Conventional Wisdom Applied to Oklahoma Gas Shales

Brian J. Cardott.  Oklahoma Geological Survey, Norman, OK  [email protected]

All hydrocarbon source rocks should be evaluated as gas shales. It remains to be seen whether all black shales should be considered as potential gas shales and what the end members should be (e.g., biogenic, thermogenic, or mixed methane; oil window vs. dry gas window; Types I, II, or III kerogen; silica-rich vs. clay-rich mineralogy). Generation and producibility of gas are equally important. Conventional wisdom from the Barnett Shale suggests that the best gas shales are organic-rich, oil-generative (Type II kerogen) black shales in the dry gas window. 

Applying conventional wisdom to Oklahoma black shales, the current gas-shale activity in eastern and southern Oklahoma is in the Woodford Shale (Upper DevonianÐLower Mississippian) and Caney Shale (Mississippian). 

The Woodford Shale and Caney Shale contain Type II kerogen. Favorable shale-gas potential is where the shales are thick (>50 ft), relatively shallow (<10,000 ft), in the gas window (vitrinite reflectance >1.4% Ro), and silica rich. A few exceptional Woodford Shale gas wells at shallow depths in the Ardmore basin are enigmatic in that they are in the oil window. Thick (<700 ft) and/or deep (>10,000 ft) Woodford Shale at high thermal maturity (>3% Ro) in the Anadarko and Arkoma basins await more risky exploration.

An Oklahoma gas-shales database contains information on gas wells completed in the Woodford Shale or Caney Shale. Wells drilled from 1939 to 2006 had initial potential gas rates of 1 to 5,312 thousand cubic feet of gas per day from 174 wells at vertical depths from 553 to 15,310 feet.