Chalk exploration, the search for a subtle trap

Abstract

Over 2 × 109STB of oil and 8.9 × 1012SCF of gas have been produced from the chalk fields of the Greater Ekofisk area, more than 50% of that being produced from the giant Ekofisk Field. Phillips Petroleum Company Norway and the PL018 licence co-venturers have undertaken a comprehensive and integrated re-evaluation of the chalk exploration potential in Blocks 2/4 and 2/7 of the Norwegian Continental Shelf. Recent exploration models indicated the potential for significant untested reserves to exist in the chalk of Blocks 2/4 and 2/7. The Chalk Exploration Project (CEP) team was given the objective ‘to define the risked value of the remaining chalk exploration potential in licence PL018’. All major structures and significant amplitude anomalies at chalk level had been drilled by 1986, leaving only the subtle field flank and stratigraphic plays for evaluation. Although many studies had been completed in the chalk over the years since discovery of Ekofisk Field in 1969, most of these studies had been focused on a single field and no regionally consistent stratigraphic framework or depositional model had been developed. As part of this project, a robust regional model for chalk prospectivity was developed covering the whole chalk basin, based on a sequence stratigraphic analysis of regional seismic and well databases, together with petrophysical, hydrodynamic, geochemical and other key technical studies. Acquisition of 3D seismic data over the chalk fields since the mid 1980s demonstrated the power of 3D seismic analysis to define stratigraphic detail within the chalk, and by 1993 the entire licensed area of Blocks 2/4 and 2/7 was covered by 3D data, giving the opportunity to undertake a semi-regional, sequence stratigraphic 3D analysis of the chalk reservoir in PL018, and determine the remaining exploration potential.