Relatively Large Mid-Ramp, Highstand, Microbialite Patch and Fringing Reefs: A New Exploration Play for Southern Alabama
Lower and middle Smackover microbialite build-ups are providing 100 to 200 ft thick porous oil reservoirs along localized flexures and paleo-bathymetric highs in a mid-ramp, upper Jurassic setting. Recent drilling and 3D seismic acquisition in Covington, Conecuh, and Escambia counties, Alabama, has delineated a trend of excellent producing wells (400 to 1300 barrels of oil per day [BOPD]) developed across the mouth of the Conecuh Embayment. In this setting greater Smackover thicknesses probably attest to increased water depths and greater accommodation space which allowed significant vertical reef build-ups in a less restrictive and possibly better environment of marine circulation than those inboard and within the embayment. Cumulative primary production from these reefs could lie on the order of one to three million barrels per well. This trend occurs basin-ward of the prolific Little Cedar Creek and Brooklyn fields which were established over the last several years. The latest discoveries appear to represent stacked, multi-cycle bioherms developed throughout the lower and middle (Smackover C and B) highstand sequences.
These recently discovered massive microbialite colonies consist of a variety of cyanobacterial growth patterns including: laminated crusts, digitate columns, crenulate clusters, and a pelleted, clotted fabric. Occasional epifaunal foraminifera, gastropods, thin shelled pelecypods, sponges, echinoid spines, red algae, and serpulid worms are observed encrusting and providing internal sediment to the reefal build-ups. Core descriptions attest to two stages of reef development with a significant lowstand exposure between and above both transgressions. Petrophysical properties of 2–5 darcys and 20% porosity are not uncommon in the mid-ramp reservoirs. Early cementation of the reef framework allowed the development of syndepositional and early burial fracturing to occur which further enhanced reservoir continuity and fluid flow characteristics.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015