WESTERN CANADA:British Columbia's geology holds promise of significant reserves

This image is a generalized geologic map of the study area. The heavy lines are faults interpreted from geological, potential-field, and seismic data. The "D"s mark the approximate location of Tertiary depocenters. The dashed line represents the minimum westward extent of Miocene plutons suspected beneath the eastern segment of QC Sound. The dotted pattern indicates the area of prime exploration interest.

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Certainly, there would have to be large oil accumulations to justify investment in this high-cost frontier environment. The Tofino, Winona, Georgia, and Juan de Fuca basins lack significant known oil-source rocks, although accessibility of the last two basins makes gas exploration plausible. The southwestern part of the QC basin, however, appears to contain a stack of source, reservoir, and caprock strata, largely at oil-window burial depths, as well as large block-fault trap structures.

Encouraging signs

Oil seeps from rocks of all ages encouraged drilling. Two dozen wells were drilled in the QC and Tofino areas, mostly in the 1950s and 1960s, eight of them in the QC basin offshore: six in Hecate Strait, two in QC Sound. The QC 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 mostly crystalline.

Economic basement in the QC basin area is massive, with thick Upper Triassic flood basalts underlain onshore by partly metamorphosed older rocks. Above, good source rocks exist in the 1,000-m-thick Upper Triassic-Lower Jurassic assemblage, with Type I and II kerogen and total organic carbon up to 11%.

Geochemical evidence suggests these rocks provided most of the basin's oil; a major pulse of oil generation and migration was in the Tertiary. The overlying Upper Jurassic-Upper Cretaceous clastic succession, at 3,000 m thick, has negligible source potential but contains high-quality reservoirs with (largely secondary) porosity of 5-15%. Above, mostly offshore, lie Tertiary mudstone, sandstone, and volcanic deposits, up to 6,000 m thick in some fault-bounded depocenters.

Tertiary deposits have Type III and II kerogen, with up to 2.5% TOC locally, but clay products of feldspar decomposition greatly degrade their permeability, especially at basal levels. Reservoir-quality sandstone facies lie mostly near the top of this unit, where migration routes from below and the seal above may be inadequate. This leads to the conclusion that the Tertiary deposits are predominantly caprock, perhaps with 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 QC islands and northwestern Vancouver Island, with uplands to the east shedding detritus. Western QC Sound was probably part of this basin, while eastern QC Sound and Hecate Strait largely lost their source rocks and received few Cretaceous deposits. Tertiary caprock, with thickness varying block to block, then blanketed Hecate Strait and QC Sound. Western QC Sound could thus contain a favorable source-reservoir-seal stack.

Gravity data also indicate a great thickness of low-density (sedimentary?) rocks is present beneath western QC Sound, but not elsewhere in the basin.

Caprock-breaching faults are sparser in QC Sound than in Hecate Strait. Regional geological and geophysical correlations suggest Cenozoic reactivation of major Mesozoic block-fault networks. Seismic and gravity data show the faults to be comparatively sparse, and depocenters and raised blocks comparatively broad, in western QC Sound.

Fault-block structures in the QC Sound as seen in a late-1980s seismic reflection profile. The inset shows the location of seismic lines and offshore wells.

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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 in the deepest depocenters may be overmature. Some traps may be breached by Neogene faults. Evidence of this possibility is the results of the Sockeye B-10 offshore well, which encountered oil staining, suggesting oil passed throught these Tertiary rocks and escaped.

A major influence on local hydrocarbon-maturation levels on QC islands is proximity to the Jurassic and Tertiary igneous plutons. Similar potential-field anomalies suggest massive igneous bodies could be present beneath eastern QC Sound. And correlations with mainland igneous suites put the age of these suspected plutons at Miocene. Pluton-related magnetic anomalies do not seem to significantly extend into western QC Sound.

To improve basin delineation, further exploration in this prospective area could include seismic and potential-field surveys, plus re-examination of relevant aspects of onshore geology.

References are available from the author upon request.

Author
Henry Lyatsky, PhD, is a Calgary-based exploration consultant, with experience in Canada and internationally. He specializes in basin analysis, prospect identification, potential-field methods, and geological-geophysical integration. He is the author or co-author of three books and numerous exploration and research papers.