Abstract

An integrated field study of the microbial carbonate and associated reservoirs at Little Cedar Creek field in southwest Alabama, eastern Gulf coastal plain of the United States, provides an excellent opportunity to examine the spatial distribution of the sedimentary, petrophysical, and productivity trends in microbial reservoirs. This study includes characterizing the sedimentary, petrophysical, and hydrocarbon productivity characteristics of microbialites, developing a three-dimensional geologic reservoir model, and evaluating the hydrocarbon potential of these reservoirs. The lower reservoir comprises subtidal thrombolitic boundstone associated with microbial buildups oriented in a southwest to northeast direction over an area that encompasses 32 mi2 (83 km2). These buildups developed in clusters in the western, central, and northern parts of the field and attained thicknesses of 43 ft (13 m). The clusters are separated by interbuildup areas of microbialites of 7–9 ft (2–3 m) in thickness that are overlain by a thick section of nonreservoir microbially influenced lime mudstone and wackestone. Porosity in the microbial reservoirs includes depositional constructed void (intraframe) and diagenetic solution-enhanced void and vuggy pore types. This pore system provides for high permeability and connectivity in the reservoir beds. Permeability ranges to as much as 7953 md and porosity to as much as 20%. The microbial boundstone beds have high potential as hydrocarbon flow units; however, the buildup areas are separated by interbuildup areas associated with a thick section of low-permeability to nonreservoir beds that serve as potential baffles or barriers to flow. Much of the 17.2 million bbl oil produced from the field is from the microbial lithofacies. The results from the Little Cedar Creek field study have application in the design of improved development strategies for other fields producing from microbial carbonate reservoirs.

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