Differentiating between biogenic and thermogenic sources of natural gas in coalbed methane reservoirs from the Illinois Basin using noble gas and hydrocarbon geochemistry

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 C1/C2+, heavier δ13C-CH4, more radiogenic noble gases (4He, 21Ne*, 40Ar*), and lower abundances of atmospherically derived gases (20Ne, 36Ar). Biogenic gases displayed lighter δ13C-CH4, higher C1/C2+, 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.

Helium in the Australian liquefied natural gas economy

“…The elemental composition of uranium ( 235 U and 238 U), thorium ( 232 Th) (both are the radiogenic parents of 4 He) and lithium ( 6 Li) (the nucleogenic parent of 3 He) in the rock forming minerals will ultimately determine the 'average' R/Ra. However, R/Ra ratios can vary considerably within subsurface reservoirs, with R/Ra ratios in the crust varying from <0.01 to 0.05 (Ni et al 2014;Darrah et al 2015;Moore et al 2017; and references therein), upper mantle~2 to 8 R/Ra (Darrah et al 2015) and lower mantle up to 40 R/Ra (Poreda et al 1986). Evolutionary mantle models predict an initial R/Ra ratio >100 (Tolstikhin and Marty 1998), while a R/Ra ratio of 230 is assumed for the initial 'un-degassed' (i.e.…”

“…Helium is readily released from minerals and transferred to underground fluids (magmatic/hydrothermal/pore water) where the helium isotopic composition is naturally averaged and can take on regional characteristics (Hunt et al 2012;Darrah et al 2014Darrah et al , 2015Moore et al 2017). During a magmatic event, injection of hot silicate-rich fluids carrying dissolved excess 3 He from the deep crust results in elevated R/Ra ratios that are primarily associated with the magmatic event, although the timing of the injection of molten fluid is not necessarily the age of the tectonic movement (Polyak and Tolstikhin 1985).…”

“…Therefore, exploration areas where high helium contents could accumulate following either fluid migration or the extensive removal of the dominant carrier phase (i.e. hydrocarbon oxidation, CO 2 dissolution) are favoured in reservoirs at shallow depth, with low temperatures and high salinity because helium is least soluble in this aqueous phase (Darrah et al 2014(Darrah et al , 2015Harkness et al 2017;Moore et al 2017). Maximum exposure to pore water, as encountered in long distance migration, also assists in the concentration of helium.…”

Geochemical applications in petroleum systems analysis: new constraints and the power of integration

“…Techniques and approaches presented herein focus on addressing three key hydrocarbon systems issues: (1) source-rock identification and the temperature/ timing of hydrocarbon generation Gao et al 2017;Pedentchouk & Turich 2017;Stolper et al 2017); (2) the mechanisms and timescales associated with hydrocarbon migration, trapping, storage and alteration Moore et al 2018); and (3) the impact of fluid flow on reservoir properties (MacDonald et al 2017).…”

“…For example, the carbonate clumped isotope technique applied in MacDonald et al (2017) could also be used to study the cementation history of aquifers targeted for managed aquifer recharge (MAR), a strategy of using aquifers to store and improve the quality of water for future use (Maliva et al 2015). The noble gases discussed in Byrne et al (2017), and applied in Moore et al (2018), are already a well-established technique studying geological sequestration of carbon dioxide and other gases (e.g. Gilfillan et al 2009).…”