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Part 1, Page 5
A logging tool that could measure lithology and produce a “lithology Log” would be a valuable tool! When software is applied to multiple logs in a well defined area, methods have been demonstrated that give lithological representations.
One tool that is considered by many to measure lithology is the Photoelectric Density tool. The measurement of bulk density when plotted with a measurement of atomic cross-section comes very close to providing rock type identification.
Lithology is associated with certain mineralogy. Sandstone is composed largely of quartz minerals. Limestone formations are composed of calcite and other related calcium minerals. Dolomite is another common type of lithology.
Major physical differences in these mineral types allow analysts to identify the mineralogy.
Mineral: Density: Cross-section:
Dolomite 2.850 gm/cc 4.78 Sigma matrix (x1021 Barns/cc)
Sandstone 2.655 gm/cc 8.66
Limestone 2.690 gm/cc 8.72
Anhydrite 2.950 gm/cc 12.30
Mineral densities – Table 1
Mineral densities – Table 2
One of the first logging measurements ever recorded, Spontaneous Potential, or SP provides information that infers lithology. In addition, SP can infer permeability. It is possible to perform Lithology identification using multiple logs.
From Schlumberger, C. & M., (1934), Doll, H. G., (1948), Wyllie, M. R. J., (1949) (1951) & others
Spontaneous potential is a measurement of the natural voltage that is created from current produced in the earth because of electrochemical action. It is normally recorded in wells drilled with water.
Formations having permeability are invaded by mud filtrate from the drilling mud. The result is electrochemical action that causes current flow in the formation. Shale formations have very low or non-existent permeability and therefore no current flow and low spontaneous potential.
The SP curve is recorded in track 1 (left-hand track) of the well log. The intensity of the Spontaneous potential can be determined by charts using the resistivity of the mud filtrate (Rmf) and the Formation water resistivity (Rw).
SP is expressed as:
SP = -(60 + .133T) log10 (Rmf/Rw)
T = temperature
Rmf = Resistivity of the mud filtrate
Rw = resistivity of the formation water.
SSP = -(K) log10 (Rmfe/Rwe)
T = temperature
Rmfe = Resistivity of the mud filtrate effective.
Rwe = resistivity of the formation water effective.
Rmfe and Rwe are obtained from charts
Generate an MS excel spreadsheet with 4 columns. Calculate SP in the 4th column.
Use the following values:
Temp: Rmf: Rw: SP:
110 100 .5 ?
110 100 .5 ?
View a copy of an example spreadsheet (sp.xls).
Since SP is not a zero based curve, its deflection is measured from a “shale base line” or predominant right most deflection.
Shale formations have little or no permeability. Sandstone, limestone and dolomite do have some degree of permeability. The SP is useful in detecting permeable beds, locating bed boundaries, determining water resistivity, and as a shale indicator.
In formations containing hydrocarbons, SP is depressed because of the reduction of conductive ions.
SP curves may be calibrated using a fixed voltage calibrator.
Charts are used to predict SP from Rwe.
Clean sandstones and carbonates are low in gamma radiation. In contrast formations containing shale are higher in gamma radiation.
Gamma radiation is statistical in nature because the radioactive decay of radioisotopes is random.
Because radioactive isotopes tend to concentrate in shale or clay formations and clean sandstone and carbonate formations are low in radioactive isotopes, the Gamma ray tool may be used to infer lithology.
Gamma tools should be calibrated with a reference test source in order to perform in a standardized manner.
Spontaneous Potential and Natural gamma ray curves are positioned in track one of the log and indicate sandstone or carbonate formations when at the extreme left of the scale and indicate shale or clay at the extreme right side of the scale.
CORRECTION FOR SHALE:
Certain porosity logs require correction for “shale volume” (Vsh). Neutron porosity is optimistic in shale. Acoustic porosity is optimistic in shale. When shale is present, effective porosity, (phi subscript e), can be used to more accurately determine water saturation (Sw) .
Using information from the natural gamma log;
Shale Volume = Vsh = (Gr – Grcs) / (Grsh – Grcs)
Gr = Gamma ray counts in the zone of interest
Grcs = Gamma ray counts in a clean sand
Grsh = Gamma ray counts in a shale zone
Using information from the SP log;
Shale Volume = Vsh = (sp – spcs) / spsh – spcs)
SP = SP in zone of interest
Spcs = sp in clean sand
Spsh = sp in shale zone
Which shale volume equation should be used?
Use SP for shale volume calculation for instances of high Rmf/Rw.
Use Gamma ray for shale volume calculation for instances of low Rmf/Rw.