Utilizing Noble Gas Geochemistry to Trace Hydrocarbon and Fluid Migration in the Northern Appalachian Basin
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Date
2016-12
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The Ohio State University
Abstract
Unconventional gas accumulations within the northern Appalachian Basin have become economically viable energy resources in the United States, largely due to recent advances in drilling technologies. While horizontal drilling and hydraulic fracturing have significantly improved the recoverability of hydrocarbons from unconventional reservoirs, the success rate of any given petroleum well is still highly variable. Therefore, efforts to understand better the fundamental properties of tight reservoirs and crustal fluid migration have increased. The use of noble gas geochemistry is emerging as a geochemical technique capable of characterizing the dynamics of subsurface fluid flow. This study evaluates the radiogenic noble gas isotopic ratios and major gas composition of production gases collected from 15 natural gas wells throughout the western and central regions of New York State. For the purposes of this study, samples were split into two groups based on the thermal maturity of their respective reservoir host rocks as determined by conodont alteration index values. Group 1 includes six samples taken from Middle to Upper Devonian-age low maturity rocks. Group 2 includes nine samples taken from Lower Devonian-, Silurian-, and Ordovician-age rocks that are post-mature. Group 1 displayed 4He concentrations ranging from 232.5 ccSTP/cc at sample BR (GGC-1343) to 2974 ccSTP/cc seen at sample F(GGC-1344), with an average concentration of 995.5 ccSTP/cc and 21Ne* had concentrations ranging from 10.0 ccSTP/cc at BR (GGC-1343) to 32.6 ccSTP/cc seen at sample F(GGC-1344) with an average concentration of 19.0 ccSTP/cc. Group 2 concentrations for 4He ranged from 245.3 ccSTP/cc at EE (GGC-1335) to 5068 ccSTP/cc seen at BG (GGC-1338) with an average concentration of 1846 ccSTP/cc and 21Ne* had concentrations ranging from 10 ccSTP/cc at EE (GGC-1335) to 42.7 ccSTP/cc seen at FW (GGC-1336) with an average concentration of 26.6 ccSTP/cc. Measured 4He/21Ne* values of produced gases systematically increase with increasing distance from the Appalachian Structural Front (ASF) for both sample groups, but do not correlate with modern reservoir depth, longitude, or conondant alteration index. These results suggest that deeper fluids originating from Ordovician shales have traveled farther distances and are more highly fractionated than Devonian fluids that have remained in-situ following maximum burial and show an enrichment of 4He relative to 21Ne*.
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Keywords
Appalachian Basin, Noble Gas Geochemistry, Conodont Alteration Index, Appalachian Structural Front, Unconventional Hydrocarbon Reservoirs