A Diagenetic Study of the Wolfcamp Shale, Permian Basin, West Texas

Abstract

The Permian Wolfcamp Shale is a major unconventional resource play in the Permian basin of west Texas and southern New Mexico. Petrographic methodologies were used on two cores in the Midland basin to compile a diagenetic history. The Wolfcamp shale is comprised of calcareous and siliceous shale interbedded with calclithites and wackestones/ packstones deposited as sediment gravity flows. The paragenesis is complex with predopositional diagenesis in the lithoclasts partially overprinted by postdepositional diagenesis (dolomitization). Some lithoclasts contain an apparent early marine cement followed by blocky calcite cement. Partial chert replacement in the lithoclasts is evident, however the timing is unknown. Other phases include authigenic replacement of allochems by albite, dolomite, and pyrite. The packstones contain patchy chert and ferroan dolomite which is associated with porosity. In the calcareous and siliceous shales, early diagenetic phases include framboidal pyrite, small dolomite rhombs, some with an iron rim, and calcite concretions. Later burial diagenetic events include ferroan dolomite and barite/celestine. Authigenic sphalerite occurs as thin laminations and within allochems. The timing of sphalerite precipitation is either during early diagenesis, likely related to bacterial sulfate reduction, or remobilized zinc and sulfate during later burial diagenesis. Organic matter is present, including filling porosity in pyrite framboids. Horizontal fractures, interpreted as a result of overpressuring are filled by ‘beef’ type calcite. Some vertical fractures are cemented by blocky equant calcite, quartz, and dolomite with ferroan alterations on the edges. Some fractures cut the calclithites and shale whereas others terminate at the calclithites and packstones. These preliminary observations suggest that the Wolfcamp shale behaves as a closed system where the diagenetic phases were sourced internally. The fractures cross multiple facies and could have acted as fluid conduits for internally sourced fluids. A combination of clay diagenesis and shale dewatering are inferred to be two of the main mechanisms for authigenic mineral formation during burial diagenesis. Investigating the diagenetic stages in the Wolfcamp shale helps constrains which mineral phases can hamper or improve reservoir quality, which is important for fracability, porosity distribution, and the location of ‘sweet spots’.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016