Influence Of High Thermal Background On Deep Water Reservoir Quality In The Baiyun Sag,Pearl River Mouth Basin

The deep water area has become a hotspot field of global hydrocarbon exploration.In northern South China Sea deep water area,a complete set of source rocks,reservoir and caprock shows a good oil and gas exploration prospect.In the Pearl River Mouth basin,the heat flux changes from normal value to high heat flux from north to south,which is basically consistent with the variation trend of lithospheric thickness and geothermal gradient.However,the influence of geothermal gradient on reservoir quality under the background of high geothermal gradient is still unclear.In addition,the effect of fault activity,diapir structure and associated deep CO₂ injection on reservoir properties in Baiyun Sag is still not clear.The influence of high geothermal gradient on diagenesis and reservoir performance is still a weak point,which restricts the next exploration deployment severely.Based on the previous research,depending on the different heating mechanism of stratum,the reservoir in the deep water area of Baiyun Sag is divided into two types for the first time:?1?Areas affected by magmatism,faulting,and diapirism in geological history are reservoirs affected by the"abnormal heat"and?2?areas that have not been affected by magmatism,faulting and diapir in geological history are reservoirs affected by"normal heat".The maximum temperatures of reservoirs in the type?1?areas during their geological history are greater than the present formation temperature.According to the difference of geothermal gradient,the type?2?areas are divided into:low geothermal gradient zone?<4?/100 m?and high geothermal gradient zone?>4?/100 m?.The results were based on the microscopic observation,identification and quantitative statistics of thin sections,fluorescence thin sections,the thin section of casting sand and cathode luminescent thin section.Scanning electron microscopy,XRD analysis,porosity and permeability,pore-throat observation and measurement,electron probe microanalysis,seismic profile interpretation methods were used.The following three comparative studies are made:?1?differences in sedimentation between high and low geothermal gradient zones;?2?differences in diagenesis and reservoir quality between high and low geothermal gradient zones;?3?the diagenesis difference between the reservoir affected by CO₂ and the reservoir unaffected by CO₂ and its influence on reservoir physical properties.Firstly,it is recognized that the physical properties of the same formation in different geothermal gradient zones are not significantly different due to the diversity of sedimentation.In areas with high and low geothermal gradients,the sandstone types of the same formation are basically the same.The porosity and permeability of the reservoir increase with the increase of the content of rigid particles,decrease with the increase of the content of plastic particles,and increase with the decrease of the content of plagioclase.In the area of high and low geothermal gradient,the relationship between sandstone grain size and physical properties of the same formation is basically consistent.In the area of high and low geothermal gradient,the correlation between the content of heterozygote and the maximum surface porosity is inverse.Secondly,for reservoirs affected by"normal heat",decrease rate of the maximum porosity and permeability of sandstone with the increase of buried depth/formation temperature is more rapid in high geothermal gradient zone.The formation temperature affect the process of diagenesis and the reservoir quality by changing the particle compressive strength,changing the solubility of minerals,changing the clay mineral transformation,changing the reaction rate of water-rock and influencing the formation of carbonate cement.Combined with the comparison of burial history and crustal tension-thinning history of different geothermal gradient zones in the study area,it is proposed that temperature-burial time index controls the decrease rate of reservoir physical properties.These findings explain the different evolutions of reservoirs in areas with different geothermal gradients.Thirdly,most CO₂ found in Baiyun Sag is of inorganic origin,while a small amount is of organic origin.Magma,diapir and fracture activities that lead to"abnormal heat"make the deep hydrothermal flow to the upper reservoir.The deep hydrothermal and the heat carried by it modified the sandstone reservoir,among which CO₂ is a typical representative of the deep hydrothermal.Diapir structures control the distribution of CO₂ in Baiyun Sag.The precipitation of minerals such as dawsonite,ankerite and kaolinite,as well as the dissolution of silicate minerals?such as feldspar?and carbonate minerals?such as calcite?caused by deep thermal fluid and CO₂ injection into the reservoir are the direct reasons affecting the reservoir quality in the"abnormal heat"area.In a word,when sedimentary conditions are basically the same,a lower geothermal gradient is conducive to the preservation of reservoir pores,so that high quality reservoir can be formed and preserved in deep strata.The higher the formation temperature is,the worse the pore preservation will be.The constructive effect of CO₂ injection is mainly the dissolution of minerals such as feldspar and calcite,while the destructive effect is mainly the cementation of minerals such as dawsonite,ankerite and kaolinite.The constructive effect of CO₂ injection on the reservoir is greater than the destructive effect in this study area.