Council Run field in north central Pennsylvania is enigmatic because it lies near the eastern edge of the Appalachian Plateau where many potential reservoir rocks have low porosities and permeabilities. Source rocks are post-mature. Reservoir sandstones at Council Run, however, have porosities as high as 16%, gas saturations up to 83%, and EUR's exceeding 1.0 bcf in some wells. d 13C and d D of the produced gases indicate an oil-associated accumulation.
The principal reservoirs at Council Run are Upper Devonian in age. They occur within a fourth-order Type 1 stratigraphic sequence. Sandstone stacking patterns define lowstand, transgressive, and highstand systems tracts. Productive sandstones are lithic to highly feldspathic with hybrid pore textures comprised of reduced primary intergranular voids and secondary oversized fabric-selective voids. Early petroleum emplacement helped to preserve primary pores.
The Burket Member of the Harrell Formation and the Marcellus Formation are probable sources of the gases. The generative potential of these rocks is exhausted, but geochemical data suggest that these black shales originally contained oil-prone kerogens that generated liquid hydrocarbons in the past. The isotope chemistry of gases produced at Council Run implies that they are mixtures of 1) hydrocarbons generated by primary cracking of kerogens while the source rocks resided within the oil window and 2) gases generated by secondary cracking of oil during deeper burial of the Devonian strata. Modeling of burial and petroleum migration history in the subsurface rocks of the region helps to constrain the timing of petroleum accumulation in the field.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana