Contrasting Styles of Late Neogene Deep-Water Sandstone Depostition Offshore Texas

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Middle Miocene and middle Pliocene deep-water reservoirs of the western Gulf Coast Basin are associated with failed shelf margins and subregional unconformities referred to as submarine pediments. The submarine pediments formed broad, convex-landward arcs along nondeltaic slopes and on the southwestern flanks of subjacent delta systems. They were created by retrogressive failure and were later enlarged by erosion during periods of lowered sea level. At times of lowered sea level, the deep embayments carved into the continental platform funneled nearshore sands downslope to basin-floor fans.

The pediments were first backfilled by deep-water mudstones deposited by mass transport processes. These slump blocks and high-energy turbidites exhibit mounded to chaotic seismic reflections that dip landward. Later, sand-rich channel-levee complexes were deposited above the basal mudstones and near the seismic fades transition from chaotic reflections to overlying horizontal or wavy reflections. The pre-entrenchment morphology of the shelf margin was finally restored by coalescence of small, prograding deltas that are recorded as clinoform reflections.

Unconfined lower slope and basin-floor fans, associated with the submarine pediments, are generally sand-poor. The sand-rich lowstand fan deposits are restricted to highly elongate, dip-oriented leveed channels that mark the principal pathways of sediment transport.

Sandstones confined to leveed channels of the upper fan and pediment fill are the most prolific hydrocarbon reservoirs within each stratigraphic sequence. These channel sandstones exhibit high vertical continuity but low lateral continuity because interbedded turbidite mudstones increase away from the channel axes. Thin sandstones of the lower fan may exhibit high lateral continuity, but they typically have poor reservoir properties because of high concentrations of original muddy matrix.


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