Geothermal field condition of coal reservoir and its genetic mechanism of low geothermal anomaly area in the Qinshui Basin

The geothermal field conditions significantly affect the occurrence and production of coalbed methane (CBM). In terms of the current production situation of the CBM wells in Qinshui Basin,it is understood that the gas production of the CBM development wells in the low geothermal abnormal area of coal reservoirs is generally low.Therefore,studying the geothermal field conditions of coal reservoirs and revealing the formation mechanisms of the low geothermal abnormal area are extremely important for the CBM development. Based on the measured data of ground temperature from the CBM wells in the Qinshui Basin,the distribution characteristics of geothermal gradient and terrestrial heat flow of both No.3 and No.15 coal seams have been systematically analyzed,and the distribution law of the geothermal field is revealed. Meanwhile,the geothermal field classification standard of the Qinshui Basin is pro- posed,the range of low anomaly geothermal area is delineated with geothermal gradient less than 1.6 °C / hm of Carbon- iferous-Permian coal reservoirs in the Qinshui Basin,and the control mechanism of low anomaly area distribution is re- vealed. The results show that the temperature of the thermostatic zone in the Qinshui Basin is gradually increasing from northwest to southeast. The depth of the thermostatic zone is gradually shallow from north to south. The tempera- ture range of the thermostatic zone is 13.2-15.2 °C,and the depth range of the thermostatic zone is 27.4-33.1 m. The ground temperature and its gradient of coal reservoirs in the Qinshui Basin increase with the increase of depth. The ground-temperature of No.3 coal reservoir ranges from 14.61 to 100.9 °C with an average of 30.58 °C ,and the ge- othermal gradient ranges from 0.008 to 3.770 °C / hm with an average of 1.6 °C / hm. The ground-temperature of No. 15 coal reservoir varies from 15.3 to 111.8 °C with an average of 33.28 °C ,and the geothermal gradient varies from 0. 046 to 5.350 °C / hm with an average of 1.87 °C / hm. In addition,the terrestrial heat flow of the Qinshui Basin ranges from 0.93 to 94.60 mW / m2 with an average of 41.5 mW / m2 ,which is lower than the average heat flow of China main- land (63 mW/m2),all of which reflect that the basin is in a stable tectonic thermal state. According to the geother- mal gradient value,the geothermal low abnormal areas are delineated with geothermal gradient less than 1.6 °C / hm in No.3 and No.15 coal reservoirs which are mainly distributed in strips in the northeast and east of the basin,accounting for 25.2% and 27.9% of the total basin area respectively,and the amount of CBM resources is nearly 1.0 trillion cu- bic meters. The geothermal gradient decreases linearly with the increase of floor elevation and increases linearly with the increase of effective burial depth of coal reservoir. Furthermore,the control mechanism of crust uplift and de- nudation and groundwater recharge and seepage on the low geothermal anomaly area of coal reservoir in study area is revealed.