ABSTRACT: Deltaic Depositional Systems on an Unstable Shelf Margin, Vicksburg Formation, South Texas

LANGFORD, RICHARD P., Bureau of Economic Geology, University of Texas at Austin, Austin, TX

The deltaic deposits of the early Oligocene lower Vicksburg Formation were deposited on an unstable continental shelf margin where interplay between growth faulting and sedimentation resulted in the complex geometries of present-day reservoirs. The deltaic deposits are easily recognizable in dip-oriented seismic lines by their distinctive wedge shape. Wedges are shingled and backstepping, with older wedges lying basinward of younger wedges. Wedges roll over into the underlying growth fault, with dips commonly exceeding 45 degrees. Internally, wedges are arranged in parasequence sets, bounded by angular unconformities that formed when deposition slowed but fault movement continued. Individual wedges may be over 1000 ft (300 m) thick along their contacts with the growth fault, but they inch out, or are truncated by unconformities, within 3 mi (5 km) down depositional dip. Correlation of well logs and interpretation of seismic lines reveal abrupt changes in depositional environment over short distances. The deltas are strike elongate, extending up to 5 mi (8 km) along faults. Individual deltaic sandstones exhibit a dramatic downdip transition from distributary channel facies to delta front and prodelta facies. Sedimentary structures and facies relationships indicate that deltas were deposited in a mixed fluvial/wave-dominated system where there was extensive wave reworking of the fluvial component. There is a transition, repeated in numerous stacked deltaic sandstones, from fluvially dominated deposits in the south to more wave-dominated deposits in the north, suggestin a northerly longshore current. Understanding of this system allows better delineation of productive reservoirs and reservoirs with incremental production potential, possibly through well recompletion. This study was funded as part of the Secondary Gas Recovery Project by the Gas Research Institute, the U.S. Department of Energy, and the State of Texas.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)