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Integration of Surface Seismic, Microseismic, and Production Logs to Characterize Lacustrine Shale Plays in Upper Triassic Yanchang Formation, Ordos Basin, China

Abstract

A huge volume of organic-rich, deep and semi-deep lacustrine shale was developed in Upper Triassic Yanchang Formation, Ordos Basin, China. Drillings in the shale have discovered high hydrocarbon production potential after hydraulic fracturing. Due to strong heterogeneity of the lacustrine shale, the production varies significantly even among multilateral horizontal wells on the same platform. To characterize the shale reservoir and understand hydraulic fracturing performance, a high-density, wide-azimuth seismic survey was acquired and processed. We adopted the simultaneous prestack seismic inversion to calculate Young's modulus and Poisson's ratio, the crossplot of which was successfully employed to quantitatively grade shale rocks. A frackability discriminator was defined according to Young's modulus and Poisson's ratio. Specifically, the prestack seismic gathers were reorganized into six azimuthal stacks to investigate azimuthal anisotropy. A 90°-shift attribute was calculated independently for each azimuthal stack to exploit the variations inside reservoir rather than interfaces. Azimuthal variations in the attribute were then quantified through ellipse fitting to predict anisotropy azimuth and anisotropy intensity. We validated these results using production logs recorded in three horizontal wells and microseismic data acquired while fracturing these wells. The correlation of the magnitude of microseismic events and frackability index revealed that more large magnitude events occurred in more frackable zones. And the regions of moderate anisotropy intensity tend to form complex fracturing networks, which contribute to more successive and stable production. In summary, the integration of seismic data, microseismic data, and production logs could improve shale reservoir characterization with a diverse suite of measurements to optimize horizontal well locations and hydraulic fracture treatments.