A Study On Hydrocarbon Generation Evaluation And Distribution Characteristics In The Zifusi Sag

The tectonic position of Jianghan basin is in the central north part of Yangtze Block, which is a typical inland salt lake fault basin in eastern part of China. Jianglin depression is in the west southern part of the Jianghan basin. Cretaceous-Paleogene of the Jiangling depression are terrestrial river-lakes sandstone sedimentary system. The Xingouzui group is main source rock. There are three sags that called the MeiHuaiQiao sag、Zifusi Sag、Jiangkou Sag, hydrocarbon generation of the MeiHuaiQiao sag is the most favorable, emphases of exploration is focus on Jingzhou anticline blet, which resulting lost balance in exploration, evaluation of the Jiangling Sag is not deeply, especially on the hydrocarbon generation potential of the sag. Reacquainting and reappraise of depression exploration potential are important for find the gas enrichment area and appraises of hydrocarbon generation potential in Jianglin.This paper based on collecting predecessors’hydrocarbon source rocks results of geochemical analysis in the study area. For regional and layer which data is insufficient, appropriately complement the analysis of samples for testing in order to set up important wells of different zone organic source rocks geochemical characteristics, comprehensive analysis the lower of Xingouzui Group、the upper of Shashi Group source rock distribution characteristics and organic geochemical characteristics, and has evaluated the hydrocarbon-generating potential of hydrocarbon source rocks According to the result of geochemical characteristics of source rock, combined with the analysis results of maturation history of effective source rock, dynamically evaluating hydrocarbon generation potential of effective source rock, and pointing out the favorable hydrocarbon generation area.The source rocks of the Zifusi sag is the upper Shashi Group and the lower Xingouzui Group which growing in the first major tectonic-evolutionary stages, while the qianjiang formation on the second tectonic evolution stage, because of its low maturation becoming the potential source rock. The source rock is single type which mainly is lacustrine sludge shale rock, and salinization plaster mudstone in lake and etc. Geological and geochemical analysis show that Xingouzui group lower segment mud interlayer and Ⅱ oil group has higher organic matter abundance, reach to poor-middle grade, the Xingouzui group lower segment Ⅲ oil group and Shashi group upper segment are poor source rock, Shashi group upper segment are poor source rock, Shashi group upper segment, Xingouzui lower segment source rock organic matter are mainly Ⅱ2-Ⅲ type, Ⅰ、Ⅱ oil formation with lower maturity is in early period, Ⅲ group enter later mature period. According to the geochemical index in this area,evolution of organic matter is divided into four stages,immature stages:depth<1800m, Ro <0.65%.Early mature stage:1800to2700m depth, Ro is0.65to1.0%. Mature late stage:the depth of2700to3250m, Ro1.0to1.3%.High mature stage:depth>3250m, Ro>1.3%. Source rock evaluation showed that the Nige formation and Ⅱ oil formation of the lower Xingouzui Group has a relatively high hydrocarbon potential.Semi-deep lake facies and delta front sub-facies sedimentary mainly developed in the Zifusi sag and its surrounding of paleogene.The Dark mudstone from the work area is mainly distributed in the Shashi group to the Xingouzui Group.Include the upper Shashi Group,The Xingouzui Ⅲ formation,The Xingouzui Nige formation,The Xingouzui Ⅱ formation and The Xingouzui Ⅰ formation.The dark mudstone of The Xingouzui Ⅱ formation is the thickest among them.Dark mudstone higher organic carbon content place is located in the Fuxingchang-Mituosi-Zifusi, which decreased in northwestern and southeastern. The TOC content of Nige formation and Ⅱ oil formation of the lower Xingouzui Group is much higher, which reach to1.0%, other formations are basically less than0.6%, the changing tend of higher TOC contents area is going to the south from the upper Shashi group to the Ⅰ oil formation of the Xingouzui group.The Zifusi sag is almost at lacustrine facies to half deep lacustrine facies, which developing from northeast to southwest. The central thickness area is at the Fuxingchang to Zifusi, which northwestern and southeastern side becoming thinner. The thickness of Ⅱ oil group is the most thichest, which is much more than150M. The second one is upper Shashi Group, which centeral thickness is100M, and other three oil groups are less thicher than Ⅱ oil group. Source rock higher organic carbon content place is located in the Fuxingchang-Mituosi-Zifusi, which decreased in northwestern and southeastern. The TOC content of Nige formation and Ⅱ oil formation of the lower Xingouzui Group is much higher, which reach to1.0%, other formations are basically less than0.6%, the changing tend of higher TOC contents area is going to the south from the upper Shashi group to the Ⅰ oil formation of the Xingouzui group. Hydrocarbon generation area is divided into three levels. The first class hydrocarbon-generating area of Ⅱ oil group which belong to the lower of the Xingouzui Group distribution in the center of Zifusi depression area, for outward is class Ⅱ of hydrocarbon generation area. Class Ⅲ major southern distribution of hydrocarbon generation area; Ⅲ oil group and the upper part of shashi group in the Jingsha anticline belong to Ⅱ class of hydrocarbon generation area, the structure of Fanjiatai and Zifusi sag area and other areas are distributed in Ⅲ class of hydrocarbon generation area.