Fracture development in Paleozoic shale of Chongqing area (South China). Part one: Fracture characteristics and comparative analysis of main controlling factors

摘要

Natural macro- and micro-fractures in the Longmaxi shale (Lower Silurian) from Yuye-1 well and the Niutitang shale (Lower Cambrian) from Yuke-1 well are characterized. Regional geological structural evolution and shale geochemistry (including TOC and mineral composition, etc.) are analyzed. The controlling factors of natural fractures in Palaeozoic shale in the southeast of Chongqing have been confirmed, which have been analyzed qualitatively or semi-quantitatively. The results show that fractures are abundant, in both the Longmaxi Shale and Niutitang shale, due to the multiphase strong tectonic movement in the southeast of Chongqing. Therefore, tectonic factors are the principal factors that control fracture development in the Paleozoic shale of this region, followed by the mineral composition of the shale. Vertically, the mineral composition of shale in Yuke-1 well has a high dispersion degree and a strong heterogeneity (the dispersion coefficients of quartz and dolomite were 0.48 and 1, respectively), therefore, the development and distribution of fractures are controlled by quartz and dolomite content in the Niutitang shale, moreover, the quartz content shows positive correlation with the fractures quantity, and fractures are most abundant where dolomite content up to 64.7%. While the mineral composition of shale in Yuye-1 well has a low dispersion degree and a strong homogeneity in a vertical position (the dispersion coefficients of quartz and plagioclase were 0.15 and 0.26, respectively), which shows unconspicuous control effect for fractures, therefore, the development and distribution of fractures are mainly controlled by the TOC content, moreover, the TOC content shows positive correlation with the fractures quantity. Fractures are sealed by calcite, quartz and pyrite. Although sealed fractures do not contribute to the permeability of the reservoir, they are important planes of weakness that tend to be reactivated by hydraulically induced fractures. SEM images show that intergranular pore is connected by microfractures, thereby, the validity of porosity has been improved. The mixed network of microfractures and micropore provide enhanced permeability or storage capacity for the reservoir.