Millican field, located in northwest Coke County, Texas, is part of an extensive trend of Pennsylvanian carbonate buildups that exists on the flanks of the Midland basin, just seaward of the shelf edge. The Millican buildup is 900+ ft thick and is primarily a bank deposit composed of mud-baffling organisms.

Four cored wells in a section from the front to the back of the buildup were chosen for detailed study. Fusulinid-age dating of the cores shows a major unconformity dividing the Millican buildup. Primarily, the buildup is of early Missourian age except for its pinnacle which is of early Virgilian age. Eight limestone facies were also recognized across the buildup. Labeled according to their distinctive components, they are: (1) crinoidal, (2) foraminiferal, (3) bryozoan, (4) mollusk-phylloid algal, (5) peloidal, (6) foraminiferal-fusulinid, (7) high energy (intraclastic and/or oolitic), and (8) blue-green algal. From the facies distribution across the buildup, a depositional model can be formed.

Porosity development within the Millican buildup is also facies controlled. Virtually no primary porosity exists, and the majority of secondary porosity generation occurs in rocks that are almost entirely composed of phylloid algae and mollusks. This understanding of the facies, environments, and diagenesis across the Millican buildup should be an important tool in exploring for similar and more subtle traps.

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