Page 92 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -
LITHOLOGY

DOLOMITE (argillaceous) 5 0 GAMMA 1 RAY API 1

Y (saliferous

POTASSIUM SALTS
i) a
(some halite)
(m)
HALITE (salt)
thickness

ANHYORITE
a oO
with dolomite bands

Figure 7.21 Potassium salts giving very high peaks of radioactivity in an evaporite sequence. (The lithology comes from an
interpretation of combined logs and cuttings). Permian, North Sea.

Coal and organic rich shale cyclic deltaic sequences, a low gamma ray coal is imme-
(source rock) radioactivity diately overlain by a high gamma ray, organic-rich shale
The relationship between organic matter and uranium (Figure 7.23). It appears that uranium, which is adsorbed
enrichment is the basis for being able to identify organic by organic matter to cause the high gamma ray values in
rich shales (source-rocks) using the gamma ray log and is the organic rich shales, is not adsorbed by organic matter
discussed under Section 7.10 (Organic matter and source in terrestrial swamps where no clay is present. Clay
rocks: uranium content). In practice, although high seems to be a catalyst to the adsorption. Thus, pure coals
gamma ray values often correspond to organic matter have the typical low gamma ray response while shaly
rich, source-rock intervals (Figure 7.22), such intervals coals have a gamma ray value which depends on the shale
do not always have a high gamma ray value (i.e. (or ash) content {Kayal and Christoffel, 1989).
Schmoker, 1981 and Section 7.10 below).
o—@eo % ORGANIC CARBON 200
Coals have low gamma ray values (Figures 7.1, 7.23). Igneous and voicanic rock radioactivity
The contrast in this response between pure coals and Igneous rocks are not volumetrically important in petro-
organic shales is remarkable, especially when, in typical leum wells, but occur sufficiently frequently to be a

UTHOLOGY
10
8
6
GAMMA RAY API
&
100

ORGANIC MATTER TYPE
sapropel 50%
(m)
depth

gamma tay
a
Z-organic ni¢ ¢ carbon sapronel 90%

sapropel 20-50%
cuticules
sapropel 50-30%

Figure 7.22 High organic carbon values and the total gamma ray giving good correlation, in this case due to uranium associated
with organic matter.
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