Diagenetic Fluid Activity And Its Influence On Diagenesis-pore Evolution Of The Reservoir Under Abnormally High Pressure Background

Yinggehai Basin,with the strongly Cenozoic tectonic activity,frequently tectonic thermal events,and abnormally high pressure development,is a typical high temperature and high pressure Basin in China and one of the important natural gas producing areas in the South China Sea.At present,the medium-deep burial Miocene Huangliu Formation and Meishan Formation in the Ledong X area of the Yinggehai Basin have achieved breakthrough in natural gas exploration.Its developed the marine gravity flow channel-submarine fan sedimentary sandstone reservoirs have characteristics of the multi-stage diagenetic fluid activity,coupled with high temperature and high pressure background,it has a significant impact on the reservoir diagenesis and pore evolution.However,at present,the diagenesis-pore evolution law of the reservoir is not obvious.In order to study the Diagenetic fluid activity and its influence on diagenesis-pore evolution of the medium-deep buried sandstone reservoir in the Ledong X area of the Yinggehai Basin under abnormally high pressure background.The Miocene Huangliu Formation and Meishan Formation reservoir in Ledong X area are analyzed,based on identification of blue epoxy resin-impregnated thin sections under microscope,scanning electron microscope,XRD analysis of clay minerals,measurement of pore throat image,measurement of granularity image,high pressure Hg injection,physical properties,micro beam fluorescence,cathodoluminescence,electron probe,inclusion homogenization temperature,laser raman spectral composition analysis,micro area and in-situ stable isotopes and other analysis and testing methods,combined with the previous research results,guided by the theories of sedimentary petrology,petroleum geology and isotope geochemistry.Based on the analysis of diagenetic fluid type,stage,scale and genesis,the influence of diagenetic fluid activity on the Huangliu Foramation and Meishan Formation reservoir diagenesis under abnormally high pressure background is studied,the diagenesis-pore-evolution characteristics of the reservoir is clarified,the differential process of the burial-diagenesis-fluid activity-pore evolution of the reservoir is analyzed.Finally,the development model of the favorable reservoir is established.The results show that,the diagenetic fluid types of the Huangliu Formation and Meishan Formation in the Ledong X area are mainly hydrocarbon fluid and deep thermal fluid.The reservoir experienced three stages hydrocarbon charging process.The first hydrocarbon charging occurred at the end of A stage of eodiagenesis with a small scale（The filling time of the Huangliu Formation is 5.0～4.5 Ma,and that of the Meishan Formation is 7.5～7.0 Ma）;The second and third stages were continuous hydrocarbon charging,which occurred in the A～B stage of mesodiagenesis with large scale（The filling time of the Huangliu Formation is after 3.0 Ma,and that of the Meishan Formation is after 4.0 Ma）.The reservoir experienced first stage large-scale CO₂and small-scale H₂S thermal fluid charging.Theδ¹³C_CO2value of CO₂in natural gas of the Huangliu Formation and Meishan Formation are range from-24.90‰to-0.40‰,among them,theδ¹³C_CO2value more than-8.00‰are above 80%,mainly inorganic origin;Theδ¹³C_PDBvalues of late-stage carbonate cement are range from-10.10‰to 1.80‰,theδ¹⁸O_PDBvalues are range from-21.90‰to-8.20‰,theδ¹⁸O_SMOWvalues are range from 8.30‰to 22.50‰,the genesis are related to the deep CO₂thermal fluid and the decarboxylation of organic acids;Theδ³⁴S values of late-stage pyrites are range from31.89‰to 58.05‰,theδ⁵⁶Fe _IRMMvalues are range from-1.28‰to 0.41‰,theδ⁵⁷Fe _IRMMvalues are range from-1.56‰to 0.02‰.It is mainly hydrothermal pyrite formed by the reaction between H₂S thermal fluid and Fe²⁺in diagenetic system.After B stage of mesodiagenesis（after 1.9 Ma）,driven by strong overpressure,deep CO₂and H₂S thermal fluids migrated upward along abyssal fault to the second member of the Huangliu Formation-Meishan Formation below 3900m,resulting in the transformation rate of authigenic clay minerals is accelerated,the vitrinite reflectance is mutated,the salinity of formation water is reduced,and the homogenization temperature of some inclusions is higher than the maximum temperature of the formation,and formed many typical hydrothermal genesis minerals,such as ankerite,late-stage siderite and pyrite,authigenic rutile,authigenic apatite,sulphate minerals,chalcopyrite,sphalerite and so on,in the reservoir.The Huangliu Formation and Meishan Formation reservoirs in the Ledong X area can be divided into two types of the overpressure reservoir affected by hydrocarbon fluid（mainly developed in the overpressure zone of the first member of the Huangliu Formation,abbreviation overpressure reservoir）and the strong overpressure reservoir affected by hydrocarbon fluids and thermal fluids（mainly developed in the strong overpressure zone of the second member of the Huangliu Formation-Meishan Formation,abbreviation thermal strong overpressure reservoir）.Among them,the diagenetic stage of the overpressure reservoir reaches A₂sub-stage of mesodiagenesis,and experienced pore evolution of three stages,including weak compaction decreased porosity,strong cementation decreased porosity and organic acid dissolution increased porosity,with the high porosity reduction rate and the low present porosity;the diagenetic stage of the thermal strong overpressure reservoir reaches B stage of mesodiagenesis,and experienced pore evolution of four stages,including strong compaction decreased porosity,weak cementation decreased porosity,organic acid dissolution increased porosity and inorganic acid dissolution increased porosity,with the low porosity reduction rate and the high present porosity.Influenced by sedimentation and burial-diagenesis-fluid activity-pore evolution,favorable reservoirs are mostly formed in the main channel of the second member of the Huangliu Formation with well preserved primary pores and relatively developed secondary pores.The research results provide reference and ideas for the research of fluid activity characteristics under the background of abnormal high pressure and its influence on diagenesis-pore evolution of the medium-deep buried reservoir in the Yinggehai Basin,the Northern South China Sea.