Identification, evolution and geological indications of solid bitumen in shales: A case study of the first member of Cretaceous Qingshankou Formation in Songliao Basin, NE China

The literatures on solid bitumen (SB) in shales are reviewed. Then, taking the organic-rich shales in the first member of the Cretaceous Qingshankou Formation (Qing 1 Member) in the Songliao Basin as an example, the definition, classification, occurrence and evolution of SB in shales are investigated, and the indications of SB on maturity and the influence of SB on reservoir space are discussed. The difference in primary maceral types is primarily responsible for the different evolution paths of SB. Pre-oil bitumen is mostly in-situ SB, while post-oil bitumen and pyrobitumen are usually migrated SB. In the immaturity to early oil generation stage, bituminite, vitrinite, and inertinite can be distinguished from SB depending on their optical characteristics under reflected light, and alginite can be differentiated from SB by their fluorescence characteristics. Under scanning electron microscope (SEM), in-situ SB and migrated SB can be identified. The SB reflectance increases linearly with increasing vitrinite reflectance, as a result of a decrease of aliphatic structure and the enhancement of aromatization of SB. Within the oil window, three types of secondary pores may develop in SB, including modified mineral pores, devolatilization cracks and bubble holes. In the high maturity stage, spongy pores may develop in pyrobitumen. SEM combined with in-situ analysis techniques (e.g. Raman spectroscopy) can further reveal the structural information of different types of SB, thus providing crucial data for research at micro-scales such as organic matter migration paths and dynamics. © 2023 Science Press. All rights reserved.