Systems Tracts: Variations in Thickness and Lithofacies with Paleobathymetry

WORNARDT, WALTER W., JR., Micro-Strat Inc., Houston, TX, and PETER R. VAIL, Rice University, Houston, TX

Well log/seismic sequence stratigraphy dip sections located offshore Texas and Louisiana, Gulf of Mexico, permit the recognition of numerous sequences and systems tracts based on the integration of high-resolution biostratigraphy, paleobathymetry, well logs, and seismic profiles. The identification of sequence boundaries, maximum flooding surfaces, and lowstand systems tract boundaries within each sequence permits the recognition of the thickness and lithofacies variations of systems tracts with paleobathymetry in a downdip direction. The lowstand systems tract, basin floor fans are usually found in interslope basins in the middle to lower bathyal environment. In the Plio-Pleistocene lowstand systems tract, slope fan complexes are very well developed in upper to middle bathyal environ ents. Here they commonly consist of sand-filled channel and overbank deposits and attached lobes. The slope fan complexes typically pinch out by onlap in an updip direction near the outer neritic to upper bathyal boundary. In the lower bathyal environment the slope fan complexes usually consist of turbidite mudstones. The late Miocene and older slope fans are similar except they are commonly deposited in deeper (lower upper bathyal to middle bathyal) environments. The lowstand systems tract prograding complexes are commonly thin, hemipelagic shales in the bathyal environments that become very thick, shallowing-upward prograding deltas and shorelines in the upper bathyal to outer neritic environments. In the middle and inner neritic environments they are typically thick interbedded shoref ce sands and neritic shales. The updip equivalent is commonly fluvial incised valley fill. In certain areas with high deposition rates and a relative steep slope on top of the slope fan complex, shingled turbidites will commonly develop in outer neritic to upper bathyal environments. The transgressive systems tracts are very thin hemipelagic shales in the bathyal environments, develop basal shoreface sands with overlying shales deposited in the outer neritic environments, and become interbedded shoreface sands and neritic shales in the middle to inner neritic environments. If the incised valleys are not filed with sediments by the fluvial lowstand prograding complex, they are commonly filled with estuarine sediments during the transgressive systems tract. The highstand systems tracts are also thin in the bathyal environments, become thick prograding packages of sediments in the outer neritic environment, and have well-developed shoreface sands interbedded with marine shales in the middle to inner neritic environments. The variations in systems tracts with paleobathymetry downdip provide the biostratigrapher, geologist, and geophysicist a means to anticipate the overall thickness and lithofacies of the systems tracts on well logs and seismic profiles. Thick hydrocarbon reservoir rocks may be present in the deep-water basin floor fan, slope fan, channel fill, and overbank deposits and shingled turbidites. Shallow-water reservoirs occur in the prograding coastal belt and incised valley fill sands.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)