Sedimentary System And Reservoir Rharacteristics Of Chang 7 Member Of The Triassic Yangchang Formation In Ordos Basin

This paper uses sedimentology,high-resolution sequence stratigraphy,oil and gas reservoir geology and petroleum geology,and other theoretical methods,combined with core observations and the second interpretation of logging data to characterize the distribution characteristics of sedimentary microfacies and sand body and pore throat characteristics of reservoir of the Chang 7 key tight reservoir Member in the Ordos Basin.On this basis,the characteristics of reservoirs and the distribution of reservoirs in the study area were systematically analyzed.finally,we completed the selection of the Chang 7 favorable zones in study area.Two major sedimentary systems developed in Chang 7 member of Ordos Basin,The delta sedimentary systems is developed in the northeast,northwest,and southwest,and the deep-half-deep lake systems is developed in the south-central region,respectively.there are three main types of sedimentary facies in study area.the meandering river delta is developed in the northeast of study area,the braided river delta is developed in the northwest and southwest of study area,and the turbidite fan is developed in the middle and south of study area.The scale of sand body development in the study area is mainly controlled by the deposition system.Sand bodies are generally the result of multi-stage overlay.The thickness of sand bodies is in the range of 5m to 30 m.The direction of the distribution of sand bodies on the surface is mainly from NE to SW.The grain size of the sandstones in the study area is mainly fine-grained,and the sorting is mainly medium,and the roundness of the sandstone is mainly secondary.Feldspar and quartz are the main rock debris components in the study area,followed by rock debris,and lithology is mainly feldspathic sandstone;hydromica is the main filler component of the Chang 7member of the study area,followed by iron calcite.,dolomite and siliceous,but also a small amount of kaolinite,chlorite film and soon.Sandstones in the study area experienced various degrees of compaction,cementation,metasomatism,dissolution and fracture in the process of diagenesis.According to the diagenetic stage division scheme and its signage,the sandstone of Chang 7 member enters the late A of middle diagenetic phase.During the period,part of the sandstone entered the B stage of middle diagenetic phase.The pores of the Chang 7 Member in the Ordos Basin are dominated by intergranular pores and feldspar-soluble pores.Intergranular dissolved pores and debris dissolved pores can also be seen.Intercrystalline pores,micro-cracks and mold pores are less developed in the study area.The maximum pore size distribution range of reservoir rocks in the study area is wide,the minimum pore size is less than 5?m,and the maximum pore size can reach 200?m,and the face rate varies from 0.1% to 14.5%,showing strong heterogeneity.There are four types of reservoir pore structure in the study area.Among them,type III is the most important type of pore structure,followed by type II and type IV,and type I is less.The porosity of the Chang 7Member in the Ordos Basin is mainly distributed in 6%~14%,and the permeability is mainly distributed in 0.01×10-3?m2~0.4×10-3?m2.The reservoirs with different permeability grades have similar pore size distribution,but the distribution of throat is difference.The shape of nano-order pore throats are mainly tube bundles,which are mostly developed within the grains.The connectedness of micron pores is better.The existence of small throats is the main factor of extremely low permeability and tight reservoir formation.Each section of Chang 7Member has a common feature:The physical properties of the reservoir gradually deteriorate from the main body of the river to the flanks of the river.In the plane,due to the sedimentary differentiation and the mutual deposition between different sedimentary phases,the physical properties of the reservoir are also constantly changing in the direction perpendicular to the river.Combining with characteristics of the plane distribution of Chang 7 sedimentary microfacies and sand body and the results of reservoir characteristics,a new classification standard for reservoirs in the study area was formulated,and the reservoirs in the study area were divided into four categories.Among them,Class I is the best reservoir in the study area,but its distribution range is small.The most widely distributed reservoir in the study area is Class II+III reservoir.The Chang 7 reservoir in the Ordos Basin is dominated by non-structural oil reservoirs and is a typical tight reservoir;the process of its accumulation is preceded by accumulation and then compaction,and the enrichment patterns are mainly controlled by reservoirs.Among the Chang 7 members,the Chang 71 reservoir group is the most enriched,followed by the Chang 72 reservoir group,The reason of less oil reservoirs developed in Chang 73 is The formation of the Chang 73 reservoir is poor.Reservoirs are mainly developed in the I+II reservoirs in the area,and few see water.