A complete set of downhole measurements along with Formation MicroScanner (FMS) electrical images of the borehole surface was recorded in the basement of Hole 896A during Ocean Drilling Program Leg 148. This complete geophysical data set offers, largely because of the quality of the recorded data, the first opportunity to integrate core results, meter-scale logs, and centimeter-scale borehole wall images in a structural sense for oceanic basement. At an even larger scale, extremely low resistivity values and consistently high acoustic velocity were measured in the hole. These values indicate that the basement has been altered to a large extent at this topographic high and heat-flow maximum, but also that cracks and fractures once conveying fluid to alter fresh basalts are now sealed.
Nearly 7700 features were mapped over 225 m of images (34.3 per meter on average) and described in terms of geometry and aperture. The penetrated basement section consequently appears to be intensively fractured. In the laboratory, Archie’s formula (1942) is found to provide a reasonable first-order estimate of porosity for this severely fractured basement section. Once applied to downhole measurements at meter-scale, this relationship yields a continuous porosity profile through the penetrated section. The combination of meter-scale (with the Dual LateroLog) and centimeter-scale (with FMS images) investigations of the rock structure outlines the presence of four narrow and subvertical fracture zones in the analyzed interval.
At 356 mbsf, one of these narrow intervals appears to separate two different fracture sets, as well as stress regimes in terms of borehole shape and rupture mode. This might indicate the presence of a presently active fault zone at this depth, which divides the upper basement and may explain the surprisingly high permeability values inferred deeper in the hole from packer experiments. Borehole elongations are very small throughout, but coherent in azimuth and distribution with those obtained only 1 km away at Site 504. In the near future, a detailed description of upper basement structures both in terms of geometry and volume should arise from the combined analysis of standard downhole measurements and fracture sets obtained from FMS image analysis.
Date of initial receipt: 22 August 1994
Date of acceptance: 22 February 1995
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