Hartshorne coal rank applied to Arkoma Basin coalbed methane activity, Oklahoma, USA

Coal rank, generalized for all coal beds at or near the surface, in the eastern Oklahoma coalfield has been known since 1915. Near the surface (depths < 305 m, < 1000 ft), coal rank increases from high volatile bituminous to low volatile bituminous from west to east in the Arkoma Basin in Oklahoma. The rank of subsurface (depths > 305 m, > 1000 ft) coals in Arkoma Basin coalbed methane (CBM) prospects was previously unknown. A new Hartshorne coal (Middle Pennsylvanian) rank map is based on mean maximum vitrinite reflectance values of 0.76% to 2.41% Rmax from 70 coal samples from vertical depths of 0 (surface) to 1327 m (4355 ft). Hartshorne coal rank in the subsurface increases from high-volatile bituminous to semianthracite from west to east in the Arkoma Basin in Oklahoma. Bituminous rank boundaries changed considerably from earlier surface rank maps (primarily by increasing the area of medium volatile bituminous rank coal in the subsurface) and revealed a previously unknown semianthracite rank area. This discrepancy is significant because the rank of deep (> 305 m, > 1000 ft) coal resource assessments and CBM exploration projects have been based on shallow coal rank assignments.A total of 2635 Hartshorne (Hartshorne, Lower Hartshorne, and Upper Hartshorne) CBM wells have been completed in Oklahoma since 1988. Most (1610) wells are horizontal with lateral lengths ranging from 4 to 1498 m (14 to 4914 ft; an average of 669 m [2195 ft]). Coal in most Hartshorne CBM wells is medium volatile bituminous rank. Hartshorne coal is semianthracite rank in about 160 CBM wells in Le Flore County. The four Hartshorne CBM wells with the highest initial potential (IP) gas rates (48 to 65 thousand cubic meters per day, Mcmd; 1.7 to 2.3 million cubic feet per day, MMcfd) are medium volatile bituminous rank from horizontal wells in Haskell and Pittsburg counties. IP gas rates < 28.3 Mcmd (< 1 MMcfd) in low volatile bituminous and semianthracite rank Hartshorne horizontal CBM wells may be low due to complications in drilling into high rank coals with cleat spacing < 1 cm, to stimulation differences, or to gas migration into adjacent sandstone channels.

► Oklahoma Hartshorne coal rank is from high-volatile bituminous to semianthracite.
► Mean maximum vitrinite reflectance values from 70 coal samples are 0.76% to 2.41%.
► Rank in most Oklahoma Hartshorne CBM wells is medium-volatile bituminous.
► Highest initial potential gas rates are from medium-volatile bituminous rank coals.