Characteristics and genesis of fillings in fracture-cavity space in first member of Permian Maokou Formation, Sichuan Basin: a case study of well A1

Abstract

In recent years, natural gas discoveries have been continuously made in the first member of the Permian Maokou Formation in the Sichuan Basin, bringing attention to the genesis mechanism of marlstone reservoir space. Centimeter-scale fracture-cavity type reservoir space is one of the newly discovered types in the first member of the Maokou Formation, and relevant research has not been publicly reported. Taking well A1 in the Sichuan Basin as an example, based on core observation and description, the characteristics and genesis of fracture-cavity fillings were studied through comparative analysis of microscopic petrology, carbon-oxygen isotopes, strontium isotopes, and trace elements. The similar distribution characteristics of carbon-oxygen isotopes and strontium isotopes between calcite cement and mudstone matrix in the fractures of the first member of the Maokou Formation indicated that calcite mainly came from the dissolution and redeposition of surrounding rocks. Compared with marlstone, the fracture calcite cement showed lower contents of valence elements (V, Cr, U, Mo) and rare earth elements (REE), while the content of micronutrient elements (Ni, Cu, Zn) increased, revealing the differential migration pattern of trace elements in this process. The differential variation characteristics of Mn and Sr contents between calcite and marl matrix revealed the marine affinity of diagenetic fluids, further indicating their independence from atmospheric freshwater and deep hydrothermal fluids. Since the Late Jurassic, lateral tectonic thrusting pressure has formed detachment faults and related fold systems in southeastern Sichuan, which potentially caused deformation of the Maokou Formation marlstone and the generation of centimeter-scale fracture spaces. Diagenetic fluids formed during subsequent compaction-dissolution processes (including lateral dissolution) provided material sources for quartz and calcite cement in the fractures.

Keywords

• diagenesis
• clay mineral
• marlstone
• maokou formation
• permian
• sichuan basin