Core-Based Geochemical Study of Mudrocks in Basinal Lithofacies in the Wolfberry Play, Midland Basin, Texas
Baumgardner, Robert W.; Hamlin, H. Scott
Wolfberry production (including Leonardian, Wolfcampian, and underlying Upper Pennsylvanian formations) totals 232 million barrels of oil and 592 billion cubic feet of gas from 1998 to 2011 (>50 million barrels of oil in 2011, alone). The Lower Permian Wolfcamp and Leonard are part of the Wolfberry play in the Midland and northern Val Verde Basins of Texas. Core-based study provides ‘ground truth' about the source rocks and carrier beds in this unconventional reservoir. Analysis of more than 1,000 feet of core from three wells near center of the Midland Basin in northern Reagan County shows that these rocks can be divided into four facies: 1-siliceous mudrock, 2-calcitic mudrock, 3-muddy carbonate-clast conglomerate, and 4-skeletal wackestone/packstone. These facies are interpreted as hemipelagic deposits and sediment gravity-flow deposits reworked, locally, by bottom currents. Facies are interbedded on scales ranging from centimeters (predominantly) to meters. Siliceous mudrocks contain relatively high total organic carbon (up to 6.3 percent), low manganese content, rare burrows, and common phosphatic nodules and pyrite framboids. Collectively, these features indicate that anoxia prevailed during deposition of these fine-grained sediments. Siliceous mudrocks display values of Corg/N <10, indicating that the associated organic matter has a large marine component. Measurements of total organic carbon and geochemical proxies (obtained by hand-held ED-XRF scans on 1-foot spacing) for marine productivity, reducing conditions, and organic matter accumulation do not co-vary consistently, suggesting that production, accumulation, and preservation of organic matter are multivariate processes that operate independently. Measurements of unconfined compressive strength show that most wackestone/packstones are more brittle than all siliceous mudrocks. Even so, mineralized fractures are present in all facies. The combination, in close vertical proximity, of abundant organic carbon, brittle mudrock, and thin, potentially ‘frackable' beds in the basinal Wolfcamp and Leonard makes these intervals attractive targets for fracture stimulation and horizontal well completion.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013