State of lithospheric stress and borehole stability at Deep Sea Drilling Project Site 504B, eastern equatorial Pacific

Hole 504B in the eastern equatorial Pacific is the deepest hole to penetrate oceanic basement, extending more than 1500 m beneath the seafloor. Two borehole televiewer (BHTV) logs were obtained in this hole, one through the upper 1000 m of basement during Deep Sea Drilling Project leg 83 and the other extending an additional 260 m deeper into the crust during Ocean Drilling Program leg 111. The two BHTV records have been combined and processed in terms of both acoustic amplitude and travel time in order to evaluate the extent and distribution of rock failure along the borehole wall. A histogram of borehole enlargements versus azimuth depicts a dominant breakout azimuth of N122.5°E which corresponds to the direction of minimum principal stress Sh and correlates well with stress orientations inferred from solutions to regional earthquake focal mechanisms. Furthermore, the bimodality of this histogram, with a secondary mode orthogonal to Sh, indicates that a significant number of enlargements are coalesced tensile fractures occurring along the orientation of SH, the maximum principal stress. The appearance of this orthogonal, bimodal distribution suggests that the regional horizontal stress field is highly anisotropic, a condition supported by seismic data which show all of the earthquakes in the area to be strike-slip. The frequency of borehole enlargements increases with increasing depth and depicts a systematic structural deterioration of the well bore. The tensile fractures along the SH azimuth contribute to this degradation and appear to be induced by thermal stresses due to the injection of cold water into hot rock. The frequency of these extensional features does not increase with depth. Rather, their appearances can be directly correlated with shipboard efforts at deliberately cooling the well and/or with the sudden resumption of drilling after the hole had been allowed to reequilibrate thermally for several days. These latter borehole enlargements are more pronounced then those commonly associated with hydraulic fracturing. The mechanisms for fracture initiation and growth, based upon temperature contrasts between the well bore fluid and the adjacent rock may enhance rock failure.

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