In theory, from the high temperature and pressure during the coal generating gas to the present low temperature and pressure of coalbed methane reservoir, the accumulation of coalbed methane was from oversaturated to undersaturated. The gas content of the coalbed methane reservoir in the south Qinshui basin was 12 35.7 m 3 /t. According to the isotherm and measured gas content of No. 3 coal, the adsorbed gas content in some wells was highly saturated and oversaturated, which was hard to theoretically understand. In addition, there were no thermogenic and biogenic gases at the late stage in the south Qinshui basin. This article proposed that the overpressure was the main reason for the present high saturation of the coalbed methane reservoir. In early Cretaceous, the coalbed methane reservoir was characterized by overpressure and high saturation caused by gas generation from coal measure source rocks. In late Cretaceous, the coalbed methane reservoir was rapidly uplifted, and with the temperature and pressure decreasing, the pressure condition of adsorbed gas changed from overpressure to normal-under pressure, which resulted in the high saturation and gas content in the present coalbed methane reservoir.
IssuesThe coalbed methane reservoir in south Qinshui basin is located in the Jingcheng of south Qinshui basin, with the biggest proven reserve of the coalbed methane in China. The total proven reserve in Zaoyuan, Fanzhuang and Panzhuang is 754.44 10 8 m 3 . The coalbed methane reservoirs are mainly distributed in No. 3 coalbed of Shanxi Formation of Permian (P 1s ) and No. 15 coalbed of Taiyuan Formation of Carboniferous (C 2t ). The vitrinite reflectance of coal is 3.0% 4.5% [1] . The boundary of the reservoir is Sitou fault in west and oxidation zone in east and south [2] . The north boundary is groundwater divide. The depth of reservoir is 300 700 m, with the temperature ranging from 18 to 35 . According to the results of the injection/pressure drop and DST well testing , the pressure gradient of the gas reservoir in depth of 292 888 m is 0.0038 0.012 MPa/m with the average of 0.0071 MPa/m, indicating the normal-under pressure conditions [3] .The gas content in the coalbed methane reservoir of the south Qinshui basin was mainly 12 35.7 m 3 /t, of which the high value occurred in the stagnation belt of groundwater. According to the adsorption isotherm map of No. 3 coal, in some wells the gas saturation was over 90%, up to or near saturated (Fig. 1). In the coalbed methane bearing basins of the US, such as San Juan basin, the high gas saturation also often appeared. Fig. 1. The gas content of No.3 coalbed in the south Qinshui basin under the present-day pressure. The dashed part was the change scope of adsorption isotherm of No.3 coal. The data were quoted from refs. [3, 5 7]. The pressure data of Jinshi-1, CQ-9 and Pan-1 were measured by the injection/pressure drop while the other data was calculated with pressure gradient. The gas content of CQ-9 and Jinshi-4 was over the theoretical value, probably resulting f...