Relationship between gas reservoir distribution and structural system of upper Triassic Xujiahe Fm in the Sichuan Basin

Abstract

The gas reservoir of Upper Triassic Xujiahe Fm in the Sichuan Basin is characterized by “accumulation in the early stage, entrapment in the middle stage and activation in the late stage”. In order to provide guidance for the prediction of hydrocarbon enrichment zone of Xujiahe Fm, we first plotted the regional structure map based on the 2D and 3D seismic merging data of the basin. Then, we displayed and described the structural characteristics by means of the 3D visualization technology of low-angle vertical backlight irradiation. In addition, the structural system was classified according to the dynamic direction of regional structure and the structural interrelationship, and the formation stages of the structure were confirmed. Finally, based on drilling and testing data, the hydrocarbon enrichment zone of Xujiahe Fm was predicted. And the following research results were obtained. First, five structural systems are developed in Xujiahe Fm in the Sichuan Basin, including EW-oriented arc structure, NE-oriented linear structure, NE-oriented arc structure, SN-oriented to NW-oriented brush structure and NW-oriented arc structure. Second, the EW-oriented arc structure is formed due to the uplift and extrusion of the northern section of Longmenshan thrust belt in the Indosinian period, successively developed in the Yanshanian and stabilized in the Himalayan period. And it is widely distributed in the basin. Third, the NE-oriented linear structure in the north of Western Sichuan Depression is resulted from the Anxian movement of Longmenshan thrust belt in the Indosinian period. Fourth, the NW-oriented arc structure before the Dabashan and the NE-oriented linear structure before the Huaying Mountain are stabilized in the Yanshanian and successively developed in the Himalayan period. Fifth, the SN-oriented structure in the Western Sichuan Depression is formed due to the extrusion of the southern section of Longmenshan thrust belt in the Himalayan period. Sixth, the combination of SN structure to the east of Longquanshan fold belt and NW-oriented linear structure in the basin is a brush structure belt with Jiangyou paleo-uplift as the mainstay, which is resulted from the extrusion of the southern section of Longmenshan thrust belt in the Himalayan period. Seventh, the high-yield wells are usually distributed in the arc anticlines of Indosinian–Yanshanian superimposed with the faults of Himalayan period. The structures of the Himalayan period are usually dry traps, and their fracture development zones are mostly water producing layers. The Indosinian synclines are also ineffective traps even though they are uplifted to anticlines in the Himalayan period. In conclusion, the confirmation of structural system stages can provide a technical support for the prediction and description of hydrocarbon enrichment zone of Xujiahe Fm in the Sichuan Basin.