Queen Charlotte Basin
Encyclopedia
The Queen Charlotte Basin is a structural basin mostly beneath the continental shelf offshore, between the Queen Charlotte Islands
, Vancouver Island
, and the British Columbia
mainland, roughly coincident with the physiographic region named the Hecate Depression.
The term Queen Charlotte Basin normally refers to the Tertiary
rocks, but these are underlain by what seems to be a thick Mesozoic
succession. The Queen Charlotte Basin was formed by periods of extension, including thinning and volcanism during the mid-Tertiary period. The large Tertiary pluton
s that magnetic data suggest exist in the southeastern part of the Queen Charlotte Basin seem to be related to the Anahim hotspot
.
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 seep
s 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
basin 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 basalt
s, underlain onshore by partly metamorphosed older rocks. Above, high-quality source rocks exist in the ~1000 metre (0.621372736649807 mi)-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, ~3000 metres (1.9 mi) 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 ~6000 metres (3.7 mi) 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 pluton
s. Similar potential-field anomalies suggest massive igneous bodies may be present beneath eastern Queen Charlotte Sound, and correlations with mainland igneous suites of the Anahim Volcanic Belt
put their age at Miocene. The pluton-related(?) magnetic anomalies do not seem to significantly extent into western Queen Charlotte Sound.
Queen Charlotte Islands
Haida Gwaii , formerly the Queen Charlotte Islands, is an archipelago on the North Coast of British Columbia, Canada. Haida Gwaii consists of two main islands: Graham Island in the north, and Moresby Island in the south, along with approximately 150 smaller islands with a total landmass of...
, Vancouver Island
Vancouver Island
Vancouver Island is a large island in British Columbia, Canada. It is one of several North American locations named after George Vancouver, the British Royal Navy officer who explored the Pacific Northwest coast of North America between 1791 and 1794...
, and the British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
mainland, roughly coincident with the physiographic region named the Hecate Depression.
The term Queen Charlotte Basin normally refers to the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
rocks, but these are underlain by what seems to be a thick Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
succession. The Queen Charlotte Basin was formed by periods of extension, including thinning and volcanism during the mid-Tertiary period. The large Tertiary pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
s that magnetic data suggest exist in the southeastern part of the Queen Charlotte Basin seem to be related to the Anahim hotspot
Anahim hotspot
The Anahim hotspot is a volcanic hotspot in central British Columbia, Canada. It is situated on the Interior Plateau, a large region that lies between the Cariboo and Monashee Mountains to the east, and the Hazelton Mountains, Coast Mountains and Cascade Range to the west...
.
Geology
Renewed interest in western CanadianWestern Canada
Western Canada, also referred to as the Western provinces and commonly as the West, is a region of Canada that includes the four provinces west of the province of Ontario.- Provinces :...
shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
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
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
reservoirs in the southwestern part of the Queen Charlotte Basin, in western Queen Charlotte Sound.
With widespread oil seep
Seep
A petroleum seep is a place where natural liquid or gaseous hydrocarbons escape to the earth's atmosphere and surface, normally under low pressure or flow. Seeps generally occur above either terrestrial or offshore petroleum accumulation structures...
s 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
Source rock
In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including...
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
Juan de Fuca Plate
The Juan de Fuca Plate, named after the explorer of the same name, is a tectonic plate, generated from the Juan de Fuca Ridge, and subducting under the northerly portion of the western side of the North American Plate at the Cascadia subduction zone...
basin 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
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, based on kinematic indicators and cross-cutting relationships of major faults and dated dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
; 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 basalt
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
s, underlain onshore by partly metamorphosed older rocks. Above, high-quality source rocks exist in the ~1000 metre (0.621372736649807 mi)-thick Upper Triassic-Lower Jurassic assemblage, with oil-prone Type I and II kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
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
Clastic wedge
In geology, clastic wedge usually refers to a thick assemblage of sediments--often lens-shaped in profile--eroded and deposited landward of a mountain chain; they begin at the mountain front, thicken considerably landwards of it to a peak depth, and progressively thin with increasing distance inland...
succession, ~3000 metres (1.9 mi) thick, has negligible source potential but contains high-quality reservoirs with largely secondary porosity of ~15% or more. Above, mostly offshore, lie Tertiary mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
, sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
and volcanic deposits, up to ~6000 metres (3.7 mi) thick in some fault-bounded depocenters.
The Tertiary deposits have gas-prone Type III and II kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
, with up to 2.5% TOC locally. However, clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
products of feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
decomposition greatly degrade their permeability, especially at basal levels. Reservoir-quality sandstone facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
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
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
and sedimentological
Sedimentology
Sedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...
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
Hecate Strait
Hecate Strait is a wide but shallow strait between the Haida Gwaii and the mainland of British Columbia, Canada. It merges with Queen Charlotte Sound to the south and Dixon Entrance to the north...
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
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
. 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
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
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 pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
s. Similar potential-field anomalies suggest massive igneous bodies may be present beneath eastern Queen Charlotte Sound, and correlations with mainland igneous suites of the Anahim Volcanic Belt
Anahim Volcanic Belt
The Anahim Volcanic Belt is a long volcanic belt, stretching from just north of Vancouver Island to near Quesnel, British Columbia, Canada. The Anahim Volcanic Belt has had three main magmatic episodes: 15–13 Ma, 9–6 Ma, and 3–1 Ma. The volcanoes generally become younger eastward at a rate of to ...
put their age at Miocene. The pluton-related(?) magnetic anomalies do not seem to significantly extent into western Queen Charlotte Sound.