A preliminary investigation of pore development mechanisms in highly matured solid bitumen: Implications from the Ediacaran Dengying Formation solid bitumen in the central Sichuan Basin, Southwest China

Abstract

Although pores in solid bitumen are crucial for shale reservoirs, their development mechanisms have not been fully understood partly due to the small size of solid bitumen in shales and the difficulty in its discrimination from residual kerogen. However, solid bitumen in conventional reservoirs generally has a larger size and definite origin, and therefore the examination of their pores can provide some insights into the mechanisms of pore formation in shale solid bitumen. In this study, solid bitumen samples from conventional reservoirs of the Ediacaran Dengying Formation in the central Sichuan Basin were collected for petrographic and geochemical analyses to investigate their pore characteristics and formation mechanisms. The results indicate that pores are commonly identified in solid bitumen grains that are derived from the cracking of tar pads and in solid bitumen grains that have experienced gravity-driven component separation during their formation. The development of pores in solid bitumen is controlled by the combined effect of gas expulsion and the viscosity of solid bitumen during its movable stage. The gas expulsion from movable bitumen creates pores, and they can be preserved or healed depending on the component-controlled viscosity of movable bitumen. The enrichment of sulfur- and oxygen-containing compounds and heavy components such as asphaltenes in movable bitumen can enhance its viscosity and thus inhibit the healing of pores, ultimately facilitating pore development in the present overmature solid bitumen grains.