Characteristics and Dominant Factors for Natural Fractures in Deep Shale Gas Reservoirs: A Case Study of the Wufeng-Longmaxi Formations in Luzhou Block, Southern China

Abstract

In southern Sichuan Basin, the main production layers are characterized by deeply buried, high stress difference, and complex structural conditions. The Luzhou area is far from large faults, and natural fractures are greatly important for shale gas storage and production. Multi-scale natural fractures control the migration, enrichment, and preservation conditions of shale gas, and facilitate the formation of complex fracture network under the action of hydraulic fracturing. In this study, based on the outcrops, drilling cores, geochemical tests, thin section, and other experiments, the development characteristics of the Wufeng-Longmaxi shale in the Luzhou block, Southern China, are statistically analyzed, and the controlling factors (e.g., tectonic factors, organic matter, mineral components, mechanical properties) are discussed in details. Fractures observed in the outcrops are mainly regional fractures with two groups of orthogonal joints. Similarly, fractures observed in the core are also mainly joints fractures perpendicular to the laminae with three typical features (high-density, high-angle, and unfilled). In detail, steeply dipping fractures (75-90°) account for 78.1% of all fractures, with 85.1% being unfilled, 78.1% having a longitudinal extension less than 4 cm, and 65.1% having a spacing less than 2 cm. In brief, there exist cross-scale similarity among outcrops, cores, and microscopic thin sections, which is critical to the shale gas preservation conditions. Based on this understanding, further research is conducted on the relationship between the fracture density and gas content, which shows that (i) when the fracture density is less than 122 number/m, TOC content and fracture density together positively dominate gas content; (ii) when fracture density exceeds 122 number/m, gas content appears negative with fracture density, and TOC content is not the critical factor anymore. The above study establishes quantitative limits for shale gas preservation in the study block. It may assist in providing references for determining the sweet spot area and further deep shale gas exploration and development in the southern Sichuan Basin.

Geological Characteristics of Lower Paleozoic Shale Gas Accumulation in the Yuxi Region, Southern Sichuan Basin: In View of High-Density Methane Inclusions

“…The fluid inclusions trapped in the uplift period confirmed that the Himalayan compression movement of Paleogene led to the opening of fracture vein and the adjustment and migration of shale gas. The early similar opening time with well H202 of fracture vein indicate that the damage degree was also similar, while the late stage opening of fracture vein led to the partial loss of shale gas. − Therefore, the current pressure coefficient is 1.8. Although it is still in the overpressure state, its gas content is worse than that of well H202 (Figure ).…”

“…The early similar opening time with well H202 of fracture vein indicate that the damage degree was also similar, while the late stage opening of fracture vein led to the partial loss of shale gas. 56 − 59 Therefore, the current pressure coefficient is 1.8. Although it is still in the overpressure state, its gas content is worse than that of well H202 ( Figure 12 ).…”

Qualitative–quantitative multiscale pore characteristics of black shale from the Upper Ordovician and Lower Silurian black shale in the Southern Sichuan Basin, China

“…Shale composition is an important factor affecting fractures, including mineral composition and content and TOC content (Ding et al, 2012;Zeng et al, 2013;Yin and Wu, 2020;Li, 2022). The statistical results show that with increasing quartz content, the fracture density increases first when the quartz content is less than 50%, while the fracture density decreases when the quartz content is over 50% (Han et al, 2022). Because of the positive relationship between TOC and quartz content, the relationship between TOC and fracture density also shows a similar trend (Han et al, 2022).…”

Study of Wellbore Instability and Collapse Mechanism for a Layered Continental Shale Oil Horizontal Well

“…The development of continental shale oil in China is still in its infancy. The shale oil horizontal wells in Block G in the Songliao Basin are affected by a lack of mature theories, technologies and experiences in the direction of wellbore stability [4][5][6][7][8]. The continental shale oil in the northern Songliao Basin has huge potential resources and has become an important strategic replacement resource in Daqing.…”