Natural fractures in the deep Sinian carbonates of the central Sichuan Basin, China: Implications for reservoir quality

Abstract

The deeply buried carbonates in the Sinian Dengying Formation are one of the crucial hydrocarbon exploration targets in the central Sichuan Basin of China, and are characterized by strong heterogeneity in reservoir quality, which is closely related to natural fractures. To reveal the impacts of natural fractures on the reservoir quality of Deng-4 Member carbonates in the Gaoshiti-Moxi area, the types, characteristics, and factors controlling the development and effectiveness of fractures, were systematically analyzed based on core samples, thin sections, image logs, petrophysical measurements, and drill stem tests. Tectonic shear fractures are the dominant fracture type affecting the petrophysical properties of the Deng-4 Member reservoirs. The distribution and development of shear fractures are mainly controlled by lithology, mechanical layers and faults. Factors influencing the effectiveness of shear fractures include the timing of fracture formation, cementation, dissolution and current in-situ stress. Early-formed shear fractures are more susceptible to filling during subsequent cementation, whereas fractures formed at later stages are rarely filled and, also, have the potential to be solution-enlarged by dissolution. The relatively large apertures of unfilled shear fractures parallel to the orientation of SHmax are associated with the small effective normal stress exerted on fracture surfaces. Shear fractures effectively improve the overall permeability of Deng-4 Member carbonate reservoirs by connecting relatively isolated pores but have only minor impacts on reservoir porosity. The development degree and effectiveness of fractures together determine the contribution of fractures to reservoir permeability. In the Gaoshiti-Moxi area, shear fractures in the NWW-SEE and nearly E-W strikes provide the major flow pathways and contribute the most of permeability of the tight carbonate reservoirs in Deng-4 Member. These results provide insights for deeper understanding of the impact on reservoir quality of natural fractures in tight carbonates, and are significant for the evaluation of high-quality carbonate reservoirs in the Sichuan Basin.