Effect of lithofacies assemblages on multi-scale fractures in the transitional shale and its implications for shale gas exploration

Abstract

Shale gas exploration and development have significant potential in the Upper Permian Longtan formation. Muti-scale fracture characteristics and its controlling factors of transitional shale reservoirs within the complex lithofacies combination are still not sufficiently understood. The fracture characteristics of Longtan Formation shale were conducted by using cores, thin sections, and SEM. On this basis, the controlling factors of fractures and their impact on gas properties were analyzed using XRD analysis, TOC, N2 adsorption, breakthrough pressure experiment and two-dimensional finite element simulation. The results indicate natural fractures above the micron scale are mainly tectonic fractures. Tectonic fractures are high-angle shear, parallel slip, and near-vertical tensional fractures; while non-tectonic fractures mainly are bedding fractures; and there are numerous interlayer, intraparticle and interparticle micro-fractures at the nano scale. The density and extent of fractures in the same structural setting are primarily influenced by lithofacies and rock mechanical properties. Fractures are well developed in organic-poor calcareous and organic-containing siliceous lithofacies because of relatively more brittle minerals. However, 50% and 45%, respectively, are the threshold for the effects of brittle and clay minerals on the density of micro-fractures. The development of micro-fractures increases with these minerals under the content threshold. While beyond this threshold value, micro-fractures development are significantly constrained. The lithology assembles greatly influenced the characteristics of fractures because of variable stress barrier and rock mechanical properties. The fractures of shale interbedded with sandstone with the lower deviation are more developed and much longer than those of shale interbedded with coal and limestone. Micro-fractures can considerably contribute to the enrichment of shale gas, which will enhance the amount of gas that can be stored. However, intense construction of macro-fractures with high dip-angle leads to loss of sealing ability in the floor or roof. Our research establishes a foundation for the analysis of transitional shale fractures in the Longtan formation or other regions, which helps to reduce the risk of exploration and development.