Natural fractures in deep to ultra-deep tight reservoirs: Distribution and development

Natural fractures serve as effective storage spaces and primary seepage pathways in deep to ultra-deep tight reservoirs, affecting the hydrocarbon migration and enrichment, single-well productivity, and exploitation methods and outcomes of the reservoirs. Based on the summary of latest research results and literature review on fractures in tight reservoirs, this study delves into the distribution characteristics and developmental patterns of natural fractures in deep to ultra-deep tight reservoirs. The results show that the natural fractures are of large, meso, small, and micro scales, following a power law distribution. In other words, a larger scale corresponds to a smaller number of fractures, and vice versa. Large- and meso-scale fractures primarily facilitate seepage; small-scale ones mainly enable seepage and storage; and micro-scale ones principally serve as storage spaces. The type, occurrence, and mechanical properties of the natural fractures formed across different periods are determined by the evolution of stress regime during stratigraphic burial. The formation, distribution, and developmental degree of multi-scale fractures are subjected to the magnitude of tectonic stress, the mechanical properties of rock mechanical stratigraphy, and the thickness differences in mechanical layers. Structural deformation results in varied local stress and strain distribution at different structural locations, increasing fracture heterogeneity. Thrust faults control the distribution of faulted fracture zones by controlling the deformation of strata on the hanging walls. The combination style and movement mode of strike-slip faults, along with rock mechanical stratigraphy, jointly dictate the three-dimensional spatial distribution of related fractures. Furthermore, the crack-seal patterns of the fractures during formation and evolution determine their storage spaces and record the evolutionary history of their effectiveness.