| Based on previous exploration data and the actual geological conditions of Ordos Basin, coal-forming pattern of main coal seams, reservoir formation conditions and process of Coal Bed Methane reservoir have been researched deeply by multi-item technical means, such as document retrieval, outcrop section observation, the industrial analysis of coal-rock, identification and quantity of coal’s maceral concentrates, microscope section analysis , mercury injection test and so on. The general Geology law of CBM’s enrichment and reservoir formation was definite through the researchment.The major coal-bearing series of Ordos Basin are Benxi formation of Carboniferous, Shanxi Formation of Permian and Yan’an formation of Jurassic.the result show that there were six marine coal accumulating environments existed under the sedimentary background of epicontinental sea sedimentary system and shallow delta sedimentary system in Benxi formation and Taiyuan formation of Permo-Carboniferous,which are barrier land peat fiat,lagoon peat fiat,tidal peat flat ,marine shallow delta peat flat,carbonate plateau peat flat and neritic shelf peat flat. And there were four coal accumulating environments of continental facies developed in Shanxi formation under the sedimentary background of fluvial- delta sedimentary system, which are terminal fan peat swamp of alluvial fan , flood plain peat swamp of fluvial face, delta plain peat swamp and lakeside peat swamp. Delta face and lake face developed in the Yan’an formation of the Jurassic, and delta plain peat swamp developped in upper delta plain, medium delta plain and lower delta plain was the majoy coal accumulating environment in this Period, the middle delta plain of them is the most suitable area for coal-accumulating. The distribution of coals are controled obviously by the sedimentary system.The main coal of Permo-Carboniferous was mainly distributed in the east area of Ordos Basin,Yan-8 main coal and Yan-9 main coal of Yan’an formation enrich in Qingyang-Huangling area which is in the south of basin, Yan-6 main coal and Yan-7 main coal enrich in Wushenqi area which is in the north of basin.There are advantageous conditions for the CBM’s reservoiring of the main coals(8# coal and 5# coal)of Permo-Carboniferous in east area of Ordos Basin, such as the large thickness of coal source rock,the high thermal evolution degree, the coal ranks are mainly middle-high metamorphism coal, vitrinite of maceral is more than 70%, crack is abundant in this coal reservoir, distribution of the rich coal center is definite, the single coal thickness is large, and material base of gas generation and gas storage is abundant. The CBM reservoir is mainly controled by structure, roof and floor lithologic characters and hydrogeological characteristics. The structure controls form of seam gases, roof and floor lithologic characters restrict the gassiness,and hydrogeological characteristics play a key role in lateral sealing of CBM reservoir. The CBM reservoir of Jurassic have good exploration prospect because of their large thickness,wide distribution and the very thick, multiple coal seams in structure, but also of high exploration risk because that thermal evolution of the coals is poor,whose coal bank is long flame coal, gas generation ability is poor and hydrocarbon generating quantity is finite, and petrographic characteristics and coal characteristics is worse than Permo-Carboniferous’s.The result of the research on formation process shows that CBM accumulation process of Permo-Carboniferous coal is complicated because of coal-bearing series’s multiple uplift and denudation after their formations, and the mutual dispose relation between tectonic evolution history, deposit-buried history and thermal-hydrocarbon generation history is the key to CBM’s Enriching and reservoiring.The gas accumulation process of the low bank coal in Yan’an formation of Jurassic is relatively simple,and preservation conditions are the important factors of CBM’s accumulation, especially present structure characteristics and Groundwater’s occurrence condition play key roles in seam gas formation. The model of reservoir formation of coal gas enrichment is built for high, medium and low rank types,and the reservoir forming feature of each model is detailed analyzed and described.The geological factors of CBM’s Enriching and reservoiring are detailed analyzed based on the resaerch of CBM reservoir formation conditions and accumulation process, which mainly include coal seam thickness, burial depth of coal-bed, petrographic and coal characteristics, thermal evolution of coal, reservoir property, structure conditions, surrounding rock properties and hydrodynamic condition. It is proved that the local structural high part and the region of low angle slope whose direct roof is safe are range of profitability for CBM’s Enriching and reservoiring, tectonic heat event, lithologic association of seam roof and floor rocks whose lithology are mudstones and the thick coal seam of shallow and Low bank beneficial to CBM’s Enriching and reservoiring.The understanding mentioned above can be utilized in evaluating and selecting the blocks for coal bed methane exploration,which has guiding significance for CBM’s further exploration practice in Ordos Basin. |
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