Characteristics of Permian Igneous Rocks and their Petroleum Geology Implications in the Northern Part of Central Sichuan, Sichuan Basin, China

Based on data from cores, cuttings, thin sections, and logging and seismic activities, the lithofacies, geophysical response, and spatial distribution characteristics of the Permian igneous rocks in the northern part of central Sichuan were analyzed and summarized. Furthermore, the eruption and filling processes of the igneous rocks were restored, and the hydrocarbon geologic significance of the volcanic eruption events was investigated. The results reveal that the igneous rocks in this area are volcanic overflow facies and sedimentary facies, with volcanic lava and pyroclastic rocks developed. Only one eruption cycle occurred vertically. The igneous rocks exhibit low natural gamma ray, low acoustic, high compensation neutron logging, high density, and high resistivity logging values. The seismic responses are characterized by multiaxial, flexural, and upward arching. The plane distribution shape is an inverted U, with a nearly east–west linear distribution. The igneous rocks in the area were emplaced via a fissure eruption along the basement faults in the early Longtan. The eruption-filling process can be roughly divided into four stages: the deposition of the Maokou Formation, the geomorphological embryonic stage, the geomorphological differentiation stage, and the volcanic eruption and filling stage. The underdevelopment of primary pores and the lack of dissolution in the later stage led to poor igneous reservoir performance, which limits the igneous reservoir hydrocarbon exploration prospect. However, the early faulting activity resulted in the differentiation of the Maokou Formation sedimentary and karst landforms. The thermodynamic conditions created by the later eruption of the igneous rocks, sufficient Mg2+ sources, and favorable geomorphological units laid a good foundation for the development of the dolomite reservoirs in the Maokou Formation. Among them, the karst slopes and highland areas of the Maokou Formation along the two sides of the strip of igneous rocks are dominant areas for the development of the dolomite reservoirs in the Maokou Formation.