Structural and Stratigraphic Controls on Reservoir Quality and Distribution of the Oligocene Vicksburg formation, Brooks County, Texas.

The Vicksburg formation, composed mainly from fluvial and deltaic sandstones, spans the western and northern Gulf of Mexico. Upper Vicksburg formation (lower Oligocene) represents a significant gas producing horizon in South Texas. Gas productivity and quality from this horizon is believed to be affected by the stratigraphic and/or the structural settings of the Vicksburg formation. Brooks county is selected as a test site to examine the structural and stratigraphic controls on Vicksburg's reservoir quality and distribution. This site is part of the Vicksburg flexure growth fault system where it is bounded by the Wilcox fault in the northwest and the Frio fault in the southeast. Gas reservoir in this site is represented by anticlinal features composed of fluvial/deltaic sand channels draped over immense rollover anticlines formed downdip of the Vicksburg fault system. Integrated approach using 3D seismic data (area: 5 mi2) as well as 82 well logs were used to map subsurface distribution of six horizons within the Vicksburg formation namely V14, V15, V16, V17, V19, and V20. Results indicate that the main growth fault system is the Vicksburg flexure that formed anticlinal rollovers. These rollovers represent the main structural highs within the Vicksburg formation. Synthetic and antithetic faults associated with the main growth fault system are compartmentalizing reservoirs within the Vicksburg formation and affecting the reservoir distribution and quality. Understanding the nature and the spatial distribution of these faults is key to improving our understanding of the quality and distribution of gas reservoirs within the Vicksburg formation in Texas.