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Microfacies Analysis of the Microbial Thrombolite Buildup in the Oxfordian Smackover Formation, Little Cedar Creek Field, Alabama


The discovery of hydrocarbons in the Smackover Formation in the Little Cedar Creek Field, Alabama in 1994 piqued interest with researchers. The established stratigraphic framework for this region allows the exceptional opportunity to carry out detailed microfacies analysis of the microbial thrombolitic component in the Jurassic (Oxfordian) buildups through core description, thin-section, and isotopic analysis to identify the succession of organisms, microbial carbonate deposition, and diagenesis that contributed to formation of these thrombolite buildups. The Little Cedar Creek Field parallels the up dip limit of the Smackover Formation approximately 3.2 km (2 miles) away. Based on the cores used in this study, the Smackover was intercepted at depths ranging from 3,321.4 to 3,567 meters (10,897-11,703 feet) and contains buildups 2 to 13 meters (7-42 feet) thick. Four microfacies were defined A: Black Renalcis-like Layer, B: Digitate, C: Chaotic, and D: Brown Laminated Centimeter Scale Cycle. In a few instances small sponges form a primary framework, but in most of the buildup, distinct layers of microbially precipitated micrite forms in succession one on top of the other. The first layers, or primary framework, was sampled for isotopic analysis. Values range from -2.4 to -5.1 ‰VPBD for δ18O and 3.1 to 4.0 ‰ VPBD for δ13C, suggesting a marine origin. Permeability and porosity values were determined for each microfacies. Overall, porosity values range from 2% to 16.83% and permeability values range from 0.087 to 930 millidarcies. It was found that microfacies A (Black Renalcis-like Layer) was the least porous and permeable presenting itself as a potential baffler or barrier to flow, since it sandwiched microfacies that were relatively more porous and permeable. Because Jurassic microbial carbonates are important hydrocarbon reservoirs of global interest, understanding these complex thrombolitic buildups, can help explain variation in reservoir quality.