Page 133 - Basic Well Log Analysis for Geologist

‘MAPTER VII

Ceenneeeeeereeeeeer ree ee reece ee ener ee cee cece eee cee eee a

LITHOLOGY LOGGING AND MAPPING TECHNIQUES

General

Petrophysical logs provide most of the subsurface data Neutron-Density Log), lithologies can be determined.
available to an exploration geologist. Besides their Figure 35 in Chapter [Vis a schematic illustration of how
importance tn completion decisions, they are also Gamma Ray Neutron-Density Log responses are related to
invaluable tools for mapping and identifying lithologies. rock type. Figure 45 is a Gamma Ray Neutron-Density Log
Six techniques are presented here which can assist through the Ordovician Stony Mountain Shale and Red
geologists with lithologic determination and mapping. They River Formation in Richland County, Montana. Note in
are: (1) Gamma Ray Neutron-Density Log. (2) M-N* Figure 45 how the different rock types are related to log
lithology plot, (3) MID* thology plot, (4) Alpha mapping responses. As a quick look method, where there are a
from SP log, (5) clean sand or carbonate maps from gamma limited number of lithologies, this log works well for basic
ray log, and (6) rock typing and facies mapping. These lithologic and facies mapping. Whenever lithologies are
techniques are especially important to a geologist when more complex, additional logging devices and techniques,
lithologic data from cores or samples are unavailable. such as the M-N* and MID* plot, must be used. Of course.
after you determine lithology, you can prepare lithology or
Combination Gamma Ray Neutron-Density facies maps.
Log
M-N* Lithology Plot
The gamma ray log (Chapter V) measures the natural
radiation of a formation, and primarily functions as a The M-N* plot requires a sonic log along with neutron
lithology log. It helps differentiate shales (high and density logs. The sonic log is a porosity log (Chapter

radioactivity) from sands, carbonates, and anhydrites (low IV) that measures interval transit time. Interval transit time
radioactivity). The neutron log is a porosity device that is (At) is the reciprocal of the velocity of a compressional
used to measure the amount of hydrogen in a formation sound wave through one foot of formation. A sonic log,
(Chapter IV). The density log is a porosity device that neutron log, and density log are all necessary to calculate
measures electron density (Chapter IV). When these three the lithology dependent variables M* and N*. M* and N*
logs are used together (i.e. Combination Gamma Ray values are essentially independent of matrix porosity

Table 10. Matrix and Fluid Coefficients of Several Minerals and Types of Porosity
(Liquid-filled Boreholes).

_ _. _ Atya —Pma___(Psnp*)ma___ (PcLN*)ma

Sandstone (1) (Vina = 18,000) 6 > 10% 55.5 2.65 —0.0357 —0.05T
Sandstone (2) (V_, = 19,500) 6 < 10% 51.2 2.65 -—0.0357 | —0.005
Limestone 47.5 2.71 0.00 0.00
Dolomite (1) (@ = 5.5 to 30%) 43.5 2.87 0.0357 0.085+
Dolomite (2) (@ = 1.5% to 5.5% & > 30%) 43.5 2.87 0.02T 0.0657
Dolomite (3) (b = 0.0% to 1.5%) 43.5 2.87 0.0057 0.047
Anhydrite 50.0 2.98 -0.005 —0.002
Gypsun 52.0 2.35 0.4977
Salt 67.0 2.03 0.04 —0.01

From Schlumberger Log Interpretation Manual/Principles. Courtesy Schlumberger Well
Services: Copyright 1972, Schlumberger.
tAverage values
+? Based on hydrogen-index computation

118