GEOLOGIC STRUCTURAL CONTROLS ON COALBED METHANE CONTENT OF THE NO. 8 COAL SEAM, GUJIAO AREA, SHANXI, CHINA
Applied Ecology and Environmental Research
Gas contents are highly heterogeneous in coalbed methane (CBM) reservoirs of the Gujiao area, Shanxi province, China. Diverse geologic data and stimulate methods were used to provide insight into the origin of this variability and the consequences of gas content in the No. 8 coal seam. The results show that the No. 8 coal seam experienced two gas-generating periods and one fluctuant period, and has stepped into migration period since the Late Cretaceous. The distribution of gas content of the
... . 8 coal seam in the Gujiao area is mainly dominated by structural form, water level and burial depth, all of which are structural related parameters. The gas content in the subarea A is much lower than that in the subarea B because the subarea A is deformed severely and the subarea B is a gentle monocline. The gas content first increases and then decreases with increasing burial depth, and the turning burial depth is about 600.0 m. Strong runoff area is harmful for preserving gas due to the washing effect in it. The analyzing results show that: gentle and simple monocline blocks with burial depth ranges from 500.0 m to 800.0 m, water level lower than 900.0 m is the most important "sweet spots" for CBM development in the Gujiao area. content continuously increases with increasing R o, max (the maximum vitrinite reflectance) (Qin et al., 1997) . Tectonic subsidence is helpful for preserving gas, but uplifting movement will cause the migration of gas (Song et al., 2012) . Structural deformation, burial depth and hydrodynamic condition are key parameters controlling the gas distribution (Meng et al., 2014a) . Despite the great number of papers and reports, important questions about controls of geologic structure on gas content remain unanswered, and what have mainly mentioned by former researchers are the distribution and evolution of gas content and permeability, which are the most important factors affecting gas production. However, previous resource evaluations haven't taken the geologic structure into consideration. Moreover, the researches of resource evaluation and geologic structure in China mainly have concentrated in the famous southern Qinshui basin (SQB) and eastern Ordos basin (EOB) (Lv et al., 2012; Meng et al., 2014b; . Related research is necessary but absent in many CBM-producing basins in China, especially in the Xishan coal field (XCF). Compared with the SQB and EOB, the XCF has very short CBM exploration and development history (the CBM development was started in 2011) and much more complicate structural conditions, in which Gujiao is the only one area has been put into CBM development (Wang et al., 2015a) . However, the CBM development in the Gujiao area is unsuccessful (the average gas production is about 277 m 3 /d per well) due to its' complicate geologic structure and the absent of resource evaluation (Xia et al., 2016) . Herein, the effects of geologic structure on the distribution of the gas content in the No. 8 coal seam were analyzed based on the coal core testing data, logging interpretation results, coalmine geology data, field geology data and water chemical measurements. Then the favorable geologic structural conditions for preserving gas were recognized. This study will be helpful for the CBM development, and will give instructions for adjustment of CBM development plan in the Gujiao area. APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 15(1): 51-68.