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Sequence Stratigraphic Framework of Upper Jurassic Oxfordian Smackover Equivalents Illustrated by the Humble McKean #12 Core, Buckner Field, Southern Arkansas, Central Gulf of Mexico, USA
The Humble McKean #12 core illustrates important aspects of the sequence stratitgraphic framework of the Jurassic Oxfordian reservoirs of the central Gulf of Mexico. In this region we can recognize 6 Jurassic sedimentary sequences. Economically, the most important sequences are Jurassic sequences II and III which contain the majority of the Jurassic hydrocarbon reservoirs in the Gulf Coast, the Smackover and Buckner formations. The Humble McKean core encompasses all of sequence II and its bounding unconformities with sequences I and III. The LST of Jurassic sequence II are continental redbeds (Norphlet Formation) sitting unconformably on sequence I (Louann Salt). The TST of sequence II (lower Smackover Formation) consists of varved, organic lime mudstones deposited in relatively deep, anoxic conditions under a density stratified water column forming the primary Jurassic source rock of the Gulf of Mexico. The HST of sequence II consists of an uninterrupted shoaling upward sequence from toe-of-slope to high energy ooid grainstone beach. The high energy shore-face-beach complex is the main reservoir facies of the Jurassic fairway. Soil breccia, caliche and siliclastics mark the unconformity. Extensive meteoric diagenesis during exposure modified the pore systems of the underlying reservoir facies from primary intergranular to oomoldic porosity. The TST of the overlying sequence III (Buckner Formation) is marked first by marine sabkha deposition and, as transgression progresses, extensive evaporative lagoonal conditions. Reflux of post-anhydrite brines from this lagoon down into the HST of sequence II, dolomitized the oomoldic grainstones of the beach-shoreface complex, enhancing the reservoir quality of these rocks. The massive anhydrites of sequence III ultimately form the seal for structural traps across southern Arkansas. To the south, Jurassic sequences III and IV (both equivalent to the Buckner at the Humble McKean well) prograde southward into the Louisiana Salt Basin. Each sequence mimics the lime mudstone to ooid grainstone shoaling upward sequence of the Smackover in the Humble McKean well. Fields developed in sedimentary sequences III and IV in southernnost Arkansas and northern Louisiana, however, are stratigraphic traps consisting of up-dip coastal plain silt and shale seals. The lack of evaporites and short duration of the bounding unconformity seperating sequences III and IV result in associated reservoir pore systems being dominated by primary intergranular porosity without dolomite.
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