A Regional Erosion Surface and Its Effect on the Smackover Reservoir-Seal System along the South Arkansas – North Louisiana Jurassic Shelf

The Upper Jurassic Smackover Formation displays three shoaling upward cycles in the north-central U.S. Gulf Coast (Louisiana-Arkansas-Mississippi). From the base, they include the Smackover “C”, Smackover “B”, and Smackover “A” cycles. The basal “C” cycle consists of an upward succession of lithofacies indicating a progradation of beach-to-basin environments. The overlying “B” cycle encompasses thick grainstones formed as Bahamian-type marine sand bars. The “A” cycle includes isolated shelf-margin ooid shoals.

The reprocessing of 3D-seismic data with enhanced high frequency attributes and a high signal to noise ratio clearly depicts a linear marine sand bar belt of the Smackover “B” cycle in southern Arkansas. These NW – SE trending bars were formed parallel to the South Arkansas Jurassic shelf edge. One particular bar that was drilled and conventionally cored is 2 – 3 miles long, at least 80 feet thick, shows a classic porosity signature, provides structural drape and local thinning of overlying sediments, and exhibits an obvious velocity sag on the underlying seismic reflectors.

In the two wells which tested the marine sand bar, the contact between the Smackover Formation and the overlying Jones Sand Member of the Haynesville Formation is abrupt and appears locally scoured. A detailed investigation of conventional cores shows evidence of a major erosional surface at the top of the Smackover Formation. In fact, cemented oolitic grainstone clasts are incorporated into the siliciclastic fluvial and paralic sediments of the overlying Jones Sand. The seal to this otherwise very attractive Smackover reservoir was breached by an erosional event at the top of the Smackover “B” cycle. The apparent seismic seal that exists today is a product of late burial diagenesis, and was not present during hydrocarbon migration.