Marita Gading
IKU Petroleum Research
Trondheim, Norway
The results of IKU Petroleum Research studies support the assessment of a considerable potential for the Voring basin in the Norwegian Sea as a hydrocarbon province.
Several critical factors concerning generation and preservation of hydrocarbons remain beyond our present understanding. Although some of these factors may be evaluated based on existing data, more firmly grounded results cannot be expected until exploration drilling begins.
Large deepwater areas of the mid-Norwegian margin are expected to be opened for exploration for oil and gas in the 15th concession round. Of particular interest are Voring basin I and II and More basin exploration areas, referred to as the Voring I and II and More basin (Fig. 1).
Great water depths, exceeding 1,500 m in large portions of these areas, and unknown geological conditions represent huge challenges for the oil industry. Risks must be balanced against the prospects of major discoveries in the many large structures occurring in these areas. The Voring I area is the northernmost of these areas at 66-68 N. Lat., 3-8 E. Long. (Fig. 1).
NEW GEOLOGIC PROVINCE
During the Cretaceous, the Voring basin was located closer to Greenland than to Norway, and an understanding of the geological evolution of Greenland is necessary to properly evaluate the potential resources of this basin. Traditional play models, with Middle Jurassic sandstone reservoirs and Upper Jurassic source rocks, are improbable for parts of the Voring I area because the Jurassic is situated at too great depths in large portions of the area.
Scientific drilling in the Voring basin and on the Voring marginal high west of the basin was undertaken on Deep Sea Drilling Project Leg 38 in 19741 and Ocean Drilling Program Leg 104 in 1985.2 Quaternary and Neogene sections were cored in the Voring basin, and on the Voring marginal high also Paleogene strata were cored. Ash layers and thick layers of basalt document that the Voring margin has been a volcanic margin.
HYDROCARBONS
High volumes of methane were recorded in Hole 341 (DSDP Leg 38),3 and the total organic content (TOC) increased rapidly below 183 in and decreased below 343 M.4 Drilling was terminated in Miocene strata at 456 in below seabed due to a petroliferous odor and bright yellow fluorescence, although significant amounts of hydrocarbons were not detected by pyrolysis.4 Morris 3 on the other hand, found ethane and propane in addition to methane, and also a small amount of (migrated?) oil in its earliest generation state. These observations, combined with an increased understanding of the structural evolution of the area, form the basis of the relatively high prospectivity now accorded to the Voring I area.
INCREASED ACTIVITY
The forthcoming opening for exploration has led to increased activity in the area. Norwegian Petroleum Directorate and Geco Prakla have acquired considerable seismic data in the Voring I area during the last few years, and several oil companies have carried out surface geochemical studies. During the past year, the Exploration Geology Department at IKU has undertaken two large projects in this region. High resolution multichannel seismic has been collected and interpreted, and these results were used to select localities for a subsequent shallow drilling program that is now completed (Fig. 2). Additionally, a basin analysis program has been completed.
The basin analysis has been sponsored by Elf Petroleum Norge and was carried out to provide an improved framework for understanding of the geological history and hydrocarbon potential in the area. Several other oil companies have later joined as participants. The study comprises six parts: seismic interpretation and structural mapping, velocity analysis, gravity and magnetic modelling, lithology interpretation,
thermal modeling, and hydrocarbon generation modeling.
Ten key horizons within the entire sedimentary succession were delineated in the seismic interpretation and structural mapping sub-study. A velocity model was obtained for the area in the velocity analysis part, and the interpreted horizons were depth converted, paleo burial depths were estimated, and the lithology in the area was indicated. Basin configuration and depth-to-Moho and depth-to-basement were analyzed in the gravity and magnetic modeling. Areas and levels where potential reservoir rocks may occur under closure have been identified. Thermal models have been employed to assess possible hydrocarbon generation at these levels. Uncertainties of source rock maturation result from our limited knowledge of present day subsurface temperatures, paleo-heat flow effects as well as the effects of volcanic processes and crust-mantle processes. In large portions of the Voring I area, younger alternatives to the traditional Upper Jurassic source rocks are needed, as the Upper Jurassic in these areas is too deeply buried.
IKU carried out shallow stratigraphic drilling in the Voring basin during summer 1993 in a project supported by seven oil companies. A grid of approximately 810 km of high resolution multichannel seismic was collected prior to drilling (Figs. 2, 3). This kind of seismic is particularly well suited for studies of the upper layers below seabed. Three stratigraphic holes were drilled in localities chosen from the shallow seismic: two on the Nyk high and one on the Naglfar dome (Fig. 1). Special drilling techniques were required because of the great water depth in the area. Aluminum pipe and wire line operated drilling and sampling equipment (piston core, turbine, collapsible drill bit) were used, in addition to rotation of the entire drill string. The cores were wire line retrieved in 4.5 m lengths as the hole was drilled. A multinational group, with several Russians, participated in the drilling operations, and the equipment and methods used are the same as those developed by the Russians for drilling of the world's deepest hole, on the Kola peninsula. Sediments of Cretaceous and Tertiary age were penetrated, despite difficult conditions and different technical problems. Approximately 368 m of Mesozoic and Cenozoic rocks were drilled and sampled from subbottom depths of up to 240 m.
REFERENCES
1. Talwani, M., Udintsev, G., et al., Initial Reports Deep Sea Drilling Project 38, 1976, 1,256 p.
2. Eldholm, O., Thiede, J., Taylor, E., et al., Proceedings of the Ocean Drilling Program, Scientific Results 104, 1989, 783 pp.
3. Morris, D.A., Organic diagenesis of Miocene sediments from site 341, Voring Plateau, Norway, in Talwani, M., Udintsev, G., et al., eds., Initial Reports DSDP 38, 1976, pp. 809-814.
4. Kvenvolden, K.A., Organic geochemistry, Leg 38: Introduction to studies, in Talwani, M., Udintsev, G., et al., eds., Initial Reports DSDP 38,1976, pp. 809-814.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
Issue date: 11/07/94
