Comparative Diagenesis of Smackover Formation, Northern U.S. Gulf Coast: Arkansas, Louisiana, Mississippi, and Alabama

Abstract

The Upper Jurassic Smackover Formation is one of the most prolific hydrocarbon producers of the northern U.S. Gulf Coast. Studies indicate distinct lithological and diagenetic patterns between the central region (Louisiana, Mississippi, and Arkansas) and eastern area (Alabama and Florida). Although highly porous, permeable, and productive in both regions, the formation shows major differences in thickness, lithology, and diagenesis across northern U.S. Gulf Coast.

The Smackover Formation is up to 1800 feet thick in the central region whereas its maximum thickness is less than 400 feet in the eastern province. Abiotic precipitation of grains in the central region formed mile-long and 100s feet thick porous and permeable sand bodies. However, the Smackover Formation in eastern region was built dominantly by microbial precipitation which led to the formation of 10 – 150 feet thick, porous, and productive microbial reefs. Despite differences in thickness and modes of precipitation, the Smackover Formation in both regions consists of three shoaling upward cycles named, from base to top, Smackover “C”, “B”, and “A”.

Of particular importance is the difference in marine diagenesis between the two areas. In the central region, diagenesis at the seafloor was minor and limited to sparse circumgranular isopachous cement. In the eastern province, however, marine diagenesis appears pervasive which includes abundant precipitation of micritic, micro-mosaic, bladed pore fill, and circumgranular cements. The variation in marine diagenesis between the two regions is attributed to a difference in seawater salinity which was higher in the eastern region. The two regions show similar meteoric and burial diagenetic overprints which include minor early meteoric dissolution and moderate burial calcite cementation. Although, platform areas in the central and eastern regions remained mostly limestone, major dolomitization affected the entire Smackover Formation carbonates over salt highs in both regions. Here, seepage of evaporated Mg-rich Buckner brine into the Smackover carbonate through exposure surfaces created thick, highly porous, permeable, and productive Smackover dolomite.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018