Analysis and Interpretation of the Borehole Televiewer Log: Information on the State of Stress and the Lithostratigraphy at Hole 504B

Abstract

Hole 504B in the eastern equatorial Pacific has been the focus of several scientific expeditions during the past few years, where a series of re-entry, drilling, and coring operations has yielded important data regarding the structure of the upper kilometer of the oceanic lithosphere. As part of the extensive downhole experiments program planned for ODP Leg 111 at this site, a borehole televiewer (BHTV) log was obtained across a 355-m vertical section of crustal base ment, extending upward from 1531 to 1176 m below the seafloor. The BHTV record was analyzed in terms of both acoustic amplitude and traveltime in order to accurately identify zones of structural failure in the surrounding basement rock. A subsequent examination of breakout frequency vs. azimuth helped identify the orientation of in-situ horizontal principal stresses according to the well-substantiated hypothesis that breakouts occur coincident with the direction of minimum principal stress (S h ) where compressive forces are at their greatest. This S h orientation is found to be NI22.5°E, a direction that agrees closely with the results of a previous BHTV study of breakouts in the upper section of this same borehole and also with stress orientation estimates inferred from regional intraplate earthquakes. The distri bution of breakout frequency vs. azimuth shows the data to be bimodal and orthogonal to a degree that is statistically significant. The S h direction is inferred from the primary mode. The orthogonal pattern indicates that a considerable number of wellbore enlargements were detected coincident with the direction of maximum principal stress (S H ). These are interpreted as extensional fractures produced by localized tensional stresses parallel with the S H orientation. Consid eration of various hydraulic fracturing schemes and attendant analyses indicates that these fractures are likely to be in duced when the ratio of effective horizontal principal stresses {S H /S h ) is very large in a cylindrical wellbore or is some what smaller in a hole exhibiting a slight degree of ellipticity. This hypothesis is supported by the solutions to source mechanism studies associated with local earthquakes that indicate a strike-slip sense of faulting and a relative order of stress magnitude where S H » S h . The bimodal, orthogonal distribution of wellbore enlargements also results in a slight correction to the method used for estimating the principal stress orientation, one based upon the population modes rather than the mean.Inspection of three-dimensional images generated from the BHTV record in the lower portion of the borehole re veals the surprising appearance of pillow lavas at a depth previously assumed to be composed of sheeted dikes. When the ophiolite analogy is applied to contemporary oceanic crust, the transitional unit between the upper pillow lavas and minor flows and the variably metamorphosed diabase dikes below seems to be a complex, gradual, and gradational interlayering of lithologies that extends over a thicker vertical inter...

Borehole Televiewer Data Analysis from the New Hebrides Island Arc: The State of Stress at Holes 829A and 831B

“…Reduction of the stress concentration occurs at θ = 0° and 270, with respect to S Hmax . Therefore, under elliptical borehole conditions it is more likely that stress induced tensile fractures (θ = 0°, 180°) or shear failure (θ = 90°, 270°) will occur (Morin et al, 1989). However, neither tensile fractures nor breakouts were observed in the data of Hole 829A and Hole 83 IB from Leg 134, indicating that additional processes must be assumed, which reduced the applied stress.…”

Constraints on the Geometry and Fracturing of Hole 894G, Hess Deep, from Formation MicroScanner Logging Data

“…Second, the orientation of breakouts remains consistent between different formations within a well and between wells on a basin scale. Third, the Shmin direction deduced from breakouts coincides with that deduced from other stress indicators such as hydrofracturation directions or large-scale geological features (Bell and Gough, 1979;Bell, 1981, 1982;Fordjor et al, 1983;Zoback et al, 1985;Hickman et al, 1985;Plumb and Hickman, 1985;Morin et al, 1989Morin et al, ,1990. As a consequence, the term breakout now tends to indicate stress concentration-induced shear failure (Newmark et al, 1984;Zoback et al, 1985).…”

“…An older tool, the BoreHole Tele Viewer (Zemanek et al, 1970a;Zemanek et al, 1970b), turned out to be more suited to these studies (Newmark et al, 1984;Newmark et al, 1985b;Zoback et al, 1985;Morin et al, 1989) because it provides higher resolution measurement of the borehole shape and was used to confirm elongations inferred from caliper data (Plumb and Hickman, 1985).…”

Analysis of stress‐induced oval fractures in a borehole at Deep Sea Drilling Project Site 504, eastern equatorial Pacific

“…0148-0227/99/1998JB900086 $ 09.00 cording to Barton [1988] Oval-shaped features seemingly etched into the wall of this borehole were detected in the BHTV log obtained from ODP Leg 111 [Morin et al, 1989]. Approximately 12 completely formed ovals were clearly identified, all occurring across a relatively narrow depth range of-400 m (1350 mbsf _+ 200 m).…”

“…Approximately 12 completely formed ovals were clearly identified, all occurring across a relatively narrow depth range of-400 m (1350 mbsf _+ 200 m). Morin et al [1989] initially postulated that these unexpected forms were perhaps pillow basalts haphazardly intersected by the well. However, no supporting evidence could be found for the existence of pillow basalts at these depths, and upon further examination it was discovered that all of the ovals had roughly the same size and azimuthal orientation, a condition unlikely to be achieved by the penetration of a drill bit through a random series of pillow basalts.…”