The Mississippian "chat," present at the unconformity between the Pennsylvanian and Mississippian in north-central Oklahoma and south-central Kansas, is a weathered and/or detrital interval of highly porous or hard, tight chert at or near the top of the Osagean. Hydrocarbon exploration and development of these rocks has been going on for more than 50 years. Study of well logs and completion records of more than 6600 wells in north-central Oklahoma shows that the chat is widespread but not continuous and that chat res ervoirs are very heterogeneous.

The depositional environment and diagenetic alteration of the chat suggest it formed through a combination of uplift and either erosional detritus or weathering-in-place of Osagean Mississippian cherty limestone. Fossiliferous clasts found in chat cores were likely eroded in a high-energy environment such as that found above wave base in Mississippian shallow seas. The limestone clasts were trans ported by small-scale debris flows into a lower energy environment and deposited in a lime mud matrix. The chat-producing trend be ginning in T25N, R3E in Osage County, Oklahoma, and extending eastward beyond the study area for about 20 mi suggests a rela tionship between probable shelf-edge erosion and the development of a chat reservoir. Other chat deposits formed from the weathering-in-place of Mississippian limestone on structural highs.

Thin sections reveal that diagenesis resulted in partial replacement of calcite shells and cement with silica following the debris flow. Preserved original fossil structures suggest molecule by molecule silica replacement of calcite. Dissolution of remaining calcitic material, possibly by meteoric water, created secondary porosity and a potential hydrocarbon reservoir.

The chat appears on well logs as a low-resistivity zone having low density and high porosity that, by normal interpretation methods, would calculate wet and nonproductive. Oil and gas produced from such zones are generally accompanied by salt water. Examination of drill cuttings can indicate whether the zone is highly porous or dense chert, with the former needed for a quality reservoir. (Begin page 116) Commercially productive zones generally have a minimum porosity of 25-30% and water saturations less than 80%. Micrologs provide indications of permeability that in core analysis is low (0.1 to 50 md; average < 20 md). Completion techniques should be designed for a siliceous reservoir containing detrital clays and no carbonate. Fracture treatment can generate additional permeability. Ultimate recovery from unitized fields having tens of wells ranges from 1 to 4 million bbl of oil and 1 to 3 bcf gas. Individual wells in good-quality reservoirs have produced more than 150,000 bbl of oil and varying amounts of gas from depths of 3000 to 5000 ft (914-1524 m).