Queen Charlotte Basin

From Wikipedia, the free encyclopedia

Queen Charlotte Basin lies mostly beneath the continental shelf offshore, between the Queen Charlotte Islands, Vancouver Island, and the British Columbia mainland. The term Queen Charlotte Basin normally refers to the Tertiary rocks, but these are underlain by what seems to be a thick Mesozoic succession.

Renewed interest in western Canadian shelf basins results from widespread expectations that the long-standing government moratorium on offshore exploration there may soon be lifted. The best oil prospects seem to exist in Cretaceous reservoirs in the southwestern part of the Queen Charlotte Basin, in western Queen Charlotte Sound.

With widespread oil seeps from rocks of all ages, two dozen wells were drilled in the Queen Charlotte and Tofino areas before the 1970s; many land areas were mapped in the 1980s and 1990s. However, Vancouver and Queen Charlotte islands mostly lack caprock. Hecate Strait seems to lack adequate source and reservoir rocks, and the offshore wells did not significantly test the Mesozoic horizons. Rocks on the mainland are crystalline. The Tofino, Winona, Georgia and Juan de Fuca basins lack significant known source rocks. By contrast, southwestern Queen Charlotte Basin seems to contain a stack of source, reservoir and caprock strata, largely at oil-window burial depths, as well as large block-fault trap structures.

While some workers (e.g., Lyatsky and Haggart, 1993; Lyatsky, 2006) regard the Mesozoic horizons to be the primary oil-exploration targets, others (e.g., Dietrich, 1995; Hannigan et al., 2001) focus more on the overlying Tertiary rocks. Rohr and Dietrich (1992) considered the Queen Charlotte Basin to have formed largely by strike-slip movements in the Tertiary. On the other hand, Lyatsky (1993, 2006) considered significant strike-slip movements to be impossible in this basin since at least the Late Oligocene, based on kinematic indicators and cross-cutting relationships of major faults and dated dikes; instead, he viewed the basin's Cenozoic evolution to be a product of reactivation of older networks of block-bounding faults.

Economic basement in the Queen Charlotte Basin area is massive, thick Upper Triassic flood basalts, underlain onshore by partly metamorphosed older rocks. Above, high-quality source rocks exist in the ~1,000-m-thick Upper Triassic-Lower Jurassic assemblage, with oil-prone Type I and II kerogen and TOC (total organic carbon) up to 11%. Geochemical evidence suggests these rocks provided most of the basin’s oil, and a major pulse of oil generation and migration was in the Tertiary. The overlying Upper Jurassic-Upper Cretaceous clastic succession, ~3,000 m thick, has negligible source potential but contains high-quality reservoirs with largely secondary porosity of ~15% or more. Above, mostly offshore, lie Tertiary mudstone, sandstone and volcanic deposits, up to ~6,000 m thick in some fault-bounded depocenters.

The Tertiary deposits have gas-prone Type III and II kerogen, with up to 2.5% TOC locally. However, clay products of feldspar decomposition greatly degrade their permeability, especially at basal levels. Reservoir-quality sandstone facies are found largely near the top of this unit, where migration routes from below and the seal above may be inadequate. The Tertiary deposits thus seem to be predominantly caprock, perhaps with some secondary exploration targets.

Stratigraphic and sedimentological studies indicate the Triassic-Jurassic source rocks were deposited in a broad shelfal basin encompassing this entire region and beyond. However, the Cretaceous basin was confined to western Queen Charlotte Islands and northwestern Vancouver Island, with uplands to the east shedding detritus. Western Queen Charlotte Sound was probably part of the same Cretaceous basin, while eastern Queen Charlotte Sound and Hecate Strait largely lost their pre-existing source rocks and received few, non-marine, Cretaceous deposits. Tertiary caprock, with thickness variable block to block, then blanketed Hecate Strait and Queen Charlotte Sound.

Western Queen Charlotte Sound should thus contain a favourable source-reservoir-seal stack. Gravity data also indicate a great thickness of undrilled low-density (sedimentary?) rocks is present beneath western Queen Charlotte Sound but not elsewhere in the Queen Charlotte Basin.

Caprock-breaching faults are sparser in Queen Charlotte Sound than in northern parts of the basin; the Queen Charlotte Basin is not overpressured. Regional geological and geophysical correlations suggest the major Mesozoic block-fault networks were reactivated in the Cenozoic. Seismic and gravity data show the fault-bounded Tertiary depocenters and raised blocks to be comparatively broad in western Queen Charlotte Sound.

The caveats are several. Cretaceous rocks, deposited near their provenance areas, tend to be petrologically immature, and secondary porosity in them may be hard to predict. Buried source rocks beneath the deepest depocenters may be overmature. Some traps may be breached by Neogene faults: one offshore well encountered oil staining, suggesting oil passed through these Tertiary rocks and escaped. A major influence on local hydrocarbon-maturation levels on Queen Charlotte Islands is proximity to the mostly Jurassic and Tertiary igneous plutons. Similar potential-field anomalies suggest massive igneous bodies may be present beneath eastern Queen Charlotte Sound, and correlations with mainland igneous suites put their age at Miocene. The pluton-related(?) magnetic anomalies do not seem to significantly extent into western Queen Charlotte Sound.

[edit] References

Dietrich, J.R., 1995. Petroleum resource potential of the Queen Charlotte Basin and environs, west coast Canada; Bulletin of Canadian Petroleum Geology, v. 43, p. 20-34.

Hannigan, P.K., Dietrich, J.R., Lee, P.J., and Osadetz, K.G., 2001. Petroleum Resource Potential of Sedimentary Basins on the Pacific Margin of Canada; Geological Survey of Canada, Bulletin 564, 72 p.

Lyatsky, H.V., 1993. Basement-controlled structure and evolution of the Queen Charlotte Basin, west coast of Canada; Tectonophysics, v. 228, p. 123-140.

Lyatsky, H.V., 2006. Frontier next door: geology and hydrocarbon assessment of sedimentary basins offshore western Canada; Recorder (Canadian Society of Exploration Geophysicists), v. 31, no. 4, p. 66-75.

Lyatsky, H.V. and Haggart, J.W., 1993. Petroleum exploration model of the Queen Charlotte Basin area, offshore British Columbia; Canadian Journal of Earth Sciences, v. 30, p. 918-927.

Rohr, K.M.M. and Dietrich, J.R., 1992. Strike-slip tectonics and development of the Tertiary Queen Charlotte Basin, offshore western Canada: evidence from seismic reflection data; Basin Research, v. 4, p. 1-19.