Officer Basin facts for kids
The Officer Basin is a huge, bowl-shaped area in the ground where layers of rock and sediment have built up over millions of years. It covers about 320,000 square kilometers (that's bigger than many countries!) across the border of southern and western Australia. Scientists are interested in this basin because it might hold large amounts of hydrocarbons, which are what we call oil and natural gas.
This basin has a very long history of collecting sediments, with some layers being more than 6 kilometers thick. These layers formed over roughly 350 million years during a time called the Neoproterozoic Era. Because of its deep and long history, the Officer Basin is a good place to look for oil and gas. It's also thought to have once been part of a much larger ancient basin, called the Centralian Superbasin, along with other basins like the Amadeus Basin and Georgina Basin. This superbasin later broke apart due to movements in the Earth's crust.
Contents
What is the Officer Basin?
Australia has many very old landmasses called cratons. These cratons tell us a lot about the Earth's long and complex geologic history. One of the biggest events that shaped Australia was when it started to split away from Antarctica during the Mesozoic Era (the time of dinosaurs!). This splitting, called rifting, created many large basins both on land and under the sea. These basins are where much of Australia's oil and gas are found today.
However, the Officer Basin is special. Much of its potential for oil and gas comes from events that happened long before Australia and Antarctica split. The Officer Basin refers specifically to the rock layers that formed during the Neoproterozoic Era. Younger layers, like those of the Gunbarrel Basin, sit on top of it. The thickest parts of the Officer Basin are in the north, with the layers getting thinner towards the south.
How the Basin Formed
The first sediments in the Officer Basin were laid down on top of much older, harder rocks. Scientists call this first major set of layers "Supersequence 1". This supersequence includes the Buldya Group, which might hold oil and gas, and other layers like the Hussar, Kanpa, and Steptoe groups that act like seals, trapping the oil and gas below.
After these layers formed, the basin experienced big changes. The ground folded and eroded, and then sank again during a cold period called the Marinoan glacial period. More sediments were added due to other major Earth movements, like the Petermann/Paterson orogeny. This long period of sediment collection in the Officer Basin ended around 490 million years ago.
The main layers that are most likely to contain oil and gas are found in the Buldya Group. These layers are made of coarse sands and silts, mixed with mudstone, shale, and other rocks. The thickest parts are in the north, thinning out to the south and west. Movements in the Earth's crust during the Neoproterozoic Era created different zones within the basin. These movements also caused salt layers to move and form dome-like structures called diapirs, which can trap oil and gas.
Layers of Rock: Supersequence 1
Supersequence 1 contains some of the most important rock layers in the Officer Basin for finding oil and gas. Let's look at some of them, starting from the oldest.
Browne Formation
The Browne Formation is the deepest and thickest layer in the Officer Basin's Supersequence 1. It can be several kilometers thick in the central and eastern parts of the basin. This layer is mostly made of shale and mudstone, but it also has dolomite and very thick layers of evaporites (rocks formed when water evaporates, like salt). This suggests that the Browne Formation formed in a shallow, calm environment, like a lagoon or a tidal flat.
Because it has so much salt, the Browne Formation often shows signs of salt movement, like twisted or broken rocks near the salt domes (diapirs). While it's very thick, the Browne Formation probably produced most of its oil and gas very early in the basin's history, so there might not be much left to extract today.
Hussar Formation
Above the Browne Formation is the Hussar Formation. It's much thinner than the Browne. Unlike the Browne, the Hussar has almost no evaporites. Instead, it's mostly made of sandstone and shale, sometimes mixed with dolomite. The bottom part of the Hussar Formation is a very clear, thick mudstone layer that helps scientists identify it using seismic logs (like underground sonar).
The Hussar Formation shows signs that it formed in a changing environment, from shallow ocean shelves and shorelines to tidal flats and even river areas.
Kanpa Formation
The Kanpa Formation sits on top of the Hussar. It continues the trend of younger layers being thinner. The Kanpa is special because it contains a very dense layer of basalt (volcanic rock) called the Kenne Basalt, with very little sediment mixed in. Generally, the Kanpa is a mix of dolomite, shale, evaporites, and chert.
Scientists have studied tiny crystals called detrital zircons from the top sandstone layer of the Kanpa. Using a method called U-Pb dating, they found that this layer is no older than 725 million years. The presence of stromatolites (layered rock structures made by ancient microbes) in the Kanpa suggests it formed in a shallow ocean or tidal flat where carbonates were common.
Steptoe Formation
The Steptoe Formation is the youngest layer in Supersequence 1, sitting above the Kanpa. Its exact thickness isn't well known, as it has only been seen in two rock cores so far, but it can be up to 500 meters thick. The Steptoe is mostly sandstone and dolomite, which often changes into siltstone. Its age is not precisely known, but it must be younger than 725 million years (the age of the top of the Kanpa). Because it's similar to the Kanpa, scientists think the Steptoe formed in a similar environment, but perhaps closer to the coast.
| Formation name | Age | Lithology | Thickness | Source, seal, reservoir |
|---|---|---|---|---|
| Browne | ~ 840-830 Ma | Mostly shale, thick evaporite sequences, dolomite | ~ 2.4-4 km | Source |
| Hussar | ~ 830-790 Ma | Sandstone and shale with interbedded dolomite, minimal evaporites | ~ 500-800 m | Source |
| Kanpa | ~ 790-725 Ma | Siliciclastic-carbonate sequence interbedded with dolomite, minor shale, chert and evaporite | ~ 250-400 m | N/A |
| Steptoe | < 725 Ma | Sandstone and massive dolomite, grading into siltstone | ~ 100-500 m | Source, reservoir |
Finding Oil and Gas
Looking for oil and gas in the Officer Basin has been limited. By 2020, fewer than 20 exploration wells (holes drilled to find oil) had been dug. Even so, many of these wells show that there could be oil and gas deep underground, especially in the northern parts of the basin.
No new exploration wells have been drilled since the late 1990s. However, some of the older results suggest that oil and gas might exist within Supersequence 1, particularly in the Kanpa and Hussar formations. These layers show good amounts of organic material, which is what turns into oil and gas over time.
Scientists have used computer models to understand when oil and gas would have formed in different layers. For example, in the older Browne Formation (around 800 million years old), most of the oil and gas would have formed about 750 million years ago. But in the younger Kanpa and Hussar formations, the peak time for oil and gas production shifted to around 300 million years ago. This big difference is due to the many tectonic events (Earth's crust movements) that the Officer Basin has experienced. These events helped to keep the oil and gas forming for a longer time in the younger layers.
Where Oil and Gas Might Be Trapped
Because the basin is so old, much of the oil and gas that formed in the "source rocks" (where it originates) might have already moved out. This leaves "reservoir rocks" as the main places where oil and gas could be found and extracted. Supersequence 1 is the most promising part of the Officer Basin for finding petroleum. Many of its layers are likely a mix of "seals" (which trap the oil) and "reservoirs" (which hold it).
The best reservoir rocks are thought to be in the Hussar Formation because it's mostly sandstone, which has tiny spaces that can hold oil and gas. The best "seal" rocks are in the Browne Formation (below the Hussar) and the Paterson Formation (above the Hussar). Scientists estimate that these reservoir rocks could have more than 20% porosity (empty space) and good permeability (how easily liquids can flow through them).
Thin layers rich in organic material are found throughout Supersequence 1. These layers can produce excellent amounts of oil and gas. Even though these layers are spread out, the different ways oil and gas can be trapped in Supersequence 1 mean that significant amounts could still be found. The amount of organic material in these layers is usually between 1% and 3%, but some samples have as much as 21%! So far, six oil shows (signs of oil) have been found in rock samples from Supersequence 1. The moving salt layers in the Browne Formation could create many different types of traps for oil and gas, such as folds in the rock or traps along faults. This makes it possible for oil and gas to collect in many smaller areas, especially in the deeper, central part of the basin.
How Heat Affects Oil and Gas
When the Officer Basin first formed, the sediments in the Browne Formation got hot enough to create oil and gas very early on. So, most of that potential was used up during the Neoproterozoic Era. However, the layers that formed later, like the Hussar, Kanpa, and Steptoe formations, were not buried as deeply at first. This allowed oil and gas to continue forming in these younger units for a much longer time, even into the Phanerozoic Eon (the time of complex life).
After Supersequence 1 was laid down, around 700 million years ago, the basin was affected by at least seven major tectonic events. How these events exactly influenced the formation of oil and gas is still being studied, as this region hasn't been explored much. However, a section of Neoproterozoic rock about 2000 meters thick in the central Officer Basin is currently at the right temperature for oil to form. The main type of organic material found in the source beds of the Browne, Hussar, Kanpa, and Steptoe formations is called type II kerogen, which produces both oil and gas. The discovery of bitumen (a tar-like substance) and oil in the drilled cores strongly suggests that a system for producing petroleum exists within Supersequence 1.
Images for kids
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Subsurface structure and various trap types present within Supersequence 1 in the Officer Basin.
