Characteristics of natural fractures in carbonate reservoirs and their impacts on well productivity in the Sinian Dengying Formation, central Sichuan Basin

Carbonate reservoirs in the Sinian Dengying Formation, central Sichuan Basin, are a hot spot for hydrocarbon exploration and development in deep and ultra-deep sequences in China. Multi-type and multi-scale natural fractures are widely seen in the Dengying reservoirs, and of significant impacts on the seepage flow pattern and well productivity. In this study, the genetic types and developmental characteristics of natural fractures in the fourth member of Dengying Formation (Deng 4 Member), Gaoshiti-Moxi area, are clarified utilizing the cores, thin sections, image logs, experimental tests and production testing data. Major geological factors controlling the development of natural fractures are used to analyze the impact of natural fractures on well productivity. Results show that natural fractures in the Deng 4 Member reservoirs in Gaoshiti-Moxi area are mainly of tectonic and diagenetic types. The tectonic fractures can be divided into shear fractures and tension fractures, and the diagenetic fractures are mainly bedding fractures and stylolites, among others. The shear fractures dominate the Deng 4 Member reservoirs, and are mainly of high-angle ones and striking in the NNW-SSE, nearly EW, NE-SW and nearly SN directions. Factors controlling the shear fracture effectiveness include the timing of fracture formation, cementation, dissolution, fracture occurrence and current in-situ stress. The shear fractures in NNW-SSE and nearly EW orientation are more effective than others. The development degree of shear fractures is closely related to the lithology, mechanical layer thickness and faults. The shear fractures are more highly developed in micritic dolomites, and their density decreases and scale enlarges along with the increasing thickness of mechanical layers. The shear fractures near the main strike-slip faults, in particular within the tip and superimposed part of faults, are better developed. The development degree and effectiveness of fractures jointly determine gas well productivity. The effective fractures in different occurrences and scales can form fracture network, and get relatively isolated pores interconnected, which greatly improves the seepage capacity of reservoirs and elevate well productivity. In the Deng 4 Member reservoirs of Gaoshiti-Moxi area, the shear fractures in NNW-SSE and nearly EW orientation are of stronger ability to enhance reservoir permeability, and make greater contribution to the improvement of well productivity.