Abstract

The main structural features affecting the Mesozoic sequences of the Gulf rim are a series of interior salt basins extending from south Texas to Alabama. These basins formed during the rifting stage of the formation of the Gulf of Mexico. The early Oxfordian Smackover Formation, characterized by organic-rich, carbonate source rock intervals, represents the initial phase of the Late Jurassic marine transgression in the Gulf of Mexico Basin, where a density-stratified shallow sea was developed. It is predominantly composed of carbonates and calcareous shales and can easily be correlated by its lithology for considerable distances along its depositional strike. Over most of the Gulf coast, it can be subdivided into two distinct units. The Lower Smackover Formation (Brown Dense) is composed of dark-colored, organic-rich carbonate mudstone and dense argillaceous limestone deposited in a low-energy setting, while the Upper Smackover Formation is typically composed of coarser, grain-supported, porous carbonates formed in a high-energy shallow-water environment. Oil and gas resources have been produced from the more porous upper unit on which most geologic works have mainly focused. The Brown Dense has been, however, less studied, although it could become a viable resource play with the utilization of modern drilling techniques.

Recently collected 450-ft (ca. 137 m) thick core and full-suite well logs have been studied to delineate geologic features, the results of which would be significant impact on exploration programs. The detailed geologic studies have revealed that 1) the Brown Dense interval comprises the multi-stacked cyclic succession, having the coarsening-upward trend, in which the basal part is dominated with organic-rich deposits, grading upward into algal laminite-like deposits, 2) each coarsening-upward succession (interpreted as parasequence), sharply demarcated by flooding surface, would be deposited during a regression period, 3) the porosity-permeability trend is characterized by organic-rich deposits that have relatively high effective porosity (average > 3%) and permeability (up to 5 µD), respectively 4) the presence of framboidal pyrites is mostly associated with organic-rich layers, where relatively high porosity is evident, 5) porosity is mainly comprised of the shelter type with some amounts of interparticle and intraparticle pores, and 6) cementation might be severe throughout the entire interval, but it would be limited within the organic layers.

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