Evolution of organic matter and nanometer-scale pores in an artificially matured shale undergoing two distinct types of pyrolysis: A study of the Yanchang Shale with Type II kerogen

Two distinct pyrolysis experiments were performed to investigate variations in the content and composition of organic matter (OM) and their relation with the growth of nanometer-scale pores during the thermal maturation of shale. One was conducted in glass tubes under vacuum (closed system), and the other was performed in a semi-closed system in which generated oil can flow out of the system under an 80 MPa pressure compressing the sample. The maturation of OM, especially the secondary cracking of residual bitumen, occurs much slower in a semi-closed than in a closed system. However, greater carbon loss (up to 6%) was shown by pyrolyzed samples in the semi-closed system relative to those in the closed system (< 3%), which is related to differences in hydrocarbon expulsion and consolidation of retained bitumen. During the main oil generation stage, declining pore volume in both systems is mainly attributed to bitumen infilling that masked some primary and secondary pores. A significant increase in total pore volume by 2.5 cm3/100 g was observed in pyrolyzed samples from the semi-closed system after the oil peak. In contrast, a much smaller pore growth of 1.1 cm3/100 g was detected in samples within the closed system. A relatively high pore pressure in a block of compressed shale, resulting from a large amount of retained bitumen, can generate, in addition to a large carbon loss, substantial pore growth in pyrolyzed samples within the semi-closed system. Current results suggest a close, but not straightforward, relationship between the transformations of OM and pore development.