Formation Mechanism And Vertical Distribution Of Ankerite In Tertiary Reservoir And The Relationship To The CO₂ Fluid Activities,Baiyun Sag,the Pearl River Mouth Basin

Baiyun sag,located in the south of Pearl River Mouth Basin,is the key region of deepwater hydrocarbon exploration in China.Ankerite is the mainly diagenetic mineral in the tertiary reservoir in baiyun sag.Its vertical distribution and isotope characteristics are different from the common ankerite in sandstone which is related to thermal maturity of organic.Recently,ankerite associated with CO2 injection has been found in many basins around the world.Baiyun sag has undergone several times of tectonic movement,fault and magmatic activities since cenozoic.Many mantle-derived magmatic CO2 pools have been found.A large amount of ankerite is observed in those CO2 pools and their around.It is worth studying the formation mechanism of ankerite in Tertiary reservoir and the relationship to the CO2 fluid activities.Petrologic feature of sandstone,vertical distribution and microscopic characteristics of ankerite has been studied in this research by using a number of analytical methods such as observation of casting thin sections,electron microscope scanning,X-ray diffraction analysis.on the basis of the study on the distribution and genesis of CO2,combining with isotope data of carbonate cement,seismic profile analysis,natural gas composition and homogenization temperature of fluid inclusion,and taking two typical CO2 gas wells(well D2 and B1)as a case study,the forming mechanism of ankerite and the relationship to the CO2 fluid activities have been discussed.Moreover,the genetic model of ankerite in the study area was established.The ankerite in study area was formed in two stages.It mainly distributed in the high geothermal gradient area at shallow-medium burial depths from 1000 to 3000m(corresponding temperature between 60~130?)and in the low geothermal gradient area at burial depths deeper than 2500m(corresponding temperature above 120?).The ankerite cements also occurred in the CO2 thermal fluid activities area.The ?13C(-2.83‰~-1.83‰)value of the ankerite shows an inorganic origin characteristics,which is consistent with the ?13C value of magmatic origin CO2 in the gas pool.The values of ?18O and 87Sr/86 Sr also indicate that precipitation of the ankerite was affected by the deep hydrothermal fluid activity.Taking two typical CO2 gas wells as a case study and the research result shows that the homogenization temperature of fluid inclusions and evolution of montmorillonite to illite proved that the reservoir underwent thermal fluid invasion,the ankerite cements are concentrated in the CO2 thermal fluid activity regions,typical authigenic mineral assemblage of dawsonite+ankerite+siderite which can trace CO2 migration and accumulation are also found.The content of ankerite and kaolinite in the formation is positively correlated with that of CO2 in the gas pool,and the content of chlorite is negatively correlated with that of CO2 and ankerite.CO2 charging in reservoir caused the precipitation of ankerite.The abnormal transformation of clay minerals(such as chlorite)caused by CO2 thermal fluid charging provided the necessary Fe2+,Mg2+ ions for precipitation of the ankerite.It can be speculated that abundant CO2 hydrothermal activity was the main reason for the precipitation of developed ankerite in the high geothermal gradient area.However,in the low geothermal gradient area,the precipitation of ankerite was influenced by thermal maturity of organic and deep magmatic CO2 activities.