Upper Jurassic Smackover Reefs -An Example from Walker Creek Field, Arkansas
Coral-algal reefs were important shelf environments along the U. S. Gulf Coast during deposition of the Smackover Formation during the Late Jurassic. Conventional cores from the ARCO (Atlantic Richfield) No. 1 McFadden, located in Walker Creek Field, Arkansas, illustrate a reef framework, 7.6 m (25 ft) thick, of stromatolitic algal - Tubiphytes boundstone and numerous corals, in addition to containing a diverse skeletal non-framework component. The boundstone fabric contains lighter-colored internal sediment filling shelter pores and growth voids. The base of the reef comprises packstones with bivalves and scattered stick-like corals. Diversity increases upward with numerous small coral heads and fingers, skeletal algae and sponges. The climax community of primarily corals is overlain by skeletal-peloidal packstones and subsequently reservoir-quality oolitic grain-stones. Primary porosities were reduced to 5-15% by marine cements and binding stromatolitic algae. Compaction, stylolitization and later cementation further reduced porosity to only a few percent with permeabilities less than 0.1 millidarcy.