MISSISSIPPIAN CHAT RESERVOIRS, SOUTH KANSAS - LOW-RESISTIVITY PAY IN A COMPLEX CHERT RESERVOIR

Mississippian chert of variable character and diagnetic history define s a unique and prolific series of reservoirs in south-central Kansas a nd northern Oklahoma. Known collectively as the ''chat,'' these reserv oirs occur beneath, and are partly associated with, a major regional u nconformity separating Mississippian and Pennsylvanian deposits. Chat fields occur in an arcuate fairway extending approximately 160 km alon g the flanks of the southward-plunging Pratt anticline, a southern ext ension of the Central Kansas uplift. Initial discoveries were made in the 1920s and 1930s, with major expansion of the trend in the 1940s an d 1950s. Chat wells have been among the most productive in the souther n midcontinent, yielding oil and gas at rates up to 1500 bbl, or 40 mm cf per day from depths of 909-1515 m. The bet ter reservoirs have exce ptional porosities (30-50%), but moderate permeabilities (20-100 md); fracturing is assumed to play an important role in reservoir performan ce. Few detailed characterizations of chat reservoirs exist to date. R ecent study has identified three reservoir types: (1) in-situ spiculit ic chert, (2) in-situ brecciated and partly weathered spiculitic chert , and (3) highly weathered, transported chert conglomerate. These type s can be broadly categorized according to paleostructural position. Cu rrent depositional models interpret the chert as mainly primary in ori gin, related to subtidal sponge spicule bioherms along a subtle shelf break. Entrapment is predominantly stratigraphic in nature; bioherms g rade laterally into impermeable carbonate and are truncated updip bene ath 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 inf ill drilling, field expansion, and enhanced recovery projects in the 1 990s have helped revitalize interest in the ''chat.'' Future explorato ry and development efforts should add significant new data to the char acterization of these complex and highly profitable reservoirs.