Abstract: A sequence of organic-rich shale of transitional facies can be observed in the Longtan Formation of eastern Sichuan Basin.In order to investigate the characteristics of the organic pores and their controlling factors in the shale, we performed experiments and analyses on shale samples to study TOC, mineral composition, vitrinite reflectance, and organic petrological parameters through low-pressure nitrogen adsorption and scanning electron scanning combined with argon ion milling and etc.The results show that the mineralogy of shale samples from coastal-swamp, lagoon-tidal flat and shallow shelf facies is dominated by clay minerals with low content of quartz.The organic matters in the samples are mostly vitrinites with poorly-developed organic pores.However, some micro-cracks were found to occur within or along the rims of organic matter grains.In contrast, the shale of deep shelf facies was characterized by high quartz and low clay mineral contents and orgnic matters with well-developed pores being dominated by sapropelinite and solid bitumen.There is an inverse relationship or insignificant correlation between the microscopic pore volume and TOC of shale of the coastal-swamp, lagoon-tidal flat and shallow shelf facies.In contrast, the pore volume is positively correlated with TOC for shale of deep shelf facies.In combination with previous works, we propose that vitrinite and solid bitumen are not the main contributors to organic pores during the high maturity stage of the samples, while sapropelinite is the most favorable for orga-nic pore development.Thereforoe, the organic maceral composition should be taken into consideration when studying shale reservoir properties of this transitional facies in eastern Sichuan Basin as well as other areas.

Key words: organic pore, TOC content, maceral composition, Longtan Formation, Eastern Sichuan Basin