--> Characterization and Integration of the Ellenburger, Viola, and Barnett Sections in the Northern Fort Worth Basin: Thoughts on Flow of Water From the Ellenburger Into Barnett

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Characterization and Integration of the Ellenburger, Viola, and Barnett Sections in the Northern Fort Worth Basin: Thoughts on Flow of Water From the Ellenburger Into Barnett

Abstract

The Barnett Shale system in the Fort Worth Basin continues to be an active exploitation target. Natural faults and fracture, as well as hydraulically induced fractures, can create communication between the Barnett and the Ellenburger resulting in loss of fracking fluids and induction of Ellenburger sourced water into the Barnett productive zone. A 498-ft core from a producing well (Devon No.7 Adams Southwest) in Wise County, TX presents a continuous section from deep within the Ellenburger, through the very thin Viola, and into the upper part of the Upper Barnett Shale. The significance of this study is to present a continuous Ellenburger to Barnett section and use this cored section as background for understanding the relationship of the induced-Ellenburger-water problem for Barnett fracking and production. The Ellenburger limestone and dolostone were deposited in peritidal environments. During the Middle Ordovician, the Sauk unconformity developed. Exposure during this unconformity karsted the upper several hundred feet of Ellenburger forming extensive cave systems that collapsed during burial. Extensive brecciation took place, but all the breccia-related fractures are occulted with sediment and cement. Preserved Ellenburger porosity (storage) is composed of intercrystalline micropores in dolomites. Through-going, large-scale fractures associated with cave-collapse and tectonic processes probably exist but are high angle and were not encounter in core. The Viola section is eroded where 100-million-plus years of strata are missing. In the core, only five feet of the Viola carbonate is preserved and displays soil sediment-filled fractures and solution features that are associated with the unconformity. The Viola section is tight and is a vertical seal. The dominant lithofacies in the Barnett section is organic-rich siliceous mudstone with various amounts of carbonate. During the approximate 25 million years of deposition of the Barnett muds, water depth appears to have remained deep (well below storm-wave base) as no evidence of shallow-water deposition has been documented. Pore network is composed predominantly of organic-matter pores. This core presents an opportunity to review a long stratigraphic section in the heart of the Barnett shale-gas play. This continuous core shows the producing Barnett mudstone section as well as the deeper, tight Viola and porous Ellenburger fractured sections that sources migrated water into the Barnett during production.