The Comprehensive Assessment On Chang—7 Tight Reservoir Of Triassic Yanchang Formation In Zhenyuan-huachi Area Of Ordos Basin

The Ordos Basin contains rich resources of tight oil.Especially,in the southwest of Ordos Basin the Yan-Chang formation of Upper Triassic,an array of industrial oil wells have been explored in Chang-7 area displaying a great potential.However,the data from ongoing research for oil development shows that these tight reservoirs could not be effectively developed,which restricts further explorations.The research was mainly focused on basic characterization and qualitative description.The classification of the sedimentary microfacies oftight reservoirs was still obscure,and the diagenesis difference of tight reservoirs and its possible roles were largely unknown.The pore throat features of tight reservoirs and its distributions were not accurately described,and the research on its fracture was not systematic or quantitative.To address above concerns,this study applied rock identification,cast slice,conventional mercury injection,rock mechanical test by triaxial compression experiment,combined with the test data from field production analysis.The sedimentary marks were further clarified and its contrast was strengthened both vertically and horizontally.Hence,the sedimentary micro-facieswere more effectively identified and classified.Both diagenesis and diagenetic facies were fully recognized with compaction calculation and image analysis.Through super-accuracy nanometer CT scanning,the pore throat characteristic of tight reservoirs and its distributionwas more accurately represented.The binary fracture prediction method was established by applying FMI,three-dimensional geological modeling,and finite element numerical simulation.With the help of this new method,the fracture distribution was further recognized and quantitatively described.Through complexed statistical analysis,the fundamental differences between conventional and tight reservoir were clearly uncovered.Although tight reservoirs are usually classified as unconventional reservoirs,they have both the conventional and unconventional properties.Through the study of pore throat characteristics and physical properties,which were controlledand affectedby sedimentary,diagenesis and fracture,weselected evaluation parameters based on reservoir property,flowing capacity,controlling factors as wellas validation of the results.In addition,the comprehensive assessment standards were set up by applying fuzzy evaluation,which provided foundation for reservoir optimizing.This study not only clarified the key characteristics of Chang-7 tight sandstone reservoir of the southwestern Ordos Basin,but also concluded that type I and II reservoirs are superior over others.Four oil enrichment regions were selected with a favorable area around 1500 km~2 for further tight oil exploration.The main knowledgeacquired from this studyareas follow:(1)From the tight reservoir property:Chang-7 tight sandstone reservoir of the southwestern Ordos Basin is still the fine sandstone reservoir body,and neither the mud shale nor the carbonate rock reservoir body.Compared with the conventional low permeability reservoirs,the coarse sandin sediment is less,and very fine grained and mud is relatively higher.All these lead to apoorer physical property.Hence,considering its conventional feature of reservoir property,the Chang-7 sandstone tight reservoir is classified as non-conventional reservoir with conventional characteristics.(2)From the sedimentary property:a prominent characteristic is gravity flow,which plays a decisive role in the control of reservoir deposit.Sufficient and continuous sedimentary source recharge led to a high speed deposit accumulation after slide.The original steep slope became relatively gentle,which favors the advancement of the next deposit into deep lake basin and enables sedimentary sand extending to a broader area.Compared with conventional low permeability reservoir deposit,the influence from the river is weaker,and the turbidite deposits of slump body are the main characteristic.Hence,sandy debris flow is the typical and major sedimentary microfacies,which determines the distribution of relatively fine sandstone.Differently,the conventional low permeability reservoir sandbody is determined by sedimentary microfacies of deltaic front subaqueous distributary channel sand body or channel mouth bar.By comprehensive analysis of rock facies,profile facies,logging facies,Chang-7 mainly develops four types of sandbody structure,among which thick sand body accumulated by multiple stage(Type I)and thick sand body accumalated by sandbody and thin mud(Type II)are high quality sandbody structure.Compared with conventional low permeability reservoir deposit,tight reservoir accumulated faster due to multiple stage deposit.In addition,because it was at the relatively deeper level of the lake,the deposit was less affected by wave elutriation.Accordingly,the partial structure of the deposit sand body was mixed,which lead to a poor sorting features and high heterogeneity.The sediment is mainly contented with fine silty sand and high shale content.On the contrary,conventional reservoir is contented with medium coarse sand and low shale content.This is the most important deposit controlling factor resulting an unusual nonconventional characteristic of the tight the tight reservoir.Compared with conventional low permeability reservoir deposit,the sediment of tight reservoir sand body is of the characteristics of fine grained and high shale content.The content of plastic component such as feldspar,cuttings and interstitial material with weak resisting compaction is higher.It was in the middle area of the lake basin displaying a larger bed thickness and buried depth.The overburden pressure exceeds that of other places in basin.As a result,mechanical compaction diagenesis plays an important role.The calculation and comparison showed that the porosity loss of Chang-7 led by diagenesis is above 55%,15%higher than conventional reservoir(the porosity loss around 40%).This is another controlling factor leading to a unconventional characteristic of the reservoir.(3)From diagenesis:Compared with conventional low permeability reservoir deposit,reconstruction of tight reservoirs is affected by dissolution much more strongly.The proportion of dissolved pore in the area is 1.3%,about 65%of total pores comparing with 30%of that in conventional reservoirs.The increased porosity of dissolution is 19.6%comparing with 9%of that in conventional reservoirs.These dissolved pores dramatically improved the reservoirs'storage and seepage ability,which leads to the formation of Chang 7 oil reservoirs and makes effective exploration possible.Hence,dissolution is the most favorable and decisive diagenesis.Compared with conventional low permeability reservoir deposit,the storage space in tight reservoirs is mainly ranging from1-7?m porosity(74.0%total of porosity volume),compared with 6~20?m of that in low permeability reservoirs.The throat of tight reservoirs is narrower(0.01~0.08?m),and coordination number per pore throat issmaller(1-3).On the contrary,the throat of low permeability reservoirs is wider(0.12~0.23?m),and coordination number per pore throat is bigger(3-7).(4)From pore throat:Influenced by sedimentation and diagenesis,the pore throat of tight reservoir is small which was belonged to micro-pore and micro-throat.The porosity is 10.4%which show the tight reservoir has the fine storage performance like the conventional reservoir.The permeability of the research area is 0.19mD which indicate that the tight reservoir permeability is limited.So the seepage characteristic of tight reservoir is same as the conventional reservoir and the seepage characteristics is not normal.(5)From the fracture:The high angle tectonic fracture was the major fracture type developed in Chang-7 tight reservoir with a preferred orientation towards north-east and north-west.These fractureswere less filled,and filledmainly by calcite.Influenced by lithology and thickness,these fractures were largely distributed in mid-east and southeast of the area,while they were less than that of southwest and northeast.Conclusions:Chang-7 tight reservoir displayed an obvious characteristic of gravity flow deposits.Sandy debris flow is the favorable sedimentary facies zone and under its control,type I and typeII sandbodies were the most favorable reservoir bodies.After violent destructive diagenesis,constructive diagenesis is the dissolution of feldspar greatly improved reservoir property and percolation ability.The reservoir in its entirety is composed of microporous and micro laryngeal,which has favorable pore space for hydrocarbon accumulation.However,the percolation ability is weak,the overall tight reservoir and fracture development improved the percolation ability and created favorable conditions for exploration and development.In general,Chang-7 is a nonconventional debris rock reservoir,but displaying conventional reservoir property.Here,both its nonconventional and conventional characteristics were clarified.According to these new findings,dense reservoir deposition region with good reservoir physical properties and developed fractures should be targeted as a priority area for exploration.Next,the sandy debris flow deposit zone,dissolved pore developing zone and fracture developing zone need to be further explored.