Sequence Stratigraphy and Sedimentology of a Shelf-Margin Lowstand Wedge in the Deep Wilcox Flexure Trend of South Texas

SNEDDEN, JOHN W., Mobil Exploration Norway, Inc., Stavanger, Norway, JAMES C. COOKE, Mobil Exploration and Producing Services, Inc., Dallas, TX, and ROBERT K. JOHNSON and KEITH T. CONRAD, Mobil Exploration and Producing, U.S., Houston, TX

An integrated sedimentologic and biostratigraphic study of 15 wells and over 1400 ft (430 m) of core facilitated establishment of a sequence stratigraphic framework for the deep Wilcox Group of south Texas. This analysis also revealed the presence of a dip-restricted, sand-prone sediment wedge that produces hydrocarbons in growth-fault structures.

A sequence stratigraphic framework for the Wilcox was constructed via the use of faunal-increase markers, thin intervals present in well cuttings characterized by rises in the relative abundance of planktonic foraminifera. These marine flooding horizons can be utilized to subdivide the Wilcox Group into four depositional sequences termed P(aleogene)-8, P-7, P-4, and P-3, in descending order. Identification of standard sequence-bounding unconformities is hampered by the poor seismic expression of the Wilcox and the structural complexity of the area.

The Paleocene-age P-3 depositional sequence is unusual as dip correlation indicates that it is restricted to the flexure trend, tapering rapidly up depositional dip. This aggradational sediment wedge has no preserved coeval fluvial/coastal-plain system and can be miscorrelated with the Massive Upper Wilcox of the P-8 sequence. The wedge has a maximum thickness of 2000 ft (620 m) but a dip-length of less than 10 mi (16 km).

In cores, the section is dominated by fine-grained, burrowed to flat-laminated sandstones formed in wave-dominated shoreface and shelf environments. Evidence of deposition at the shelf margin, with attendant high levels of wave energy, may explain the abundance of wave-generated stratification in these sandstones.

P-3 sequence sandstones commonly occur in stacked, coarsening-upward parasequences. The P-3 sequence is interpreted as a shelf-margin lowstand wedge developed just after a rapid sea-level fall at 60 Ma. The lowstand wedge can be linked with an unconformity present within the Lobo (Lower Wilcox) interval of the updip stable platform area.

Recognition of the P-3 shelf-margin lowstand wedge also has important implications for exploration in the Wilcox flexure trend. It demonstrates that thick packages of sandstone may be found basinward of the apparent shale-out of updip sandstones. Shelf-margin lowstand wedge sandstones have greater strike-continuity and higher net/gross ratios than downdip basin-slope and basin-floor submarine fan complexes.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)