Study On Hydrocarbon Accumulation Mechanism In Deepwater Areas Of Qiongdongnan Basin

The Qiongdongnan basin, which lies in the northern continental margin of South China Sea, has a great potential of hydrocarbons and is one of large rich oil and gas basin on the offshore of China. By the guidance of modern petroleum geology theory, through macro-analysis for the hydrocarbon accumulation conditions and features of Qiongdongnan basin, using hydrocarbon accumulation features, enrichment regularity and exploration achievement in shallow water area for reference, this paper analyzes hydrocarbon accumulation elements and process in deep water area, depicts fluid dynamic field features and hydrocarbon migration-accumulation regularity, discusses the favorable exploration direction. Some achievements have been getting as below in this thesis:(1) The Qiongdongnan basin has favorable conditions of hydrocarbon. Yacheng Formation and Lingshui Formation source rocks were identified in the basin, which have high abundance of organic substances with typesâ…¡2 andâ…¢organic matter, maturre and over-mature state in the thermal evolution. Sandbodies which deposited in delta, turbidite, shoal, littoral, neritic sea and carbonatites are main reservoirs.The major factors that influence the reservoir physical property are sedimentary environment, diagenesis and abnormal pressure. Multiple good regional and local seals are developed in the basin. There are two reservoir-seal combinations, whose reservoirs are Ling-â…¢Member/Sanya formation sandstone and seals are Ling-â…¡Member/Sanya and Meishan formation mudstone, which have great significance on hydrocarbon exploration. The major factors controlling the hydrocarbon accumulation are the conditions of source rocks, reservoirs, seals and migration in the Qiongdongnan basin.(2) It develops three passage system including connected sand body, fault and unconformity in study area. Connected sand body and unconformity are the main path for hydrocarbon migration in the parallel direction. The Paleogene sand body is cutted through by fault, which has great significance on the migration of late fluid along the parallel passage system. Fifteen local structures are found in study area, of which the large and reliable are Beijiao-4, Beijiao-6, Beijiao-9 and Huaguang-3. Structural trap is the main trap, by contrast, the lithologic trap is the assistant.(3)High temperature and overpressure are commonly found in Qiongdongnan basin deep water area. The higher paleo/present-geothermal gradient have had strong influence on the thermal evolution and hydrocarbon generation of the source rocks. The strong overpressure existed in the depression center strengthens the dynamic of hydrocarbon migration, the episodic hydrocarbon expulsion of source rocks, and the rapid hydrocarbon migration. The strong overpressure existed in mudstone enhances the sealing of cap rocks, and is advantageous to the preservation of natural gas.(4) The deposition-subsidence curves and burial history of Cenozoic appears as three-stages in Qiongdongnan basin deep water area. Since Late Miocene, the high deposition-subsidence rate is advantageous to the maturity and hydrocarbon generation of source rocks rapidly, and the cracking from liquid hydrocarbon to natural gas. During Eocene and Oligocene, the higher deposition-subsidence rate creates perfect structural-sedimentary conditions for the development of source rocks in study area. The main source rocks is in the stage of extensive maturation, which have high ability of hydrocarbon generation-expulsion, and the intensity of gas generation-expulsion intensity is bigger that of oil generation-expulsion.(5) The main local structures have good hydrocarbon generating condition and good hydrocarbon migration condition. Three hydrocarbon accumulation models are established, which are "allogenic source-one way-short to mid distance-lateral and vertical migration", "allogenic source-two way-short distance-vertical and lateral migration", and "self source and allogenic source-one way-short distance-vertical and lateral migration". Comprehensive evaluation indicates that the favourable local structures are Beijiao4, Beijiao9, Huaguang3 and Beijiao6 in turn.