Structural And Hydrologic Controls Of Gas Content In The Zhengzhuang CBM Field,Southern Qinshui Basin

Based on the seismic subtle structure interpretation,tectonic evolution history analysis,fault sealing evolution,and isotope hydrogeology tracer analysis,this thesis investigated the structural and hydrogeological control on gas content of No.3 coal seam in the Zhengzhuang CBM field.The development and distribution characteristics of fault,fold and collapse column in the Zhengzhuang CBM field were interpreted by using the well logging and seismic data.In Zhengzhuang CBM field,the main structure is in the direction of the NE-SW.Normal faults are well developed,whereas thrust faults are locally distributed in the northeast part of the study area.There are two large-scale normal faults,several medium-scale normal faults and a hundreds of small normal faults.Folds are concentrated in the northeast part,and most of them are anticlines with an extending length 5-10 km.Collapse column show round and ellipsoid shape in plan view,with a diameter of 50-300 m.The structural controls on gas content were systematicly analyzed in the Zhengzhuang field.It was found that the main faults were formed in the cretaceous to Tertiary and their sealing evolution history could be divided in 3 stages: opening in Late Cretaceous to Paleogen,semi-open in Neogene to Quaternary,and sealed at present.Affected by fault sealing evolution,gas content had undergone 3 stages of changes.Coal reservoir was in full saturation or over-saturation state after the key hydrocarbon generation.Then,large amount of gas escaped in the Late cretaceous to Paleogen.Finally,the distribution of the current gas content was formed in Neocene to Quaternary.In the study area,the major faults took key control on the distribution of gas content,and minor faults only had locally influence on gas content in the local area.Generally,coal seam has low gas content in the area closing to a fault with a distance less than 1.0km to fault.However,gas content is not influenced by faulting when the distance between fault and the investigated area is greater than 1km.Moreover,gas content is relative higher in the wing of fold than the axis of fold,because fractures are well developed in the axis parts of fold.There is not an obvious relationship between collapse column and gas content distribution,especially in local area and with a very close distance to the collapse column.Both methane carbon isotopic composition and mineralizationdegree of the groundwater have positive correlation with coal seam gas content in the Zhengzhuang field.In general,gas content is high in the place with weak hydrodynamic intensity and heavy methane carbon isotopes.In contrast,gas content is low in the place with strong hydrodynamic intensity and light methane carbon isotope composition.According to the characteristics of methane carbon isotopes,this study divide hydrodynamic intensity into 3 types: ?,?,and ?.Among the three types,the type-?has average gas content of 24.6m~3/t,methane carbon isotope of >-32‰,and mineralizationdegreeof > 2600mg/L.It is concluded that groundwater flowed from the structure high to structural low,which acted a well sealing for gas migration.CBM accumulation is obviously in the area with the structure of syncline,where the slow flowing groundwater took key control on sealing of CBM.