ABSTRACT: Upper Strawn (Desmoinesian) Carbonate and Clastic Depositional Environments, SE King County, TX

Todd H. Boring

The Pennsylvanian upper Strawn Group of southeast King County, Texas, provides a unique setting to study interactions between coeval carbonate and clastic deposition during the Desmoinesian. One of the most perplexing problems is the relationship of massive Pennsylvanian platform carbonates to shallow-water marine and deltaic sediments. Within the study area, carbonate facies were deposited along the northern edge of the Knox-Baylor trough on the Spur platform, and terrigenous clastics were carried toward the Midland basin through the Knox-Baylor trough.

Based on the analysis of subsurface cores, five carbonate lithofacies and four clastic lithofacies were recognized in southeast King County, Texas. The distribution and geometry of these lithofacies are related to variations in the rate of subsidence in the Knox-Baylor trough, Pennsylvanian tectonics, deltaic progradation, avulsion, and compaction. The platform carbonates within the northern region record environments within the carbonate platform complex, including middle platform, outer platform, algal mound, and platform margin. The quartzarenitic sandstones within the southern region occur in a variety of complex depositional geometries, including distributary bar fingers, lobate deltas, and offshore bars.

The upper Strawn Group provides an attractive area for exploration geology. Both carbonates and clastics provide excellent reservoirs from a depth of approximately 5000 to 6000 ft. Total production, since the early 1940s, within the area is over 100,000,000 bbl of oil. Multiple pay zones within a 600-ft interval also provide an added incentive for exploration. Areas within and around the Knox-Baylor trough deserve additional study due to these relatively shallow, unexplored, multiple pay zones.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990