Abstract
Natural fractures are effective reservoir spaces and the main seepage channel for tight sandstones, which control the migration, enrichment, and productivity of oil and natural gas. This paper systematically reviews the formation and distribution of natural fractures in the deep tight sandstone reservoirs of China over the past three decades. Based on geological origins, multiscale characteristics, effective evolution, and primary controllers of natural fractures, a quantitative method of natural fracture prediction was proposed. Four origins of natural fractures were identified in tight sandstone reservoirs, namely tectonic, diagenetic, tectonic-diagenetic, and overpressure origins. According to the scale and control boundary, natural fractures can be divided into four scales: large scale, mesoscale, small scale, and microscale. The multiscale natural fractures are influenced by paleotectonic stress, differences of rock mechanical property, and thickness. Controlling factors for the growth and evolution of natural fractures in tight sandstones are summarized as internal factors, dynamic factors, heterogeneity, and evolution factors, the controlling mechanisms of which on formations and distributions of natural fractures are analyzed. The effectiveness of natural fractures and the corresponding evolution are governed mainly by the formation period of natural fractures, tectonic-diagenetic coupling, abnormal high-pressure fluid, and in situ stress state. Based on the analyses on the above-mentioned fractures, a set of quantitative methods are proposed to predict the orientation and intensity of natural fractures in tight sandstones. First, the method simulates the paleotectonic stress field during the formation of natural fractures. Second, the actual rock fracture model based on the transversal anisotropy of rock mechanical properties should be established. Third, constraint from the main controlling factors for the formation and distribution of natural fractures, the directions, and their intensities of natural fractures are predicted.
