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Stable Carbon and Hydrogen Isotopic Composition of Produced Gases from the Mississippian Fayetteville Shale, Eastern Arkoma Basin, Arkansas
M. Ed Ratchford1, John Zumberge2, Peng Li 3, Stephen Brown4, Steve Gates5
1Arkansas Geological Survey , 2GeoMark Research, LTD. , 3Arkansas Geological Survey , 4GeoMark Research, LTD. , 5Arkansas Oil & Gas Commission
Produced gases were collected from one hundred and three (103) Fayetteville Shale gas wells and two (2) wells that contain gases from conventional reservoirs of Pennsylvanian age. The gases were collected at the wellhead and analyzed using an Isoprime gas chromatograph-isotope ratio mass spectrometer. Fayetteville gases are very dry (>98% methane) and plot well into the “Thermogenic Region” on standard methane carbon versus hydrogen isotope or gas wetness plots. The relative amounts of ethane & propane (% C2+ or “wetness”) are spatially congruent with thermal maturity trends established by vitrinite reflectance measurements. Fayetteville methane carbon isotopes are generally lighter to the northwest in Van Buren County and to the east in White County. Gases with heavier methane carbon values appear to be related to more mature source rocks proximal to the frontal Ouachita thrust belt. Similar to methane, ethane (C2) carbon isotopes are generally lighter to the northwest and to the east suggesting corresponding differences in source rock maturity. However, ethane is isotopically lighter than methane in most samples (isotopic reversal) which may be attributed to in situ (1) wet gas cracking and (2) hydrothermal influence proximal to the frontal Ouachita thrust belt. Isotopically heavier CO2 carbon values are located in zones of higher thermal maturity and appear to be linked to carbonate degradation. Lighter CO2 carbon values may be the result of aerobic methane oxidizing bacteria in the shallowest Fayetteville wells.
AAPG Search and Discover Article #90097 © 2009 Tulsa Geological Society, Tulsa, Oklahoma
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