Thin-Bed Reservoir Analysis From Borehole Electrical Images

Abstract

Abstract In very thinly bedded sand-shale sequences, sand counts and sand/total ratios can be determined from high resolution electrical images of the borehole wall. This technique is illustrated here by a case study in turbidites with sand laminae down to 1 cm in thickness. The numerical modeling of the tool response and a comparison with full bore core data help assess the accuracy of bed thicknesses derived from electrical images for this type of formations. Introduction The Formation MicroScanner* tool records high-resolution electrical images of the rocks. This is achieved by using arrays of electrodes which are pressed against the borehole wall. The first generation tools have the 8 dipmeter buttons of a Stratigraphic High-resolution Dipmeter Tool (SHDT*), plus 27 imaging buttons on two of the four pads, at a right plus 27 imaging buttons on two of the four pads, at a right angle to each other. The second generation tools have 16 imaging buttons mounted on all four pads. Displaying core photographs next to electrical borehole images usually shows that the bedding characteristics are similar in both types of images and that the electrical image is coarser, even though the data are sampled at very high rates (10 samples per inch, both vertically and azimuthally). The purpose of this study is to investigate the suitability of electrical images for thin bed studies both by comparison to actual fullbore cores arid by numerical modeling of the tool response. Once the performance of the Formation Micro-Scanner tool in front of thin beds has been determined, the goal is to get a computer-generated description from electrical imaging data which is similar to what could be obtained from cores. THIN BED CHARACTERIZATION IN ITALY Gas accumulations found in the thinly bedded Pliocene formations in the Eastern part of Italy have proven to be of a great economical interest. The example considered in this paper corresponds to the filling by turbiditic deposits of a foredeep basin created by the subduction of the Adriatic plate below the Apennine chain. These sediments rest unconformably upon an evaporite/ carbonate basement. In Fig. 1, the core and logging data available are displayed in a compressed scale for the lower part of the case study well. The reservoir consists in sand, silt and shale layers organized in thinning and fining upward sequences. Sand beds can be locally highly cemented by calcitic material, that can completely kill the porosity. The problem of interpretation in such a reservoir is twofold:detect the individual laminations down to a thickness of 1 centimeter,discriminate the different lithological and granulometric types: cemented sand, porous sand, silt, shale. The difficulties stem from the fact that the thicknesses of these laminations are below the resolution of the standard logging tools. In the past years the use of the SHDT tool and of the Electromagnetic Propagation Tool (EPT*) allowed to solve partially the problem. Five centimeter thick layers were adequately described arid simply classified into sand or shale. The continuity of the bedding could not be assessed, and lenses might have been misinterpreted as continuous beds. P. 61