Study On The Main Controlling Factors Of The Chang 6 Accumulation Reservoir In The Mafang-Shaying Area Of ??the Ordos Basin

The source rock of the Chang-7 member has been considered to be the main source rock of Yanchang Formation in Or Ordos basin. Howerer, the Chang-6 reservior is distributed in large areas of Chang-61, Chang-62,and Chang-63 with high enrichment,which is not in accord with proximal forming theory of the extra-low permeability oil-gas reservoir.The affecting factors of the reservoir in this area are still not clear at present.The clastic composition, structure and direction of the reservoir were compared to analyze the differences of the three layers in Chang-6.The source of crude oil was ascertained through oil source correlation with the source rock conditions of Chang-6 and exsiting results of Chang-7.Finally, the characteristics of petroleum migration and accumulation were analyzed and the reservoir controlling factors of Chang-6 reservoir were summarized.Through the reservoir analysis, the results shows that:the study area belongs to lacustrine delta front deposition.The sand body extends along the direction of the ancient river channel with well connectivity. However, in the direction of the cross cutting channel, the sand body is in a lens shape with poor connectivity and is obviously controlled by the river.The reservoir is mainly dominated by gray-dark gray fine-grained feldspar and debris feldspar sandstone.Tuff fillings with medium-good sorting are in high content and shape of hypo-edge angle. From Chang-63 to Chang-61, the particle size gradually becomes coarse, the cementation effect is enhanced, but the total content of tuff fillings gradually reduces.The main pore types are intergranular pores and dissolution pores. And the throat is dominated by sheet or tubular throat.Its belongs to low porosity and ultra-low permeability reservoirs with average porosity of 11.88% and average permeability of 0.5466×10-3?m2.The reservoir has fine throats with high displacement pressure, uniform sorting, and fine throat distribution.From Chang-63 to Chang-61, the physical properties gradually become better and the pore throat gradually becomes thicker, which lead to better connectivity and seepage ability.Source rocks of Chang-61, Chang-62 and Chang-63 widely distribute in the southwest study area.And its organic carbon with a content of 0.47%-14.84% is given priority to ?2? ?.It begins to enter the mature stage when R0 is about 0.65%.The oil source correlation shows that crude oil of Chang-6 comes from Chang-6 and Chang-7.However, the organic matter of Chang-6 is sapropelic-humic rot or humic type, which means that Chang-6 is in low mature stage, indicating that the crude oil of Chang 6 reservoir is mainly composed of Chang-7 in mature stage.The river sand body and cracks in the top of Chang-7 and every layer of Chang-6 are the main channels of crude oil migration.The production of Chang-7 migrates to Chang-6 in the vertical way.The residual pressure of the Chang-7 source rock and the Chang-6 reservoir is the main driving force for the reservoir formation.The formation of Chang-6 reservior in Mafang-Shayaoxian area is affected by the distribution of underlying Chang-7 source rock, reservoir distribution of Chang-6, physical properties, and cracks.The physical properties influence the enrichment degree of oil source, and cracks is the key of reservoir formation. From Chang-63 to Chang-61, the distribution range of reservoir is gradually larger and thicker, and the reservoir physical property is gradually getting better.All of these can give good explanations for the expansion of oil distribution and the increase of the degree of enrichment.