A new insight into coalbed methane occurrence and accumulation in the Qinshui Basin, China

Highlights

• Systematic analysis of the tectonic evolution in the entire Qinshui Basin.

• Characterization of the physical structure of coal in major CBM development blocks.

• Comparing gas content and composition differences in coal reservoirs.

• Revealing the gas accumulation modes and distribution types throughout the basin.

Abstract

Coalbed methane (CBM) recovery from coal seams can benefit low-carbon energy utilization and reduce greenhouse gas emissions. The Qinshui Basin, which evolved from the North China Craton Basin in the Late Paleozoic, provides abundant coal-bearing rocks and relatively stable geological conditions for CBM occurrence. To better understand CBM accumulation characteristics, various aspects of geological structure, magmatic activity, hydrodynamics, coal seam thickness, burial depth, and sedimentary characteristics of coal measures within the basin were comprehensively investigated, along with the CBM reservoir aspects of coal components, mechanical properties, pore-fracture characteristics, and gas contents. The results show that the coal-forming environment in the northern Qinshui Basin is dominated by river flood and delta plains, while the central Qinshui Basin mainly consists of delta front, inter-delta bay, and lacustrine facies. The southern part is primarily lacustrine facies with scarce delta front subfacies. In terms of ash yield, that of coal seam 3# in the center of the study area is significantly higher than that in the north and south because the center is closer to the provenance and carries a large amount of detritus during the flood season. The primary coal structure is well preserved, and the facies cleats are largely filled with dolomite and kaolinite, suggesting mineralized fluid activity after the development of the major fracture. The gas content is mainly controlled by geological structure and hydrogeological conditions. Interestingly, for the coal seams > 1000 m away from the fault, the nature of the fault has no significant effect on the gas content. This study provides insights into the efficient exploration and exploitation of CBM in Qinshui Basin.

Introduction

The Qinshui Basin of northern China has the most abundant Permo-Carboniferous coal resources and hosts one of the most prolific and long-lived coalbed methane (CBM) plays within high-rank coal worldwide. The wealth of research in this basin provides insight into the relationships between geological conditions and CBM performance (Cai et al., 2011, Liang et al., 2020, Zhang et al., 2017). CBM is a hydrocarbon gas occurring in coal seams, which is mainly composed of methane. It is mainly adsorbed on the pore surface of the coal matrix and to a lesser extent in a free state within the pores (Olgun et al., 2020, Chen et al., 2021). Complex CBM accumulation processes may include reasonable CBM preservation during the Indosinian period, regional adjustment of gas content with respect to fold formation during the Yanshanian period, and final redistribution of gas content during the Himalayan period, which determined the present gas content distribution pattern (Wang et al., 2013, Liu et al., 2018, Liu et al., 2021, Mondal et al., 2021). In addition, abnormal paleo-geothermal activity occurred within Qinshui Basin due to tectonic-thermal events that took place during the Yanshanian orogenic period. This generated a high-temperature and high-pressure environment that contributed to continued hydrocarbon generation in coal seams and enhanced their adsorption capacity (Yin et al., 2019, Zhao et al., 2021, Wang et al., 2022).

In terms of hydrodynamics, the groundwater recharge areas with strong runoff tended to flush methane, thus hindering CBM preservation (Marshall et al., 2016, Dai et al., 2016). Wang et al. (2009) found that tectonic conditions were the most critical and direct factors controlling gas content, which mainly affected the gas generation potential, reservoir performance, and permeability of coal reservoirs; hydrodynamic conditions controlled the formation, transport, and enrichment of CBM. Li et al. (2017a) demonstrated that tectonic subsidence and weak hydrodynamics in the southern Qinshui Basin created favorable hydraulic confinement for CBM occurrence. Li (2020) argued that there were different formation pressure gradients in different areas of the basin and found that the anomalously low formation pressure in the deep coal seams was related to the structural background and reservoir petrophysics. Liu et al. (2018) confirmed that the CBM components of different wells gradually changed in spatial distribution during the formation of CBM reservoirs, and the degree of inter-well interference of CBM wells across the Fanzhuang Block in Qinshui Basin was relatively weak. Zhang et al. (2017) proposed that the tectonic pattern played an essential role in controlling reservoir permeability, which caused an exponential increase in permeability with an increase in tectonic stress difference. Moderate tectonic curvature normally has high permeability, while the permeability control of vertical stress dominating after the average horizontal stress is greater than the vertical stress. Many detailed investigations have been performed on the geological aspects of CBM blocks (Li et al., 2017a), organic geochemistry (Liang et al., 2020), geomechanical properties (Mondal et al., 2021), CBM resource evaluation (Wei et al., 2021), reservoir aspects of pore/fracture structures together (Li and Tang, 2019, Li et al., 2017b), and fluid performance including gas/water adsorption/desorption, diffusion, and seepage in Qinshui Basin (Gao et al., 2021, Yao and Liu, 2012, Li, 2020). However, few studies have paid attention to the contrasting CBM concentrations of different CBM blocks from the perspective of the whole basin, especially in terms of the gas accumulation mode.

In this study, we systematically describe the geological setting of the Qinshui Basin, followed by an investigation of the coal composition, mechanical properties, and pore-fracture characteristics of the coal seams targeted for CBM development. To clarify the gas accumulation mode across the basin, gas content, composition, and differences were comprehensively evaluated, and the controlling effects of tectonic evolution, sedimentation, and hydrogeological conditions on the gas content are discussed in detail. Therefore, this study contributes to CBM exploration and development in Qinshui Basin.