./ //V)'fi~V/~///~r~ 70% SATU AiiolN Iii~;%56", 25% 20"0

2

14

Rwa(sd)Rwa (sh)

@ SchlumbergerFIG. 5 - C

Sonic-Resistivity Method for fresh muds--medium to low porosity

FIG. 6 - a

Rocky Mountain Method for porosity and porosity balance.

R

50

500

100

Rw

1 ,r I I ! ! ,=40 46 52 58 64 70 -~t

Vm = 21,000 ? 5 10 15 '"I I I 'I'400 100 45 F

(for m = 2.0)

x = DEEP INDUCTION

= RI6e

Taylor County, Texas I ,~ ~ n l ~?i ''1''4 i 15

SONIC V5. RESISTIVITY

115 0 FFIG. 6 - A

INDUCTlON- ELECTRICAL-200+1 f, 10~00 ~ov VIVS P I _.\ \. 1

Rocky Mountain Method for saturation.

RI6e/llIL =1.5 and Rmf/Rw =26 Entering t..... In Fig. 7, Swe =0.17 Thl. "chocked by knowingthe _ (16%j ond using Fig. 8. Rllftw =040/0.052 =770 Plotting thl. with Swe =0.17 showslie 19%. Either Rt or RII. IncOl78Ct .Inco 0 bolonce I. not obtolned.Tho point mull bo moved to tho correct _ (16%)

1. If RIL I. correct-move horlzontolly, Sw. 0.212. If R16c I. correc.---..-o 01 450 , Sw. 0.25

(Ri Method)

FIG a

If balorn;e does not eleiit,move from thit calculatedporosity.

E>cceplion: When Induction is used,and Rl6c/RIL < 1.2 for SFF-40,and < 1.4 for 6FF40, RtL::::: Rt.

a) along Cl 45 Itne until:=(=c~~~i.*

Beware of high Sf'

when the finol Sw ~ ~11m,

The;lane may be wet.

b) along a horl;lontal I1nert~tt'(:=~c~i~~::;r (Archie Method)i.e. FRz > R16~

This Fino! po,nt ind,'catei amore CorAct Sow.

,F POROSITY BALANCEFOR ROCKY MOUNTAIN METHOD

17SA TURATION IN %

20,000,

"IS 2.0 so "0 SO 70 100 ISO

D-"- '" I15,000---r\ ~ i\ ,10,000 1-

h 1'- .l-. --j-50.00 j-...

'"\- \ ....\ t",..

.\ '" 1-1\' I"., ~--I-''''

....: \".\ ~4\\ 0."" ~

'''''' .~"'".00~.

.00 rl...'"

.001\\ \ ~ 0"'. \\\; ~\ \-\1

.00

"-.~..\ \~".

'\,;'.\ 1\ \'00 ~.. .... .. -\--..

'\.[ r\i..POROSITY S

"'

"'1.. r\ St"

'. \1..

. ,j

Rt/R

@l Schlumberger

770

20

60

40

10

Sw

'0

'0

20

130120110100.0'070

+ '0

... 10

+ 40

SPK = 70

FIG. 7

MISSISSIPPIEXAMPLE

TAYLOR COUNTYEXAMPLE

12 100 -180 f:.0 -160'-

,- 10-30-1 ~r7

,-

~120 ~,-" -tOO ,~ ~6 '0 - .0 """ " " - .0

" ", 2

4 6- 70 '"

" "I.' ,,~

- 60

" "" 1.0

",,"'i, 4

- .0,

- 40 t\. I" f\:I" 0.' "- 1'\ s:"- "- t\. ~O;'.... "-1"- ~I2

- '0 -~2 I".... 0.4,'\1:'.... 1"- ,"- 20

"1'\. "~-l

- '0f\... ~:t

I 1 0 "- .r-.... "- "-1"- '\ -~"0 I 1"::1".... I'.... '\1'-+ 10 ':->"j

.7 .7

" " f"\ ~j+ 20RYRt I!\ "- I" ,,-'I..

+ '0.4 .4

'"'"~~+ 40 f'\+ '0 SA U ATIJN IN 'Yo 7~1 ,t7 10

"20 '0 40 50 100 ",!SO

,@l Schlumberger

_70

.20.0

.40

0

.60

Rz R z Rmf SPRmf Rw Rw K=90

Log combination for very deep invasion (Mississippi). Resistivity logs used in ~IGrand Slam~1 interpretation (Texas Panhandle).

10010\,~

5FF40

100

--;i,\ ,

----"'

')

B,:>,

" I

""

JfL.INDUCTION

'--_c /I I- _

) 'C" )

",

) ,/,

::I'

IND. - ELEC'I SONIC5aO 1090 40

-200 +

PROXIMITY MICROLOG6 CALIPER 161 10 510

"'-.l

FIG. 9 FIG. 10

Logs in anhydritic dolomite--reef structure (Abo, New Mexico).i i 10

1560NEUTRON

c-_-=--_-_- ~===-

- - ::--=-:~

-,>

c.:.-- --5~ - -I=lcJ2SQLATEROLOG . -l~~ ___ .. J

--- .,;;;;;;;;;',...::;;;;;;;

12NEUTRON .... , I 11,.1.30 I I10 '---

360

o

6t '" 10010,110 20 ohm"'" 73 Itsec./ft.

113 30" 69.5 " Y I I I 1000254 40" 66" 150 60 70 =277 22" 72.5 " 10 5 - '0 15

Porosity Is scaled using Vm =20,500 and Vf =5300At f1 =10%, RPL =60; Rmf =60/90 = 0.67 90Measured Rmf =0.68. Therefare trend line shows no oil.

Zone at 10,110, average l>t = 72 Itsec. '. f1 =16%Archie SolutionRo =FRw = 33 x 0.02 =0.66RIL = 3.0, Sw = J2 "" 0.47Rocky Mountain SolutionR1WRIL = 23/3 = 7.7; Rmf/Rw =~ = 34Fig. 7 gives Sw = 0.75

FIG. 11

Logs and interpretation--fresh formation waters (Tensleep, Wyoming)

FIG. 12-A

Logs in salt mud--high porosity sands (South Louisiana).

2000

1000

150

500010000

e.,i{ -1250IOJX:13 I RLL

: X 8~ __ XII -1500\0112 -X3

Rw =0.78

I --; .... - .",. Xi"----.- j- I dOO

The Laterolog value at point 12 Is 570 and the 10 Me value directly above Is265. The Resl.tlvlty Index I. therefare 570/26.5 or 21.5.

46 52 58 64 70 76 CO _LH10 ~ ~ II' 17 r/)

FFIG. 12-8 (for m =2.0)

'J?-{~}'),L

~"-.-. .-t

L

~~.

ML

MICROLOG4017M 10 Ii

SONIC90

INDUCTION- ELECTRICAL~ 0 10140I s p 11 ! \6" NORM.

1-" ----.J ll.;-and

Max. Rt near tcp of sand = 4.0; Rt/Rw= 114; 01= 105 Fig. 4-Cslve. Sw= 0.24,'.33%

Interpretation in salt mud--carbonate rocks {North Dakota}. Salt mud methods using Laterolog plus Microlaterolog or Neutron (Kansas).

B~f ~i ~cl~~ _i!IZE

-,,,>

--=- 13 ~ Il .-' 1""':'--

--c--"_C3

-

>

Rm BHT = .02 (cj) 215FFIG. 15

MICROLATEROLOG010 RES. 501

1000

~480 NEUTRON 1280

LATEROLOGo 20 RES. 1500 CONDo----,-

'-~ .

Logs and interpretation for gas-filled holes--gas reservoir. Permeabi Iity in oil sands.

000

4000

2000

40

FIG. 1"8

35302520

-% Porosity

15'05

Schlumberger

.10

.7

c:o

c:o

iI:(/)

'">oJ:l'---

-;.---:- 1340."i"'---"$.

Q JQ.O

INDUCTION - GAMMA GAMMA6 1/4" GAS-FILLED HOLE

I II I "",b''' I I01>L]::0~1---l----4-+--l------,4----ILI MESTONE

/ IL...t---l--....4---+-'---l----ISANOSTONE

1000 I I ,\ ~ I. I I " ..... I I I I

4001 u ~ \B'J I "k I II 1 l I

200

2000, 1\11 \ I ," I I I

>-....

u; 2.8zILlaz 2.9

...lit.Rw 1001 -11~Y\1v+~~-J--_ '{,' 'J I Ir

FIG. 17 - 8@ Schlumberger

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