Background
The Anadarko Basin is a roughly 70,000 square mile region that spans western Oklahoma and the northeastern portion of the Texas Panhandle. As one of the most prolific oil and gas basins in the United States, the Anadarko Basin region produced over 575,000 barrels of oil per day by the end of 2018. The Anadarko Basin boasts production from a variety of Devonian and Mississippian age stratigraphic units with the focus on horizontal drilling and hydraulic fracturing of these self-sourced unconventional reservoirs. Discussions with industry contacts indicated that there was interest in SwRI conducting field training courses as well as technical service projects and a multi-client joint industry project (consortium). The goals of this project were (i) identification of geologic outcrops of reservoir-equivalent Paleozoic limestone, sandstone, and shale in close proximity to the Anadarko Basin that could serve as suitable analogs for reservoir intervals and deformation styles in the subsurface plays in Oklahoma and (ii) preliminary (reconnaissance-level) field characterization activities to generate a dataset suitable for demonstrating the outcrop-analog relevance to oil industry clients.
Approach
The project was accomplished with a multipronged approach. The first task focused on developing an ArcGIS database containing publicly available data on outcrop distribution, structural features, roads, county boundaries, and land ownership (state, federal, and private). Related literature was evaluated during this task and relevant information was assimilated into the GIS database where applicable. Particular emphasis was placed on identifying potential outcrop locations of relevant analogs to the formations producing from the Anadarko Basin in central and southern Oklahoma. The second task focused on field checking locations identified in the first task, data collection and documentation of observations, and data for each location. Given the wide extent of Paleozoic outcrops in Oklahoma, staff emphasized identifying high-value outcrops (i.e., outcrops of particularly good quality of exposure or particular relevance as subsurface analogs). Reconnaissance-level stratigraphic and structural characterization was conducted at suitable outcrop exposures, and included (i) documenting lithology and thickness of lithologic beds, and (ii) measuring and photographing structural features (i.e., bedding orientations, fractures/veins, faults, folds, stylolites), and relating the deformation styles and failure modes to the mechanical layering and structural position. The final task focused on performing a structural analysis of data to document distribution and orientations of structures observed in outcrop, compiling field reconnaissance data into a summary PowerPoint presentation, corresponding ArcGIS database, and regional deformation maps, in addition to completing the final report. The project products were designed to be readily adaptable for business development activities by SwRI staff.
Accomplishments
The objectives of this project were achieved. Geologic outcrops were identified for reservoir-equivalent Paleozoic limestone, sandstone, and shale in close proximity to the Anadarko Basin that could serve as suitable analogs for reservoir intervals and deformation styles in the subsurface play in Oklahoma. Field characterization activities were conducted that generated a data set suitable for demonstrating the outcrop-analog relevance to oil industry clients. In total, 51 field locations were evaluated in the Wichita Mountains region near Lawton and the Arbuckle Mountains region near Ardmore. Structural features observed in the Wichita Mountains region include extension fractures/veins, faults, and tectonic stylolites. Extension fractures at high angles to bedding are abundant with a range of strikes. East-northeast and east-southeast striking strike-slip faults occur along with north to northeast striking reverse faults. Sub-vertical tectonic stylolites, though not abundant, generally strike north. Structures in the Arbuckle Mountains include extension fractures/veins, faults, tectonic stylolites, and mesoscale folds. Extension fractures/veins are abundant and generally at high angles to bedding. Faults at high- and low-angles to bedding are abundant, exhibiting normal, strike-slip, reverse, and oblique slip. Patterns are simpler once fault orientations are rotated by removing fold axis plunge and local layer dip. In the rotated data, faults fall in two prominent populations (i) steep conjugate strike-slip faults striking northeast and north-northwest, and (ii) gentle to moderate-dip conjugate reverse faults striking northwest and southeast. Less common, tectonic stylolites were found in the Arbuckle anticline backlimb. Mesoscale folds are best developed in the Arbuckle Group limestones with fold axes similar to the Arbuckle anticline. Our study clearly demonstrated that the Wichita and Arbuckle uplifts provide opportunity to see reservoir-scale deformation in outcrop. Mesostructures like those analyzed in this project can strongly influence hydrocarbon production, providing a mechanical fabric that, along with mechanical layering, strongly influences hydraulic fracturing in unconventional reservoirs.