ELIASSEN, PER EMIL, CHRISTIAN HERMANRUD, HEGE M. NORDGÅRD BOLÅS and EIRIK VIK, Statoil

Abstract: Petroleum Systems of the Vøring Basin - Constraints from Basin Modelling

The Vøring Basin, located on the western margin of the mid-Norwegian Shelf of Norway, is a large clastic rift basin which has recently seen the start of hydrocarbon exploration. An understanding of the petroleum system(s) which may be present in the basin is of major importance for prospect ranking in the area.

One and two-dimensional basin modelling was used as a tool to evaluate potential source rocks and reservoir plays in the area. The possible thermal histories were assessed of the basin as a means of determining whether oil could be expected in any of the reservoirs, and to constrain the stratigraphic level at which the source rock had to occur to be consistent with this expectation. The four possible source rocks are shales of Upper Jurassic, Barremian, Cenomanian / Turonian and Paleocene age.The Upper Jurassic source rock is most likely of the four to be present, but the typically great depth of burial suggests that it is likely to be post-mature with respect to oil generation. The presence of the other source rock candidates is considered less certain, with Barremian shales representing the most promising of these, while the Cenomanian/Turonian shales are the least probable. The main phase of structural trap development of the reservoirs (with exception of the Gjallar Ridge area to the west) occurred during the Eocene/Miocene (earlier in the Vema Dome area) and any hydrocarbon supply prior to this stage would have been lost.

An underplated body has been identified west of the Vøring Escarpment. This magmatic body extends below the sediments at the western margin of the prospective area. 2D basin modelling was performed along the profile shown in Figure 1 to evaluate the degree of perturbation in heat-flow associated with the emplacement of this body, and the implications for the maturation history of the source rock candidates. Sensitivity analysis was performed with respect to the volume of intruded material and the depth of the intrusion and demonstrated a maximum temperature increase of 15 - 20°C at a burial depth of 5000 m. This increase was confined to the area immediately to the east of the Vøring Escarpment and was no longer distinguishable about 50 km east of the escarpment. The thermal anomaly associated with the underplating appears to have been relatively short-lived and was reduced to ca. 25% of the original magnitude within 10 Ma after the emplacement of the underplated body. This analysis assumes instantaneous emplacement of the underplated body and may thus overestimate the thermal effect of the underplating. Modelling of source rock maturity concluded that the thermal effects of the underplated body did not significantly influence the prospectivity of the investigated area.

The combined effects of reduced post-rifting heat-flow and reduced surface temperatures, combined with modest (600 - 800 m) sediment deposition during the late Tertiary, suggest that the possible source rocks attained their maximum temperature prior to the main phase of trap and seal formation described previously. This suggests that the prospectivity of the Vøring basin is very poor. However, rapid Cretaceous subsidence would result in a significant depresssion ot the isotherms, followed by recovery of the thermal gradient during the Tertiary and into the present-day. This late thermal recovery event is large enough to support significant hydrocarbon generation in the late Tertiary, provided that source rocks exist in the right stratigraphic positions. Figure 2 shows the modelled timing of oil and gas expulsion from potential source rock intervals for the Vema Dome area, from both the shallowest (crest) and deepest (basin) parts of the source rock kitchen.

The modelling results illustrated in Figure 2 demonstrate that charging of present-day structures requires that source rocks can be no older than Cenomanian/Turonian age. Older source rocks will have passed through most of the oil window prior to formation of the structural traps, while a Paleocene source rock is likely to be thermally immature. In this situation, charging of structures with oil would require temporary storage of generated oil in deep stratigraphic traps with later remigration to present-day structural traps. These results suggest that the Vøring Basin is more likely to be a gas province, with minor chances for an oil discovery. A possible exception lies in the Gjallar Ridge area, where later maturation of the more probable Jurassic or Barremian source rocks would permit oil charging of the later structural traps.

Figure 1 - Profile over the northern part of the Vøring Basin showing the main stratigraphic and structural elements. It is emphasized that the depths to the stratigraphic boundaries are poorly constrained in the deepest parts of the basin and may be several km different from those shown here.

Figure 2 - Timing of trap and seal formation and modelled timing of hydrocarbon generation in the Vema Dome area.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England