Downhole Measurements and Electrical Images in Hole 896A, Costa Rica Rift

Abstract

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.

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

“…The deep and shallow laterologs differ not only in depth of penetration but also in overall current geometry: the shallow measurement path is predominantly vertical and the deep path is more horizontal. Pezard and Anderson [1989] and later Pezard et al [1996] used this distinction to identify basalt intervals at Hole 504B with dominantly horizontal ( R deep < R shallow ) or vertical ( R deep > R shallow ) fractures, and a similar approach has been used at nearby Hole 896A [ de Larouzière et al , 1996]. Comparison of the deep and shallow laterologs for Hole 801C tholeiites shows generally very close agreement [ Shipboard Scientific Party , 2000a].…”

“…Velocity/formation factor relationships for oceanic basalts have not been analyzed previously, but some porosity/formation factor log analyses have used sonic‐based porosities because density‐based porosities were poor or unavailable [ Jarrard and Broglia , 1991; de Larouzière et al , 1996]. At log scale, the velocity/formation factor relation may depend on alteration intensity [ de Larouzière et al , 1996]. For Hole 801C core plugs, however, the least and most altered samples lie along the same velocity/formation factor trend.…”

“…Consequently, Broglia and Moos [1988] found that both the potassium log and total gamma ray log at Hole 418A were highly correlated with qualitative ratings of core alteration, and both indicate variation in content of potassium‐rich alteration minerals at that site. A relationship between gamma ray and either velocity, resistivity, or fracture intensity logs has been observed at Holes 395A [ Moos , 1990], 765D [ Shipboard Scientific Party , 1990a], and 896A [ de Larouzière et al , 1996], but Sites 768 and 770 exhibit only a subtle association between alteration indicators and potassium [ Jarrard and Broglia , 1991]. …”

Architecture of the volcanic layer from the Costa Rica Rift, constraints from core‐log integration

“…The borehole is also relatively cylindrical below 356.0 mbsf with the upper parts of the hole being considerably more elliptical. de Larouziere et al [1996] suggest that this represents a possible change in stress field conditions and, combined with the low resistivities and maximum intensity of fractures, implies the presence of a presently active fault zone at 356.0 mbsf in Hole 896A. This may account for the presence of large thicknesses of brecciated material at approximately this depth and would be consistent with a tectonic origin for the polymict matrix‐supported breccias and jigsaw puzzle breccias.…”

“…This fault is believed to be dipping >70° to the south and hence may be expected to intersect Hole 896A at some point. de Larouziere et al [1996] note that the lowest resistivities in Hole 896A occur at ∼356.0 mbsf, which is closely associated with a maximum in intensity of fractures. The borehole is also relatively cylindrical below 356.0 mbsf with the upper parts of the hole being considerably more elliptical.…”