Organic matter pores structure and evolution in shales based on the he ion microscopy (HIM): A case study from the Triassic Yanchang, Lower Silurian Longmaxi and Lower Cambrian Niutitang shales in China

Abstract

To illustrate organic matter (OM) pores structure and evolution in shales with different maturities, the Triassic Yanchang shale (0.5% < Vitrinite reflectance (Ro) < 1.2%) in the Ordos Basin, the Lower Silurian Longmaxi shale (2.0% < Ro < 2.8%) and the Lower Cambrian Niutitang shale (3.0% < Ro < 4.2%) in the Yangtze Platform have been selected in this study. OM pores were analyzed by helium ion microscopy (HIM). The results show that OM pores are mainly developed in pyrobitumen. A large number of OM pores were developed in pyrobitumen of the Longmaxi shale samples. However, no OM pores were basically found in solid kerogen. OM pores number and size in pyrobitumen of the Yanchang shale samples are much smaller than that in the Longmaxi shale samples. OM pores are all basically undeveloped in pyrobitumen of the Niutitang shale samples. The structure that small size OM pores nested in large size OM pores is favorable to gas enrichment and seepage. Thermal maturity has more huge impact on OM pores structure than TOC content in shale. Low thermal maturity (Ro < 1.5%) will lead to insufficient thermal evolution of OM, resulting in small size and quantity of OM pores in shale. Excessive thermal maturity that ranges from 3.0% to 4.0% will leads to the disappearance of OM pores in pyrobitumen. Appropriate thermal maturity that ranges from 1.5% to 3.0% ensures a large number of OM pores in shale.