Abstract: While coalbed methane (CBM) is a significant source of natural gas production globally, uncertainties regarding the proportions of biogenic and thermogenic natural gas in CBM reservoirs still remain. We integrate major gases, hydrocarbon composition, hydrocarbon stable isotopes and noble gases in fluids from 20 producing CBM wells to more accurately constrain the genetic source of natural gases in the eastern Illinois Basin, USA. Previous studies have indicated primarily biogenic production of methane (>99.6%) with negligible contributions from thermogenic natural gases. However, by integrating noble gases, we identify quantifiable (up to 19.2%) contributions of exogenous thermogenic gas in produced gases from the Seelyville and Springfield coal seams. Thermogenic gases are distinguished by a positive relationship between methane, ethane and helium-4, lower C 1 /C 2 +, heavier δ 13 C-CH 4 , more radiogenic noble gases ( 4 He, 21 Ne*, 40 Ar*), and lower abundances of atmospherically derived gases ( 20 Ne, 36 Ar). Biogenic gases displayed lighter δ 13 C-CH 4 , higher C 1 /C 2 +, higher levels of atmospheric gases and lower abundances of radiogenic noble gases. Our data suggest that natural gases from a deeper, exogenous thermogenic source likely migrated to the Pennsylvanian-aged coals at an unknown time and later mixed with biogenic methane diluting the geochemical signature of the thermogenic methane within the Springfield and Seelyville coal seams.

Abstract Noble gas isotopes, major element isotopes, and gas composition were obtained from the shut-in Butler #3 (API 32-105-00008) and Simpson #1 (API 32-105-00007) wells, drilled in 1998, and sample gas from the Cumnock Formation of Late Norian age. This is the first gas chemistry compilation of these wells. The wells’ gas, sampled in 2009 and in 2014, had a strong “fruity” light petroleum odor, a visible condensate plume when the wells were flowed, and are in the oil and wet gas window. Shutin well pressures were ~900 psi (Butler #3), and ~200 psi (Simpson #1); both had a substantial initial gas flow. Limited data are from the 1982 Dummitt-Palmer #1 CBM well (API 32-105-00002), now plugged and abandoned. Helium concentrations were ~0.20% to 0.24% from the noble gas analysis, neon ranged from 0.11 to 0.04 ppm, and argon was approximately 33 ppm. The measured noble gas composition contains very low atmospheric contamination with helium isotopes (0.07 R/R A ) clearly defined by a crustal origin, while neon and krypton and are mainly attributed to atmospheric origin ( 20 Ne/ 22 Ne ~9.8, 86 Kr/ 84 Kr ~0.3). Argon isotopes are mixed between crustal and atmospheric origins with 40 Ar/ 36 Ar values ~ 418 to 520. The F 20 Ne/ 36 Ar (~0.9 to 2.6), F 84 Kr/ 36 Ar (~0.8) and F 132 Xe/ 36 Ar (0.6-0.7) in the gas show enrichment in the light isotope associated with multi-stage fractionation processes with gas and fluid interaction. The methane content (range ~58 to 64%) is inverse to the nitrogen content from denitrification of very thin ammonium-bearing units (also rich in oil), and likely from illite in overlying strata.

The Shatsky Rise is an oceanic plateau consisting of three main massifs that were constructed in the Pacific Ocean by intense volcanism during the Late Jurassic to Early Cretaceous. In order to explore the sources of this oceanic plateau, we present noble gas compositions from fresh quenched glasses cored by ocean drilling at Integrated Ocean Drilling Program Site U1347 on the Tamu Massif and Site U1350 on the Ori Massif. The studied glasses are normal-type basalts, the most abundant of four types of basalts defined by trace element compositions. Possible disturbances of noble gas compositions by posteruption radiogenic ingrowth in aged glasses are assessed by extraction of gases from glass vesicles by stepwise crushing. The 3 He/ 4 He ratios in glasses from Site U1347 are lower than atmospheric 3 He/ 4 He, presumably owing to magma degassing coupled with radiogenic ingrowth of 4 He. In contrast, glasses from Site U1350 exhibit a limited range of 3 He/ 4 He (5.5–5.9 Ra). Uniform 3 He/ 4 He cannot be achieved if gases in glass vesicles have been affected by secondary contamination or posteruption radiogenic ingrowth. Therefore, the uniform 3 He/ 4 He in the normal-type basalts from Site U1350 is ascribed to their source characteristics. Relatively low 3 He/ 4 He among oceanic basalts suggests the involvement of recycled slab material in the source of the normal-type basalts. However, the depleted radiogenic isotope signatures are inconsistent with recycled slab being a distinct melting component. Instead, we propose that the normal-type basalts of the Shatsky Rise were sourced from a domain where subducted fertile material is dispersed in the mantle.