Characteristics And Genetic Mechanism Of E₃² Mixed Lacustrine Carbonate Reservoirs In Yingxi Area Of ??Qaidam Basin

The upper part of Lower Ganchaigou Formation(i.e.E32)in Yingxi area of Qaidam Basin is featured by the mixed lacustrine deposits with source and reservoir rocks in the same formation.For a long period,the extramely complicated rock compositions and textures of which have made it difficult to deep research of the sedimentology,reservoire genesis and efficient exploration and development of tight oil in this area.Based on cores,thin sections,scanning electron microscope,whole rock X-ray diffraction,well logging,mercury injection and physical properties analyises,the characteristics and genetic mechanism of diamictite lacustrine carbonate reservoir are comprehensively studied.Generally,the;32 formation in Yingxi area in dominated by salinizing lacustrine deposits with multiple stages of sedimentary cycles derived by high frequency lake-level fluctuations,and it is divided into six oil groups from bottom to top.Four types of reservoir rocks are discriminated by mineral composition,rock textures and imaging logging,including layered argillaceous dolomite reservoir,massive argillaceous dolomite reservoir,massive aleuropelitic dolomite reservoir and chaotic gypsum lumps-bearing dolomite reservoir.These reservoirs,containing different amounts of carbonate,evaporite,terrigenous clastic and clay minerals,are manifested by the characteristics of micro-fracture-pore type reservoirs with medium-low porosity and extra low permeability.The n32 formation in Yingxi area have experienced complicated deposition and diagenesis with three types of reservoir pore spaces can be identified,including primary dolomite micropores,enlarged dissolved micropores and dissolved pores.Reservoir genesis and pore evolution have affected by the comprehensive diverse factors.To be specific,the primary pores within unconsolidated sediments were significantly adjusted and transformed into primary dolomite micropores during the penecontemporaneous evaporation-reflux dolomitization.These primary dolomite micropores,generally oily,develop widely and serve as the foundation of reservoir in study area.It is also the major contribution for stable tight oil production by using development method of horizontal wells and massive volume fracturing.Subsequently,the early intercrystalline micropores of dolomite can be partly optimized and transformed by transient exposed dissolution in the local microgeomorphologic highlands to form the enlarged dissolved micropores and half-filled dissolved pores.The penecontemporaneous exposure and resulting dissolution significantly enhance the physical properties of primary dolomite reservoirs,which contribute to the locally medium-high oil production in Yingxi area.Additionally,the vertical evolution of reservoir pore types and physical properties distribution are mainly controlled by high-frequency lake-level fluctuation cycles,displaying a pore assembly shown by primary micropore,slightly dissolved micropore and dissolved pore from bottom to top.Reservoir physical properties,accordingly,shows an upward-enhancing characteristic.During the transient lake transgression,the basin was dominated by terrigenous clastics and argillaceous sediments with fewer pore space deposited from relatively low salinity environment.During the early-middle lake regression,the basin was dominated by carbonate sediments with abundant primary dolomite micropores formed after the penecontemporaneous dolomitization derived by continuous evaporation and increasing salinity.During the late lake regression,local microgeomorphologic highlands in study area were likely to exposed and suffered the dissolution.As a result,some enlarged dissolved micropores and dissolved pores developed,with optimized and improved reservoir properties.Generally,the fourth and fifth oil groups developed during the early-middle stage of the whole E32 lake regression when carbonate is dominated with massive primary dolomite micropores.And some high-quality reservoirs types of enlarged dissolved micropores and dissolved pores also occur in this stage.Thus the fourth and fifth oil groups can be favouable develpoment interval of the lacustrine carbonate reservoir in study area.