A Research On Hydrocarbon Accumulation Periods Of The Cambrian In Tazhong Area

Tarim Basin,located in western of China,is the largest inland petroliferous basin in China.It belongs to large-scale superimposed basins and has typical petroliferous features characteristics of superimposed basins.Its distribution features and enrichment regularity of hydrocarbon reservoirs have been receiving much attention.As the focus of petroleum exploration in Tarim Basin,carbonate reservoirs in lower paleozoic are predicted to be rich in hydrocarbon resources in Tazhong area.Because wells which drilling through and encountering the Cambrian are less and having less oil-gas shows,the previous researches on the hydrocarbon accumulation mechanism in Tazhong area are mainly focusing on the Ordovician and Silurian,the research on hydrocarbon accumulation process of the Cambrian is relatively weak.This paper on the basis of previous researches,from the perspective of researching the period of accumulation,the research area is Tazhong area of Tarim Basin,with the Cambrian as the target layer.Integrating the theories of petroleum geology,fluid geochemistry,and hydrocarbon accumulation chronology,guided by effective reservoir-forming conditions in the research area,the fluid inclusion analysis technique was used to determine the hydrocarbon-filling process of the Cambrian in Tazhong area,and made comparative analysis with the Silurian.Focusing on micro-macro and time-space matching,the hydrocarbon accumulation process have been discussed.This research will deepen the understanding of hydrocarbon accumulation process of the Cambrian in Tarim Basin and provide evidence for deep petroleum exploration in Tarim Basin.In order to determine the hydrocarbon accumulation period of the Cambrian in Tazhong area,the fluid inclusion samples of the middle-lower Cambrian dolomite reservoirs in the research area were systematically tested.Based on the observation of petrographic characteristics under ultraviolet light and transmitted light microscopy,micro-beam fluorescence spectroscopy analysis of crude oil composition maturity,homogenization temperature and freezing point temperature test,laser Raman spectrometer analysis of gas composition and other technical methods,combined with the burial-thermal history of the research area,the characteristics of hydrocarbon accumulation period of the Cambrian have been researched.The results indicate that:(1)During the Late Caledonian-Early Hercynian period,two periods of hydrocarbon charging occurred of the Cambrian in Tazhong area: in the late Caledonian period,the source rocks of the Yuertus Formation began to generate hydrocarbons at the maturity stage,and the resulting oil and gas was blocked by the Awatag Formation of the Cambrian dolomite reservoir.In the late Caledonian-Early Hercynian period,a refill of crude oil occurred again.Early tectonic events in the Hercynian period led to the formation of large faults.The crude oil from the source rocks of the Cambrian was adjusted along the fault to the Silurian system,and then converted to bitumen by the action of atmospheric freshwater leaching and biodegradation.In the Himalayan period,the source rocks of the middle-lower Cambrian were no longer capable of producing oil and began to produce gas,and the generated natural gas is filled into the trap to form gas reservoir.(2)There are three stages of hydrocarbon charging times of the Cambrian in Tazhong area: the first and the second stages are the charging process of crude oil in late Caledonian period,and the corresponding homogenization temperatures are 90°C-110°C and 125°C-135°C;the third stage is the charging process of nature gas in Himalayan period,corresponding homogenization temperature is 150°C-160°C.(3)The model of hydrocarbon accumulation from well ZS1-ZS5 in Tazhong area is summarized as the Late Caledonian Convergence-Himalayan gas-washing phase fractionation reform.In the Himalayan period,the Shailbulak Formation from the well ZS1 C in Tarim Basin developed a lateral transport channel for dry gas.The nature gas migration form a secondary condensate gas reservoir,which is the result of phase fractionation reform.