oa Identification, Evolution and Geological Indications of Solid Bitumen in Shale- a Case Study of the Qingshankou Formation in Songliao Basin

Solid bitumen (SB) in shale contains valuable information on kerogen transformation and hydrocarbon generation, crucial for accurately assessing shale maturity and understanding the evolution of shale reservoir space during thermal progression. Additionally, comprehending the impact of SB on shale reservoir space is essential for evaluating the sealing ability and cap rock integrity of such reservoirs. In this study, we focus on organic-rich shale from the first member of the Cretaceous Qingshankou Formation in the Songliao Basin. Through a comprehensive investigation of SB, including its definition, classification, occurrence forms, and evolution pathway, we explore the indicative significance of SB reflectance (BRo) on maturity and its influence on reservoir space development. Our findings indicate that the primary maceral type significantly influences the evolution pathway of SB. Most pre-oil bitumen is in situ SB, with only a small portion being migrated SB. Conversely, post-oil bitumen and pyrobitumen predominantly consist of migrated SB. During the immature to early oil maturity stage, SB can be distinguished from bituminite, vitrinite, inertinite, and alginite by their optical characteristics under reflected light and fluorescence. The scanning electron microscope facilitates effective identification of in-situ SB and migrated SB. BRo demonstrates a linear increase with vitrinite reflectance (VRo), attributed to the reduction of aliphatic structures and enhancement of SB aromatization. In the oil window, SB develops three types of secondary pores: modified mineral pores, devolatilization cracks, and bubble holes, while spongy pores form at higher maturity stages in pyrobitumen. Utilizing scanning electron microscopy in combination with other in-situ analysis techniques, such as laser Raman spectroscopy, provided valuable insights into the structural information of various SB types, supporting micro-scale investigation of organic matter migration paths and how it evolved in such distances. Overall, this study enhances our understanding of SB in shale formations, shedding light on the significance of SB reflectance in evaluating maturity and its role in the development of shale reservoir space during thermal evolution.