Geologic and Baseline Groundwater Evidence for Naturally Occurring, Shallow Source, Thermogenic Methane Gas in Northeastern Pennsylvania
Chesapeake Energy Corporation; Oklahoma City, Oklahoma, [email protected]
The study documents the presence and the sources of naturally occurring thermogenic methane that predates natural gas drilling activity in Bradford, Sullivan, Susquehanna, Tioga and Wyoming counties of northeastern Pennsylvania. Pre-drill baseline groundwater and surface water samples collected from nearly 17,000 locations in the study area, with more than 13,400 representing water wells. Methane was detected in 26.4% (1 in 4) of the water wells tested, 6.4% (1 in 15) exhibited methane concentrations greater than 3 mg/L, 2.6% (1 in 40) above 10 mg/L, and 0.5% (1 in 200) above 28 mg/L – the average saturation limit of methane in groundwater at atmospheric pressure. A comprehensive geologic investigation was undertaken by Chesapeake Energy to better characterize natural, shallow subsurface conditions in northeastern Pennsylvania in order to establish a coherent baseline to distinguish alleged incidents and impacts from natural regional conditions.
The origin of thermogenic natural gas in the shallow subsurface is associated with numerous organic-rich beds composed of carbonaceous, woody plant material, deposited during the Late Devonian. Widespread organic seams were observed throughout the stratigraphic section of the Catskill and Lock Haven formations at nearly 50 surface locations across a 2,600 square mile study area, which included bedrock outcrops, road cuts, quarries and excavation sites. Samples collected exhibit considerable gas source potential with total organic carbon as high as 44.4% by weight and are thermally mature with calculated vitrinite reflectance ranging from 1.9% to 3.3%. Source potential is further supported by mud gas shows observed while drilling through shallow organic beds.
Since 2007 there has been increased drilling of natural gas wells in northeastern Pennsylvania. The press and other media outlets have heightened awareness of methane presence in water wells of the region, which is supported by pre-drill baseline water testing. There are no water well construction or testing standards in Pennsylvania and past water studies have been very limited. Presumed liability regulations place natural gas drillers at fault for existence or increased methane concentrations found in water wells. Published geologic and groundwater data for the area is sporadic and mostly dated. Drilling and fracing impact studies have been proposed, commenced and even concluded, but lacked an essential rudimentary understanding of natural pre-existing environmental conditions. Results of Chesapeake’s comprehensive geologic investigation confirm thermogenic methane occurrence in water wells and other groundwater sources to be naturally occurring and common in the region.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012