21. IN SITU STRESS AT THE CÔTE D’IVOIRE-GHANA MARGINAL RIDGE FROM FMS LOGGING IN HOLE 959D1Maria Ask2 |
ABSTRACTThe orientation of the in situ stress between 550 and 925 meters below seafloor in Hole 959D has been derived from the analysis of caliper and electrical images of Formation MicroScanner data. Because of a high hole deviation (5°-13°), drill pipe wear has enlarged the hole in a northerly direction. As a result, the four-arm caliper analyses cannot distinguish between mechanically and stress induced borehole elongations. The FMS data show that one of the principal stresses is parallel to the borehole axis and that mainly tensile wall fractures have formed in the borehole wall. The tensile wall fractures demonstrate a 168° ± 8°N weighted mean orientation of the maximum horizontal stress. The upper 750 mbsf of the borehole comprises the majority of the tensile wall fractures, whereas they only occur sporadically below this depth. The change in the downhole distribution of fractures coincides with the transition from weaker to more compacted, stronger sedimentary rock. Furthermore, the integration of seismic and physical properties data suggests that this depth corresponds to the boundary between passive margin evolution and continent/ocean transform faulting. The orientation of the maximum horizontal stress in Hole 959D is almost perpendicular to the east-west maximum horizontal stress orientation of the first-order stress province in western and north central Africa. Through comparisons with (1) the orientations of the maximum horizontal stress in Hole 959D, (2) the maximum horizontal stress of three focal mechanisms from earthquakes in Ghana, and (3) the strike of the continental margin, it is suggested that lateral strength contrasts across the Côte d’Ivoire-Ghana Transform Margin are the most probable source for the observed stress orientations. It is further suggested that sediment loading and lateral variation in crustal thickness and density may only have a minor influence on the stress field in this area. |
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