Elliptic borehole breakouts are usually used to determine the orientation of in situ stress in deep sedimentary basins. The long axes of borehole breakouts are generally perpendicular to the maximum horizontal principal compression stress (SHmax). However, the azimuth of borehole breakouts is found perpendicular to the chief strike (but not to SHmax) of natural fractures in tight reservoirs, Anpeng field of Nanxiang Basin, China. Based on the core data and acoustic and resistivity borehole image logs, the natural fractures are intensively striking at east-west orientation where borehole breakouts occurred in north-south. If the borehole breakouts are induced only by in situ stresses surrounding the well and the borehole breakouts are known at north-south, SHmax should be perpendicular at east-west, but according to analyses of the earthquake focal mechanism in circumjacent regions, hydraulic fracturing data, drilling-induced fracture data, and the production performance data, SHmax is in the northeast-southwest direction. This contradiction indicates that the influence of natural fractures may cause a serious deviation of the azimuth of borehole breakouts. Therefore, in this case, it is unreliable to determine the orientation of in situ stress only by borehole breakouts in fractured tight reservoirs. In addition, the main fluid flow direction is not parallel to the dominant natural fracture, which is controlled by in situ stress in tight reservoirs.

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