Page 258 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

bounded by coals and flooding surfaces. These are in the hierarchy above the parasequence set is the
parasequences. They are equivalent to the three electrose- systems tract, a ‘linkage of contemporaneous deposition-
quences evident on the logs and the flooding surfaces are ai systems’ (Posamentier et a/., 1988), where depositional
seen as high gamma ray, high neutron responses immedi- systems are three dimensional successions of facies.
ately above the coals, although the responses are masked There are essentially three systems tracts, lowstand,
to some extent by the coals themselves. transgressive and highstand. A combination of systems
In Exxon terminology, these three parasequences form tracts forms the Exxon sequence (Figure 15.1) and is
a parasequence set. That is, they show progressively deposited during one major rise and fall of sea level, typ-
changing characteristics which, in this case are: they get ically 0.5-3 Ma (Vail e¢ al., 1977). The schematic SP or
progressively thinner upwards, the sands get cleaner and gamma ray log characteristics of each of these systems
more porous (shown by the neutron-density separation), tracts are shown as published (Vail and Wornardt,
their depositional environment gets shallower, with both 1990)(Figure 15.13). A sequence may be made up of all
the seat earths and coals getting thicker and the fining-up three tracts or some combination, especially a transgres-
successions get more important. The environment seems sive followed by a highstand systems tract. In the
to have been shallowing by steps, each parasequence preceding log example (Figure 15.12), the prograding
being deposited closer to the shoreline. This is a pro- parasequence set is bounded at the base by a condensed
grading parasequence set (Van Wagoner et ai., 1990) sequence and maximum flooding surface: it therefore
and the progressive thinning of each set suggests that makes up a highstand systems tract (cf. Figure 15.13).
deposition was faster than accommodation. The key surface at the top should theoretically be a
In the Exxon sequence stratigraphic scheme, the next sequence boundary, but it has not been identified. This

SYSTEMS TRACTS - LOG CHARACTER (GR, SP}

SP or GR
KEY
TST SP or GR
HST HST highstand systems tract
TST transgressive systems tract
BC eVe_ MFS — ~- —-|— LST lowstand systems tract
MFS maximum flooding surface
SB sequence boundary
HST TST TSE transgressive surface of erosion
mes minor condensed section
af abandonment facies

~-4_J
MFS — > >=. SB — er erosion
condensed HST —-— MFs
sectlon — ss
=
TSE
HIGHSTAND TRANSGRESSIVE

multistorey
SP or GR SP or GR SP or GR SP or GR
7 voce cafe aoc

elma
channe!
rer
er
fm
fl Sir -—eye c
unamal-
oe ee gemores | ate
turbidite
a) PROGRADING COMPLEX {PC) b) SLOPE FAN COMPLEX (SF) ¢c) BASIN FLOOR FAN COMPLEX (BF)
sands
sands
channelf
ovarbank

hemi-
pelagic
shale

TRACT
LOWSTAND
SYSTEMS

after Vail & Wornardt, 1990
Figure 15.13 Model log patterns of sequence tracts, inciuding deep water deposits (from Vail and Wornardt, 1990).
248