--> Porosity, Permeability, Pore Characterization and Rock Mechanics of the Triassic Cow Branch and Walnut Cove Formations: A Continuous Gas Assessment Unit, Dan River Basin, Stokes and Rockingham Counties, North Carolina, USA

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Porosity, Permeability, Pore Characterization and Rock Mechanics of the Triassic Cow Branch and Walnut Cove Formations: A Continuous Gas Assessment Unit, Dan River Basin, Stokes and Rockingham Counties, North Carolina, USA

Abstract

The Late Triassic (Norian) Dan River basin is a continuous gas assessment unit (AU) and a total petroleum system. The source rocks (Walnut Cove and Cow Branch Formations) are thick grey and black freshwater shales; the stratigraphically lower Walnut Cove Fm. has a thin basal coal. These lacustrine strata were deposited in a rift basin near the paleo-equator after Pangea's breakup. The AU has an estimated mean gas content of 49 BCFG, and a natural gas liquids content of 0 MMBNGL (USGS Fact Sheet 2012 - 3075) based on limited 2011 data. The Walnut Cove Fm. is up to 600 feet thick with an outcrop strike of ~22 miles and a width of several miles. The Cow Branch Fm., up to 1,500 feet thick, is exposed in broad patches in Stokes and Rockingham counties, NC, and isolated localities in southernmost Virginia. The hydrocarbon potential of these two formations as shale gas reservoirs was characterized from diamond drill core hole SO-C-2–81 (Walnut Cove Fm.), and the Cow Branch Fm. exposed continuously in the Ararat (aka Solite or Cemex) quarry. Characterization results reported herein include substantial new data not available for the 2011 USGS assessment. They are: 1) organic geochemistry and thermal maturation data (doubling the 2011 data set); 2) down hole XRD whole rock mineralogy for Walnut Cove Fm., n= 34, outcrop whole rock XRD for the Cow Branch Fm., n = 13; 3) rock mechanics including triaxial compressive strength tests with acoustic velocities, pre- and post CT scans, and Young's Modulus and Poisson's Ratio (Walnut Cove Fm., n = 7, Cow Branch Fm., n = 13); 4) mercury injection capillary pressure data obtained to characterize porosity and permeability in the both formations (Walnut Cove Fm., n = 14, Cow Branch Fm., n = 27) with maximum pressure of 60,000 psia providing a pore aperture frequency distribution down to nanometer-scale diameter; and 5) pore characterization using SEM and ion beam milled samples. This is the third report characterizing the continental Triassic rift / lacustrine deposits in NC. Previous reports on the time equivalent Cumnock Fm., Deep River basin, NC, are available on ‘Search and Discovery’. The Cumnock Fm. consists of black organic-rich mudrocks with significant porosity. Initial results from characterizing these formations show them as petroleum source rocks (wt% TOC > 2.0). Mineralogical composition varies among the three different mudrocks, which likely has implications for inter- and intra-particle porosity trends.