| Baiyun sag,located in the typical "hot basin"-Pearl River Mouth Basin in the north of South China Sea,is regarded as the vital region of oil-gas research and exploration in deepwater area in China.According to the present geothermal gradient(Gra),Baiyun sag can be divided into low geothermal gradient region(LGR,Gra?4.5?/100m)and high geothermal gradient region(HGR,Gra>4.5 ?/100m).The little disparity between deposition rate and sedimentation rate indicates that the way of Sedimentation-deposition in LGR is "Compensation" type.While in HGR,the sedimentation rate is apparently higher than the deposition rate,so the way of Sedimentation-deposition is"Hunger"(non-compensation)type.The governing factor of the basin evolution was transformed from fault to fault-thermal collectively during the post-breakup subsidence stage,especially after the generation of the Baiyun movement(23.8Ma).Since then,the thermal effect is known as one of the crucial elements in controlling the diagenetic evolution of the sedimentary strata in Baiyun sag.However,previous researchers barely consider the high heat flow background and it's influence on diagenesis evolution of the strata in Baiyun sag.Petrologic feature,physical property,burial thermal evolution,and diagenetic evolution of sandstone in Zhuhai 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 sandstone in Zhuhai Formation,the burial-diagenesis-porosity evolution process has been built in both LGR and HGR.Finally,the reservoir quality differences between sandstone reservoir in these two area has been explored,combining materials of pore-throat image analysis,mercury-injection and physical property test.The diagenetic processes of sandstone in Zhuhai Formation in LGR and HGR are different.The compaction of sandstone in LGR is controlled mainly by lithostatic effect,while in HGR it is controlled by both thermal and lithostatic effect so that the isoporosity limit buried depth is more shallow than that in LGR.The increase of heating rate accelerates the transformation progress of clay minerals in stratum,the depth that indicates the vanish of the kaolinite in HGR is more shallow than that in LGR,and the ordering process of I/S(illite/smectite mixed layer)in HGR is more mature than that in LGR.Deep-sourced fluid is involved in the diagenetic processes of sandstone in Zhuhai Group in HGR,which is influenced by the tectonic heating event,so that the thermal equilibrium of dissolution-precipitation of carbonate minerals is changed.The exchange process in interstitial water of the stratum is complex and active,and the authigenic hydrothermal minerals are produced in this process.It is the distinction in burial diagenetic evolution that generates the difference in porosity evolution process,pore structure and physical property of sandstone in Zhuhai Formation,by the results of quantitative reconstruction in key stages of porosity evolution.The pore and throat of the sandstone in Zhuhai Formation in LGR is tiny and the connectivity of pores is poor,so that the interstitial fluid can't flow smoothly.While the pore and throat of the sandstone in Zhuhai Formation in HGR is large and the connectivity of pores is ample.Generally speaking,the physical property of sandstone in Zhuhai Formation in LGR is better than that in HGR.The content and occurrence of authigenic minerals,such as kaolinite,illite and carbonate cements,and the scale and strength of dissolution are two main reasons in reservoir quality affection in different areas in Baiyun sag. |
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