| In recent years,as a hot spot in unconventional resource exploration,tight oil has attracted great attention from experts at home and abroad.Yanchang Formation is a typical tight oil enrichment horizon in Ordos Basin,but the reservoir has strong oilbearing heterogeneity,and there is a widespread phenomenon of "reservoir without oil".The unclear of oil-bearing control mechanism directly restricts the exploration and development of tight oil.For the structurally stable Ordos Basin,the reservoir physical property and pore-throat structure are mainly controlled by diagenesis,and the reservoir physical property and pore-throat structure are the key factors affecting the effective enrichment of oil.Therefore,revealing the control mechanism of diagenetic evolution characteristics of tight sandstone on oil-bearing property is of great significance to enrich the accumulation mechanism of tight oil and improve the exploration and development of tight oil.In this paper,the tight sandstone of Yanchang Formation is taken as the main research object.Using whole rock analysis,thin section observation,element scanning electron microscope,isotope,physical property test and inclusion methods to clarify the lithologic characteristics,physical property characteristics,diagenetic mechanism,and to divide the diagenetic facies of reservoir;The diagenetic facies crossplot was established based on logging curves,and the pore throat structure characteristics of different diagenetic facies reservoirs were clarified by combining image analysis method and fluid injection method;Based on diagenetic dynamics,the evolution process of reservoir physical property is reconstructed by combining forward and inversion methods;The hydrocarbon generation pressurization model,production data and physical simulation experiments were used to determine the lower limits of charing and development of tight oil.Finally,the controlling mechanism of diagenetic evolution on oil bearing is revealed.The main achievements are as follows:(1)The content of quartz and chlorite in Chang 7 reservoir is lower than that in Chang8 reservoir,while the content of feldspar,illite and carbonate is higher than that in Chang8.Chang 7 and Chang 8 tight sandstone reservoir porosity distribution in 6%～12%,but Chang 7 reservoir permeability is mainly less than 0.1 m D,when Chang 8 reservoir permeability is mainly greater than 0.1 m D.The pores of Yanchang Formation tight sandstone reservoir can be divided into primary intergranular pores,dissolution pores and clay mineral intergranular pores.The pores of Chang 8 reservoir are mainly primary intergranular pores,while of Chang 7 reservoir are mainly dissolution pores.The pore throat structure of Chang 8 reservoir is better than that of Chang 7 reservoir,which is mainly characterized by larger pores and throats and lower displacement pressure;(2)Chang 7 reservoir has strong compaction due to its fine particle size and high matrix content,so its primary pores and throats are smaller,leading to low permeability.The early carbonate cements in Chang 7 reservoir are related to microbial methanogenesis and decarboxylation of organic matter,and the late carbonate cements are mainly related to decarboxylation of organic matter.Montmorillonite transformation and feldspar dissolution are the main reasons for the formation of secondary illite in Yanchang Formation.The reason for the high illite and illite-montmorillonite mixed layer in Chang7 reservoir is that on the one hand,Chang 7 belongs to lacustrine deposits,and the content of primary illite in sediments is high.On the other hand,rich volcanic materials provide the basis for the formation of montmorillonite of Chang 7 reservoir.High content of cement and strong compaction are the main reasons for small primary pores,low permeability and high displacement pressure in Chang 7 reservoir;(3)The diagenetic facies of Chang 7 reservoir can be divided into three types: feldspar mineral-strong dissolution-weak compaction-weak cementation face,plastic minerals-strong compaction-weak cementation-weak dissolution facies and carbonate minerals-strong cementation-weak compaction-weak dissolution.The pore throat radius of feldspar mineral-strong dissolution-weak cementation-weak compaction facies is mainly distributed 2 ～ 50 μm.The pore throat radius of plastic minerals-strong compaction-weak cementation-weak dissolution reservoir is less than 10 μm.The pore throat radius of carbonate mineral-strong cementation-weak compaction-weak dissolution facies reservoir is less than 1μm.The densification and oil accumulation of feldspar mineralstrong dissolution-weak compaction-weak cementation facies reservoir occur simultaneously,when the other two diagenetic facies reservoirs had become dense before oil accumulation;(4)The charging lower limits of properties for Chang 7 tight oil were8% and 0.014 m D,respectively.The development lower limits of properties for Chang 7tight oil are 6% and 0.01 m D.Combined with the physical property evolution curve,the development lower limits of physical property during the accumulation period are 9.74 %and 0.032 m D.The coupling relationship between diagenesis and physical evolution and double lower limit determines the oil content of the reservoir.The porosity of feldspar mineral-strong dissolution-weak cementation-weak compaction facies reservoir is 14.9%before oil accumulation,which is much higher than the charging and development lower limit,so the oil content of the reservoir is good.The pore throat structure of the feldspar mineral-strong dissolution-weak cementation-weak compaction facies reservoir is good,and the permeability of the water phase increases slowly during development,the oilwater co-permeability area is large,and the water production rate is low,so the oil production is high.The results are consistent with production data.The evolution of reservoir physical property and pore-throat structure is the result of sedimentary and diagenetic processes,of which deposition is the basis and diagenetic evolution is the key factor.Under the background of the same charging force,the oil bearing of the reservoir depends on the physical properties and pore-throat structure of the reservoir during tight oil charging.The development effeciency mainly depends on oil content and pore throat structure of reservoir.The research results will better reveal the diagenetic evolution,hydrocarbon charging history and oil-bearing difference mechanism of Chang 7 reservoir,and provide suggestions for tight oil exploration. |
