Characteristics Of Diagenetic Fluids In The Lower Triassic Baikouquan Formation Clastic Reservoir In The Mahu Depression,Junggar Basin

As one of the most important hydrocarbon-bearing basins in the western interior of China,the Junggar Basin is rich in hydrocarbon resources.The Ma Bei slope area,located in the western ring of the Mahu hydrocarbon-rich depression in the basin,is adjacent to the Wuxia and Kebai fracture zones and is the most promising hydrocarbon producing area in the basin.Benefit from the large thrust faulting and through-source fracture system developed in the northwest margin of the Triassic,the Ma Bei slope area has superior reservoir forming conditions.In this paper,the Baikouquan Formation in the Mahu Depression of the Junggar Basin is used as a research object.Based on data review and field research,we applied polarized light microscopy,thin section staining,carbonate cement carbon,oxygen and strontium isotopes,fluid inclusions microthermometry,compositional analysis,trace elements,rare earth elements and other indoor analysis methods to investigate the lithological characteristics of the reservoir sand and conglomerate of the Baikouquan Formation in the Mahebei area,the carbonate cement fugacity and formation period,and the carbon and oxygen content of the reservoir.The lithological characteristics of the reservoir gravels,the state and formation period of carbonate cement,the isotopic characteristics of carbon,oxygen and strontium,the homogeneous temperature,salinity and composition of fluid inclusions,and the geochemical characteristics of the elements are investigated,and the origin and characteristics of the rock-forming fluids during the formation of the reservoir are discussed.The rock type of the Baikouquan Formation in the Mabei slope area is mainly rock chip sandstone and a small amount of rock chip feldspar sandstone.The surface fracture of the clastic particles such as quartz and feldspar and the concave-convex contact between the clastic particles can be seen,indicating a high degree of compaction in the early stage of rock formation.The surface and edge of feldspar particles are significantly dissolved,providing dissolution secondary pore space,which provides conditions for fluid transport and preservation such as oil and gas.There are three phases of carbonate colloids in the study area,which are classified as Phase I calcite colloids,Phase II iron calcite colloids and Phase III iron dolomite colloids according to the deposition state and precipitation form.δ¹³C_PDB values range from-47.23‰to3.88‰,with an average value of-17.59‰,andδ¹⁸O_PDB values range from-23.64‰to-17.98‰,with an average value of-20.89.The paleosalinity ranges from 19.79 to124.06,which is freshwater deposition,and the paleotemperature ranges from 111.83°C to 146.75°C.The high temperature indicates the involvement of deep thermal fluids,and the diagenetic stage is the middle diagenetic stage A.The ⁸⁷Sr/⁸⁶Sr ratio ranges from0.705827 to 0.707244,with an average value of 0.706513,which is lower than the⁸⁷Sr/⁸⁶Sr ratio of seawater during the same period,indicating the involvement of mantle-derived strontium and fluids carrying volcanic materials in the diagenesis process,and erasing the high value of strontium of terrestrial origin.The comprehensive analysis suggests that the injection of hydrocarbon fluids,along with the maturation of organic matter at elevated temperatures and thermal decarboxylation,caused the fluid environment to become acidic,which produced secondary pores by dissolution of unstable minerals such as feldspar and authigenic carbonate minerals,and provided conditions for the transport of rock-forming fluids.Fluid inclusions in the study area mainly exist in gas-liquid two-phase inclusions,single-phase inclusions,and three-phase inclusions containing subminerals.There are mainly two peaks of 80～90℃and 100～120℃in the homogeneous temperature,corresponding to two phases of hydrocarbon fluid filling.Laser Raman spectroscopy of the inclusions shows that the gas-phase components of the inclusions are CO₂,N₂,and CH₄,and the liquid-phase components are H₂O.The inclusion composition and temperature indicate the existence of two stages of low and high mature hydrocarbon fluids in the study area,revealing that the two stages of hydrocarbon fluids originated from early low mature oil and late high mature oil in the late Triassic and early Cretaceous.The upper crustal standardization and chondrite standardization of trace elements and rare earth elements in the study area reveal that some trace elements are relatively enriched and deficient;rare earth elements show a right-leaning light rare earth enrichment pattern.The paleodepositional environment is characterized by oxidation,low salinity,and humid climate by various geochemical elemental parameters.