100787724 lecture 01 wellsite geology part 2

LITHOLOGY RECONSTRUCTION FROM LOGS

There are two independent sources of lithology data available from oil wells:

One set of data coming directly from the drilling.1- Cuttings “chips”. 2- Cores.3- Recorded drilling parameters

# MWD log (Measurement While Drilling).

# LWD log (Logging While Drilling). One set from the wireline logging.

1- Wireline.2- Geophysical log suite.3- Sidewall cores.

lithology from drill data – the mud log

lithology from cores – direct physical sampling

Sidewall coring as a method of lithology sampling should be used essentially for verification (shale laminae).

Oil formation volume factor is defined as the ratio of the volume of oil at reservoir (in-situ) conditions to that at stock tank (surface) conditions. This factor, is used to convert the flow rate of oil (at stock tank conditions) to reservoir conditions. It is defined as:

Oil formation volume factors are almost always greater than 1.0 because the oil in the formation usually contains dissolved gas that comes out of solution in the wellbore with dropping pressure.

Oil Formation Volume Factor

Gas Formation Volume Factor

Gas volume at reservoir conditions divided by gas volume at standard conditions. This factor is used to convert surface measured volumes to reservoir conditions, just as oil formation volume factors are used to convert surface measured oil volumes to reservoir volumes.

Mud Circulating System

Results from Mud logging

Typically the mud-logging unit will produce a daily “mud log,” which is transmitted to the oil company office on a daily basis.

Items that will be included are:

• Gas readings as measured by a gas detector/chromatograph

• A check for absence of poisonous gases (H2S, SO2)

• A report of cuttings received over the shale shakers, with full lithological descriptions and relative percentages

• ROP

• Hydrocarbon indications in samples

The mud log may be of great use to the petrophysicist and geologist in operational decision making and evaluation.

Areas in which the mud log may be particularly important include:

• Identification of the lithology and formation type being drilled

• Identification of porous/permeable zones• Picking of coring, casing, or final drilling depths• Confirmation of hydrocarbons being encountered

and whether they are oil or gas.

Results from Mud logging

Cuttings descriptionl . PurposeUnderstand the component & structure of the rock toInterpret/conclude the source, depositional

environment & position of where the cutting were obtained. To allow other observers to recognize the rock whenever it is seen again

2. FormatRock type/classification, color, hardness, grain size,

grain shape, sorting, mineralogy, cement, visual structure, visual porosity estimation, oil show, other.

Data recorded from Cuttings

Items that should be included are ;

Grain propertiesa. Textureb. Colorc. Roundness, or sphericityd. Sortinge. Hardnessf. Sizeg. Additional trace minerals (e.g., pyrite, calcite, dolomite, siderite)i. Skeletal particles (e.g. fossils)j. Non-skeletal particles (lithoclasts, aggregates, rounded particles)k. Coated particles

Porosity and permeability_ Porosity type (intergranular, fracture, vuggy)_ Permeability (qualitative as tight, slightly

permeable, highly permeable)• Hydrocarbon detection/shows(Oil/Gas)

Data recorded from Cuttings

Porosity Types

Choquette and Pray (1970)

Oil Show/Indications1. Free oil in mud – amount, color2. Petroleum odour – strong, faint, weak3. Visual staining – amount, color, distribution4. Fluorescence – color, intensity, % observed under UV(usually is best indication of an oil show) .Some minerals i.e. coal, lignite also give fluorescence5. Cut – rate, color, intensity, residueDescribe the phenomenon of oil being leached from

rock by a solvent – usually chloro-ethene6. Increase in pit volume

lithology interpretation from wireline logs – manual method

•The manual interpretation of lithology from well logs should be undertaken only using all the logs registered.

•This is often the gamma-ray (or SP) and a resistivity log, or the gamma ray and a sonic .

Principal uses of wireline logs

Horizontal routine

The interpretation is then continued, again horizontally, through the other logs – resistivity, sonic and density-neutron. If all corroborate the same interpretation, the lithology can be noted and then compared to sidewall cores or other samples (see figure 11.6).

Vertical routine

Although the horizontal routine is the basis for any lithological interpretation, individual logs should also be examined vertically for trends, baselines or, absolute values. For the gamma ray log, for instance, and also the SP, a shale baseline can be drawn but also a minimum, clean sand? Limestone? etc., line for the gamma ray and a maximum deflection (SSP) for the SP (see Figure 11.7).

Absolute values and lithology For some of the more difficult, uncommon lithologies and for beds with very high or very low readings, absolute value tables can be useful.

Abrupt peaks, which may be important in stratigraphical interpretations or diagnostic of a particular interval, are often best interpreted using absolute-value table.

Lithology Interpretation

Depositional Env. Interpretation

•Gamma ray is not showing grain-size variation.•Core analysis shows considerable variations from fine to coarse grain sizes within the sand.

Bed boundaries

Bed boundaries should be drawn concisely.Moreover, the correct log should be chosen to position a limit.The best geophysical logs for bed boundary definition are those with a moderate depth of investigation.

100787724 lecture 01 wellsite geology part 2

md habibur rahman geosciences utp

mud log

data recorded

skeletal particles

gamma ray

reservoir conditions

sidewall cores

oil show

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