Flemming G. Christiansen
Geological Survey of Greenland
Copenhagen, Denmark
The Lower Paleozoic Franklinian basin in North Greenland occupies an area of more than 150,000 sq km, of which more than half is covered by the Inland Ice cap or dissected by glaciers or permanently frozen fiords (Fig. 1).
Most sedimentary units are well exposed and hence the initial assessment of the petroleum potential of this essentially unexplored frontier area concentrates on the study of potential source rocks, mapping of the thermal maturity pattern, and bitumen occurrences.1
The information obtained suggests significant petroleum accumulations in the southern part of the basin and also has strong implications for the prospective potential of the neighboring Canadian Arctic Islands.
GEOLOGICAL FRAMEWORK
The Franklinian basin in North Greenland was deposited along the northern margin of the Greenland craton (part of Laurentia) during earliest Cambrian to latest Silurian times (Fig. 1), and probably extending into the Devonian as in the Canadian part of the basin.2
Throughout the depositional period, the basin was characterized by major east-west trending facies belts, controlled by deep fault zones or flexures.3 In the southern part of the basin dominantly carbonate shallow-water deposition took place, bordered to the north by outer shelf and slope mudstones. in the north mainly siliciclastic deep-water sedimentation prevailed. The total sediment thickness is at least 3 km in the southern shelf facies and in excess of 10 km in the northern deep-water facies.
During the Ellesmerian orogeny in the Late Devonian to Early Carboniferous the northernmost part, roughly corresponding to the deepwater trough sediments, was folded. In Late Paleozoic and Mesozoic time deposition was restricted to minor fault-bordered areas of the Wandel Sea basin' whereas most of North Greenland was probably a positive, inactive areas.
SOURCE ROCK DISTRIBUTION
Source rock deposition took place mainly during two time intervals (Fig. 1). In the early to middle Cambrian outer shelf shales and limestones were deposited in a wide belt under mainly anoxic conditions and good source rocks formed (Fig. 2).' A wide outer, anoxic shelf also prevailed in the mid-Silurian and black shales which overlapped shallow-water carbonates formed good source rocks (Fig. 2) .7 Most other shales in the basin have a limited petroleum potential due to deposition under more oxic conditions, or in narrow zones.
Both source rocks have moderate to high contents of organic carbon, typically between 3 and 6%, and a good to excellent generative potential of immature samples (Table 1). The kerogen is of type II. In the Cambrian it is completely dominated by amorphous organic matter, but the Silurian contains minor amounts of graptolites, chitinozoans and scolecodonts.
The cumulative thickness of high-quality source beds averages between 20 and 25 meters for the Cambrian source rock and probably slightly more for the Silurian (Table 1).
THERMAL MATURITY HISTORY
The thermal maturity has been studied using an extensive sampling and analytical program including pyrolysis, reflectance and fluorescence, palynology, X-ray diffraction gas chromatography and spectrometry.1 The thermal maturity of the source rocks ranges from immature or early mature in the southernmost areas to postmature or metamorphic in the northern folded areas with only a narrow mature transition zone between (Fig. 3). This simple and consistent maturity pattern has been mapped across most of North Greenland.
In the northeast, the Lower Paleozoic succession is overlain by Upper Paleozoic or Mesozoic sediments with a considerably lower thermal maturity (Fig. 3). The strong similarity between the metamorphic zones and maturity pattern, abrupt changes in maturity between Lower Paleozoic and overlying sediments, and fission track datings suggest the maturation and petroleum generation to have taken place during the Ellesmerian.
HYDROCARBON GENERATION AND MIGRATION
Large amounts of hydrocarbons were generated at Ellesmerian time.
Based on most likely values of depositional area, thickness of source beds and geochemical parameters, values in the range between 25 and 100 x 109 M3 are calculated.
Evidence of such generation and migration is widespread in North Greenland, and is observed as oilstained sandstones and carbonates, as solid bitumen or as seeping asphalt (Fig. 3).
Most of the examples are closely associated with nearby source rocks, but numerous occurrences of bitumen in the southernmost part of basin where there are no known source rocks point towards significant long-distance migration of oil.
The chemical composition of these bitumens is in accordance with the Cambrian shales being the source rock (Table 2).
Biodegradation is intense whereas thermal degradation is very limited.1
MAJOR PLAYS
A number of conceptual plays are suggested based on outcrop evidence of source, reservoir, trap and generation relations. The two most obvious plays, the reefs which are enclosed by the Silurian source rock and the intraformational sandstones in the Cambrian source rocks seem disappointing due to the high thermal maturity of the first and the deep erosional level of the second play. Present accumulations are much more likely in the southern part of the basin as a result of long-distance migration. The main reservoir targets are basal Cambrian sandstones or carbonates. Traps could occur along a major transverse fault zone close to the Inland Ice cap or in gentle anticlines further north.
IMPLICATIONS
The plays recognized in North Greenland have strong implications for further exploration in this very northerly situated frontier basin. Continued exploration should be based mainly on structural analysis of aerial photographs and field data and seismic surveys should be restricted to plateaus within the most promising areas. The identification of Cambrian plays and a prolific Cambrian source rock is of great importance for other Paleozoic Arctic terrains where the Silurian and Devonian has previously been the main target of exploration.2
ACKNOWLEDGMENTS
The present study was economically supported by the Danish Ministry of Energy through EFP 83-2251-305. Published with the permission of the Geological Survey of Greenland.
REFERENCES
- Christiansen, F.G. (ad.), 'Petroleum geology of North Greenland', Bull, Gromands geol. Unders. (1989), no. 158, 104 pp.
- Rayer, F.G., 'Exploration prospects and future petroleum potential of the Canadian Arctic Islands', J. Petrol. Geol. (1981), vol. 3, 367-412.
- Surlyk, F. and Hurst, J.M., 'The evolution of the early Paleozoic deep-water basin of North Greenland', Geol. Soc. Amer. Bull. (1984), vol. 95, pp. 131-154.
- Hakansson, E. and Stemmerik, L., 'Wandel Sea basin - the North Greenland equivalent to Svalbard and Barents Shelf', In Spencer, A.M. et al (edit), Petroleum geology of North European margin (1984), pp. 97-107.
- Ziegler, P.A., 'Evolution of the Arctic-North Atlantic and western Tethys', AAPG Mem. (1 988), no. 43, 198 pp. + 30 plates.
- Christiansen, F.G., Nohr-Hansen, H. and Nykjaer, O., 'The Cambrian Henson Gletscher Formation: a mature to postmature hydrocarbon source rock sequence from North Greenland', Rapp. Gromands geol. Unders. (1987), no. 133, pp. 141-157.
- Christiansen, F.G. and Nohr-Hansen, H., 'The Silurian shales of central and western North Greenland: evaluation of hydrocarbon source rock potential', Rapp. Gronlands geol. Unders. (1989), no. 143, pp. 47-67.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
