Page 257 - The Geological Interpretation of Well Logs

- SEQUENCE STRATIGRAPHY AND STRATIGRAPHY -

trosequence analysis as anomalous. They can be used to At the equivalent hierarchichal level to the systems tract,
correlate from the deeper to shallower depositional envi- but differently defined, is the genetic stratigraphic
ronments and can also be used to divide the section up sequence (Galloway, 1989a, b) (Figure 15.1). A genetic
into sedimentologically distinct electrosequences (facies stratigraphic sequence is a package of sediment recording
successions). Moreover, their nature as condensed can be a significant depositional episode of basin margin out-
confirmed by microbiological investigation as they con- building and basin filling, bounded by sediments
tain increased numbers of pelagic fauna: this may also representing periods of widespread basin margin flooding
lead to a dated event. The more important condensed (te. condensed sequences). A genetic stratigraphic
sequences are chronostratigraphic markers. However, not sequence comprises regressive followed by transgressive
all condensed sequences are enriched in organic matter sedimentary successions, bounded by the isochronous
(Figure 15.11) and not all intervals enriched in organic marine flooding surfaces or condensed sections, other-
matter should be assumed to be condensed sequences. wise termed maximum flooding surfaces (Partington et
al., 1993 a, 8).
Facies successions: parasequence & parasequence sét, Linked depositional systems are being identified at
systems tract, genetic stratigraphic sequence and this scale and are recognised as bounded by major, key
electrosequence. surfaces such as the transgressive surface, maximum
The buitding blocks of sequence stratigraphy are hierar- flooding surface or sequence boundary. These will be
chical (Figure 15.la) and the key surfaces described illustrated, with discussion, by real examples. At this
above allow each block to be recognised (and defined). scale, large datasets are involved but are difficult to illus-
The lowest order of the hierarchy to be considered in trate in book form so that very reduced example sets are
this text is the parasequence which (for Exxon) is bound- used.
ed by flooding surfaces. A parasequence and the older To identify a parasequence on the logs, both the facies
term sedimentary cycle are almost the same. However, to succession and the bounding marine flooding surfaces
avoid confusion, the term cycle has been replaced in this must be diagnosed. A facies succession (cycle) is nor-
text by ‘succession of facies’ (Walker, 1992) to indicate mally seen as an electrosequence, and will be identified
a sequence in purely sedimentological terms (i.e. not as such in the electrosequence analysis (Chapter 14) as
sequence stratigraphic). In the Exxon scheme, several the persistent, upward change in log parameters, both in
parasequences build up to form a parasequence set the fine grained shale and coarser grained, sand intervals,
which, together form a systems tract deposited during a on all the logs. The core used in the example (Figure
particular stage in the cycle of sea-level rise and fall. 15.12) shows three coarsening-up facies successions

SED. (core)

te
4140m
>
X X
x \ \ X \
of
|
4150m
StS
>
sh

ww
>
peanrt a @ 4160m
T aw eas
(Rar

aa
Ce
paeenseett 4170m
a ay 4180m
og
ag
«®

Figure 15.12 Example of a progradational, parasequence set. Upwards the sands become coarser grained and with higher energy
structures, the coals become thicker and the parasequences become thinner progressively upwards {rate of accommodation less
than rate of sedimentation). A maximum flooding event is interpreted at the base of the parasequence set, indicating that these are
highstand deposits.
247