Evolution of coal-bed methane in Southeast Qinshui Basin, China: Insights from stable and noble gas isotopes

Abstract

The late Carboniferous-early Permian coal seams of the Qinshui Basin in Shanxi Province are the most prolific producer of coalbed methane (CBM) in China. Methane formed in the late Triassic during deep burial and reheating in late Jurassic-early Cretaceous driven by magmatic underplating. Basin inversion brought the coal seams to 400-700 m from the surface in the mid-late Cenozoic. Here we present results of a study aimed at understanding the origin of the methane, and how it was affected by Cenozoic exhumation of the basin. Methane from a 12 km traverse perpendicular to the basin margin in the southeast part of the basin have stable isotope compositions (δ 13 C =-30.2 to-35.2‰, and δD =-155 to-194‰) indicating a thermogenic origin with limited biogenic input. They are, however, lighter than expected based on coal maturity, and C1/(C2+C3) (>1000) are significantly higher than typical thermogenic methane (<50). This is due to diffusive fractionation during commercial gas extraction. He-NeAr isotopes are a mixture of crustal-radiogenic gas with air-derived noble gases. 4 He concentrations (0.52 to 33.25 ppmv) and 4 He/ 40 Ar * ratios (0.06-1.74) are unusually low. He-NeAr concentrations are consistent with the open system Rayleigh fractionation of noble gases derived from air-saturated water with 4 He/ 40 Ar * = 1 during gas extraction. The low 4 He/ 40 Ar * , compared with average crust (5) or local production (13) values, implies that more than 90% of the radiogenic 4 He produced in the coals has been lost prior to equilibrium between gas and water phase in the reservoir. This likely occurred in response to gas loss process during rapid exhumation in Cenozoic, showing that the He and Ar content of natural gases is a sensitive indicator of gas loss event caused by recent basin inversion. The event may have led to the loss of up to 44% of the methane from the coal seams. This study demonstrates the importance of basin inversion on gas preservation in shallow CBM, and shows that, in contrast to δ 13 CCH4, the light noble gases are essential for tracing such a process. Keywords Coalbed methane (CBM), Southeast Qinshui basin, noble gas isotopes, gas loss, high coal rank Highlights  Gas compositions and stable isotopes indicate diffusive fractionation during commercial gas extraction.  There is no indication of mantle volatiles in gas samples, no evidence of mantle heating during the Yanshanian Orogeny.  The low radiogenic He and He/Ar ratio indicate loss of free gas during basin exhumation.

Noble gases confirm plume-related mantle degassing beneath Southern Africa

“…Bulk gas content as a percentage was determined using a Pfeiffer Vacuum QMS 200 quadrupole mass spectrometer with all seeps sampled exhibiting concentrations of >99% CO 2 . δ 13 C CO2 were measured using a VG SIRA II dual inlet isotope ratio mass spectrometer following established procedures 58 . Precision and reproducibility are typically better than ±0.2‰ for δ 13 C (Supplementary Table 2).…”

Mesozoic Tectono-Thermal Event of the Qinshui Basin, Central North China Craton: Insights From Illite Crystallinity and Vitrinite Reflectance

“…At the same time, the exposed rocks around the Qinshui Basin, which include Taershan, Huyanshan, Zijinshan, Erfengshan, Gufengshan, etc, started to form (Yan et al, 1988;Shanxi, 1989;Wu et al, 1996Wu et al, , 2008Ying et al, 2011;Si, 2015;Huo, 2016;Yang, 2016;Chang et al, 2017;. It can be seen that there was a tectono-thermal event represented by intense magmatic activity in the Late Jurassic to Early Cretaceous, which is generally considered to be closely related to the secondary gas peak of coal seams in the basin at the same time (Figure 11) (Zeng et al, 1999;Xu et al, 2004;Jiang et al, 2005;Ren et al, 2005;Sun et al, 2005;Duan et al, 2011;Chen et al, 2019Chen et al, , 2019Yu et al, 2020a;Gao et al, 2021).…”

“…Among the geological factors, thermal field, tectonic stress field, hydrodynamic field, and formation conditions have an important influence on the formation of coalbed methane (Wu et al, 2014). The coal deposits in the Qinshui Basin began to form in the Late Carboniferous and gradually ceased when the delta deposits were formed in the early Permian (Sun et al, 2005;Ma et al, 2016;Chen et al, 2019;Yu et al, 2020a). The subsidence of the basin lasted until the Late Triassic when the temperature of the coal seam reached 135 °C (the burial depth was up to 4 km).…”

Coalbed methane geology and exploration potential in large, thick, low-rank seams in the Bayanhua Sag of the Erlian Basin, northern China

“…Industrial exploitation of high to medium-rank coalbed methane (CBM) has been realized in China. However, the annual production of CBM is far below the set targets (Chen BY et al, 2019; Liu et al, 2014; Tao S et al, 2014 Zhao et al, 2017). Recently, low-rank CBM has been successfully developed in the Powder River Basin, Uinta Basin, Alberta Basin and Surat Basin (Ayers, 2002; Hamilton et al, 2014; Tang et al, 2018), indicating that new blocks with high resource potential may exist to be discovered and researched in China (Lau et al, 2017; Sun et al, 2008; Yun et al, 2012).…”