Characteristics Of Mixed Rocks And Formation Mechanism Of High-Quality Reservoir In Saline Lacustrine Basin Affected By Volcanic Ash

The second member of the Permian Lucaogou Formation（P₂l²）is an important tight oil reservoir in the Santanghu Basin.Influenced by volcanic activity,the P₂l² is characterized by variable sedimentary environment,complex lithology and rapid change of lithofacies.Affected by volcanic ash,the formation environment and distribution characteristics of the mixed rock,the features of the reservoir and the origin of the high-quality reservoir are still unclear.Based on seismic and logging data as well as a large number of core test data,the P₂l² lithology and lithofacies are finely divided,the formation environment,distribution characteristics and controlling factors of various lithologies and lithofacies are revealed,the pore structure characteristics,distribution rules and diagenetic differences of different types of reservoirs are analyzed,and the origin of high-quality reservoirs is defined.The origin of the P₂l²mixed sedimentary rocks is different from that of mixed sedimentary types established by predecessors.A set of saline lacustrine interlayer sedimentary systems are developed in the P₂l²,which are highly mixed with external volcanic ash and carbonate in the basin.In this paper,the P₂l² is divided into four types of lithology and ten types of lithofacies based on the three-level nomenclature of rock and the abundance of organic matter and sedimentary structure,which are mainly controlled by the volcanic eruption cycle and sedimentary environment.Among them,tuff and dolomite with relatively pure lithology are mostly massive structures,while tuffaceous dolomite and dolomitic tuff are mainly laminated structures,and the abundance of organic matter and hydrocarbon generation potential increase with the increase of tuffaceous content.In order to avoid the interference of volcanic ash to the reconstruction of the sedimentary environment,the principle of combining organic and inorganic geochemical indexes,and screening,correcting and optimizing the inorganic geochemical indexes to restore the paleoenvironment is adopted in this study.During the sedimentary period of the P₂l²,the climate was hot and arid,the water salinity was extremely high and partial reduction,and the lake basin was very closed.There was an obvious response relationship between sedimentary environment and volcanic activity.During the period of intense volcanic activity,the P₂l² is characterized by semi-hot and semi-arid climate,low salinity and formation of tuff and dolomitic tuff,while the period of weak volcanic activity is characterized by hot and arid climate,high salinity,and formation of dolomite and tuffaceous dolomite.The distribution of mixed rock is controlled by volcanic eruption period,palaeotopography and palaeoenvironment,and has zonal characteristics.From sag to slope and basin margin,lithology changed from tuff and dolomitic tuff to tuffaceous dolomite and dolomite,organic matter abundance changed from high to low,and structure changed from laminar to massive,forming a unique sedimentary model of volcanic ash and carbonate in saline lacustrine basin.The P₂l² reservoir belongs to the type of low porosity and low permeability,and the physical property decreases first and then increases with the increase of tuffaceous content,which is mainly controlled by lithofacies.The reservoir pore network is complex,mainly composed of nano-scale dissolution pores,dolomite intercrystalline pores and devitrification pores,and the connectivity is poor.The reservoir pore network is complex,mainly composed of nano-scale dissolution pores,dolomite intercrystalline pores and devitrification pores,and the connectivity is poor.With the change of rock type from pure lithology to transitional type,organic matter abundance from low to high,sedimentary structure from massive to laminated transition,reservoir physical property,pore structure and fluid mobility gradually deteriorate.Based on a variety of evaluation parameters,three types of reservoirs are divided.There are five types of diagenesis in the P₂l² reservoir,and the diagenetic transformation is different in different positions and different types of reservoirs.The cementation content is low,and the quality variation of reservoir is mainly controlled by burial compaction.Dolomitization,which is the main cause of secondary pores in dolomite reservoirs,continues from the penecontemporaneous to the middle diagenetic stage and mainly occurs in the early diagenetic stage.Devitrification began to develop in the early diagenetic stage and reached its peak in the middle diagenetic stage,which is the main factor of secondary pores in tuff reservoirs.A large amount of organic acid was discharged continuously from the end of Jurassic to the end of Cretaceous,which dissolved feldspar and dolomite along microfractures and secondary pores,and played a significant role in improving reservoir physical properties.Compaction and cementation occurred earlier than dolomitization,devitrification and dissolution,and dolomitization and devitrification earlier than dissolution are the main reasons for the formation of high-quality reservoirs.Three kinds of formation models of high-quality reservoirs are proposed,and the high-quality reservoirs are mainly distributed in the northeastern edge of the basin and the slope area near the edge.