Pliocene-Pleistocene Hydrocarbon Plays of Offshore Texas

Robert A. Morton

Pliocene-Pleistocene depositional sequences of the Texas continental shelf and slope contain an estimated 1.5 billion bbl of oil equivalent in at least 75 different fields. Principal sandstone facies and associated hydrocarbons are products of repeated sea level fluctuations, shelf-margin progradation, and salt mobilization. Initial deposition was by deep-water fans that constructed an apron on the upper slope and elongate lenses in intraslope depressions. Later, large delta systems occupied the outer shelf, where accommodation space was created by evacuation of salt. The fields, which are located near diapirs or faults associated with late salt movement, can be divided into six plays. Play 1 is characterized by small fields (< 10 million bbl of oil equivalent), minor tructural deformation, poor seal development, and thin objective section above the basal unconformity. Play 2 produces gas and some oil where Globoquadrina altispira and Lenticulina 1 slope sandstones are structurally inverted. These sand-rich submarine fans were superimposed by consecutive sea level lowstands. Play 3 contains moderately large fields within a salt withdrawal basin where the Angulogerina B section is greatly expanded. Typical reservoirs are thick, upward-coarsening sandstones deposited by shelf-margin deltas. Play 4 is located on the westernmost flank of the depocenter; consequently, it has limited potential because of small field size and lack of significant reservoir facies other than thin Lenticulina 1 slope sandstones. The largest Pliocene-Pleistocene fields occur in lay 5, where stacked reservoirs (Globoquadrina altispira to Globorotalia flexuosa) and broad rollover anticlines formed within a graben between the Trimosina regional and counterregional fault systems. Hydrocarbons are also anomalously enriched beneath the modern slope (play 6) where Globoquadrina altispira and Lenticulina 1 slope sandstones as well as Trimosina lowstand wedges produce gas and some oil. Fields in this deep-water play are downthrown on large salt-cored faults exhibiting recent movement. All six plays offer some potential for either new field discoveries or reserve growth.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.