| Xihu sag,which is one of the most potential sag in the East China Sea Basin for oil and gas exploration,is located in the central of Zhedong depression.Previous studies have been done a lot in this area,but the contribution of the main source rock to the reservoir remains unclear.The distinguishing characteristics of crude oil and reservoir formation system study remain to be summarized and further studied.To solve this problem,a new geochemical technique has been used.On the basis of a comprehensive study of petroleum geology,a deep understanding of the geochemical characteristics,hydrocarbon source and the characteristics of hydrocarbon migration has been done.Meanwhile,the demarcation criterion of accumulation system is established,and the accumulation process is analyzed in important accumulation system.Then the paper summarizes the main controlling factor in the Xihu sag,and sets up a reservoir model.The obtained knowledge as follows:The crude oil in Xihu sag is mainly derived from coal-bearing source rocks,and the oil component is dominated by saturated hydrocarbon.The raw labeled chemical analysis shows that the maturity of crude oil is humic-type(? type)kerogen,which is formed in oxidation environment,and crude oil has reached from maturity to high maturity.The saturated hydrocarbons of crude oil is rich in diterpenoids,and high molecular weight steranes have a low content.There are some differences in the causes of crude oil of difFerent tectonic belt.For example,terrestrial higher plants is main organic source in Pinghu slope,and in Huangyan tectonic belt the terrestrial higher plants is also main organic source,but aquatic organism organic make a contribution to the hydrocarbon precursor.The crude oil of Xihu sag can be divided into three categories,namely a,b,c.In fact,c only appears in one well,a and b are the two main categories.The main oil of Pingbei area which is near the western slope of the sag edge is category a.There are mixing category a and category b in Pingzhong area and there is only category b in Huangyan area which is a far away from the slope.Category a with greater depth,has a higher crude oil maturity.Its sources are derived from the deep formation and category a has obvious advantages in isopimaric alkyl.Compared to category a,category b distributes in shallow depth,maturity is relatively lower and flat sticks alkyl content is relatively higher.Both coal and shale have contribution to oil and gas of Xihu sag.In the pllane,the main contribution to Pingbei area which is near the Pinghu slope of the sag edge is made by coal.Both coal and shale have contribution to Pingzhong area.And the main contribution to Huangyan area is made by shale.Xihu sag can be divided into five reservoir system,namely western slope accumulation system,the west sub-concave accumulation system,the central inversion structure accumulation system,the east sub-system and off eastern Long accumulation system.The analysis about the controlling factors of hydrocarbon accumulation have been done,and the "source-break-cover" theory of common control reservoir has been established.The theory shows that source rock is the foundation,fracture is the hub,preservation is the key and these three factors constraint the enrichment and distribution of oil and gas.At last,a typical accumulation mode has been established. |
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