South Harmony Church Field, Southwest Louisiana--Further Insights on Uppermost Wilcox Shelf-Margin Trend

Paul G. Belvedere

The uppermost Wilcox Group (Eocene) in the South Harmony Church field is a 2,000-ft thick sequence of single and multistory sandstones and shales. Cores of three separate intervals from four different wells display sedimentologic and lithologic features characteristic of inner shelf and shoreface deposition, representing an offshore bar complex. Typical lithofacies assemblages within an interval include, from top to bottom, (1) bioturbated, muddy sandstone--inner shelf, (2) less bioturbated, arenaceous sandstone--mostly inner shelf, (3) structureless to faintly horizontally laminated sandstone--mostly lower shoreface, and (4) horizontal to cross-laminated, and commonly deformed sandstone with shale and siltstone interbeds--lower shoreface.

Sandstones are fine to very fine-grained, poorly to moderately well-sorted sublitharenites. Optimum reservoir quality is observed in the bioturbated, clean sandstones of the shelf-shoreface transition zone, where porosity ranges from 5 to 23% and permeability from less than 0.01 to 31 md. Porosity is mostly secondary and attributed to the leaching of labile constituents.

The South Harmony Church field is part of the uppermost Wilcox paleoshelf-margin trend (the downdip Wilcox) that extends across south Louisiana and parallels the underlying Cretaceous carbonate-reef trend. The geologic parameters associated with the uppermost Wilcox in the study area (Allen Parish) resemble those in the Fordoche and Lockhart-Crossing fields in the central and eastern part of the trend, respectively. These similarities include (1) rollover anticline production, (2) shelf to nearshore depositional setting, and (3) lithology. In light of these observations, similar deposits can be expected along this trend, thereby suggesting further exploration for the development of downdip Wilcox fields.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.