An eolian facies within the Upper Jurassic Smackover Formation, Tchula Lake field, Mississippi

Abstract

The Late Jurassic upper Smackover Formation in Mississippi is primarily composed of an overall regressive sequence of marine carbonate ramp deposits. In central Mississippi, however, carbonate deposition was interrupted by a large influx of siliciclastic sediment. Tchula Lake field is unique not only because it produces gas and condensate from dolomitic quartz sandstone deposited in a mixed siliciclastic-carbonate environment but also because it produces from what we interpret to be an eolian dune sand facies. Throughout the Gulf Coast Basin no previous literature has cited an eolian facies within the Smackover Formation. Several characteristics of the producing zone sandstones have led to interpretations of this unit as an eolian coastal dune facies deposited on and around a Jurassic paleohigh. Facies and paleoenvironmental interpretations are based on compositions and textures described in cores and thin sections. The eolian facies consists of fine-grained, dolomitic quartz sandstone with a mean porosity of 10-20% and permeability of 0-800 md. The sand grains are rounded to subrounded and well sorted. The sandstone exhibits a sequence of upward-steepening planar laminations capped at the top by a set of small-scale wedge planar cross-laminations. The sharply defined, parallel lamina are less than 5 mm thick and are commonly inversely graded and contain probable vadose meniscus carbonate cements. This facies is interpreted to have been deposited in a backshore coastal dune environment. The unit is characterized by low-angle (<20 degrees dip) climbing translatent strata commonly produced by the migration of wind ripples along a dune. No fossils or trace fossils are found within this facies. Eolian-deposited sandstone is a potential reservoir rock along the updip limit of the Smackover Formation in west-central Mississippi. Other paleohighs along the Jurassic shoreline may have undergone similar eolian deposition during a regional sea level fall. Eolian-deposited sandstones on these paleohighs should have (Begin page 486) similar reservoir properties as the sandstone at Tchula Lake field. The updip Smackover Formation in west-central Mississippi is an underexplored horizon, and eolian-deposited reservoirs have strong potential as a new exploration target. The sea level fluctuation event associated with deposition at Tchula Lake field also has implications for exploration in Upper Jurassic sedimentary rocks across the entire United States Gulf Coast Basin and prompts a reevaluation of these reservoirs.