No abstract

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 (η) 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.

In recent years, the shale gas in the southern Sichuan Basin has achieved great commercial development, and the Silurian Longmaxi Formation is the main development stratum. In order to solve the problems of great difference production and inaccurate gas content of the Longmaxi Formation shale gas field in the southern Sichuan Basin, based on thin section identification, argon ion polishing-field emission scanning electron microscopy, high pressure mercury injection, low temperature nitrogen adsorption and the fractal method, the micropore structural heterogeneity of the siliceous shale reservoir of the Longmaxi Formation has been studied. The results show the following: The pores of siliceous shale are mainly intergranular pores and organic pores. Image analysis shows that there are obvious differences in size and distribution of shale pores among different types. The micropore structural heterogeneity is as follows: intragranular pore > intergranular pore > organic pore. In the paper, the combination of low temperature nitrogen adsorption method and high-pressure mercury injection method is proposed to characterize the micropore size distribution and fractal dimension, which ensures the credibility of pore heterogeneity. The shale pores are mainly composed of mesopores (2–20 nm), followed by macropores (100–300 nm). For different pore sizes, the fractal dimension from large to small is mesopore, micropore and macropore. Shale pore structure and fractal dimension are correlated with mineral composition and total organic carbon (TOC) content, but the correlation is significantly different in different areas, being mainly controlled by the sedimentary environment and diagenesis.