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Integrated Geologic and Petroleum Engineering Case Study of a Microbial Carbonate Reservoir: Upper Jurassic Smackover Microbial Carbonate and Associated Reservoir Facies in Little Cedar Creek Field, Southwest Alabama

Ernest Mancini, Sharbel Al Haddad, and Moetaz Mostafa
[email protected]

With the recent discovery of microbial carbonate reservoirs in the South Atlantic, industry is keenly interested in the predictability of the spatial distribution of microbialite reservoir facies and their sedimentary, petrophysical and production characteristics and particularly the ability to model trends in microbial reservoir heterogeneity and to simulate hydrocarbon productivity. In this regard, Upper Jurassic Smackover microbial buildups have been known to be productive petroleum reservoirs in the eastern Gulf for decades. These Smackover buildups commonly developed on Paleozoic crystalline paleotopographic features. These microbialites are usually directly overlain by high energy, nearshore facies forming the Smackover boundstone-grainstone/packstone reservoir. This Smackover reservoir is overlain by Buckner anhydrite beds of the Haynesville Formation. The anhydrites serve as the petroleum seal beds for these combination structural-stratigraphic traps. The trapped hydrocarbons are sourced from Smackover basinal beds rich in amorphous and microbial kerogen. The exploration strategy to locate and delineate these microbial buildups focused on identifying the paleotopographic highs associated with Paleozoic crystalline basement rocks on which the microbialites nucleated and developed. The use of 3-D seismic reflection data provided the technology to identify the paleotopographic anomalies as exploration targets. The data were further used in field development. Unfortunately, these fields, such as Appleton and Vocation fields in southwest Alabama, contained production of less than 3 million barrels of oil from 10 or less wells. Thus, the ability to study variation in the reservoir properties of microbialites and associated facies was limited. However, with the discovery and subsequent continued development of Little Cedar Creek field in Conecuh County, southwest Alabama, an excellent opportunity to study the spatial distribution of sedimentary, petrophysical, heterogeneity and productivity trends in microbial carbonate reservoirs is available. The State Oil and Gas Board of Alabama established the field in 1995. On January 1, 2005 the western part of the Little Cedar Creek field was unitized. In October 2007, the unit operator implemented a pressure maintenance program involving gas-injection in the unitized portion of the field. This enhanced recovery project targeted the upper grainstone-packstone reservoir in the field only. The lower microbial boundstone reservoir was not part of this project. To date more than 100 wells have been drilled in the field area, and production from the field totals 12.5 MMBLS of oil and 14.8 MMCF of natural gas. The objective of this paper is to present the results from an integrated geologic-petroleum engineering field case study of the microbial carbonate and associated reservoirs at Little Cedar Creek field to further the understanding of the spatial distribution of the sedimentary characteristics of microbial carbonate facies, the petrophysical properties of microbial reservoirs, and the variability in the heterogeneity and productivity of microbial reservoirs. This integrated field case study includes characterizing the sedimentary, petrophysical and production characteristics of the reservoirs, developing a 3-D geologic reservoir model, conducting reservoir simulation, and evaluating strategies to maximize production from and development of these reservoirs.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013