Thermodynamic Impact on Deepwater Sandstone Diagenetic Evolution of Zhuhai Formation in Baiyun Sag, Pearl River Mouth Basin

Li Chi; Luo Jinglan; Hu Haiyan; Chen Shuhui; Wang Daifu; Liu Baojun; Ma Yongkun; Chen Liang; Li Xiaoyan

Abstract：Baiyun sag, located in Pearl River Mouth basin, South China Sea, is regarded as the major region for oil-gas researchand exploration in deepwater area in China.The current heat flow of Pearl River Mouth basin is from 24.2 to 121.0 mW/m2, and the maximum geothermal gradient of single well is measured as 6.64 ℃/100 m, which proves that the basin is hot.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 (23.8 Ma).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, which is demonstrated by some petrological and heating records in sedimentary layer, such as the vitrinite reflectance, the homogenization temperatures of secondary fluid inclusions in sandstone and the authigenic minerals of hydrothermal genesis.Baiyun sag can be divided into low geothermal gradient region (LGR, Gra≤4.5℃/100 m) and high geothermal gradient region (HGR, Gra＞4.5℃/100 m).Analytical results of observation of casting thin sections, electron microscope scanning, and X-ray diffraction analysis, show the differences between these two regions in diagenetic processes of sandstone in Zhuhai Formation. (1) 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. (2) The increase of heating rate accelerates the transformation progress of clay minerals in stratum, the depth that indicates the vanishment 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. (3) Deep-sourced fluid is involved in the diagenetic processes of sandstone in Zhuhai Formation in HGR, which is influenced by the tectonic heating event obviously, 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.