Applications Of Reservoir Fluid Analysis To The Study Of Hydrocarbon Migration And Accumulation

During petrogenetic process, the composition, temperature, salinity and pressure of the reservoir fluid were recorded, which directly reflects the distribution and generation evolution of hydrocarbon in sedimentary basin. Longdong area located in southwest of Ordos Basin with its subject reservoir Triassic Yanchang formation and its depositional system was controlled by the tectonic setting of broad slope. The conditions of hydrocarbon migration and accumulation is much complex while the research of that is weak. Therefore it is meaningful to study the history of hydrocarbon migration and accumulation and the features of its distribution and diversification. Based on the analysis method of reservoir fluid, it can be helpful to understand the regular pattern of hydrocarbon accumulation, which can be used to guide oil exploration.Based on the research about the burial history and digenesis sequence of the Yanchang formation in the area, the characteristics of temperature and pressure fields was discussed during the period of the hydrocarbon charging, with the systematic fluid inclusion analysis and the technique of fluid inclusion thermodynamics modeling. And then the source of the hydrothermal fluids which related to the hydrocarbon charging was determined by means of the C and O isotopic analysis in the carbonate of Chang 8 reservoir. On the basis of results of the rauthigenic illite isotopic dating and the study of the fluid inclusion, the different periods of hydrocarbon filling and accumulation were ascertained and the paleo-fluid potential was reconstructed. Finally, the hydrocarbon migration and accumulation influenced by the fluid activity were discussed.The main understandings were listed as follows:①Two events of hydrocarbon charging had occurred here, and the overpressure is generally developed in the study area. The first hydrocarbon charging happened in the early stage of the early Cretaceous,while the homogenization temperature of fluid inclusions were predominantly 85～115℃, the average excess pressure was 2.1MPa.The second in the end stage of the early Cretaceous, the range of the homogenization temperature of fluid inclusions is 110～130℃, the average excess pressure approximately was 5.5MPa.②The generation of the carbonate of Chang 8 reservoir was related to the organic acid decarboxylation, and during migration, the oil source fluids were associated with the interaction of the hydrothermal fluids which were derived from magma intrusion or precipitation of the metamorphic water.③In the end stage of the early Cretaceous, the oil potential was the maximum in the area of Baibao-Huachi-Qingcheng-Ningxian, and to the east and west sides were reduced. The advantage hydrocarbon accumulation zone pointed to the Zhenyuan area, Heshui area and north and northeast of the Qingcheng area, the oil potential gradient in above area were all greater than 8m/Km.