The importance of rock characterization in North Sea reservoir description

Abstract

Reservoir sandstones may be described in terms of a range of compositional and textural parameters, providing an expanded data base for poroperm and capillary pressure studies, and wireline-log interpretation. The resultant improvement in reservoir description is likely to lead to better development planning and to increased recovery factors. Utilising fundamental geological skills, the initial core examination is carried out with an eye to significant lithological change within or between rock units. A record of structures, texture and composition is supplemented by photography. Having selected representative material with care, samples are subjected to particle-related and pore-related tests in the laboratory. Thin section and scanning electron microscopy, X-ray diffraction analysis, electron microprobe and cathodoluminescence studies together provide a clear picture of authigenic and allogenic mineral constituents, their composition, distribution and concentration. Pore properties such as porosity, permeability and pore-size distribution are measured by means of conventional core laboratory instruments. Particle properties of a sandstone reflect rock history, from source through deposition and diagenesis, and their distributions may be modelled; reservoir quality is, however, a function of pore space properties. A fundamental objective of rock characterization is the establishment of correlations between particle and pore properties to enable modelling and prediction of the latter. Wireline logs are universally available, whereas core samples are not. Consideration must therefore be given, when both are available, to the effect of observed lithological changes on log response, so that improved lithological interpretation is possible where core is absent.