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AbstractHerein, integrated vertical variation characteristic analysis was conducted by measuring total organic carbon (TOC), major and trace elements in Upper Ordovician-Lower Silurian (Wufeng-Longmaxi formations) black shale in Weiyuan, Luzhou, and Changning areas, southern Sichuan Basin to clarify the control of organic matter (OM) enrichment in different sublayers. According to the cycle boundary, it was divided into member 1 (submember 1 and submember 2) and member 2 from early to late Longmaxi formation, while it was divided into 4 sublayers from early to late during submember 1 depositional period. Under the warm and humid paleoclimate, high paleoproductivity, and gradually oxic paleoredox conditions, the TOC content (0.22-3.27 wt% in Luzhou area) in Wufeng formation increased gradually from early to late under the cocontrolling of tectonic framework and stagnant sea. After the glacial period, the warm and humid climate of Wufeng formation continued in Longmaxi formation, and TOC content reached the highest in sublayer 1 (2.02-4.02 wt%, 4.78 wt%, and 6.45 wt% in Weiyuan, Luzhou, and Changning areas, respectively), due to the high paleoproductivity and high sea level caused by melting glaciers, causing the extremely anoxic environment. The OM was preserved best. It remains higher in sublayer 2 (2.39 wt%, 3.69-4.18 wt%, and 3.5-3.86 wt% in three areas, respectively) because of the anoxic environment caused by deep water and the high paleoproductivity, but the temporary hot and dry environment has a certain negative impact on the OM preservation. Under the stable paleoproductivity, sublayer 3 has a character of water decreasing slowly, and the suboxic environment led to the decrease of TOC content (3.82-5.28 wt% and 3.77-4.13 wt% in Luzhou and Changning areas, respectively). Meanwhile, that in the yuan area became deeper and the TOC content became relatively higher (1.92-3.33 wt%). Under the open environment, sublayer 4 was controlled by lower sea level caused by regression, resulting in lower TOC content (1.31-4.57 wt%, 1.64-3.52 wt%, and 3.88-4.49 wt% in three areas, respectively). However, the global regression of the submember 2 period, high terrigenous debris dilution, and oxic environment lead to the no preservation of OM. Generally speaking, the enrichment of OM is not only affected by the changes of global sea level and paleoproductivity to some extent (Longmaxi shale) but also controlled by the tectonic framework in Sichuan Basin (Wufeng shale). The findings of this study can help for a better understanding of the OM enrichment mechanism and provide a theoretical basis for the evaluation of high-quality source rocks.

AbstractIn 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.