Heterogeneity of organic matter composition and structure and its controls on pore development in the Longmaxi shale of Sichuan Basin
Shale gas exploitation in the Sichuan Basin of China is concentrated in the overmature marine Longmaxi Formation.As the main gas storage space,the development of organic matter(OM)-hosted pores shows great heterogeneity at micro-scale,which further controls the mechanism of shale gas storage and migration.To date,the origins of OM-hosted pore heterogenerity are still unclear,due to a poor understanding of OM compositional and structural complexity.In this paper,the organic petrology,OM-hosted pore type and its correlation with OM chemical structure of a typical overmature Longmaxi shale in the Sichuan Basin were systematically characterized and quantified,by combining optical microscopy,field emission scanning electron microscopy,and laser Raman spectroscopy.Results show that the OM maceral of examined siliceous Longmaxi shale is dominated by pyrobitumen,followed by graptolites,which display light-gray to grayish color under oil-immersion incident light and have no fluorescence.On the basis of the different OM occurrence under electron microscopy,the pore development types can be divided into bioclastic pores,structural pyrobitumen pores,and amorphous pyrobitumen pores.The apparent transformation rate(ATR)of bioclastics is between 0～2％,with only a small number of isolated pores found on the surface and the pore size distribution mainly of 10～50 nm.The ATR of structural pyrobitumen is in the range of 8％～11％,with relatively uniform sponge-like or irregular pores developed and the peak of pore size about 60 nm.The ATR of amorphous pyrobitumen was the highest,with an average of about 16％,and the peak of pore size is shifted to larger size.In order of graptolites bodies,structural and amorphous pyrobitumen,the OM aromatization decreases and the corresponding pore development increases.The study suggests that the primaty OM composition and chromatographic fractionation during hydrocarbon generation and expulsion are key factors in pore characteristics heterogeneity.
Overmature marine shaleLongmaxi FormationMicroscopic componentsOrganic carbon structurePore development
万方数据
维普
