Application of FMS images in poorly recovered coring intervals: examples from ODP Leg 129

A thick sedimentary section was penetrated at three sites in the West Central Pacific during the Ocean Driling Program (ODP) Leg 129. Although average core recovery ranged from 17.3 to 29.5%, recovery in some intervals falls as low as 0–5%. Furthermore, the recovered portion of the core is conventionally assigned to the topmost part of the cored interval, although this is now known to be incorrect in most instances in consolidated parts of the section. These drawbacks have led to incomplete and even erroneous sedimentological interpretations. High-resolution electrical (Formation MicroScanner) images of the borehole obtained by scanning the borehole wall with arrays of small electrodes pressed against the borehole surface can provide:
(1) detailed sedimentary structure and texture of missing sections of each core, through calibration with visual data obtained from recovered cores;
(2) the original sedimentary features, where these have been disturbed or brecciated during drilling, as well as drilling artifacts on the borehole wall;
(3) recognition of sedimentary facies (e.g. slump units in hole 801B) in non-recovered intervals;
(4) correct location of recovered portions within the cored interval, through matching of specific sedimentary features.

Examples of these applications of the FMS images to the poorly recovered ODP Leg 129 cores are presented. The use of FMS logging has widespread application in any borehole where conventional cores have not been taken or where core recovery is limited. However, log calibration with core samples is essential.