Chalk exploration, the search for a subtle trap

Abstract

Over 2 x 10 9 STB of oil and 8.9 x 10 12 SCF 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. This paper describes the results of the 'Chalk Exploration Project' (CEP) a study performed by Phillips Petroleum Company Norway together with their Norwegian PL018 coventurers and concludes by denning the seven key risk factors for evaluating subtle chalk stratigraphic prospects and leads.In order to leverage the extensive chalk expertise available within the licence group, a team of geoscientists was assembled from PL018 co-venturer companies in early 1995. The study area was centred on the Ekofisk Field and included the whole of PL018 (Fig. 1). The project had three main phases; database compilation, development of a regional exploration model and lead/prospect analysis and risking. The development of a regionally standardized well and seismic database was a significant challenge, but this provided the foundation for the project and where possible all well data were captured in digital format. The well database was extensive, comprising exploration and development data from over 500 wells. The main components of the database were: (a) a regional, phase-corrected, 2D seismic grid and a semi-regional 3000 km 2 merged 3D seismic survey; (b) a regional velocity database; (c) a regional biostratigraphic...

Characterization and zonation of a marly chalk reservoir: the Lower Cretaceous Valdemar Field of the Danish Central Graben

“…The reservoir properties of the Lower Cretaceous chalks differ considerably from those reported from the better known Upper Cretaceous-Danian chalks in the Central Graben area and onshore Denmark (Jørgensen 1992;Bramwell et al 1999;Frykman 2001). The purest Lower Cretaceous chalks have an IR content of 6% and the bulk volume of the reservoir consists of marly chalks with IR content up to 30%.…”

“…The purest Lower Cretaceous chalks have an IR content of 6% and the bulk volume of the reservoir consists of marly chalks with IR content up to 30%. For comparison, the IR content usually seen in Danish North Sea chalk reservoirs of the Tor and Ekofisk formations (Shetland Group) is less than 5% and often in the range 1-3% (Bramwell et al 1999).…”

Mass-transport deposits and reservoir quality of Upper Cretaceous Chalk within the German Central Graben, North Sea

“…After deposition, pelagic chalk was subjected to relocation by various gravity-driven mass-transport processes and thus can be protected from early porosity-reducing cementation processes and enhances thereby the reservoir quality (Schatzinger et al 1985;Taylor and Lapré 1987;Bramwell et al 1999;Anderskouv and Surlyk 2012). After transport 1 3 of the MTDs to a new site, bioturbation has no or only little time to act (Hardman 1982;Schatzinger et al 1985).…”

“…In contrast, in areas where chalk deposits represent an important hydrocarbon-producing interval such as in the Norwegian, Danish and Dutch North Sea sectors, the Chalk Group is studied in more detail. The Maastrichtian and Danian Chalk in the Danish, Dutch and the Norwegian North Sea sectors revealed favourable structural and petrophysical conditions, and some of them are proven, oil-bearing reservoirs (Megson 1992;Jeppesen 1994;Bramwell et al 1999;Surlyk et al 2003; Van der Molen et al 2005;Gennaro et al 2013). It is yet an open question whether the Chalk Group in the German Central Graben is a prolific exploration target.…”

Towards an improved lithostratigraphic subdivision of the Chalk Group in the Netherlands North Sea area - A seismic stratigraphic approach

“…Such indicative sequences cannot be identified in pelagic environments, making it impossible to reconstruct a sequence stratigraphic scheme of the Chalk Group (Bramwell et al, 1999). Therefore, a more general allostratigraphical subdivision is presented in this paper, following Bramwell et al (1999). In this approach sedimentary rock units are purely defined by their boundary unconformities rather than their sedimentary make-up.…”