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Characteristics of Organic Matter Particles and Organic Pores of Shale Gas Reservoirs: A Case Study of Longmaxi-Wufeng Shale, Eastern Sichuan Basin

Heterogeneity of organic matter (OM), including size, type, and organic pores within OM, is being recognized along with increasing study using SEM images. Especially, the contribution of organic pores to the entire pore system should be better understood to aid in the evaluation of shale reservoirs. This research observed and quantitatively analyzed over 500 SEM images of 19 core samples from Longmaxi-Wufeng Shale in the eastern Sichuan Basin to summarize the features of OM particles and OM-hosted pores and their evolution during burial. The features of organic pores as well as the embedded minerals within OM particles enables to recognize four different type of OM particles. The organic pore features of each type of OM particles were quantitatively described using parameters such as pore size distribution (PSD), pore geometry, and organic porosity. The PSD of weakly or undeformed porous pyrobitumen indicates that the large organic pores (usually 200 nm to 1 um) is less common than small pores but the major contributor to organic porosity. The organic porosity of OM particles covers a large range of 1–35%, indicating a high heterogeneity among OM particles. Based on analysis of 81 OM particles, the average of organic porosity of the five samples were calculated and ranges from 3% to 12%. In addition, samples from well JY1 have higher organic porosity than JY8. These results helped to reveal how significant the organic pores are for shale gas reservoirs. In addition to presenting many examples of OM particles, this research should significantly improve the understanding of type and evolution of OM particles and contribution of OM-hosted pores to the entire pore system of high to over mature shale.