Study On The Formation And Enrichment Of Coalbed Methane In No.8 Coal Seam In Daning-Jixian Area

The Daning-Jixian area is rich in coalbed methane resource reserves,and the coalbed methane gas content in the area shows a strong heterogeneous distribution characteristic.This paper focuses on the geological control mechanism of coalbed methane occurrence and differentiation in the study area to carry out reservoir formation simulation work.At the same time,combined with hydrogeological conditions and gas composition,isotope and other geochemical indicators,the coalbed methane enrichment characteristics of No.8 coal seam in the study area are analyzed together.Get the knowledge as follows:(1)studied the "three history" evolution rule of coal seam in the region.The simulation of buried subsidence history,thermal evolution history,and hydrocarbon generation history is carried out around the clue of coalbed methane generation.the results show that settlement area coal seam buried history,thermal evolution history and hydrocarbon generation that would become divided into six stages:?-the Late Carboniferous-Permian(306.5?251Ma):coal measures strata and cap rock sedimentary-immature-biogenic gas phase;?-Early and Middle Triassic(251 Ma-203.6 Ma):the first hot-thermal metamorphic plutonic-rapid subsidence stage;?-Late Triassic-Late Jurassic(203.6 Ma?145.5 Ma):after lifting fluctuate-stagnation of stagnation of thermal evolution and hydrocarbon generation stage;IV-Early Cretaceous(145.5 Ma?130 Ma),secondary sedimentation-regional metamorphism and secondary hot gas phase,?-Late Cretaceous(130 Ma?65.5 Ma):rapid uplift-thermal evolution end stop-hydrocarbon generation;?-Cenozoic(65.5 Ma?present):stop continue to rise-hydrocarbon generation.(2)Revealed the evolution law of coalbed methane accumulation in the area.Combined with the history of tectonic evolution,the dynamic processes of coalbed methane generation,migration,accumulation,and escape were simulated,and it was considered that the differential distribution pattern of gas content in the study area was affected by five tectonic events:the first tectonic event occurred in the Late Triassic The overall uplift and erosion in the study area affected by the Indosinian movement led to the cessation of deep metamorphism and the dissipation of the pre-preserved protozoal gas and the first thermal genesis gas;the second tectonic event occurred in the middle and late Jurassic.The influence of strong northwest-west squeezing stress in mid-Yanshan formed folds and fault zones in the B3 zone in the middle of the area,resulting in a large amount of coalbed methane near the fault zone.The third tectonic event occurred in the Early Cretaceous,due to the influence of abnormal thermal events in the mid-Yanshan Sea,a large amount of thermogenic gas was generated in the coal seam,and the subsequent structural inversion caused the opening of the fault system to dissipate a large amount of the gas generated in the early stage and generate a lot of gas Emissions and differences have a decisive effect on gas accumulation in the study area.The fourth and fifth tectonic events occurred in the late Yanshan period and the Xishan period,respectively.The two-stage tectonic uplift controls the differential escape process of regional coalbed methane.(3)Analyze the influence of hydrogeological conditions on coalbed methane enrichment in the area.Through hydrochemical analysis and groundwater dynamic field research,it is believed that the gas content of No.8 coal seam in the study area is strongly affected by groundwater activities,and the study is divided into two independent groundwater dynamic systems with Xueguan fault as the boundary.In the east of the fault zone,the groundwater as a whole runs from east to west,and the gas content of the coal bed gradually increases in the direction of groundwater flow;in the west of the fault zone,the entire groundwater is in a stagnation groundwater environment,which forms a hydraulic sealing effect on the coalbed methane,causing coalbed methane enrichment and groundwater There is a positive correlation between salinity and gas content.(4)The main controlling factors of coalbed methane enrichment in the area are analyzed.Coalbed methane in No.8 coal in the study area is mainly of thermal origin,and has generally experienced migration and diffusion in the later period.The ?13C1 value of coalbed methane is generally lighter.Although the ?13C1 value and reflectivity show a certain positive correlation,they are not Obviously,the desorption-transport-diffusion effect and the secondary biogas mixing and water dissolution based on it control the distribution characteristics of the current coalbed methane ?13C1 value.Combining tectonic evolution,current structural morphology,gas isotope index and hydrogeological conditions,it is considered that there are two types of coalbed methane in the study area:"thermal origin-hydraulic migration-fault sealing type" and "slope-hydraulic sealing-fault sealing type" An enrichment mode of coalbed methane.