Little Cedar Creek Field: Reservoir Characterization to Simulation. A Geological and Engineering Case Study of an Upper Jurassic Microbial Carbonate Reservoir in Southwest Alabama

Sharbel Al Haddad¹ and Ernest Mancini^2
1ExxonMobil
²Texas A&M University

Little Cedar Creek field is a mature oil field located in southwestern Alabama, in the onshore area of the northeastern Gulf of Mexico. The main reservoirs are microbial carbonate facies and associated nearshore carbonate bank facies of the Upper Jurassic Smackover Formation that overlie conglomerate and sandstone facies of the Norphlet Formation and underlie argillaceous, anhydritic and carbonaceous facies of the Haynesville Formation. The lower reservoir is comprised of subtidal thrombolitic boundstone associated with microbial buildups oriented in a southwest to northeast direction over an area that encompasses 83 square kilometers (32 square miles). These buildups attained thicknesses of 13 meters (43 feet) and developed in clusters in the western, central and northern parts of the field. The inter-buildup areas of microbialites are 2-3 meters (7-9 feet) in thickness and are overlain by a thick section of non-reservoir microbially-influenced lime mudstone and wackestone. These beds are potential barriers or baffles to flow and serve to separate the microbial boundstone flow units recognized in the western, central and northern parts of the field. 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 and high productivity. Permeability ranges up to 7953 md and porosity up to 20%. The upper reservoir consists of a series of progradational ooid and peloid sand bodies in a carbonate bank setting. The carbonate bank complex extends from the western part of the field to the central part in a southwest to northeast direction. These marine carbonate sand belt buildups are comprised of up to six wackestone-packstonegrainstone sequences and attain thicknesses of 8 m (26 ft). In inter-buildup areas associated with the carbonate sand bodies have a thickness of 1-2 m (4-8 ft) and are underlain by a thick section of wackestone. Porosity consists of primary interparticle and secondary solution-enhanced interparticle, intraparticle, vuggy and grain moldic pore types and ranges 0- 33%. Permeability is critical to the low productivity of this reservoir and ranges 0-452. Carbonate sand belt buildup areas serve as potential heterogeneous hydrocarbon flow units and the inter-buildup areas containing a thick section of low permeability to non-reservoir rock serve as potential baffles or barriers or baffles to flow. The petroleum trap in the field is stratigraphic being controlled primarily by changes in depositional facies. The trapped hydrocarbons are sourced from Smackover basinal beds rich in amorphous and microbial kerogen. 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. The study provides a sound framework in the establishment of a field/reservoir-wide development plan for optimal primary and enhanced recovery for these reservoirs. Moreover and with the recent discovery of microbial carbonate reservoirs in the South Atlantic, such a reservoir-wide development plan has broad applications to other fields producing from microbial carbonate reservoirs particularly in the ability to model trends in microbial reservoir heterogeneity and to simulate their hydrocarbon productivity.

AAPG Search and Discovery Article #90185 © AAPG Geoscience Technology Workshop, Revisiting Reservoir Quality Issues in Unconventional and Conventional Resources, Austin, Texas, November 12-13, 2013