Page 239 - The Geological Interpretation of Well Logs

- FACIES, SEQUENCES AND DEPOSITIONAL ENVIRONMENTS FROM LOGS -

and mineral mix} and, more importantly, there are natu-
rally radioactive minerals in the sand-sized grain fraction
(Rider, 1990). For example, sands which contain potassi-
um feldspars have a natural (potassium) radioactivity.
Gamma ray log variations wilt therefore reflect the distri-

crevasse
splay es bution of the feldspars, which may vary with grain size,
and not clay. Other radioactive grains include micas,
awamp

glauconite, heavy minerals and rock fragments in gener-
delta wr
fringe (
al, all of which will disturb gamma ray log shape
interpretation. These grain type variations are normal and
are the basis of the often used classification of sands
devised by Folk (1954) which shows quartz (non-
radioactive), feldspars (radioactive) and rock fragments
Strandplain
(frequently radioactive) as the compositional elements

shoreface
which vary independently against clay content which
istal dalta front

gives the rextural element (Figure 14.8).
shorefaca
Secondly, the relationship between clay volume and

analysis in the example (Figure 14.9) is of two sand-
grain size. There is no doubt that a grain size/clay
content relationship exists (ie. Figures 14.2, 14.3).
However, it is by no means a constant one. The textural

SERRATED
FUNNEL
BELL

AGRADATIONAL
PROGRADATIONAL
relationship between grain size and clay content, and
stones, one an alluvial molasse showing a consistent
the other a well-winnowed, marine sandstone with no
Figure 14.6 A palaeogeographic reconstruction based on SP
log shapes in a deltaic environment, the Cayce sandstone relationship at all (Pettijohn et af, 1972). Using the
(modified from Finley and Tyler, 1986). gamma ray tog itself and comparing it to grain size
fractions over cored intervals, brings out similar relation-
ships (Figure 14.10): in some sandstones where clay
(generally gamma ray), indicating facies relationships
content varies with grain size, so do the gamma ray
within parasequences (Figure 14.7), parasequence stack-
values (Figure 14.10) in others there is no relationship
ing patterns or facies changes at sequence boundaries
(Figure 14,10a). Frequently, good relationships are
(Chapter 15). Indeed, often a heavy reliance is put upon
seen in deltaic and fluvial environments, where deposi-
log shapes. However, from a sedimentological viewpoint,
tion is largely controlled by flowing current energy.
relating a particular log shape to a particular facies should
However, in many cases, the changes in grain size
not be done (Cant, 1992), there are too many overlaps of
which are essential to the identification of sedimentary
different facies giving similar shapes. But the questions
structures or sequences do not involve changes in clay
which arise from a logging view are more fundamental. Is
content or at least changes sufficient to affect the gamma
there a consistent reason why log shapes exist? Do they
signify what is claimed of them? The reliability of using
log shapes to supply grain size, and ultimately, facies
upper lower
information is examined below.
foreshore A shoreface shoreface offshore
The gamma ray - grain size relationship; ——
——S
a critical examination
parasequence
The gamma ray log does not vary because of changes in

grain size; it varies (often) because of changes in clay
content (the same is true for the SP). However, sedimen-
tologica] interpretations based on gamma ray log shapes
require the log to vary with grain size. This involves
undeclared assumptions. 1: gamma ray variations are
related to clay volume changes and, 2: clay volume
changes are related to grain size differences. These two
assumed relationships will be examined. seaward dipping planar beds BSS trough cross-beds
First of all the gamma ray relationship to clay volume.
hummocky beds Bay bloturbated marine mud
This has been discussed previously (Sections 7.6, 7.8).
It was pointed out that care must be taken in using the
Figure 14.7 The gamma ray log used to show facies
gamma ray to indicate clay volume, because clay radio- differences in a sequence stratigraphic analysis (from
activity is not constant (it depends on the clay mineral] Yan Wagoner ef af., 1990).
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