Types, Evolution of Fractures, and Their Relationship with Oil and Gas Migration of Permian Changxing Formation in Yuanba Gas Field of Sichuan Basin, China

Yuanba gas field of Sichuan Basin is characterized by weak-tectonic deformation, and the fractures have had a significant effect on oil and gas migration. This paper investigates the types, evolution of fractures, and their relationship with oil and gas migration of the Permian Changxing Formation of the Yuanba gas field according to geological and geochemical analyses. Four genetic types of fractures with different occurrences were distinguished: intercrystal fractures, pressure solution fractures (stylolites), structural fractures, and overpressure fractures. Intercrystal fractures present multiple straight edges between the dolomite particles and no fixed direction. Pressure solution fractures are in the form of zigzags. Structural fractures have straight edges and a certain direction and are usually distributed in groups. Overpressure fractures have irregular shapes and irregular directions and occur in multiple quantity. According to the comprehensive analyses, the intercrystal fractures, pressure solution fractures, and structural fractures formed in shallow-intermediate burial environments during the Early Yanshanian period and acted as paleo-oil migration pathways; thus, paleo-oil is widely distributed in the Permian Changxing Formation reservoir, as indicated by the presence of pyrobitumen. The overpressure and structural fractures formed in deep burial environments during the Late Yanshanian and Himalayan periods and served as the dominant pathways for the migration and remigration of cracked gas. This study is significant for understanding the formation of oil and gas fields in the weak-tectonic deformation zone.