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Mechanical Stratigraphy Controls on Vertical Fracture Patterns in the Upper Triassic Tight Oil Sandstones, Ordos Basin, China


Mechanical stratigraphy subdivides stratified rock into discrete mechanical units and mechanical interfaces by rock properties such as elastic stiffness and tensile strength. Vertical opening-mode fractures are widely developed in the relatively undeformed Upper Triassic tight oil sandstones of the Ordos Basin, China. Vertical opening-mode fractures developed within mechanical units and terminate against mechanical interfaces. The determination of mechanic interfaces and the reasonable division of the mechanical units play an important role in the deeply understanding of vertical fracture patterns.

The outcrop of the Upper Triassic Yanchang Formation, with 10 km length and 20 m height, is well exposed along the Yanhe river, and is used to evaluate the tie among fractures, mechanical stratigraphy and sedimentary stratigraphy. According to the outcrops and Schmidt Hammer rebound values collected in each bed, a proxy for unconfined compressive strength, the mechanical interfaces are mainly divided into 8 types: bedding interface, corresponding to the second interface defined by Miall, 1988; depositional discontinuity interface, rhythmically lithologic change interface, mudstone interbed interface and calcium interbed interface, corresponding to the third interface defined by Miall, 1988; calcium barrier layer, corresponding to the third or fourth interface defined by Miall, 1988; mudstone barrier layer and depositional scour surface, corresponding to the fourth or fifth interface defined by Miall,1988. According to the statistics of the ratio of the number of fracture terminations and the sum of the number of fracture terminations and crossings in the interfaces, the interface that terminates more than 20% of fractures can act as the mechanical interface. The mechanical units are constrained by mechanic interfaces.

According to the mechanical interfaces, which constrain fracture propagation, and fracture sizes, vertical opening-mode fractures within reservoirs are divided into micro-scale, small-scale, meso-scale and macro-scale ones. The apertures and lengths of fractures at different scales are present power law distributions. The average spacing of fractures presents a positively linear correlation to the thickness of mechanical stratigraphy. Study on the types of mechanical interface and the characteristics of mechanical stratigraphy can guide fracture pattern prediction thus providing a geological basis for tight oil exploration and efficient development.