Study on tectonic fracture characteristics and stage evolution of Longmaxi shale reservoir in Yongchuan, southeastern Sichuan Basin

doi:10.13745/j.esf.sf.2023.3.2

Abstract

Abstract:

Development characteristics and formation stages of natural fractures are of great significance to shale gas accumulation in the complex structural area of southeastern Sichuan. It also significantly influences the effects of hydrofracturing reconstruction and high gas production of shale reservoirs. Currently, there is no systematic study on the characteristics and evolutions of natural fractures with various origins in deep shale gas blocks far from the basin margin in the southeastern Sichuan Basin, which hinders the efficient exploration and development of deep shale gas reservoirs in the basin. Based on field outcrops, drilling cores, thin sections, geochemical data, and apatite dating, the characteristics and formation stages of natural fractures in the Longmaxi Formation shale reservoirs in the Yongchuan area have been studied. The results indicate that the Longmaxi Formation shale mainly developed tectonic fractures, including shear fractures with medium to high angles, low angle slip fractures, and vertical tensile fractures. Additionally, bedding fractures and overpressure fractures are also observed in the cores, with hydrothermal dissolution fractures mainly developed near the main fault zones. Vertically, the density of fracture development gradually decreases from deep to shallow due to variations in mineral compositions. Horizontally, fractures with high angles, distinct single fracture groups, and high vertical penetration capacity are densely developed in the anticline core or wing steep slope area, while fractures with low angles, small scales, and various fracture groups are developed in the gentle areas. Three tectonic movements—the late Yanshan (86-47 Ma), early to middle Himalayan (47-24 Ma), and late Himalayan to present (24-0 Ma)—contributed to the tectonic fractures in this area. The fracture development and filling characteristics in different evolutionary stages are notably different. The study establishes a tectonic fracture development and evolution model of shale in three phases in the Yongchuan area, providing valuable insights into the study of shale gas enrichment and preservation, as well as a deeper understanding of the evolution of shale fractures in the Longmaxi Formation, southeastern Sichuan Basin.

Key words: fractures characteristics, evolution stages, Wufeng-Longmaxi shale, Yongchuan area, southeastern Sichuan Basin

CLC Number:

HE Jianhua, LI Yong, DENG Hucheng, WANG Yuanyuan, MA Ruolong, TANG Jianming. Study on tectonic fracture characteristics and stage evolution of Longmaxi shale reservoir in Yongchuan, southeastern Sichuan Basin[J]. Earth Science Frontiers, 2024, 31(3): 298-311.

Figures/Tables 12

Fig.1 Tectonic units in southeastern Sichuan Basin and the structural characteristics of Yongchuan area

Fig.2 Genetic types and characteristics of fractures in the Longmaxi Formation outcrops around the Yongchuan area

Fig.3 Genesis types of fractures in the first member of Longmaxi Formation cores in the Yongchuan area

Fig.4 Core fracture parameters in the Yongchuan area

Fig.5 Stages analysis and development characteristics of joints in the outcrop

Fig.6 Cutting characteristics and analysis of fractures in the cores in Yongchuan area

Fig.7 Thin-section images and cathodoluminescence images of the fracture cements in the first member of Longmaxi Formation shales in the Yongchuan area

Fig.8 Characteristics of inclusions in quartz and calcite veins in the first member of Longmaxi Formation in the Yongchuan area

Fig.9 Histogram of homogenization temperature distribution of fracture filling inclusions

Fig.10 Cross plot anlaysis of carbon and oxygen isotope of fracture filling

Fig.11 Rock acoustic emission test curve of the first member of Longmaxi Formation shales in the Yongchuan area

Fig.12 Fracture evolution model in the Yongchuan area

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