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ABSTRACT: Sequence Stratigraphy of Plio-Pleistocene Strata in the South Additions: Offshore Louisiana

Bruce Bowen, Bernie Shaffer, John Beard, Jory A. Pacht

TGS Offshore Geophysical Company and GECO Geophysical Corp. have conducted a regional sequence stratigraphic study of the Plio-Pleistocene in offshore Louisiana south additions. Strata are deposited along an unstable progradational continental margin and systems tracts differ greatly from those described for stable progradational continental margins. Delineation of these units required iterative analysis of seismic, wireline, and biostratigraphic data.

Condensed intervals were interpreted from 45 wells. These intervals were identified by major increases in abundance and diversity of nannofossil and planktonic foraminifera. Biostratigraphically identifiable condensed intervals occur at the maximum flooding surface and on top of the slope fan systems tract within each depositional sequence. Since biostratigraphic resolution is finer than the time interval over which the sequences are deposited, systems tracts can be regionally correlated on seismic sections with a high degree of accuracy.

The systems tracts exhibit unique seismic characteristics. Basin-floor fans commonly show a single reflector that downlaps in both directions onto the sequence boundary. Lowstand slope-fans exhibit concave-upward reflections associated with reflections that downlap away from them (channel-overbank facies), chaotic reflections, and thin parallel units. Reflections in lowstand prograding-wedge systems tracts exhibit divergence toward downthrown sides of growth faults. Unlike stable passive margins, progradational patterns are not observed. Both transgressive and highstand systems tracts generally exhibit concordant reflections. Oblique progradational patterns, common in highstand strata along stable progradational margins, are rare.

These systems tracts also exhibit characteristic patterns on SP and resistivity well-logs. Basin floor-fans commonly exhibit well-defined blocky sandstones and excellent reservoir potential. Good reservoir sands are also present in the lowstand prograding wedge system tract, characterized by sand-rich coarsening-upward sequences. Lithology in the lowstand slope-fan systems tract is highly variable. However, prolific reservoirs are present in channel facies, which exhibit blocky or fining-upward sequences. Reservoir quality is generally poor in the transgressive systems tract (generally characterized by blocky mudstones) and highstand (mud-rich coarsening-upward sequences) systems tract.

This approach allows systems tracts to be correlated throughout the Gulf Coast Basin and effectively used to time geologic events in the basin and to predict location of reservoir and seal strata.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990