The Control Mechanism and Application of Sedimentary Environment for the Shale Gas Accumulation
According to shale depositional environment conditions,organic-rich gas shale can be divided into three types:Marine facies,Transition facies and the Continental facies.The Marine shale Wufeng-Longmaxi Formation and the Continental shale in Dongpu Depression are the main research objects.By comprehensive utilization of Geochemistry and mineral analysis testing technology,based on analysis of a large number of test data and using the methods of statistical comparison,the author discussed the formation conditions and control factors of Marine and Continental organic-rich shales.We studied the constraint function of different sedimentary environment for the shale oil and gas development and distribution,then it is the anatomical study on the typical region,obtaining the following understanding:(1)Regardless of Marine and Continental sedimentary,the rich organic matter shale formation are mainly the restriction of the organic production and burial conditions.Found through the analysis of structure and paleogeographic environment,Wufeng Formation high-quality hydrocarbon source rocks is controlled by rising ocean currents,the influence of Longmaxi Formation control factors of hydrocarbon source rock development is because of the sea level and tectonic plate extrusion.In Guizhou province it formed a wide range of residence environment;The Northern Dongpu Depression shale is mainly due to ancient water column stratification caused by high salinity,forming a strong reducing environment to form dark mud shale.The control action of sedimentary shale development is relatively more obvious.(2)By petrographic analysis,in Guizhou,when the carbonate mineral content in shale<30%,the higher quartz content,the lower clay mineral content,than the higher TOC,which means the siliceous shale(quartz + feldspar>50%,clay minerals<40%carbonate minerals<30%)has higher TOC than the clay shale(quartz +feldspar<50%,clay minerals>40%carbonate minerals<30%);when the carbonate mineral content in shale>30%,the higher carbonate mineral content,means the lower quartz and clay content,the lower TOC,which means the carbonate shale has low TOC.Overall,the organic material content:siliceous shale>clay shale>carbonate shale.The main lithic facies in Donpu depression are lamellar calcareous mudstone,massive silty mudstone and gypsum mudstone.Gas&oil test and fluorescence flakes test show that lamellar calcareous mudstone of semideep-deep lake facies is the advantage lithic facies for shale oil&gas exploration.(3)The organic content,organic material types,mineral content,lithic facies and reservoir space characteristics of marine and terrestrial organic-rich shale have the"annulus features" matched with sedimentary facies.In Guizhou,deep shelf shale contain more organic material which provided by authigenic organisms.From the center to the edge of the rift sedimentary basin,from the bottom to the top of the formation,the organic material types of terrestrial shale change from sapropelic to humic.With the depth increasing the maturity become higher.There are difference of hydrocarbon organic evolution and products between sapropelic and humic.All of these factors cause the shale oil&gas concentrated in the rift lake basin by some rules.(4)On the side of the sedimentary facies control the distribution of organic-rich shale,the deep shelf shale in Guizhou have these characteristics:high TOC,stable distribution,high brittle mineral content,large gas content;the high pressure fractured,brittle interlayer content shale with-in salt layers in northern Dongpu depression are the advantage shale oil type.Under the standard of favorable zone optimization,Xishui-Tongzi areas are selected to be the favorable zone in Guizhou province;The optimization result in Dongpu depression matched with sedimentary facies well,most of favorable zone are in the lake facies,and the Type I area matched with the sedimentary center.The relationship between the favorable zone and structural sedimentary center reflect the shale oil&gas distribution controlled by sedimentary environment.