Prediction of fracture fluids in shale oil reservoirs of southern Ordos Basin based on AVAZ dispersion characteristics

Fracture systems significantly influence the storage capacity and seepage characteristics of oil and gas reservoirs, and accurately identifying fracture fluid types is crucial for reservoir evaluation and development. To address the challenge of predicting fracture fluids in the Triassic Yanchang Formation shale oil reservoirs of the Ordos Basin, a fracture fluid prediction method based on frequency-dependent Amplitude Variation with Azimuth (AVAZ) inversion was proposed to enhance the qualitative discrimination accuracy of reservoir fluids. By integrating Chapman's multi-scale fractured rock physics model with AVAZ theory, the HTI medium reflection coefficient approximate expression was extended to the frequency domain through Taylor series expansion. The frequency-dependent attributes of AVO intercept, isotropic gradient, and anisotropic gradient were systematically derived, and the frequency dispersion characteristics from numerical simulations were analyzed to establish a fracture fluid indicator. Based on actual seismic data from the third submember of the seventh member of Yanchang Formation (Chang 73 member) in the southern Ordos Basin, spectral balancing inversion was applied to obtain frequency dispersion attributes, and fluid discrimination criteria were calibrated using well logging and production test data. Numerical simulations demonstrated that the fracture fluid indicator exhibited the most significant response to gas-filled fractures. Although the value ranges for oil and water saturation were close, subtle differences could serve as a basis for distinction. The influence of background lithology variations on the fracture fluid indicator was minimal, confirming the robustness of the method. Practical applications showed that the consistency rate between fracture fluid indicator predictions and oil-bearing reservoirs reached 71%. This study not only addresses the practical problem of distinguishing oil and water in the exploration and development of shale oil in the Ordos Basin, but also provides important references and promotion value of its technical approach and methodological system for fluid identification in other fractured reservoirs.