Tectonic Fracture Formation and Distribution in Ultradeep Marine Carbonate Gas Reservoirs: A Case Study of the Maokou Formation in the Jiulongshan Gas Field, Sichuan Basin, Southwest China

Tectonic fractures are the key factors affecting hydrocarbon migration and accumulation in ultradeep marine carbonate gas reservoirs. Taking the Maokou Formation in the Jiulongshan Gas Field as an example, tectonic fracture formation and distribution are quantitatively characterized by the outcrops, cores, Fullbore Formation MicroImager (FMI) imaging logging, acoustic emission experiments, fluid inclusion experiments, and burial-thermal evolution history analysis. The formation stage of the tectonic fractures in the study area can generally be divided into three stages: the Indosinian stage, the early middle Yanshanian stage, and the late Yanshanian-Himalayan stage. The key stages are the early middle Yanshanian stage and the late Yanshanian-Himalayan stage. According to the theory of tectonic geomechanics, the evolution pattern of different stages of tectonic fractures and faults in the Maokou Formation is established. The finite element method was used to simulate the three-dimensional paleotectonic stress field during the key stages of fracture formation, and a rock failure criterion (eta) was used to quantitatively predict the development and distribution of the tectonic fracture. In the early middle Yanshanian stage, the fracture degree was relatively small, and the highly fractured areas were mainly concentrated in the areas near the northern fault zone and the high part of the anticline, with the highest rock failure proximity of 1.118. In the late Yanshanian-early Himalayan stage, the highly fractured areas are distributed in the northeast and northwest, near the E-W fault rupture zone, the high parts of the Jiulongshan and Tadongping areas, and the local tectonic high parts. The degree of rock failure mainly concentrated between 0.890 and 1.127. There is a good positive correlation between the fracture density and the degree of rock failure.