The Development Model Of Xujiahe Continental Shale In Western Sichuan Depression And Its Reservoir Characteristics

There are many types of continental basins in China.The sedimentation facies of continental shale changes significantly,causing the shale reservoir characteristics to change quickly.It is hard to predict continental shale gas favorable region and evaluate sweetspots.These have restricted the exploration and development of continental shale gas.Thus the research of continental shale development model and its controlling effects on reservoir characteristics was conducted.This paper studies continental shale from Xujiahe Formation in western Sichuan Depression by major and trace element analysis,heavy-mineral analysis,organic geochemistry analysis and other experimental methods.After these studies,the development model of continental shale was established and the shale reservoir characteristics were evaluated.On basis of these,the controlling effect of continental shale development model on reservoir characteristics was analyzed.The provenance of Xujiahe Formation in western Sichuan Depression is felsic rock of upper crust and the tectonic setting is continental island arc.The vertical development of Xujiahe Formation is controlled by multiple structural evolutions.Organic-rich shale was deposited in reducing sedimentary environment with high paleo-water depth,low paleosalinity and paleoproductivity,when the climate in western Sichuan Depression was warm and humid.On basis of these,the comprehensive development model controlled by tectonic,palaeoenvironment and palaeoclimate is established for Xujiahe continental shale.Under the control of the development model of continental shale,four main types of shale lithofacies are developed in Xujiahe Formation,including organic-rich sand-rich argillaceous shale lithofacies,organic-containing sand-rich argillaceous shale lithofacies,organic-containinginterbedding mixed shale lithofacies and organic-poor mud-rich siliceous shale lithofacies.Xujiahe shale is mainly composed of clay mineral and quartz.The clay mineral compositions of Xujiahe shale are dominated by illite smectite mixed layer and illite.On the whole,shale has abundant organic matter,which is at mature to high mature stage.The type of kerogen is III.There are two main kinds of pores within Xujiahe shale,including clay mineral intergranular pores and intrgranular dissolved pores.There is diversity in pore structure of Xujiahe shale.Mesopores make the greatest contribution to the total pore volume,whereas micropores provide the major specific surface area.Shale physical properties are poor.In summary,the geological characteristics of Xujiahe shale in western Sichuan Depression are favorable for shale gas to form and storage.There are great differences in reservoir characteristics among different types of shale lithofacies.Comprehensive comparison in reservoir capacity shows that organic-rich sand-rich argillaceous shale lithofacies,which developed clay mineral intergranular pores,intrgranular dissolved pore and organic pores,is the most favorable shale lithofacies in Xujiahe Formation for its high TOC value,maximum pore volume,maximum specific surface area and best parameters of reservoir physical properties.