Page 240 - The Geological Interpretation of Well Logs

~ THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

Quartz
Composition

Fetdspars ae
0
Y
Rock fragments

Figure 14.8 The Folk classification of sedimentary rocks. It is based on separate
variations in composition and texture. Composition is defined by quartz (non-
radioactive), feldspar (radioactive) and rock fragments (often radioactive).
Texture concems the variation between the elements of composition and clay
(after Folk, 1954),

30

A. 100 me
x
& 3 MARINE
N 20F
¥ z ENVIRONMENT
3 ese
> -
< Ww
| & j '
Oo
R iol
0 10 50 100
a“
| DOGGER 8 oo» ° . .¥ 4 GAMMA RAY API
_
°
0 , ! , 1 1 , **

30 «6440 «660 70 100 200
GRAIN SIZE
B. 100
{log median-p) me

% x DELTAIC
Figure 14.9 Textural analysis of the clay — grain size
FREQUENCY (PRESUMED CHANNEL)
relationship. Clay content compared to grain size for an ENVIRONMENT
alluvial sandstone (the Molasse) and a marine sandstone
(Dogger B). (From Pettijohn et al., 1972; modified from
Fuchtbauer, 1964). vy. fi
Oo
10 50 106
ray (Figure 14.11). In turbidites, grain size variations GAMMA RAY API
are seldom seen on the gamma ray (e.g. Shanmugam ef
Figure 14.10 Ganuma ray value correlated with grain-size
al., 1995), only bed thickness changes, and as beautiful-
classes. A The marine sandstone shows similar gamma ray
ly demonstrated by Slatt in outcrop studies (Slatt er al.,
API values for different grain size classes, i.e. there is
1992), interpreting the gamma ray response in terms of
overlap. B The deltaic sandstone shows excellent separation
bed thickness/grain size trends can be very misleading of classes. Average grain sizes from thin sections: cse =
(Figure 14.12). The clay volume-grain size relationship, coarse, me = medium, fi = fine, v.fi = very fine. (From
therefore, is sufficiently common to be thought universal, Simon-Brygoo, 1980).
230