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The pool under study is part of the Sailor Springs Consolidated field, Clay County, Illinois. The 23 wells within the pool have produced more than 350,000 bbl of oil from the McClosky limestone (Mississippian-upper Valmayeran) since 1981. The average depth of the wells and the pay zone is 3,000 ft.
The trap is predominantly stratigraphic in nature. Examination of core, thin sections, and geophysical logs indicates that the producing zone is composed of a complex series of oolitic bars. The individual bars are laterally discontinuous, flat bottomed, convex upward, and composed of oolitic skeletal grainstones that grade into dense skeletal wackestones and mudstones that act as reservoir seals. The bars are commonly subtle and easily overlooked. Several wells produce more than 100 BOPD from a zone less than 3 ft thick.
Detailed mapping of total pore volume, total permeability, and facies cementation patterns is essential for successful field development and secondary waterflooding. These parameters have had a direct impact on primary and secondary production performance within the pool. Detailed reservoir mapping also reveals that the pool is subdivided into several reservoirs separated by reentrants that cut across the oolitic bars.
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