地质科技通报 (May 2024)
Characteristics and formation period of fractures in the reservoirs of Permian Maokou Formation, Zigong area, Southwest Sichuan Basin
Abstract
Objective Fractures commonly occur in the marine carbonate reservoirs of Permian Maokou Formation in Zigong area of Sichuan Basin and have important impacts on reservoir properties, seepage patterns and hydrocarbon enrichment. Methods Cores, thin sections, image logs and experimental test data were used to clarify the type of fracture genesis and development characteristics, analyse the time of fracture formation, and determine the period of fracture formation. Results The results show that the marine carbonate reservoirs of Maokou Formation in Zigong area are divided into two types, namely, tectonic fractures and diagenetic fractures, among which the tectonic fractures include tectonic shear fractures and tectonic tensile fractures, and the diagenetic fractures include horizontal bedding fractures and diagenetic sutures. Tectonic shear fractures dominate Maokou Formation reservoir and occur mainly in the NEE and NNE directions. The fracture inclination angle ranges from 20° to 80°, and the extension length is less than 60 cm. The fracture degree of filling is low, and the validity is good. Combined with the analysis and test data, the reservoir fractures of Maokou Formation in Zigong area were determined to have formed by 3 stages of structural movement. The first stage included the late Hercynian and early Indosinian periods, approximately 240-220 Ma. Under the SW stress derived from the clockwise movement of the South China Plate, a small number of shear fractures developed, and the fractures were mostly filled with minerals, representing the secondary development period of fractures. The second stage occurred in the late Yanshan-early Himalayan period, approximately 78-69 Ma, and a large number of tectonic fractures developed under the NW-trending stress generated by the Xuefeng uplift in Jiangnan, which was the main period of fracture formation in the study area. The third period was the late Himalayan period, approximately 13-0 Ma. Fractures were formed under the NEE compressive stress generated by the collision of Indian Ocean Plate, and most of the fractures were unfilled, indicating good effectiveness. Conclusion The above fracture-related research provides the basis for establishing favourable exploration zones in the study area.
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