In search of oil
Adam Vonk is a sedimentary geologist who is just about to finish his PhD at the University of Waikato to take up a new position as a petroleum geologist in Perth, Australia.
What is a hydrocarbon?
Adam Vonk explains that a hydrocarbon is a molecule made up of hydrogen and carbon. Hydrocarbons can be very simple structures of one carbon atom and four hydrogen atoms (CH4 – methane) or they can be much longer, complex chains. As the hydrocarbon molecules get longer, they move from existing in a gas state to a liquid state. The liquids become progressively more waxy until, eventually, they have the consistency of tar. This means that the longer and more complex the hydrocarbon chain is, the more difficult it is to get it out of the ground, as it cannot flow freely.
In his work, Adam has been principally concerned with the search for hydrocarbons (for oil exploration) and to see how rocks exposed at the surface can give us information about rocks below the surface (in the subsurface) that might contain oil or gas.
By studying what people have found out previously about the geology of a particular area, Adam first conducts field studies where he describes and collects the rocks that are exposed at the surface and then makes maps of where they are exposed.
From this, Adam develops a storyline of how these rocks were formed and how the area is likely to look beneath the surface. This helps him to identify places where there are rocks that might contain oil. Adam produces maps from this information that provide guidance and information for other scientists.
Seismic reflection data
Seismic reflection data traces waves travelling through the rocks (similar to earthquake waves). These waves change speed and reflect (bounce back) at the contact between different rock types. This provides clues to the geology below the surface.
Adam’s area of study is the Taranaki region in the North Island of New Zealand, which was first explored for oil resources as early as the late 1890s.
Natural gas in Taranaki
Adam Vonk explains that we are more likely to find natural gas in the Taranaki basin. This is because the source rock is mostly coal, which is generally considered to be a gas-producing source rock. There is a small amount of oil in the Taranaki region, but oil fields such as Tui oil field are often smaller than gas fields such as Maui.
Points of interest:
A source rock is also called a source bed and refers to an organic sediment or rock that contains and can release oil or gas when it is heated during burial. Note that sedimentary rocks are formed when sediments are deposited and then compacted under pressure.
Through the rapid development of technology, the ways to identify rocks that contain hydrocarbons (oil and gas) have become more and more accurate. In early days, people first explored where oil seeped out at the surface. Now, scientists often look in particular for porous rocks (rocks that contain small holes to hold oil and allow oil to seep through) buried deep below the Earth’s surface.
They also get information through two- and three-dimensional seismic reflection data, which allows them to trace how waves travel through the rocks (similar to earthquake waves). These waves change speed and reflect (bounce back) at the contact between different rock types. This provides clues to the geology below the surface.
Mapping rocks onshore
Adam Vonk’s PhD research has involved mapping where sedimentary rocks are found in the Taranaki region in New Zealand, to see how this information can help identify new fossil fuel reserves. He explains that there is quite a lot of information and research about offshore resources. However, only a little is known about onshore sedimentary rocks and how this gives clues to finding fossil fuel resources. Petroleum companies and the New Zealand government have produced a book about the geology of the Taranaki basin, but Adam found that it did not describe how the offshore and onshore geology are related to each other.
Adam explains that, by examining onshore rocks and determining their history, it is possible to predict what sort of rocks may be found beneath the land surface and how this information provides clues about offshore rock layers. This information explains how rocks are distributed, and this allows oil companies to have a better idea of likely places to explore for fossil fuels.
Jargon alert
Each field of science has its own language – here are a few of the terms Adam uses:
Sedimentary rocks – rocks formed from the deposition, consolidation and compression of fine particles (sediments of various sizes) that came from pre-existing rock, which can be igneous, sedimentary or metamorphic in origin.
Hydrocarbon – an organic compound made up of hydrogen and carbon. Fuels such as oil and natural gas are hydrocarbons.
Stratigraphic – studying the layers of rocks. A geological layer is called a strata, and stratigraphy looks at these layers to determine how old they are and what they are made from.
Monograph – a detailed book on a specific topic
Points of interest
The last large-scale mapping of the Taranaki onshore region was undertaken during World War II in an effort to find fuel to help with the war.
To drill an onshore hole, it costs around 5 million dollars, and it costs 50 million for an offshore hole.
One of the questions Adam came across very early in his career was “How it is possible that you can find shell fossils at the top of a hill?”. The formation of rocks is often a complex process, but put simply, shell fossils that are found on the top of a hill have been uplifted out of the sea, where they originally formed, because seashells usually form in the sea. It’s this sort of logic that a geologist like Adam uses when he is trying to work out the geological history of the rocks he is studying.
Marine fossils on hilltops
Adam Vonk explains how it is possible that we can find a marine fossil – remnants of an organism that lived in the sea – on the top of a hill. Originally, the fossil would have been deposited on the sea floor, and the organism would have been compacted and became fossilised. Then the rock that the fossil sits in was uplifted. In the Taranaki basin in the North Island of New Zealand that Adam has studied, there has been a great deal of uplift, with the greatest uplift occurring in the centre of the island.
Jargon alert
Each field of science has its own language – here is a term Adam uses:
Uplifted – upheaval and lifting of the Earth’s crust, which is typically caused by tectonic plates moving and pushing each other.
A key question that Adam asks himself is “How old are these rocks?”. To solve this problem, Adam uses fossils of relatively large shells (macrofauna) and relatively small shells (microfauna) that are usually the size of a grain of sand. The shells we see on beaches, seas and oceans today are only a small fraction of all the shells that have ever lived in the waters around New Zealand, and they have evolved over time, often over millions of years, into many different species. By looking at the different species of shells that are found within the rocks, Adam can work out the relative age of the rocks. The process of using fossils to work out the age of rocks is called ‘biostratigraphy
Ancient environments
By looking at fossils, Adam Vonk can determine two things. Firstly, he can work out how old the rocks are, and secondly, he can work out the paleoenvironment of the rock. Paleoenvironment refers to the ancient environment that the fossil formed in.
By looking at one family of organisms and tracing how they have changed or evolved over time, it is possible to work out the approximate age of the rock. For his research, Adam looks at scallops. He knows if he finds a scallop shell in the rock that existed 10 million years ago, then the rock must be around that age. If he finds another scallop shell that existed five million years ago, that the older one evolved into, he knows that the rock must be younger.
To work out paleoenvironments, scientists work on the basis that the Earth back in time was similar to how the Earth is today. Adam reconstructs paleoenvironments by looking at the rock the fossil is in and asking: Where would this organism have been at the time of fossilisation? What would be a current environment we can find on the planet now that would be similar?
Points of interest
Discuss the environment where we can find scallops today.
Fossils also help Adam work out what sort of environment a rock was formed in. For example, if Adam finds a rock that contains sand and shells that are usually found on modern day beaches, then it is likely that rock was deposited on an ancient beach. The use of fossils for working out the environment a rock was deposited in is called ‘paleoecology’. This technique also allows geologists to work out what the climate and sea level was like in the past.
In light of the debates around climate change, Adam thinks that, in the future, people will change how they use oil. New projects will look at ways that oil can be used without the damaging impact on the environment it has had in the past. Oil has been a product of great importance to humanity, and apart from fuelling transportation and electricity production, it is being used to produce everyday products like plastic, pantyhose, lipstick, false teeth, shoe polish, pharmaceuticals like aspirin, as well as hearing aids and heart valves.
Adam thinks that there is much future in this field of science.
