Geological contraints on the position of hydrocarbon-contacts in the Gina Krog and Sleipner East areas

Abstract

Knowledge of the geological constrains on the position of hydrocarbon-water contacts are crucial in terms of hydrocarbon exploration. The controlling mechanisms have a large impact on the in-place volumes in a potential hydrocarbon prospect. The Southern Viking Graben encompasses large hydrocarbon accumulations in both overpressured and normal pressured reservoirs. The Paleogene Sleipner East field and the Jurassic Gina Krog field contains substantial amounts of hydrocarbons, in normally and overpressured reservoirs, respectively. These areas have been studied along with corresponding satellite structures. The hydrocarbon-bearing intervals are located at several stratigraphic intervals and depths, ranging from c. 2200-4250 m. This study covers the geological constraints on the position on hydrocarbon-water contacts, in the Sleipner East and Gina Krog areas of the Southern Viking Graben, and present-day spill-routes for the two areas. A regional seismic interpretation of the Top Ty and Top Hugin reservoirs were carried out based on 3D seismic data. Spill point interpretation(s) of individual structures were further systematized in order to create regional migration routes. Furthermore, the overburden was extensively investigated for seismic amplitudes and geological features in general, in order to explain the underfilled structures. A further investigation of structure bounding faults were carried out by evaluating juxtaposition of the different lithologies along the fault plane. In addition, SSF (shale smear factor) values have been calculated along the fault planes where sand-sand juxtaposition is evident. A regional spill-route is suggested from the deeper situated Sleipner East field and further up-dip towards the Sleipner Terrace and Utsira High. The present-day hydrocarbon-water contacts can be explained by regional spill-routes and leakage. A potential leakage point above the Biotitt structure is suggested to be the termination of the spill-route in the Paleogene. Consequently, the Fosen structure located further north on the interpreted spill-route is dry without shows, most likely due to leakage at the termintation of the spill-route. Vertical migration of hydrocarbons from the underlying Jurassic/Triassic hydrocarbon accumulations in the Sleipner East and Loke structures have most likely migrated into the Paleogene reservoirs above. Based on geochemical signatures from the work of Isaksen, Patience, Van Graas & Jenssen (2002) and hydrocarbon shows below the hydrocarbon contacts, vertical migration positions from the deeper strata to the Paleogene along the Sleipner Fault Zone has been suggested. However, no evidence from seismic amplitudes in the interval between the Paleogene and Jurassic reservoirs have been observed. The Greater Gina Krog area encompasses structures interpreted to be filled to spill, underfilled and potentially overfilled. Out of the studied structures, three structures are interpreted to be filled to spill (Gina Krog Main, Ermintrude Main and Eirin) and four structures are interpreted to be underfilled (McHenry,Dougal, 15/6-4, Ermintrude West). Some of the underfilled structures contains seismic expressions for vertical leakage in the overburden. Additional structures are presumed to be underfilled based on the position of the hydrocarbon-water contact compared to the spill point of the structure. Alternative interpretations of the geometry and fault sealing of the Hugin Formation sets up several potential spill directions in the Greater Gina Krog area. The established spill routes are in accordance with present-day overpressures. When exploring for hydrocarbons in the Paleogene interval, vertical leakage points in the termination of spill routes should be further addressed. In the Jurassic interval, a large number of the studied structures have hydrocarbon contacts situated shallower than the structural spill points. However, potentially overfilled structures with hydrocarbons below the structural spill point also occurs.