Tectonic fracturing and fracture initiation in shale reservoirs—research progress and outlooks

Shale oil and gas development has gained significant progress in China with the contineous research and technological advancements in unconventional oil and gas. Extensive production practices demonstrate that natural fracture development in shale reservoirs is a crucial factor influencing oil and gas enrichment, high production and stable yield, for fractures not only improve reservoir properties but also facilitate subsequent reservoir modification during hydrollic fracturing. The formation stages of tectonic fractures and fracture initiation holds significant importance for revealing the oil and gas enrichment patterns and preservation condition. Early studies on shale reservoir fractures mainly focused on fracture classification/identification/characterization, main controlling factors of fracture development, and fracture distribution prediction and modelling, with less attention to the determination of fracture formation stages, main controlling factors of fracture initiation, mechanism of fracture opening and closing, and quantitative characterization of fracture openness—this restricts the efficient exploration and development of shale oil and gas in China. This paper highlights research progress addressing the above gaps, particularly the delineation of fracturing stages, dating of filling veins, and quantification of fracture openness based on comprehensive literature review. The classification methods for fracturing stages can be divided into two categories: qualitative geological analysis and geochemical tracing of fracture fillings. These methods however have practical limitations, where only the relative sequence of fracturing stages can be obtained, and the results are affected by the accuracy of basic geological data such as burial/thermal histories. Most of the fissure veins in fractures are carbonate minerals and quartz. With the advent of high precision in situ U-Pb microprobe dating technology, it is possible to determine the absolute ages of different veins while avoiding the problem of multiple solutions to fluid activity periods due to differences in interpreting thermal/burial histories. The initiation of tectonic fractures is controlled by many factors, not only by rocks’ intrinsic properties but also the current crustal stress and formation-fluid pressure. The petrological characteristics of fibrous fillings widely distributed in fractures record the crystal growth process. Such special crystal morphology provides the evidence of multi-stage tectonic fracturing, revealing a fracture evolutionary process with multiple fracture opening and closing. At present, fracture openness is characterized using the fracture apature parameter or inferred by the fracture initiation pressure obtained by calculation and experiment. Based on the analysis of the above results, this paper points out the key problems and development trends in the study of the fracturing stages and initiation of tectonic fractures in shale reservoirs, aiming to enrich and improve the theory and research methodology for shale oil and gas reservoirs, and provide an important scientific basis for the study of structural preservation conditions and enrichment mechanism of shale oil and gas in China. © 2024 Science Frontiers editorial department. All rights reserved.