A Window into the Proterozoic — Using High-Quality 3-D Seismic Exploration Volumes to Image the Sub-basement in the Southeast Fort Worth Basin

The Fort Worth Basin is the most fully developed shale gas field in North America. Although imaged by perhaps one hundred 3D seismic surveys, the intense competition for acreage and focus on the relatively shallow, flat-lying Barnett Shale objective, has resulted in little published work on the basement structures underlying the lower Paleozoic strata. Major tectonic features including the Ouachita thrust-fold belt, Lampasas arch, Llano uplift and Bend arch surround the southeast Fort Worth Basin. Subtle folds and systems of large joints represent almost all 3D seismic surveys in the Fort Worth Basin with little to no vertical offset. At the Cambro-Ordovician Ellenburger level, these joints are often diagenetically altered, and exhibit collapse features at their intersections. Where the overlying Viola Limestone is absent, these joints and karst represent drilling/completion hazards that will produce water and negatively impact production. A few published papers in the core (Northeast) area of the Fort Worth Basin hypothesize basement control of these joints and collapse features. Unfortunately, these surveys do not exhibit coherent basement reflectors. In support of our investigation, Marathon Oil Company has provided a high-quality, wide-azimuth survey near the southeast fringe of the Fort Worth Basin. Although the Barnett Shale is uneconomic in this area due to relatively low thickness and TOC, this high quality, state-of-the-art data volume provides a unique image of complex basement deformation.

Our initial interpretation is based on the prestack time-migrated data volume. Although no-where near the data quality at the objective level, we can identify and pick sub-basement folds and thrust faults. We combine our structural interpretation with gravity and magnetic data over the area, thereby linking our interpretation to more regional tectonic features seen in Texas and Oklahoma. We combine Euler deconvolution on the magnetic data, limited well control (3 wells) and seismic velocity analysis to determine the depth to basement and the depth of magnetic anomaly sources. The high-quality seismic data in the Paleozoic section provides an accurate means of layer-stripping the effects of the well-imaged overburden for gravity inversion. Our long-term objective is to design a workflow that can use exploit the rich 3D seismic database provided by the shale-gas industry to provide a window into the less well-understood Proterozoic world.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California