Mississippian "chat" reservoirs, south Kansas: Low-resistivity pay in a complex chert reservoir

Mississippian chert of variable character and diagnetic history defines a unique and prolific series of reservoirs in south-central Kansas and northern Oklahoma. Known collectively as the "chat," these reservoirs occur beneath, and are partly associated with, a major regional unconformity separating Mississippian and Pennsylvanian deposits. Chat fields occur in an arcuate fairway extending approximately 160 km along the flanks of the southward-plunging Pratt anticline, a southern extension of the Central Kansas uplift. Initial discoveries were made in the 1920s and 1930s, with major expansion of the trend in the 1940s and 1950s. Chat wells have been among the most productive in the southern midcontinent, yielding oil and gas at rates up to 1500 bbl, or 40 mmcf per day from depths of 909-1515 m. The bet ter reservoirs have exceptional porosities (30-50%), but moderate permeabilities (20-100 md); fracturing is assumed to play an important role in reservoir performance. Few detailed characterizations of chat reservoirs exist to date. Recent study has identified three reservoir types: (1) in-situ spiculitic chert, (2) in-situ brecciated and partly weathered spiculitic chert, and (3) highly weathered, transported chert conglomerate. These types can be broadly categorized according to paleostructural position. Current depositional models interpret the chert as mainly primary in origin, related to subtidal sponge spicule bioherms along a subtle shelf break. Entrapment is predominantly stratigraphic in nature; bioherms grade laterally into impermeable carbonate and are truncated updip beneath the sub-Pennsylvanian unconformity. Total production from the chat trend is on the order of 381 million bbl oil and 2.3 tcf gas. New infill drilling, field expansion, and enhanced recovery projects in the 1990s have helped revitalize interest in the "chat." Future exploratory and development efforts should add significant new data to the characterization of these complex and highly profitable reservoirs.