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.
AAPG Search and Discover Article #90067©2007 AAPG Mid-Continent Section Meeting, Wichita, Kansas