Gas Origin and Constraint of δ¹³C(CH₄) Distribution in the Dafosi Mine Field in the Southern Margin of the Ordos Basin, China

Abstract

Coalbed gas or coalbed methane (CBM) is a vital hydrocarbon gas which is mainly absorbed in the surface of the coal matrix and has multiple benefits as in being effective in development and utilization. To determine the gas origin, the distribution of the stable carbon isotopic composition for methane, and its constraint in the Middle Jurassic strata in the Dafosi mine area in the southern margin of the Ordos Basin, nine CBM samples were collected from pumping wells at the surface, arranged along three profiles located in different tectonic areas. The gas components and stable isotope compositions were measured, and the results show that the average methane and carbon dioxide concentrations in the coal seams measured are 77.44 and 1.04 mol %, respectively. CBM has apparent dry gas characteristics, with more than 6835 distributed C1/(C2 + C3) ratio values. The average values of the carbon and hydrogen isotopic compositions of CBM are −79.2‰ and −241.9‰, respectively. The gas origin in the Dafosi mine field is mainly composed of biogenic gas from CO2 reduction and mixed thermogenic gas. The former content accumulated in the present coal seams ranges from 68.93% to 100.00%, as calculated by the binary mixing model. A web-based learning tool also indicates mostly CO2 reduction-biogenic gas. Fold types play an essential role in controlling the distribution of different genetic CBM with different stable carbon isotope ratios. Biogenic gas with low carbon isotopic composition lies in the southern parts of the Qijia anticline blocked by the footwall of the fault. The mixed CBM comprised of biogenic and thermogenic gases with high carbon isotopic composition is distributed in the areas of Anhua and Shijiadian synclines sealed with lithology and hydrocarbon concentration, respectively.

Hydrogeochemical Characteristics and Water–Rock Interactions of Coalbed-Produced Water Derived from the Dafosi Biogenic Gas Field in the Southern Margin of Ordos Basin, China

“…9 Geofluids gas field also had lower Na + concentrations which means that these areas were not entirely closed. There may be faults in the core of the Qijia anticline that allow atmospheric (mete-oric) water to flow into coalbeds and migrate to both wings [27]. The TDS of the coalbed-produced water samples in the study area was generally high.…”

“…The 18 O drift characteristics of the produced water may be related because the oxygen content was higher than that of hydrogen in the coalbed [27]. The produced water sample (DFS-133) is located in the core of Qijia anticline plotted on the CMWL, showing meteoric water recharge characteristics.…”

“…4 coal is 11.6 m, and it is a mineable coalbed throughout the area. The Dafosi gas field is a specific biogenic gas mining area based on the gas component and its stable isotopes analysis [27].…”

“…The Dafosi gas field is located in the Binchang mining area of the Huanglong coal field along the Ordos basin's southern margin. The development potential of CBM resources is enormous, and its gas origin is biogenic gas [27]. Much research has been conducted on the occurrence and exploitation of CBM in this area.…”

“…Ma et al [2] stated that CO 2 -enhanced coalbed methane technologies are feasible and improve the recovery of CBM, while Lin et al [29] studied the pores of Dafosi coal samples and established a gas drainage model to enhance the efficiency of CBM drainage. Bao et al [27] analyzed the genetic types and distributions of gas in this area based on gas and stable isotope data, recognizing that the gas is mainly biogenic CBM. However, there are few studies focused on the water-rock interaction mechanism of the study area.…”