J. W. Haggart1, J. R. Dietrich2, and H. V. Lyatsky3
1Geological Survey of Canada, 101-605 Robson Street, Vancouver, BC V6B 5J3,
Canada, [email protected]
2Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB T2L 2A7
3Lyatsky Geoscience Consulting, 4827 Nipawin Cr NW, Calgary, AB T2K 2H8
ABSTRACT: Petroleum Geology of Queen Charlotte Islands Region, British Columbia, Canada
The offshore region of western Canada is a large, relatively-unexplored, frontier exploration province containing the landmasses of Queen Charlotte Islands and northern Vancouver Island, as well as the adjoining interior continental shelf areas of Dixon Entrance, Hecate Strait, and Queen Charlotte Sound. Limited onshore and offshore exploration drilling was undertaken in this region during the 1950s and 1960s, with hydrocarbon shows encountered in Tertiary strata; the underlying Mesozoic strata remain essentially undrilled. More recent integrated geoscience studies of the Queen Charlotte Islands region have demonstrated that this province contains all necessary components of a significant hydrocarbon producing system, with estimated total recoverable resources of 2.6 billion barrels of oil and 20 Tcf of gas.
Rocks present on Queen Charlotte Islands, and inferred from stratigraphic and geophysical considerations to be present in the adjacent shelf regions, include a thick sequence of Mesozoic and Tertiary sedimentary strata, with Paleozoic igneous, metamorphic, and sedimentary rocks exposed locally. Oil and tar seeps are known from virtually all stratigraphic units found on the islands. The widespread, massive, volcanic Upper Triassic Karmutsen Formation is considered to be the economic basement; the underlying Paleozoic rocks are largely metamorphosed. The Karmutsen Formation consists of a thick (ca. 2500 m) sequence of island-arc basalts with interstratified reefal limestones in its uppermost part. Conformablyoverlying uppermost Triassic to Lower Jurassic carbonate and clastic strata, -1 km thick, are widespread, and include good to excellent petroleum source rocks with abundant Type I and Type II organic matter; TOC values of up to 11% are noted locally. Poor-quality source rocks are present in the Cretaceous succession, -3 km thick, where only Type III organic matter is found, with TOC values around 1% or less. Tertiary clastic deposits, lying principally offshore, contain Type III and II organic matter, with TOC values averaging 1 to 1.5% (and locally higher in coal-bearing zones). Carbonaceous mudstones with good to excellent source rock potential have been identified in the Tertiary succession in almost all wells in the basin.
Stratigraphic correlations, as well as gravity and seismic geophysical evidence, suggest that the Mesozoic source-rock strata continue under western Queen Charlotte Sound on the shelf offshore. Triassic-Jurassic source rocks range from mature to overmature onshore, whereas Tertiary source rocks tend to be immature to mature, depending on local proximity to Jurassic and Tertiary igneous plutons. Oil expulsion and migration from the Mesozoic source rocks is known to have taken place during the Tertiary. Considering the apparent burial depths, and making realistic geothermal-gradient assumptions, around 40% of the offshore Tertiary basin fill is thus estimated to be within the oil window; the underlying Mesozoic source rocks are expected to vary from mature to overmature, depending on local burial depth and proximity to igneous plutons.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado