QUEEN CHARLOTTE BASIN OFF WESTERN CANADA MAY HOLD OIL, GAS POTENTIAL

Henry V. Lyatsky
University of British Columbia
Vancouver

James W. Haggart
Geological Survey of Canada
Vancouver

The queen Charlotte basin is a large Tertiary tectonic feature on the west coast of Canada. Over the last few decades it has repeatedly been the target of petroleum exploration.1

A considerable amount of drilling was carried out in the 1950s and 1960s,2 both on Queen Charlotte Islands and offshore. Six wells were drilled in Hecate strait and two in Queen Charlotte sound (Fig. 1). The maximum depth reached was 15,656 ft.

A moratorium on offshore drilling is in effect for environmental reasons.

If it is lifted, a new surge of exploration activity is anticipated.

To prepare for that activity, the Geological Survey of Canada started the Queen Charlotte Basin Frontier Geoscience Program, with the principal objective of assessing the petroleum potential of Canada's northern west coast.

In this article, the authors discuss the petroleum geology of this region and highlight what we believe are new exploration opportunities.

Until recently, the Tertiary fill of the Queen Charlotte basin has been considered the primary exploration target,23 and most of the earlier exploration effort offshore was focused on the northern part of the basin (Fig. 1).

However, the Cenozoic Queen Charlotte basin overlies older rocks and structures. Results of new geological and geophysical studies suggest that the petroleum potential is highest in the Mesozoic strata in the southern part of the region, beneath Queen Charlotte sound.

REGIONAL STRATIGRAPHY

Rocks underlying the Tertiary basin fill are exposed on Queen Charlotte Islands and northern Vancouver Island (Fig. 2).

The economic basement in the region is massive basalts of the Upper Triassic Karmutsen formation.4 These rocks are conformably overlain by a carbonate and clastic sequence (Kunga and Maude groups) that exceeds 1,000 m in thickness.5

The Upper Triassic-Lower Jurassic sedimentary rocks, which were deposited in a tectonically quiet shelf environment, are widespread across the region. They are found on Queen Charlotte Islands, northern Vancouver Island, and many small islands in Queen Charlotte strait.6 During Cretaceous time, however, these rocks were partly exposed subaerially and eroded in the Hecate strait area, as suggested by reconstructions of Cretaceous paleogeography on Queen Charlotte Islands.7

The overlying Middle Jurassic Yakoun and Moresby groups and younger, Late Jurassic strata (Fig. 2), are mostly volcanic and associated epiclastic sedimentary rocks.5 8 These rocks are several hundred meters thick and considered unimportant for petroleum potential.

The conformable uppermost Jurassic to Upper Cretaceous succession consists of as much as 3,000 m of variable elastic lithologies.7

The Queen Charlotte Islands area was a locus of marine deposition during much of the Cretaceous, and the shoreline gradually transgressed eastward across the islands at that time.

The western part of the islands, as well as the Queen Charlotte Sound area, likely experienced relatively continuous sedimentation. However, the eastern part of the islands and the adjacent Hecate Strait area were mostly a highland, possibly subject to marine deposition for only short intervals during the later part of the period.

Tertiary strata are more than 5,000 m thick and comprise sandstone, mudstone, and conglomerate, interbedded with volcanic rocks.9 10 Clastic rocks of the Neogene Skonun formation form most of the Queen Charlotte basin fill offshore.2 Older Tertiary volcanic and sedimentary strata have been mapped onshore.5

SOURCE ROCKS

Numerous oil seeps are found in Mesozoic and Cenozoic rocks on Queen Charlotte Islands,9 and dead oil has been discovered on northern Vancouver Island.11

Type II and I kerogen predominates in the Upper Triassic and Lower Jurassic strata on Queen Charlotte Islands, which locally contain up to 11% TOC (see table). These rocks are considered to be the principal oil source in the region.12

Oil stains in Neogene rocks in an offshore well in Hecate strait, as well as many seeps onshore, are sourced from Upper Triassic and Lower Jurassic kerogen.13 14 15

In contrast, the uppermost Jurassic to Upper Cretaceous succession has a low source-rock potential.12 These strata contain Type III organic matter, usually in small amounts.

Tertiary strata have a poor to moderate source-rock potential.16 They are characterized by Type II and III kerogen, with 2-2.5% TOC (Table 1).

RESERVOIR ROCKS

Reservoir facies are apparently lacking in Upper Triassic and Lower Jurassic rocks.17 However, secondary porosity up to 15% is present locally in the Cretaceous rocks on Queen Charlotte Islands.18

Tertiary strata offshore contain porous, discontinuous sandstone intervals that are separated by mudstone. However, permeability of Tertiary rocks is usually low due to blockage of fluid conduits by products of feldspar decomposition.2

HYDROCARBON SEAL

The authors suggest the Skonun formation offshore may form a regional seal because its permeability is commonly low. Nevertheless, presence of oil seeps and stains in Neogene rocks suggests that a pulse of oil migration occurred recently in the region. The authors suspect the likely conduits are the numerous Tertiary faults that are found onshore and offshore.15 19 20 21 If so, the caprock quality of Tertiary strata is degraded. Significantly, the Skonun formation offshore, where drilled, was not found to be overpressured.2

DEFORMATION HISTORY

The Mesozoic deformation history of the Queen Charlotte Islands was characterized by contractional folding and thrusting during the Middle Jurassic, followed by Late Jurassic high-angle block faulting.5 15

For most of Cretaceous time, the region experienced reduced tectonic activity.7 8 During the latest Cretaceous and Early Tertiary, compressional inversion of old high-angle faults took place.

The Tertiary Queen Charlotte basin formed largely by reactivation of the old fault networks with associated minor (about 10%) extension.19 20 Normal faulting took place offshore in the Neogene (Fig. 3).

High-angle block faulting and inversion tectonics characterized the structural evolution of the study area in post-Middle Jurassic time.6 15 19 As a result, structural traps associated with faults and anticlines are typical for the Tertiary, whereas large drape and stamp structures are expected in the Mesozoic.

ORGANIC MATURATION

In addition to burial-related heating, two distinct episodes of magmatic heating have affected the sedimentary rocks in the region.22

On Vancouver Island, Lower and Middle Jurassic magmatic activity was wide-spread, and Paleogene activity has also been noted.23 24 On Queen Charlotte Islands, magmatic episodes occurred during the late Jurassic, as well as Paleogene to early Neogene.25 26

Along the mainland coast, magmatism took place during the late Jurassic and early Cretaceous,27 and locally in the Neogene.28

In the offshore region, large plutons apparently lie beneath Queen Charlotte sound.20 They are located on trend with the Neogene track of the Anahim hot spot on the mainland,28 which points to a Miocene age for some of the plutons.

Thermal maturation of rocks was influenced by Jurassic and Tertiary magmatism.22 Rocks vary from immature to overmature within the area; consequently, some of the hydrocarbon generation may have been only local.

Burial-related heating contributed to maturation on a regional scale, possibly in late Cretaceous time, when the Triassic and Jurassic stratigraphic intervals were covered by as much as 3,000 m of younger sediments. In the Cenozoic, burial of older rocks beneath the fill of the Queen Charlotte basin likely placed those rocks into the oil window regionally.

Oil generation in the late Jurassic was induced by magmatism and thus was local. Structures related to the middle Jurassic thrusting and late Jurassic block faulting were already in place, but no reservoir was available at that time.

In the latest Cretaceous, regional, burial-related oil generation might have occurred in the western part of the area. Compressional reactivation of old high-angle faults may have produced structures favorable for entrapment.

Oil generation occurred in the Tertiary, related to both burial of source rocks and localized magmatism. Neogene faulting has produced favorable structures, but the timing of their formation with regards to oil migration is uncertain. The extent of the reservoir is also unclear.

OIL, GAS POTENTIAL

All three major sedimentary packages-the Upper Triassic to Lower Jurassic source rocks, the Cretaceous potential reservoir rocks, and the Tertiary seal-are apparently superimposed beneath Queen Charlotte sound. Therefore, that area may be prospective.

Cretaceous strata, which contain potential reservoirs on Queen Charlotte Islands, are likely a prime exploration target offshore. Secondary targets may be found in the Tertiary package.

On the other hand, source rocks may have been overheated due to magmatism, and the existence of regional reservoirs is uncertain. Juvenile faulting may postdate oil migration, and it may have degraded the quality of the regional Tertiary seal.

A different situation is found in the Hecate strait area. The Tertiary rocks in this part of the region are underlain by partly eroded Upper Triassic and Lower Jurassic strata, and a significant thickness of Cretaceous rocks is unlikely, due to non-deposition.7 The authors conclude, therefore, that petroleum prospects are low in that area.

CONCLUSIONS

Recent geological and geophysical studies reveal new potential exploration targets on the west coast of Canada.

Beneath Queen Charlotte sound, oil accumulations may be found in Mesozoic strata. This interval is still untested, however.

Some uncertainty is associated with the timing and extent of oil generation and with the presence of a suitable reservoir.

ACKNOWLEDGMENTS

The authors thank Bob Thompson for a thorough review. Glenn Woodsworth made useful suggestions.

REFERENCES

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