Microbial Promotion of Early Carbonate Cements and Stabilization of Bed Tops in the Middle Permian Brushy Canyon Formation, West Texas
Carbonate cements are common in organic-rich sedimentary rocks and significantly modify rock porosity, permeability, and fracture mechanics in both conventional and unconventional reservoirs. A variety of anaerobic microbial metabolisms are known or suspected to promote carbonate mineral precipitation. However, controls on microbially-promoted cementation are poorly understood, limiting our ability to construct process-based models of diagenetic facies or to understand interactions between diagenesis, transport, and deposition. Channel-filling turbidites cropping out in the Salt Flat Bench of the Brushy Canyon Formation (Guadalupe Mountains National Park, west Texas, USA) show evidence for incomplete erosion of laminated siltstones (Bouma D divisions) through iron- and manganese-rich partially burrowed tops down to dolomite-cemented, pyritic bases. In the relatively rare instances where D divisions were completely eroded, underlying A division sandstones were also eroded down to the next D divisions. Cemented intervals in D divisions are up to 50% cement with bulk carbonate isotopic compositions implying formation at low temperature with very little incorporation of respired organic carbon. These observations suggest that carbonate cements formed in the anaerobic zone, and most likely in the iron reducing zone, of reducing turbidite tops during intervals when the overlying water column was weakly oxic. These cements probably modified sediment transport and deposition patterns in the mid- to upper-slope. If widely developed, similar cements in other settings could play a role in determining rock properties of reservoirs deposited in Deepwater settings.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California