Heavy mineral stratigraphic analysis on the Clair Field, UK, west of Shetlands

Abstract

Hydrocarbons in the Clair Field, west of the Shetland Islands, are hosted by Devonian–Carboniferous clastic red beds deposited in a non-marine fluviolacustrine setting. The succession is almost entirely biostratigraphically barren and, hence, alternative approaches to reservoir correlation are required. Heavy mineral analysis (HMA), which subdivides clastic successions on the basis of changes in provenance and sediment transport history, has proven successful in establishing a high-resolution correlation framework for the Clair Field. Since the technique offers a reliable and rapid method for monitoring the stratigraphy of the Clair reservoir succession, HMA has been undertaken on a real-time basis at well site for virtually all development wells during Phase 1 of the Clair Field development, and for all Phase 2 appraisal wells. Heavy mineral data can be acquired in less than 2 hours from receipt of sample. Consequently, owing to the relatively slow penetration rates frequently associated with Clair drilling, stratigraphic information is usually acquired ahead of logging while drilling. Heavy mineral data are used in the decision-making process in a variety of situations, including picking of casing and coring points, whether to maintain or alter well trajectory, and when to terminate drilling. In the Clair Field, formation tops can be subtle and, since HMA can establish trends and predict formation changes before they are encountered, they are critical in aiding geosteering decisions. HMA has also been used to monitor stratigraphy and to pick formation tops when logging tools have failed, allowing drilling to continue and avoiding tripping to change the bottom-hole assembly. The application of HMA to the Clair Field development is illustrated by reference to a number of wells drilled on the field since 2005.