The Carbonate Cements And Its Formation Mechanism And Distribution Characteristics In The Deep Reservoir Sandstone In Baiyun Sag

With the discovery of the deepwater gravity flow deposition and the LW3-1 well oil gas exploration success, as well as the discovery of several natural gas business and oil-bearing structure in Baiyun Sag, Pearl River Mouth Basin, the Baiyun Sag showed good prospects for oil and gas exploration. But the study on the time and types of carbonate cements, geochemical characteristics, formation mechanism, distribution characteristics are relatively weak. Based on core observation, the identification and quantitative statistics of thin section and cast slice through microscope, the analysis of cathodoluminescence and scanning electron microscopy, the test of carbon and oxygen isotopic, fluid inclusion homogenization temperature and electron microprobe analysis, combined with the tectonic and sedimentary background, logging data, this paper points out that the types and formation periods of carbonate cement, the occurrence, the composition and content and the diagenetic evolution relationship with other authigenic in the sandstone reservoirs of Zhuhai formation and the Zhujiang formation, then analyse the source, the formation mechanism and distribution characteristics of the carbonate cements in the sandstone reservoirs. It is of great theoretical significance and practical significance to understand the source and formation mechanism of carbonate cements in the sandstone reservoirs, the effect of carbonate cements on the quality of the reservoir and the favorable reservoir prediction.The results shows that the carbonate cement（average 5.1%）, kaolinite（average 2.2%） and siliceous cement （average 1.7%） were the main cements in the sandstone of Zhuhai formation and the Zhujiang formation. The carbonate cements were formed in three stages, the early carbonate （formed in syndiagenetic stage to the early diagenetic stage A period） is mainly calcite and a small amount of siderite, calcite is characterized by high content of Ca, low content of Fe, Mg （CaO average 55.4%, FeO average 0.24%, MgO average 0.7%） and bright orange luminescence. The medium carbonate is mainly ferrocalcite （formed in the middle diagenetic stage A period）, characterized by high content of Ca, a little high content of Fe, low content of Mg （CaO average 55.7%, FeO average 1.0%, MgO average 0.3%） and dark yellow luminescence. The late carbonate cements are mainly ankerite （formed in the middle diagenetic stage late A period）, characterized by high content of Fe, Mg, a little low content of Ca （FeO average 11.3%, MgO average 10.9%, CaO average 33.3%） and non-luminescent.The formation mechanism of early carbonate cements in the Zhujiang Formation and Zhuhai formation in the study area is different. The isotopic composition of carbonate cements in the Zhujiang Formation:δ¹³C_PDB:-2.43‰～0.29‰,δ¹⁸O_PDB:-9.79‰～-3.08‰, the paleo salinity （Z） is basically greater than 120 （119.31～125.75）, it reflects that the formation of carbonate cements is closely related to the marine water, and it is the product of the precipitation from the carbonate over saturated alkaline sea water in the early stage of shallow burial. The isotopic composition of carbonate cements in the Zhuhai Formation:δ¹³C_PDB:-9.37‰～-8.13‰,δ¹⁸O_PDB:-7.11‰～-7.09‰, the paleo salinity were all less than 120 （104.58～107.11）, it reflects that the formation of carbonate cements is closely related to the fresh water. The middle carbonate cement has a wide range of carbon isotope (δ¹³C_PDB:-20.88‰～-5.29‰,δ¹⁸O_PDB:-11.1‰～-8.99‰). It is indicated that a part of carbons originated from CO₂ released by decarboxylation of organic acid, a part of carbons is closely related to deep crustal matter. The dissolution of feldspar and other clastic particles, the transformation of the clay minerals and deep hydrothermal fluid can provide a large number of Ca²⁺、Mg²⁺、Fe²⁺ ions for the formation of carbonate cements. The carbon and oxygen isotopes of late carbonate cements (δ¹³C_PDB:-2.83‰～1.83‰, δ¹⁸O_PDB:-9.45‰～-5.77‰) is basically consistent with the seawater in the same period (δ¹³C_PDB:-1‰～-2‰,δ¹⁸O_PDB:-8‰～0‰), it is indicated that the formation of late carbonate cements is the result of the dissolution and reprecipitation of the early carbonate cements and the bio clastic limestone, and late carbonate cements may also be affected by the deep hydrothermal fluid.Carbonate cemented sand strata in the Zhujiang Formation and Zhuhai formation has a low Ga （41.8API～91.4API, average 71.3API）, DT （56.5μs/ft～107.9μs/ft, average 76.8μs/ft） and CNCF (3.1%～¹⁸.9%, average 11.7%), a high ZDEN （2.2 g/cm³～2.7g/cm³, average 2.5g/cm³） and MLR2C （4.0Ω·m～17.9Ω·m, average 10.8Ω·m）. Carbonate cemented sandstone layer has obvious distribution law in the vertical direction, and it mainly distributes in the ¹⁸.5Ma -21Ma period, which is the result of sedimentary environment, hydrocarbon activity and tectonic movement.The reservoir sandstone pore of Zhujiang group and the Zhuhai formation mainly is intergranular dissolved pore （average 6.4%）, followed by intragranular dissolved pores （average 1.8%） and primary intergranular pore （average 1.4%）. There is a positive correlation between porosity and permeability, the physical property of the sandstone of Zhujiang Formation is better than the Zhuhai Formation. Carbonate cements have the overall effect on the reservoir properties of sandstone, resulting in a loss of porosity of 4.3%.