Deciphering genetic phases of natural fractures in low-permeability reservoirs through core-scale acoustic emission chrono stratigraphy

In hydrocarbon exploration, natural fractures influence gas migration and reservoir storage. Identifying these fractures is key to improving recovery rates and efficiency. Traditional assessments often overlook microstructural features like sub-resolution fissures. These features, which influence petromechanical properties, are closely associated with geohazards. This study examines the natural fracture systems within tight sandstone reservoirs in the Ordos Basin and proposes an innovative integrated evaluation framework consisting of three components: core characterization, acoustic emission chronostratigraphic monitoring (A technique that utilizes acoustic emission (AE) signals generated by stress-induced microfractures in rocks to deduce their formation periods and stress histories), and structural stress field inversion. By integrating multi-scale data spanning macroscopic to microscopic domains, this study systematically elucidates the acoustic emission response mechanisms of diverse lithologies under stress loading. A pioneering revelation of spatiotemporal differentiation patterns (Refers to the variations in the timing and spatial distribution of fracture nucleation and propagation processes) governing fracture nucleation in low-permeability reservoirs was achieved. The findings demonstrate that thermo-hydro-mechanical coupling effects and fluid percolation fields exert play a dominant role in controlling fracture propagation. These insights culminate in a quantitative characterization model of fracture evolution based on acoustic emission. Experimental analyses identify two to three principal tectonic fracturing episodes within the Chang 4 + 5 and Chang 6 reservoir units. The Yanshanian Movement Episode II-III (Jurassic-Cretaceous transition) notably produced three distinct fracture sets: NE-trending (35°±8°) compressional-shear fractures, NW-oriented (125°±10°) extensional fractures, and conjugated shear systems aligned along N-S and E-W orientations. Himalayan-phase tectonic overprint induced a 28 % reactivation of Yanshanian fractures, manifested in the partial closure of NW-SE extensional fractures and selective opening of NE-SW compressional systems. Surpassing the limitations of conventional macro-parameter rock mechanics evaluation, this research proposes an innovative quantitative methodology for engineering-geological effects of micron-scale latent fractures that are not easily observable but significantly influence the petromechanical properties of rocks. A permeability prediction model incorporating multi-phase structural modifications of fracture networks is established, which provides a theoretical framework for three-dimensional well pattern configuration and volumetric fracturing optimization in unconventional reservoirs.