Page 237 - The Geological Interpretation of Well Logs

- FACIES, SEQUENCES AND DEPOSITIONAL ENVIRONMENTS FROM LOGS -

3 GAMMA RAY
3 8 API 160

z= grain size
Gamma ray Grain size Lith.
a 1000p 100 204% Observations
fe API 360 00H 1908 ----J
amr Om

OISTRIBUTARY CHANNEL
10}
_

—P ty 2 yess
3 io

@me\e 1m 4—10m oss - grain size

iT (fA a
III
SAND BAR
aa 2?) 1S
V4 2
x
coal

30m yore silt laminae

Figure 14.3 Funnel shapes on the gamma ray log correspond- DISTRIBUTARY CHANNEL
ing to coarsening-upwards sequences, interpreted to have Figure 14.4 Grain size — gamma ray correlation: a very close
been deposited in prograding, estuarine shorelines. The relationship is possible between the two. (Modified from
gamma ray shapes in this example are characteristic, Simon-Brygoo, 1980).

reverse, a decrease in clay content. size trends and, by sedimentological association, as facies
A core cut through a Triassic fluviatile sand body in the successions. A decrease in gamma ray values will indi-
Sahara (Figure 14.2) shows a typical bell shape on the cate an increase in grain size: small] grain sizes will
gamma ray log response (Serra and Sulpice, 1975). The correspond to higher gamma ray values (Figure 14.4).
increase in gamma radiation corresponds to an increase in The sedimentological implication of this relationship
clay content regularly upwards. The increase in clay con- leads to a direct correlation between facies and log shape,
tent is correlated to a decrease in the sand-grain size. A not just for the bell shape and funnel shape as described
sedimentological analysis of the core shows a set of sed- above, but for a whole variety of shapes.
imentary structures typical of fluviatile point bar deposits Numerous publications show the log shapes expected
(Serra and Sulpice, 1975). The bell shape, therefore, can or found in various facies (Krueger, 1968; Galloway,
be interpreted as indicative of a fining-upwards, fluviatile, 1968; Fisher, 1969; Fons, 1969; Pirson, 1970; Goetz et
point bar sandstone (in this case). A second example al., 1977; Coleman and Prior, 1982; Galloway and
shows gamma ray funnel shapes with the corresponding, Hobday, 1983; Vail and Wornardt 1990; Van Wagoner e
core-derived sedimentology (Figure 14.3). Each funnelt- al., 1990; Cant, 1992). They all depend on the relation-
shape represents a succession coarsening-upwards ship between log shape and grain-size trends in sandstone
from bioturbated, offshore muds to silts to bioturbated, bodies: A bell shape indicates a fining-up sequence which
shallow marine sands capped by root beds and coals. The may be an alluvial/fluvial channel but also a transgressive
successions are interpreted to have been deposited in shelf sand. A funnel shape is a coarsening-up succession
prograding, estuarine shorelines. The funne] shape is which may be a deltaic progradation or a shallow marine
therefore indicative of coarsening-up, prograding estuar- progradation (Figure 14.5). The analogies may even be
ine shoreline successions (in this case). extended to deep sea deposits. In these cases the log
These examples show the close relationship possible shapes are those of overall successions rather than indi-
between the gamma ray log and sandstone grain size. vidua] bodies (Parker, 1977). The shapes come from the
Shapes on the gamma ray log can be interpreted as grain- diminution in bed thickness associated with diminution

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