A DIAGENETIC STUDY OF THE LOWER WOLFCAMP SHALE, MIDLAND BASIN, TEXAS: WHAT CAN DETAILED PETROGRAPHIC AND MAGNETIC STUDIES TELL US ABOUT PARAGENESIS IN THIS COMPLEX SYSTEM?

The Wolfcamp shale has emerged as a substantial unconventional reservoir in the Midland basin, Texas. This study draws upon petrographic, electron microscopy and rock magnetic techniques to characterize diagenetic events and their associated fabrics in the lower Wolfcamp. Overall, the lower Wolfcamp is comprised of alternating carbonate and organic rich shale facies. Early diagenetic events include precipitation of silica and calcite. Pyrite formation appears to have occurred throughout the burial history and is likely related to both bacterial sulphate reduction and thermochemical sulfate reduction. Sphalerite is also observed however, the relative timing of its precipitation is currently unknown. Horizontal and vertical mineralized fracture orientations are present in both facies. The mineralogy of fracture fills during burial diagenesis are largely comprised of silica. Silica often replaces other mineral phases such as calcite in these fractures. Precipitation of ferroan carbonates, albite and double terminated quartz occurred during late stage burial diagenesis. The transformation of smectite to illite and resultant mobilization of elements likely caused the precipitation of authigenic minerals. As depth increases, dolomitization of matrix and allochems becomes more pervasive. Preliminary magnetic fabric investigations indicate the presence of both primary and alteration fabrics. In general, shale facies show planar oriented, oblate magnetic fabrics and carbonates show vertical/sub-vertical oriented, prolate fabrics. Exceptions to this trend are likely due to diagenetic alteration of primary fabrics or the presence of inverse magnetic carrier minerals. Future work will include paleomagnetic investigations to constrain the relative timing of diagenetic events and advanced imaging techniques to refine the paragenetic sequence. Furthermore, we will investigate whether the Wolfcamp behaves as an open or closed geochemical system.