Geological controls on prediction of coalbed methane of No. 3 coal seam in Southern Qinshui Basin, North China

In order to better understand the geological controls on coalbed methane (CBM) in Southern Qinshui basin (SQB), North China, geological surveys and laboratory experiments, including coal petrology analysis, proximate analysis and methane adsorption/desorption, were conducted. Results show that the coals from the SQB contain 0.59–3.54% moisture, 3.5–15.54% ash yield, 73.62–88.92% fixed carbon and 2.14–4.04% hydrogen, with C/H ratios in the range of 19.96–36.25. The vitrinite reflectance (Ro,m) ranges from 1.95 to 3.49%. The coals are composed of 18.5–97.4% vitrinite and 2.4–81.4% inertinite. The geologic structures, coal-bearing strata and coal depositional environment were studied by both field geological research and laboratory tests. A positive relationship is found between CBM content and basin hydrodynamics, in which CBM easily concentrates in the groundwater stagnant zone because of the water pressure. Furthermore, integrated geographical information system (GIS) and analytical hierarchy fuzzy prediction method (AHP) were used to evaluate the CBM resources in the SQB. The results show that the amount of CBM associated with the No. 3 coal seam in the SQB is 3.62 × 1011 m3. The CBM resource concentration (gas-in-place per square kilometer) in the SQB is in the range of (0.72–2.88) × 108 m3/km2, with an average of 1.21 × 108 m3/km2, which decreases from Zhengzhuang coal district to Shitou fault and from Fanzhuang coal district to the margins of the basin. The best prospective targets for CBM production are likely located in the southwest/northwest Zhengzhuang and central Hudi coal districts.

► Multiple factors of gas controlling were studied. ► CBM easily concentrates in the groundwater stagnant zone due to the water pressure. ► Gas contents of No. 3 coal seam displays structurally controlled pattern. ► AHP method was used to evaluate CBM resources and select prospective CBM targets.