Abstract

Oil and gas reservoirs in the Cowley Formation (upper Osagean to lower Meramecian) are within a thick (up to 400 ft [122 m]) section of spiculite-dominated rocks, derived from demosponges, deposited in a low-latitude setting. These rocks are present in the subsurface for 325 mi (523 km) along paleostrike in southern Kansas and some adjoining states. They represent a stratigraphically significant lithosome that markedly contrasts thin and areally restricted spiculitic rocks present in some Mississippian reservoirs elsewhere in the mid-continent. Cowley lithologies represent a low-gradient ramp, whereon (1) bedded spiculites were deposited in moderate-energy, shallow-water, inner-ramp settings; (2) lenticular-, nodular-, or flaser (L/N/F)-bedded spiculite and shale were moderate- to low-energy, progressively deeper-water medial-ramp deposits; and (3) dark shales are deepest-water, outer-ramp facies. The internal stratigraphic architecture of the unconformity-bounded Cowley identifies it as a depositional sequence with component deepening-upward basal strata (transgressive systems tract) overlain by shallowing-upward, progradational clinoforms (highstand systems tract). Sequence deposition was punctuated by several unconformities attending short periods of subaerial exposure. Suppression of otherwise warm, shallow-water carbonate production, and instead spiculite deposition, in this low-latitude setting was likely a consequence of elevated concentrations of dissolved silica and nutrients in the ambient marine environment.

Three successive generations of silicification are recognized in the rocks. Early partial silicification is presumed to have begun in the marine environment, and ensuing silicification and attendant porosity formation were likely coincident with falling sea level as pore fluids evolved from being of mixed marine-meteoric to meteoric composition. Petroleum reservoirs mainly with vuggy porosity are present in relatively high-porosity bedded spiculites and less porous L/N/F-bedded rocks. Traps commonly are developed in structurally modified, subunconformity buried-hills and truncated, gently dipping strata. Reservoirs in the L/N/F-bedded rocks locally extend considerable distances downdip within individual clinoformal parasequences in the section, thereby locally creating thick gas-saturated reservoir columns. Because of its great subsurface extent, the Cowley section, commonly bypassed during drilling, offers considerable potential for as-yet discovered fields in the mid-continent.

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