Study On Oil Accumulation Conditions In Lower Paleozoic Of Western Margin Of Ordos Basin

The western margin of Ordos basin is situated in the northern part of South-North trending structural zone which lies between different tectonic realm of east and west China. It is surrounded by different tectonic units of the north China craton, alxa block, the qingling orogen and qilian orogen. Because of wide and thick Palaeozoic marine facies formation,good hydrocarbon source rocks,reservoir rocks and so on, systematical researched in oil geology conditions of western margin of Ordos basin in lower Paleozoic is conduce to evaluate the gas exploration objects and provide foundations to optimise objects.Based on making full use of the former research results, author studied in detail the lower Paleozoic Ordovician natural gas in western margin of Ordos Basin, and analyzed reservoir forming conditions and characteristics according to sedimentary facies, weathering crust reservoir, shale cap rock and tectonic activity, by comprehensively utilizing geologic data, log wells, outcrops, etc. Research on reservoir forming conditions contributed to recognize reservoir forming history and mechanism of natural gas, and provided a theoretical basis for future gas prospecting.Through the correlation between sedimentary facie,rock type and geochemical behavior, Majiagou Formation and Pingliang Formation are the chief source rock in Lower Paleozoic of western margin of Ordos Basin. The evolutionary phase is medium abundance overcure dry gas phase, the kerogen type is I～II1. The limestone of open marine platform in Majiagou Formation and the limestone, the dolomite along with some submarine fan reservoir in Pingliang Formation are the chief reservoir rocks in Lower Paleozoic of western margin of Ordos Basin. Based on the study, classified the reservoir-cap combination in three types, weathering crust reservoir-cap combination, inner structure reservoir-cap combination and gravity flow clasolite (submarine fan) reservoir-cap combination.There was Permo-Carboniferous system dark mudstone found everywhere in study area, that was regional cap rock,the west and south of the middle Ordovician wedge out was Pingliang formation mudstone overlay area,there was Carboniferous mudstone be the baring in Huanxian,Dingbei and.Zhengning.The way of migration in upper-Palaeozoic in study area mainly was monophase gas,migration pathway rely on unconformity surface(unconformity surface between Neopaleozoic and Eopaleozoic ), fractures was the secondly.According to the accumulation factors and their matching relationship in time and space, the play model in western margin of Ordos basin was divide into three types:○1 the play model was under the control of the structural trap○2 the play model was under the control of the non-structured trap(lithology,layer,ancient karst)○3 the play model was under the control of submarine fan clastic rock gas reservoir.