Analysis Of Sedimentary-diagenetic Evolution On Middle-deep Buried Sandstone Reservoirs Under Overpressure Background

Yinggehai Basin,located in the northern part of the South China Sea,is a typical basin with high-temperature(geothermal gradient generally greater than 4.0℃/100 m)and high-pressure(pressure coefficient greater than 1.6).It is a typical area to study the influence of high temperature and overpressure background on reservoir diagenesis.In recent years,many large and medium-sized high-temperature and high-pressure gas fields have been found in the middle and deep layers of the basin.Located in the southeastern area in the slope zone,the Ledong slope area(hereinafter referred to as the Ledong area)and the Dong Fang area,which is located in the center of the basin,are the core area of the favorable natural gas accumulation in the Yinggehai Basin.The gravity flow sedimentary strata of Miocene Huangliu Formation are the key exploration area for oil and gas development.Among them,the Dongfang area is located in the center of the diapir zone,and the thermal fluid activity in the diapir high-pressure belt is strong,which is greatly affected by diapir activity;Ledong area is located in the slope in the east of the central diapir zone,which is less affected by diapir activities.The hydrocarbon filling periods and scales in Dongfang and Ledong reservoirs in different structural positions are unequal.Both belong to the background of high temperature overpressure,but the overpressure occurrence time and intensity are also different between Dongfang area and Ledong area.The overpressure occurrence in Dongfang area(9～5 Ma)is earlier than Ledong area(5～2 Ma).According to the measured pressure data,the current overpressure intensity of Huangliu group in Ledong area(the average measured pressure coefficient is 2.10)is significantly higher than that of Huangliu group in Dongfang area(the average measured pressure coefficient is 1.90).The petrological characteristics,physical properties and diagenesis characteristics of Miocene Huangliu formation sandstone reservoirs in Dongfang district and Ledong district under the background of high temperature and high pressure are compared by means of thin section identification under microscope,fluorescence of hydrocarbon-bearing thin sections,scanning electron microscope,electron probe,stable isotope analysis of carbonate,microbeam fluorescence analysis of hydrocarbon inclusions,laser Raman composition and homogenization temperature of fluid inclusions,The hydrocarbon filling periods and characteristics of the reservoir are clarified,and finally the differential evolution process of burial diagenesis porosity of Huangliu formation in Dongfang district and Ledong district is restored.The result shows that the Miocene Huangliu Formation reservoirs in Dongfang area and Ledong area of Yinggehai basin are different in buried depth,clastic composition and content,structural maturity and petrology,the physical properties and diagenetic pore evolution process of the reservoirs are jointly controlled by sedimentation,overpressure formation time and intensity,and diagenesis.The original sedimentary conditions such as different provenance supply and transportation distance determine the clastic material composition and structural maturity of sandstone reservoir,and restrict the strength of reservoir compaction,dissolution and the type of cement.The migration of sedimentary center and subsidence center controls the burial depth of strata and the structural maturity of sandstone in Dongfang area and Ledong area to a certain extent.The sedimentary period of Huangliu formation in Ledong area is close to the source area and deeply buried as a whole,resulting in low mineral maturity and structural maturity of the reservoir,and subjected to strong static rock compaction in the early diagenetic stage.The sequence of overpressure occurrence time and overpressure intensity also have different effects on the diagenetic pore evolution of the reservoirs in the two regions.The formation of early overpressure background in the eastern region plays a role in resisting compaction and preserving primary pores;The overpressure formed later in Ledong area has limited resistance to compaction and cementation.With three phases of hydrocarbon filling with different components and scales,three phases of dissolution with different scales are developed in the study area.On the whole,Huangliu formation in Dongfang area has weak compaction and cementation,strong dissolution and good physical properties(average porosity 17.7% and average permeability11.1 m D);Although the dissolution developed under the background of late overpressure in Ledong area has improved the physical properties of Huangliu formation reservoir to a certain extent,the overall compaction and cementation of Huangliu formation sandstone in Ledong district are strong,and the physical properties are slightly poor(average porosity 8.3%,average permeability 1.6 m D).