Devonian Shale Gas Resource Assessment, Illinois Basin
Robert M. Cluff, Suzanne G. Cluff, and Catherine M. Murphy
In 1980 the National Petroleum Council published a resource appraisal for Devonian shales in the Appalachian, Michigan, and Illinois basins. Their Illinois basin estimate of 86 TCFG in-place has been widely cited but never verified nor revised. The NPC estimate was based on extremely limited canister off-gas data, used a highly simplified volumetric computation, and is not useful for targeting specific areas for gas exploration.
In 1994 we collected, digitized, and normalized 187 representative gamma ray-bulk density logs through the New Albany across the entire basin. Formulas were derived from core analyses and methane adsorption isotherms to estimate total organic carbon (r2=0.95) and gas content (r2=0.79-0.91) from shale bulk density. Total gas in place was then calculated foot-by-foot through each well, assuming normal hydrostatic pressures and assuming the shale is gas saturated at reservoir conditions. The values thus determined are similar to peak gas contents determined by canister off-gassing of fresh cores but are substantially greater than average off-gas values. Greatest error in the methodology is at low reservoir pressures (or at shallow depths); however, the shale is generally thinner in these areas so the impact on the total resource estimate is small.
The total New Albany gas in place was determined by integration to be 323 TCFG. Of this, 210 TCF (67%) is in the upper black Grassy Creek Shale, 72 TCF (23%) in the middle black and gray Selmier Shale, and 31 TCF (10%) in the basal black Blocher Shale. Water production concerns suggest that only the Grassy Creek Shale is likely to be commercially exploitable.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California