Factors Controlling Coalbed Methane Production from Helper, Drunkards Wash and Buzzard Bench Fields, Carbon and Emery Counties, Utah
Helper, Drunkards Wash and Buzzard Bench Fields are portions of a continuous CBM reservoir within the Ferron Member of the Upper Cretaceous (Middle-Late Turonian) Mancos Shale. CBM production occurs within non-volumetric heterogeneous compartmentalized reservoirs. Gas content for high-volatile B bituminous coals is anomalously high, displaying an isotopically mixed character of biogenic and thermogenic signatures. Gas content is variable throughout the trend, exhibiting a general decrease to the updip, southeastern portion of the coal belt. Correlation is noted between gas content, EUR, and salinity of formation fluids. In portions of the trend, updip saline formation fluids grade downdip into fresh water.
An inclined potentiometric surface is developed between the deeper western area (Wasatch Plateau) and the shallower eastern region (San Rafael Uplift), resulting in minor artesian overpressuring in portions of the field. Fresh water and anaerobic bacteria enter the downdip portion of the coal belt via major Basin and Range collapse grabens that were emplaced about 15 mya. Subsurface fresh water inflow to the Ferron Member has been measured at 2.4 ft3/sec, with a C-14 date of 28,000-31,000 years. Reservoir heterogeneity and compartmentalization was created by varying structural styles, forming permeability conduits and baffles that channeled artesian fresh water flow through the Ferron coals. Areas of high EUR are characterized by high rates of fluid flow, high gas content, low salinity and an increase in the fraction of isotopically light biogenic methane. Poor permeability with low fluid flow, low gas content, high salinity and a dominance of thermogenic methane characterize areas of low EUR.
It is suggested that initial thermogenic gas content within the coal was reduced as a result of disequilibrium induced by Laramide-aged uplift, which raised Ferron coals from a maximum burial depth of about 9,300 ft to as shallow as 1,000 ft. Facies equivalent Ferron marine sandstones crop out about six miles to the east of the subsurface coal belt. Spontaneous methane degasification resulted in undersaturated coals near this margin. Original formation fluid (Rw = .08) is retained in this updip region. Subsequent Basin and Range extension introduced fresh water and anaerobic bacteria into the system, re-saturating coals with isotopically light biogenic methane along avenues of enhanced permeability and within faulted compartments of the CBM trend.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009