Edwards 1991: Control of depositional environments, eustasy, and gravity and salt tectonics on sandstone distribution in an unstable shelf edge delta, Eocene Yegua Formation, Texas and Louisiana Trans. Gulf Coast Assoc. Geol. Socs. v. 41, p. 237-252.
ABSTRACT
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Predicting the locations and geometry of downdip sandstone reservoirs
in the Yegua trend of Texas and Louisiana is a key challenge to explorationists.
In order to formulate and test depositional and stratigraphic models
for this purpose, well log, seismic and biostratigraphic data were integrated
over a distance of about 350 miles along strike. The resulting correlation
framework, based on almost 4,500 well logs, extends from updip on the
stable shelf to the downdip limit of well control on the slope.
Well logs provide the highest stratigraphic resolution in this data set. On
the stable shelf, the entire Yegua-Cook Mountain sand-bearing interval is about
I,000-1,500 ft. thick. Shale markers which show tens to hundreds of miles of
continuity along strike bracket progradational cycles 100-300 ft. thick. These
log-defined cycles comprise the basic mapping units used in this study. Seismic
reflectors in this zone are essentially parallel. Downdip of the shelf, complications
were introduced by growth faulting, erosion due to slope failure, mass wasting
of sediments overlying failure-induced erosion surfaces, and complex patterns
of basin fill seaward of the shelf margin. This complex zone thickens basinward
to several thousand feet. All of these zones were further complicated by contemporaneous
and post-depositional salt movement in the Houston Salt Diapir Province. Biostratigraphic
data were used to determine general ages for downdip section, to indicate prominent
discontinuities, and to show environmental trends.
Large-scale slope failures occurred several times during Yegua deposition,
resulting in regional erosional unconformities. It is not known if their origin
is related to eustacy. Slope deposits that onlap extensive erosional surfaces
are also well represented, but their precise age relationship to updip systems
is unclear.
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Paleoenvironments
were interpreted from maps of log-derived interval isopach, net sand,
percent sand, blocky sand and log facies. Each mapping unit is bounded
by regionally correlatable shale zones interpreted to have formed during
high stands. Strata bounded by the markers represent systems tracts comprising
upper delta plain, lower delta plain, mouth bar, distal mouth bar, and
shelf. Inferred local shelf bypassing, and channel incision and basinward
shifts in facies suggest small but significant falls in sea level during
deposition. Each of these cycles resemble "depositional sequences" but
in terms of their duration they are described as high frequency fourth
order events. Small-scale stratigraphic complexity within the mapped
units suggests frequent autocyclic shifts of depocenters.
Effective sand prediction downdip to the shelf edge and upper slope is
accomplished using transgressive shale ("condensed section" or "flooding surface")
markers as a stratigraphic framework, while recognizing the significant
effects of eustatic sea-level changes. Although widespread erosional
unconformities, possibly equivalent to sequence boundaries, could be
mapped on the slope, neither unconformities nor correlative conformities
could be identified sufficiently on the shelf to enable their detailed
and regional mapping.
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©by Marc B. Edwards
Consulting Geologists, Inc