The Rule Of The Coal Accumulation And Coal-bed Methane Accumulation In Jurassic Yan’an Formation Of Ordos Basin

Based on previous exploration data and the actual geological conditions of Ordos Basin, with the direct of sedimentology and coalbed methane geology, and coal-forming pattern of main coal seams, the sedimentary system and the law of the coal accumulation and coal-bed methane accumulation 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 and so on.The paper selected favorable target areas for exploration of coalbed methane.According to the development characteristics of the Jurassic Yan’an formation in Ordos Basin, and the more stable of the four marked beds(BaoTa sandstone in the bottom of yan 10, coal, marked layer of yan 6 and marked layer of yan 9) and an auxiliary marked bed (PeiZhuang sandstone), to re-classify the Yan’an formation of more than 600 wells in the basin.The main coal seam development period is yan 9 substage to yan 6 substage.A detailed study has been processed, which is about depositional system, the facies distribution and evolution.Results show that the sedimentary system of the main coal seam (Yan 9 substage -Yan 6 substage) is big delta and small lake sedimentary systems of the Jurassic Yan’an formation in the Ordos Basin.The lake sediment was mainly developed in the southeast of basin,the areas areYan’an,Ansai,Zhidan,Ganquan,Fuxian and so on.The extensive development of delta sediment with the characteristic are large delta plain and small delta front,and the delta is multi-source rivers delta,which is from Hangjinqi-Wushenqi-Jingbian delta,and the Yanchi-Huanxian-Huachi delta is from the western,and the Qing yang–Heshui delta which is from southwest of Ordos Basin.The main coal bed of Yan 9 to Yan 6 substage are developed in all basin with the areas around the basin subsidence center, and have the similar shape and distribution to the depression. The coal of yan 9substage accumulated in the southen and southwestern of the basin,the coal thickness is 15m.The coal of yan7 to yan 6 substage accumulated in the Wushenqi area which is the north of the basin ,which upto 9.5m and 12m.The main coal accumulation environment is the marshes in delta plain, and the type of this delta provide an important venue. The law of accumulation of the main coal seam in Yan’an Formation is: from the edge of the basin to the basin center, the environment is from marsh in outer delta plain to marsh in middle delta plain to marsh in inner delta plain or lakeside marsh; the effect of coal accumulation is weak to strong to weak.Based on the basic principles of constituency (such as the depth of the coal seam, the properties of the reservoir, gas saturation, storage conditions, hydrogeological conditions). The favorable targets are Wushenqi area of the northern Ordos basin, and Longdong– Huangling area of southen Ordos basin. The two target areas had the following chatacteristics: the depth of the coal seam between 300~1500meters,the thickness of coal seam between 4~20meters,the roof of coal bed is mainly mudstone which thickness is between 2~15meters,the structure is very simple,the stratigraphic dip is between 1~8 degree,the gas content of coal bed is about 4~8 m~3/t, the ash content of coal is less than 15 percent , but the hydrogeological condition of Longdong-Huangling is better than Wushenqi area, the salinity of formation water is more than 20g/l in all area of Longdong-Hangling, but there is only local region in Wushenqi area which the salinity of formation water is more than 20g/l.