Abstract
Field observations, fractures identified from micro-resistivity logging images from six wells, and seismic anisotropy analysis all show that six sets of vertical shear fractures are developed in the middle Yanchang Formation in the southern Ordos Basin, comprising three orthogonal fracture systems. Of these, System I fracture sets predominate, trending approximately 75° and 150°, the result of NWW-SEE directed maximum compressive stress. System II fracture sets trend approximately 60° and 120°, the result of E-W directed maximum compressive stress, while System III fracture sets trend approximately 30° and 90°, the result of NEE-SWW directed maximum compressive tress. Fractures with a strike of 75° are most common in the middle Yanchang Formation because of rock heterogeneity, while three reservoir sets are present, comprising fluvial medium-to-fine grained blocky sandstones and thin interbeds of sandstone and shale from lacustrine-delta facies.
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