THE PENNSYLVANIAN LOWER STRAWN FORMATION, JACK AND WISE COUNTIES, FORT WORTH BASIN: FACIES DISTRIBUTION AND STRATIGRAPHIC ARCHITECTURE

Lower Strawn stratigraphy in Jack and Wise Counties consists of several “cycles” of shallow-marine deposits prograding onto the structurally-high Bend Arch. Given the enhanced scrutiny required for finding conventional reserves in mature basins, an in-depth stratigraphic analysis and interpretation of the Lower Strawn lithofacies distribution is presented. Previous work focused on the Upper Strawn in North Central Texas, and more distal Lower Strawn south toward the Llano uplift. However, this study examines the Lower Strawn in Jack and Wise counties where shallow-marine systems dominated. The Lower Strawn comprises 2,000 ft of strata below the Brannon Bridge Limestone and above the Caddo Limestone. Deltaic elements identified from whole core include, but are not limited to distributary-channels, channel-mouth-bars, distal-delta-front deposits, and tidally influenced shoreline. A relative lack of trace fossils combined with channelization, turbidites, and slumps suggest recurrent high discharge. Based on wireline log characteristics individual, progradational deltaic deposits in Jack and Wise Counties are approximately 30 mi long by 15 mi wide. Regional stratigraphic flooding surfaces reveal the extent and distribution of these coarsening-upward siliciclastic and carbonate units, provide a stratigraphic framework, and highlight the frequency of the repetitive stratigraphic motifs or cycles. Additionally, incorporation of a 30 mi² 3D seismic dataset allows for tying motifs to seismic data. Delineating the boundaries between siliciclastics and thin carbonate units is an essential, yet challenging step towards understanding the distribution of Lower Strawn depositional systems. The Bend Arch was a structural high throughout Lower Strawn deposition, thus paleo-topographic variation along this structure provided a focus for these prograding systems. Thickness maps demonstrate Bend Arch influence on sediment distribution, and provide a model for depositional variability along this trend, and in analogous foreland basins globally. The integration of core, wireline logs, and seismic data enables the identification of specific depositional elements in the Lower Strawn and provides a better understanding of reservoir characteristics and geometries in this area of the Fort Worth Basin.