Variation in the orientation of the maximum horizontal stress in thick channel-fill sandstones with low-permeability: A case of the Bonan Oilfield in the Bohai Bay Basin, eastern China

Abstract Measurements of in situ stress in a single well from borehole breakouts, hydraulic fracturing, multipole acoustic logs and differential strain analysis of core indicate that the orientation of the maximum horizontal stress is northwest-southeast in thick channel-fill sandstones in the central-western Bonan Oilfield, while the orientation is northeast-southwest in the thin interbed siltstones and mudstones of the floodplain in the eastern Bonan Oilfield. This result shows that the orientation of the maximum horizontal stress in the thin interbed of siltstones and mudstones is in accordance with the regional stress orientation obtained by the focal mechanism of a natural earthquake, but has a 30°–40° variation with the thick channel-fill sandstones. Finite element simulation suggests that the main reason for the variation in the orientation of the maximum horizontal stress is the diversity of overall mechanical properties, especially modulus of elasticity, of the thick channel-fill sandstones and the interbedded thin layer of siltstones and mudstones. The average Young's modulus over the stratigraphic column governs stress rotations. The transformation of the orientation of the maximum horizontal stress from the central-western to the eastern locations is at the channel boundary. This research provides a case in which the variation in the orientation of the maximum horizontal stress is affected by thick channel-fill sandstones. It shows that lithological changes can cause variations in the direction of in-situ stress.