Queen Charlotte Basin facts for kids

The Queen Charlotte Basin is a large area under the ocean, mostly on the continental shelf. It's located off the coast of British Columbia, Canada. You can find it between the Queen Charlotte Islands, Vancouver Island, and the mainland. This area is also known as the Hecate Depression.

The Queen Charlotte Basin mainly refers to rocks from the Cenozoic Era, which is the most recent geological era. However, there are much older rocks from the Mesozoic Era underneath them. This basin was created over time by the Earth's crust stretching and thinning. This stretching also caused volcanic activity during the middle Cenozoic Era. Scientists think that large rock formations called plutons in the southeastern part of the basin might be linked to the Anahim hotspot, a place where hot rock rises from deep within the Earth.

What is the Geology of the Queen Charlotte Basin?

People are becoming interested again in the ocean areas off western Canada. This is because a long-standing rule that stopped oil and gas exploration might be lifted soon. The best places to find oil seem to be in rocks from the Cretaceous period. These are found in the southwestern part of the Queen Charlotte Basin, in an area called western Queen Charlotte Sound.

Why is the Queen Charlotte Basin Interesting for Oil?

Before the 1970s, many oil seeps (where oil naturally leaks out of the ground) were found in rocks of all ages. Because of this, about two dozen wells were drilled in the Queen Charlotte and Tofino areas. Also, many land areas were mapped in the 1980s and 1990s.

However, some areas like Vancouver Island and the Queen Charlotte Islands don't have enough "caprock." Caprock is a layer of rock that traps oil and gas underground. Other areas like Hecate Strait don't seem to have good "source rocks" (where oil and gas form) or "reservoir rocks" (where they collect). The wells drilled offshore didn't fully explore the older Mesozoic rocks.

In contrast, the southwestern Queen Charlotte Basin seems to have all the right layers. It has source rocks, reservoir rocks, and caprock stacked together. These rocks are also at the right depth for oil to form. Plus, there are large "block-fault trap structures" which are like natural containers for oil.

What Kinds of Rocks are in the Basin?

The oldest important rocks in the Queen Charlotte Basin are thick layers of flood basalt from the Upper Triassic period. These are volcanic rocks that covered large areas. Beneath them are even older rocks that have been partly changed by heat and pressure.

Above these, there are high-quality source rocks from the Upper Triassic to Lower Jurassic periods. This layer is about 1,000 meters (about 0.6 miles) thick. These rocks contain special organic material called kerogen that can turn into oil. Geochemical tests suggest these rocks produced most of the oil in the basin. A lot of oil was formed and moved around during the Cenozoic Era.

Above that, there's a layer of clastic rocks (made of broken pieces of older rocks) from the Upper Jurassic to Upper Cretaceous periods. This layer is about 3,000 meters (about 1.8 miles) thick. It doesn't have much potential for creating oil, but it contains good reservoir rocks. These rocks have spaces where oil can collect.

Even higher up, mostly offshore, are Cenozoic deposits. These include mudstone, sandstone, and volcanic rocks. They can be up to 6,000 meters (about 3.7 miles) thick in some areas. These Cenozoic rocks contain kerogen that tends to produce gas. However, minerals like clay can block the flow of oil or gas through them. Good quality sandstone for reservoirs is mostly found near the top of this unit. So, these Cenozoic deposits mostly act as caprock, trapping oil and gas below.

How Did the Basin Layers Form?

Studies of rock layers and how sediments were deposited tell us a lot. The Triassic-Jurassic source rocks were laid down in a wide, shallow ocean basin that covered this whole region. However, the Cretaceous basin was smaller, mostly limited to the western Queen Charlotte Islands and northwestern Vancouver Island. Land areas to the east were eroding and shedding material into this basin.

Western Queen Charlotte Sound was likely part of this same Cretaceous basin. But eastern Queen Charlotte Sound and Hecate Strait lost their older source rocks. They received very few new deposits during the Cretaceous period, and these were not from the ocean. Later, Cenozoic caprock covered Hecate Strait and Queen Charlotte Sound. Its thickness varied from one block of land to another.

This means that western Queen Charlotte Sound should have a good combination of source, reservoir, and seal (caprock) layers. Gravity measurements also show a very thick layer of light (likely sedimentary) rocks under western Queen Charlotte Sound. This is not seen in other parts of the Queen Charlotte Basin.

There are fewer faults (cracks in the Earth's crust) that break through the caprock in Queen Charlotte Sound compared to the northern parts of the basin. The Queen Charlotte Basin is not "overpressured," meaning the fluids inside the rocks are not under extremely high pressure. Geological and geophysical studies suggest that the major fault networks from the Mesozoic Era became active again in the Cenozoic Era. Seismic and gravity data show that the Cenozoic areas where sediments collected, and the raised blocks of land, are quite wide in western Queen Charlotte Sound.

What are Some Challenges for Finding Oil?

There are a few things that make finding oil here tricky. Cretaceous rocks, which were deposited close to where they came from, might not be ideal for oil. Also, it can be hard to guess where the secondary spaces (porosity) in these rocks might be. Some source rocks buried very deep might be "overmature," meaning they've been heated too much and the oil has turned into gas or been destroyed.

Some oil traps might also be broken by younger faults. One offshore well found oil stains, suggesting oil had passed through these Cenozoic rocks and escaped. A big factor affecting how much oil has formed on the Queen Charlotte Islands is how close the rocks are to igneous (volcanic) rock formations called plutons. Similar magnetic patterns suggest that huge igneous bodies might be under eastern Queen Charlotte Sound. These are thought to be from the Miocene period, similar to rocks in the Anahim Volcanic Belt on the mainland. However, these magnetic patterns don't seem to extend much into western Queen Charlotte Sound.