A Study On Formation Conditions And Accumulation Regularity Of Teriary Reservoirs In The Western Qaidam

Based on the study of petroleum geology, geochemistry, geophysics, and by using the data of drilling, seismic and logging, the paper researches the condition for the formation and accumulation regularity of oil and gas in the western Qaidam Basin.On the basis of analysis about geochemistry characteristics of source rock, the author pointed out quality and hydrocarbon supplying ability of different source rock in the studying area. And thought that the best formation of source rock were E₃ and N₁¹ formation of the centre zone, E32 formation of the south zone, E₃² and N₁ formation of the south zone. Based all these, the author made predicate favorable reservoir, and believed that the favorable clastic reservoir distributed in YueJin No.1, Shaxi, Shizigou, Youshashan mountain and YueJin No.2; the favorable fractured reservoir distributed in Shizigou, Huatugou, Nanyishan mountain, Youquanzi, Xianshuiquan, Jianding mountain, Xiaoliangshan mountain, Dafengshan mountain and Kaitemikeli areas. In the paper, by analysising the sealing mechanism and capacity of cap rocks, the author thought that the deep regional seals (E₃², N₁¹) could effectively seal oil and gas of deep formation, but the shallow regional seals couldn't. Through the study on the formation and fixed period of structures, the author thought the formation period of structural traps had two periods which were N₂², N₂³ and Q. In addition, the pool-forming mechanism was posed, according to the mechanism, pool-forming stages of the west Qaidam Basin were classified that subsidence-increasing pressure forming petroleum stage and erosion-decreasing pressure accumulating petroleum stage. There were two main control factors for the second stage which were erosion and fracture. By studying overpressure, fluid potential and drainage system, the author made definite the migration and accumulation characteristics of oil and gas in the studying area. Making use of hydrocarbon-generation history and trap evolution history analysis methods, the author thought the pool-forming periods had two periods which were in the late deposition of N₂¹ and from the late deposition of N₂³ to Q. Based above all, the author comparted patterns of oil-poor formation to five types including single-direction oil infilling and double forming period, multi-direction oil infilling and double forming period, complicated multi-direction oil infilling and double forming period, single-direction oil infilling and late forming period, double-direction oil infilling and late forming period. In general, throught studying static and dynamic accumulation conditions, there were seven main control factors of hydrocarbon accumulation which were source kitchens, structure, lithology, third-band fracture, fluid dynamic fields, sedimentary facies and reservoir quality. And it pointed out four the favorable exploration direction and three the favorable exploration zones in the studying area.