Enrichment law of deep coalbed methane accumulation in central and eastern Ordos Basin
The Upper Paleozoic coal seams are widely developed in Ordos Basin,and deep coalbed methane(CBM)resources are very rich.To clarify the geological characteristics and gas bearing laws of deep coal reservoirs in Ordos Basin,and to select favorable areas for coalbed methane enrichment,the study focuses on the No.8 coal seam in the central and eastern Ordos Basin.By comprehensively utili-zing geological,analytical and production data,the geological characteristics and gas bearing character-istics of deep coal reservoirs were analyzed.The results show that No.8 coal seam is characterized by larger deposition thickness in the middle and east,and smaller deposition thickness in the west and south.Coal rocks are mainly composed of bright coal and semi-bright coal.Cap rocks are mainly com-posed of mudstones and limestones,with good preservation conditions.The genetic types of coal pores include plant tissue pore,bubble pore and intercrystalline pore.Tensile fractures are developed locally,and the pore size is mainly micropores and macropores.The CBM content is high in central and eastern regions and low in western regions.The gas content shows a trend of"first increasing and then decrea-sing"with increasing burial depth.The higher formation temperature promotes the conversion of ad-sorbed gas to free gas,resulting in more than 30%for free gas in coal reservoir.The evaluation system,composed of such six parameters as gas content,thickness of coal seam,buried depth,macroscopic coal-rock type,shale content and coal body structure,has been established.The favorable areas of CBM in Type Ⅰ andⅡhave been classified in the three pilot test areas.The research can provide guidance for the optimization of favorable areas and efficient development of deep CBM.
deep coalbed methanegeological characteristicsfavorable areaOrdos Basin
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