Tectonic And Structural Controls On Gas Accumulation Characteristics In The Weibei Coalbed Methane Pilot-production District

The study area is located in the south of Wei Bei uplift,which is a part of the southeast of Ordos.This area locates in the boundary of-the basin and has a complex structure.The main target seam of this study is No.5 coal.The coal-seam belongs to the top of upper Carboniferous Taiyuan formation.Based on previous research results and new collected well,logging,and 2-D seismic data,the fine geological structure was interpretated.Structural situation,structural feature and structural evolution were analyzed,and the tectonic and structural controls on coalbed methane(CBM)accumulation were summarized.Sparse pulse wave impedance inversion was made to convert seismic profile to lithological profile.Then the reservoir physical properties including the thickness and gas content of the No.5 coal seam were analyzed.The achievement of the inversion verified the results of the tectonic controls of CBM in the study area.The majority parts of the study area develop reverse fault except some part of Heyang,which develop some normal fault.Three fault zones were idenfied,i.e.the North-Xuefeng,South-Xuefeng,and Qiangao fault zones.Those fault zones divided the Hancheng area into four tectonic sub-zones:Panlong slope area,Xuefeng fault-anticline zone,Yingnan nose-like structures zone and Qiangao fault-anticline zone.Only the Gannan slope area is in Heyang area.Hancheng area developed a large angle unconformity.The unconformity was developed in the period of Cretaceous to quaternary.The formation of the uniformity was after the two main hydrocarbon generation of No.5 coal seam.Thus the uniformity contributes great to CBM accumulation and enrichment in the study area.The southern area of Xuefeng fault-anticline zone and the southeast of Yingnan nose-like structures zone,where the erosion were great,have low gas content.The development of fold also has an important effect on CBM enrichment.Results show that the syncline located in the northeast of Panlong slope area had made a good sealing process on CBM.But most anticlines and normal faults had made disadvantage effects on the sealing process of CBM.Resutls of sparse pulse wave impedance inversion reveal that the No.5 coal seam widly distributed in the study area.The thickness of coal seam is commonly between 3m to 6m,with the minimum of 1.3m and the maximum of 7.9m.The gas content of Hancheng area is high(commonly>10m3/t),with only a few areas that have low gas content of 6 m3/t.However,the gas content of Heyang area is low,with the valus commonly lower than 7m3/t.By combining the results of tectonic study with other investigations,the research reveals that deep-buried area with simple tectonic has high gas content,e.g.the Panlong slope area.However,for the deep-buried area with complex structure,the tectonic play an important role in gas content.The areas of denudation severely,anticline or normal fault have low gas content,and areas of the edge of nose-like structures or reverse fault have high gas content.