A Study On The Genetic Mechanism To Mixed Siliciclastic-carbonate Rock Tight Reservoir Of The Qiketai Formation In The Shengbei Sag

The second member of the Jurassic Qiketai Formation in the Shengbei Sag is an important exploration target for continental tight oil in the Turpan-Hami Basin.The sedimentary environment and lithology are complex and the vertical and horizontal distribution of reservoirs with different lithologies show strong heterogeneity.The mixed sedimentary environment of complex lithologies and its distribution characteristics,the microscopic pore structure and densification mechanism of different types of reservoirs are aporia that need to be solved urgently.This paper used field outcrops,cores,drilling and logging data,seismic data,and experimental analytical data to classify the rock types of the second member of the Qiketai Formation,to analyze the sedimentary environment,genesis,and development characteristics of different type rocks vertically or horizontally,and to study the microscopic pore structure,diagenesis types and diagenetic evolution sequence of reservoirs with different lithologies,thereby to reveal the genetic mechanism of tight reservoirs in the studied interval.Through core observation,thin section observation,scanning electron microscopy,and whole-rock mineral X-ray diffraction analysis,it is believed that mixed terrigenous clastic-carbonate sedimentary rocks are widely developed in the horizon.According to the rock composition,mineral content,and sedimentary textures,more than 20 complex lithologies of the second member of the Qiketai Formation are combined and classified into 4 major types and 10 subtypes,of which three are belonged to mixed sedimentary genesis including in situ mixing type,punctuated mixing type,and facies marginal mixing type.The development of mixed rocks with different genesis is controlled by paleo-climate,lacustrine level fluctuation,and paleo-water environment.Carbonate mixed rocks are developed in shallow water environments with semi-arid-arid climate,brackish water,oxidation,and weak supply of terrigenous clastic.From the slope near the provenance to the sag zone,sandy beach bars,lime mud flats,mixed beach bars,granular beaches,shallow lacustrine bays,carbonate beach bars and semi-deep lacustrine mud sedimentary microfacies are developed in sequence.The types of rock assemblages vary with microfacies and a unique mixed sedimentary model was established for shallow lacustrine terrigenous clastic-carbonate rocks.According to the analysis results of casting thin section,scanning electron microscope,joint measurements of nuclear magnetic resonance and high-pressure mercury intrusion,the physical properties,storage space types and microscopic pore structure characteristics of different mixed rock reservoirs were revealed,and the reservoirs and their combination types are classified.It is considered that the tight reservoir of the second member of the Qiketai Formation is characterized by low porosity and ultra-low permeability,the dominance of residual intergranular pores,dolomite intercrystalline pores and micro-fractures.The submicron-nanoscale pore throats of < 1μm carry substantial weight,with poor pore connectivity.The argillaceous/calcareous siltstone,silt-bearing granular carbonate rock,powdery-fine-crystalline limestone and silty limestone(dolomite)havethe best physical properties.Three types of reservoirs with different physical properties and pore structures were divided and four types of reservoir combinations were proposed as well.Based on the experimental results of thin section,cathodoluminescence,electron probes,in-situ carbon and oxygen isotopes,and inclusion homogenization temperature,the reservoir mainly experienced the diagenetic processed including compaction,cementation and dolomitization,and weak dissolution.The diagenetic environment is always alkaline-weakly alkaline,providing a rich material source for cemented minerals.There are four different diagenetic systems and diagenetic evolution sequences.Two stages of strong calcite cementation were observed in the granular carbonate rock,the powdery-fine crystal limestone has experienced continuous filling of chlorite,and argillaceous siltstone has an early analcite cementation.The silty limestone mainly shows the developments of early chalcedony and microcrystalline quartz cementation,and argillaceous mud-microcrystalline carbonate has experienced strong compaction.Dolomite is mainly formed during the burial diagenesis stage,which is beneficial to the formation of secondary pores,and four kinds of reservoir diagenetic evolution models are established.Compaction and cementation significantly reduce the porosity and permeability of the reservoirs,which is earlier and stronger than the dissolution,primarily resulting in the tightening of mixed sedimentary rock reservoirs in the study area.The tightening of the reservoirs is also significantly earlier than the hydrocarbon fluid charging.