Impact Of Co₂ Fluid Activities On Mechanism Of Diagenetic Evolution And Porosity Development

Yinggehai Basin,located in the western part of the northern continental shelf of the South China Sea,is a typical high-temperature basin(geothermal gradient generally greater than4.0?/100m)and high-pressure(maximum pressure coefficient is 2.3).It is also one of the most important natural gas producing areas in China.The content of CO₂ is high in Yinggehai Basin,the relative content of CO₂ in single wells can reach higher than 90%.The diapir structure and normal faults in the basin is the important channels of the migration of CO₂ affecting the diagenesis and pore evolution of the sandstone reservoirs in the Miocene Huangliu Formation.And there are differences in CO₂ distribution patterns between Ledong Area and Dongfang Area of the Yinggehai Basin.The distribution of CO₂in Ledong Area is related to the depth.Reservoirs below 3900m in Ledong Area have high CO₂ content,while those above 3900m do not contain CO₂.The distribution of CO₂ varies greatly between single wells in Dongfang Area.The highest CO₂ content up to 75%in some of the wells in Dongfang Area,while the lowest CO₂ content in wells is no more than10%.The sandstone reservoirs of Huangliu Formation in the study area are divided into two types:CO₂ affected reservoirs and CO₂ unaffected reservoirs.Petrologic feature,physical property,burial thermal evolution,and diagenetic evolution of sandstone in Huangliu Formation has been studied in this research,using a number of analytical methods such as observation of casting thin sections,electron microscope scanning,X-ray diffraction analysis,simulation of single well burial thermal evolution history,petrographic analysis and uniform temperature measurement of hydrocarbon inclusions.Moreover,with the quantitative reconstruction of porosity evolution process of sandstones in Huangliu Formation,the burial-diagenesis-porosity evolution process has been built in both CO₂affected reservoirs and CO₂ unaffected reservoirs.Finally,the evolution process of reservoir diagenesis,fluid charging process and porosity evolution under the influence of CO₂ are analyzed.The results shows that there are differences in the diagenetic evolution of the reservoirs of CO₂ affected reservoirs and CO₂ unaffected reservoirs.The compaction intensity and the dissolution-precipitation process of authigenic minerals is affected by the CO₂ charging process and associated overpressure.In the CO₂ unaffected reservoirs,due to the lack of overpressure in protecting primary porosities,the ratio of primary porosity decreases rapidly with the increasing of formation depth.In the Ledong Area,the proportion of primary porosity decreased from 9.0%to 0.5%with the increase of depth from 3723m to 3900m,and in Dongfang Area,the proportion of primary porosity decreases from 10.0%to 3.0%with the increase of depth from 3125.33m to 3900m.The relatively low pressure and CO₂-poor fluid make the ferric calcite(7.9%in Ledong Area and 1.8%in Dongfang Area)is easy to be precipitated and ankerite(0.2%in Ledong Area and 0.1%in Dongfang Area)is difficult to be precipitated.The dissolution is relatively weak(the rate of secondary pores:4.1%in Ledong Area and 6.2%in Dongfang Area),and the primary and secondary pores are rare.While in the CO₂ affected reservoirs,the overpressure weakens the compaction intensity,and the ratio of primary porosity is basically unchanged with the increase of buried depth.The maximum value of the primary porosity in different depth is basically maintained at 2.0%?4.0%in Ledong Area,and up to5.5%in some layers.And the maximum ratio of primary porosity can up to 11.0%in Dongfang Area.The CO₂ fluid and high pressure increase the solubility of Ca CO₃ and aluminosilicate minerals,and ferric calcite and feldspar are easy to be dissolved(the rate of secondary pores:5.1%in Ledong Area and 18.1%in Dongfang Area)and ankerite(1.1%in Ledong Area and 2.6%in Dongfang Area)and siliceous cements(0.7%in Ledong Area and 0.4%in Dongfang Area)are easy to be precipitated.The models of CO₂ charging and porosity development are different in Ledong Area and Dongfang Area due to the difference of the sedimental-tectonic background of these two areas.Ledong Area is located in the eastern slope zone of the Yinggehai Basin,where large-scale faults are developed below 3900m,which is the main channel for CO₂migration in this region.While Dongfang Area is located in the central diapir zone of the Yinggehai Basin,and the large-scale diapir structures are the main channels for CO₂migration from deep to shallow reservoirs.CO₂ dissolved in the high-salinity thermal fluid and migrated vertically to the sandstone reservoir of Huangliu Formation through diapirs or deep faults,and changed the diagenetic environment and porosity development process of the reservoir.As a result,both primary and secondary pores are well developed in the CO₂ affected reservoirs,which pore connectivity and physical properties are great.