Strength and deformation properties of clays are known to correlate with a standard geotechnical index parameter, the Plasticity Index, IP, where these correlations are commonly used for characterising top soil or in the case of the offshore oil industry seabed analysis for foundations of offshore structures. By their nature, these studies are generally limited to the top 100+ m of the overburden. From a theoretical standpoint there is nothing to hinder the extension of such methods to greater depths other than the effects of diagenesis. This hypothesis is tested using drill cuttings from two North Sea fields of Tertiary age to characterise the overburden sequence. The results of the study are encouraging when compared with more conventional techniques using borehole log correlations suggesting that the plasticity index may offer a complimentary method for charactering overburden sequences for the purposes of borehole stability and reservoir compaction-seafloor subsidence.
1. INTRODUCTION
Direct determination of strength and deformation properties of overburden clay and shale by standard rock mechanics testing are time consuming. Furthermore, rock mechanics testing implies that the relevant formations are cored; unfortunately the coring process is expensive, often difficult to perform and may jeopardize the drilling operation. Nonetheless, knowledge of the strength and deformation of the overburden may be vital to the field development, e.g. well planning and wellbore stability, reservoir compaction and seafloor subsidence.
Drill cuttings are generally not used as a source for rock property evaluation partly because most laboratory techniques require pieces of core material larger than 5 mm which are of good to high quality and partly because, in many cases, drill cuttings are not preserved for further analysis once a lithological description has been completed. In those cases where drill cuttings are used the strength and stiffness properties are generally determined indirectly either from indentation or resonant acoustic wave techniques [1].
Strength and deformation properties of clays are known to correlate with a standard geotechnical index parameter, the Plasticity Index, IP, where these correlations are commonly used for characterising top soil or in the case of the offshore oil industry seabed analysis for foundations of offshore structures. By their nature, these studies are generally limited to the top 100+ m of the overburden. To date, there appears to be no documentation on the use of such correlations for evaluating the properties of 1000+ of overburden. Moreover, from a theoretical standpoint there is nothing to hinder the extension of such methods to greater depths other than the effects of diagenesis. In order to test this hypothesis, drill cuttings from two North Sea fields of relatively young geological age (lower Paleocene & younger) were used in a study to characterise the overburden sequence. The results of the study were compared with more conventional techniques using borehole log correlations.
2. GEOLOGICAL SETTING
Field 'A' is a deep water gas field situated 120 km west of the Norwegian coast (Kristiansund) and Field 'B', an offshore oil field, is situated 185 km west of Stavanger.
