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Comparative Analysis of Microbial Carbonate Fabrics at Various Scales Within the Architecture of a Large Microbial Reef Mound; Cambrian, Wilberns Formation, Mason County, TX

Abstract

Research into lacustrine microbialites has intensified in response to recent giant oil field discoveries in pre-salt offshore Brazil. Similarities between lacustrine and marine carbonates indicate that marine microbial carbonates may provide meaningful analogues for understanding the heterogeneity of pre-salt reservoirs. The large scale of recently accessible Upper Cambrian microbial mound and reef complexes within the Wilberns Formation in private ranches of Mason County prompted a detailed outcrop study of the facies architecture, diagenesis, and porosity/permeability distributions of microbialites as potential analogues for pre-salt microbial carbonate reservoirs. In parallel with the larger project, we are undertaking a detailed study of the microbial fabrics that make up the architecture of a spectacularly exposed microbial reef mound named “Drox Rock” on Mill Creek. The facies elements are similar at multiple scales. Our analysis of the Drox Rock mound will provide useful comparisons to the fabrics found at various scales within yet larger microbial reef complexes within the Wilberns Formation. The microbial reef mound is composed of numerous individual microbial columns ∼20 cm across. Each column is vertical or has complex branching or anastomosing structures. Each column interior is thrombolitic with cm to mm scale clots which are dolomitized and tan colored. Each column is surrounded by a light gray, homogeneous undolomitized micrite rind ∼ 5cm thick. In some cases the rind is cryptalgally laminated. The columns show a vertical pattern indicating growth pulses in which pauses in growth are represented by the expansion of the rind over the top of the columns and forming a through-going micrite zone. The pauses in growth are separated by zones of vertically stacked thrombolite. In outcrop a honeycombed weathering pattern appears to represent differential erosion of the thrombolitic zones. This honeycombed weathering may accentuate original heterogeneity in porosity. The larger reef mound is itself surrounded by an undolomitized micrite rind 1 m thick. The last phase of the growth of the reef mound is succeeded by deposition of the rind over the top of mimicking the pattern in columns. We will refine our interpretation of microbial fabrics and investigate the microscopic elements using petrographic analysis of thin sections and polished slabs. The Mason Microbial Research Program is funded by Chevron, ConocoPhillips, Shell, and Statoil.