Figure 1. Depositional sequence and associated systems tracts in Plio-Pleistocene strata of the offshore Louisiana South Additions. HST= highstand systems tract, TST =transgressive systems tract, PW = lowstand prograding wedge, SF = lowstand slope fan, BFF = lowstand basin floor fan. Interpretation by TGS/GECO.

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Plio-Pleistocene strata in offshore Louisiana were deposited along an unstable progradational continental margin. Systems tracts differ greatly from those described for stable progradational continental margins.

Basin-floor fans commonly show a single reflector which downlaps onto the sequence boundary. They contain well-defined blocky sandstones which exhibit good reservoir characteristics. Lowstand slope-fans exhibit concave-upward reflections associated with reflections which downlap away from them (channel-overbank facies), chaotic reflections and thin parallel units. Good reservoirs are present in channel and nearby overbank deposits.

Reflections in lowstand prograding wedges exhibit divergence toward downthrown sides of growth faults. Progradational patterns are usually not observed. Good reservoir sands occur in sand-rich coarsening-upward sequences. Both transgressive and highstand systems tracts generally exhibit concordant reflections. Reservoir quality is generally poor in these systems tracts.

Condensed intervals, identified by major increases in abundance and diversity of planktonic microfossils, occur at the maximum flooding surface and top of the slope fan systems tract. Since biostratigraphic resolution is finer than the time interval over which the sequences are deposited, systems tracts can be accurately correlated throughout the basin. They can then be effectively used to date geologic events and predict location of reservoir and seal strata.

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