**1. Introduction**

China is a large country of coal production and energy consumption, the largescale and high-efficiency development of CBM can not only reduce mine gas accidents but also supplement the shortage of conventional natural gas [1,2]. Coalbed methane (CBM) is an important environment-friendly energy resource [3,4]. CBM includes biogenic gas and thermogenic gas, of which biogenic gas is generated dominantly by anaerobic bacteria and methanogens via coal biodegradation at low temperatures (<5 ◦C) [5–7]. Influenced by generation time and geological evolution, the biogas nowadays in coal reservoirs is secondary biogenic gas, which mainly is the reduction in carbon dioxide according to the gas component [8]. The resources of CBM are abundant in China, and commercial development for the middle-high rank CBM has achieved technological

**Citation:** Zheng, C.; Ma, D.; Chen, Y.; Xia, Y.; Gao, Z.; Li, G.; Li, W. Biogenic Methane Accumulation and Production in the Jurassic Low-Rank Coal, Southwestern Ordos Basin. *Energies* **2022**, *3*, 3255. https:// doi.org/10.3390/en15093255

Academic Editors: Mofazzal Hossain, Shu Tao, Dengfeng Zhang, Huazhou Huang, Shuoliang Wang and Yanjun Meng

Received: 16 March 2022 Accepted: 26 April 2022 Published: 29 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

breakthroughs [9–13]. Although the CBM resources of low-rank are rich [14,15], the development of low-rank CBM in western China is still not satisfactory [16–20].

Both scientific research and production practice show that it is a key step to making clear the geological characteristics of CBM accumulation and production. It was considered that CBM generation, accumulation and production are controlled by sedimentation, coalification, tectonism, hydrodynamics, as well as other geological factors [14,16,21–23]. The coalbed methane well productivity performance was a result of the strong interaction of cumulative thickness, burial depth, gas content, permeability, and reservoir pressure of fractured coal seam [24], in which the gas content is positively correlated with the coal burial depth, coal thickness [25]. The geologic structural setting and hydrogeological influenced the spatial distribution of gas content and permeability and so on [26–28]. Hydrodynamic confinement results in relatively high gas content of coal reservoir in the groundwater stagnation zone, while gas content is comparatively low in the recharge and runoff zones [11,29,30]. Characteristics of in situ stress indirectly control coalbed methane development by affecting permeability [31–33]. In a word, if the main controlling factors, such as structure, coal-forming environment, and hydrologic geology, are matched well, the enriched coalbed methane zone with high production would be formed [15].

Although much literature has been published on the geologic feature of CBM reservoirs [33–36], previous investigations on the CBM in the study area were mainly focused on geological background and CBM enrichment conditions [37–39]. Furthermore, the usual methods for the prediction of favorable areas are empirical methods, such as expert scoring method, fuzzy comprehensive evaluation method, etc., which have subjective human factors to a large extent [40,41]. Few studies on the model of coalbed methane accumulation and production based on the genetic type of coalbed methane have been conducted in this area. In particular, the detailed evaluation of low-rank coal biogas in western China. In this study, the random forest data training method was used to train and classify the sample data in order to comprehensively analyze the accumulation and production of CBM taking the Dafosi minefield, Binchang mining area as the study area. Firstly, we determine the gas compositions and the genetic types; secondly, we analyze the key factors affecting the CBM accumulation and production and then reveal the relationship between the gas content, gas production and geological conditions. Finally, the rank of factors is determined with grey correlation, and the favorable area for CBM development was predicted in the study area by using the random forest method.
