The investigation is based on analyses of planktonic and benthonic forarninifera and Bolboforma carried out in the uppermost Palaeogene and Neogene of the production well 2/4-C-11 (56°33' N, 3°13' E) on the Ekofisk Field and the exploration well 15/12-3 (58°14' N, 1"53' E) on the Sleipner Field. The analyses are based primarily on ditch cuttings, but in well 2/4-C-ll, 49 sidewall cores and six short conventional cores were also available. Strontium isotope and paleomagnetic analyses were also carried out in well 2/4-C-11. A 420 m-thick accumulation of Lower Miocene sediments is present in the Ekofisk area. The Middle Miocene comprises approximately 180m, and the Upper Miocene is represented by an approximately 500 m-thick interval. A hiatus is present in the Middle Miocene, and the middle part of the Middle Miocene is missing. A thin interval (approximately 120m) of Lower Pliocene sediments is found overlying the Upper Miocene. Uppermost, a thick accumulation of Upper Pliocene-Pleistocene sediments (approximately 1000 m) is in place. More than half of this was deposited during the Pleistocene. The entire Upper Pliocene is represented in this area. This contrasts to the northern North Sea, as well as areas further north, where sediments of this epoch are partially missing in a hiatus comprising the lowermost part of the Upper Pliocene. In the Sleipner area the Lower and Middle Miocene are approximately 160m and 180m thick, respectively. The Upper Miocene (approximately 100 m) and the Lower Pliocene (approximately 40 m) thin from the Ekofisk towards the Sleipner Field. Uppermost, an approximately 900 m-thick succession of Upper Pliocene-Pleistocene sediments is present. Nearly two-thirds of this accumulation are Pleistocene in age.

Tor Eidvin & F. Riis, Norwegian Petroleum Directorate, P. O. Box 600, N-4001 Stavanger, Norway;
Y. Rundberg, Saga Petroleum ASA, P. O. Box 490, 1301 Sandvika, Norway