Limestone as an oil reservoir
The world’s largest oil fields are contained in limestones. By contrast, New Zealand’s oil deposits are mostly sandstone based. However, limestone associated with the Tikorangi Formation in the Taranaki Basin is a hydrocarbon reservoir.
Oil and gas reserves
There is no such thing as a ‘pool’ of oil or gas trapped underground that can be tapped into and pumped to the surface. Almost all oil and gas is found within the tiny spaces in deeply buried sedimentary rocks such as sandstones and coarse-grained limestones. These porous rocks can be likened to a hard sponge, full of tiny holes, but not compressible. The oil or gas is trapped inside the rock’s porosity.
Oil formation
This animated video demonstrates the formation of hydrocarbons over millions of years in the depths of the Earth.
Oil and gas forms in sedimentary ocean basins. Dead plankton (organic plant and animal material) gets deposited together with mud, sand and other sediments. In this early stage the plankton/sediment layer is called sapropel. Through an increase in temperature the sapropel is converted into kerogen. Folding or faulting forms oil traps where oil and gas can accumulate in the pore space of a source rock below the trap otherwise it will escape to the earth's surface as seepage. The liquid or gas that forms is known as 'hydrocarbons
When the well bore penetrates into the reservoir, the great pressure that the gases and fluids are under, due to deep burial, is released. This then pushes fluids into the well bore and lifts them to the surface.
Oil and limestone
The world’s largest oil and gas fields are mostly contained in porous limestones, formed millions of years ago in tropical marine environments. New Zealand limestones are mostly of the cool-water or temperate category, and because of deep burial in their conversion from sediment to rock, they have very low porosity. However, these hard rocks have been fractured by compression caused by tectonic plate movements and have developed a fracture porosity. These fractures can be infilled with gas, oil and/or water depending on the make-up of the deeper rock layers.
Oil and gas in rocks
Adam Vonk explains that, to find hydrocarbons, scientists first need to identify a source rock that has been buried deep enough so that the temperature and pressure drive off or separate the hydrocarbons from the kerogen – the organic material the hydrocarbons were formed from. Once they have separated from the rock, the hydrocarbons will move into an oil trap to form a reservoir. A trap is an area that the oil collects in.
This process is called migration. Reservoirs are porous rocks. Once the hydrocarbon is in the reservoir rock, it is then trapped by an impermeable rock layer, called a seal rock. The seal rock stops the hydrocarbons migrating further. All this must happen deep within the Earth to avoid the hydrocarbons degrading in quality.
Jargon alert
Each field of science has its own language – here are a few of the terms Adam uses:
Source rock – a rock that contains the raw materials from which hydrocarbons will eventually form. They are rich in organic matter.
Kerogen – the type of organic material in the source rock – the chemical compounds that make up the organic matter.
Hydrocarbons – a molecule made up of hydrogen and carbon. Fuels such as oil and natural gas are hydrocarbons.
Porous – having tiny holes or pores that a fluid or gas can enter. Porous rocks allow fluids to seep into them.
Points of interest
Hydrocarbons can also migrate along fault lines, as well as traps – why is this so?
Taranaki Basin oil and gas
Taranaki Basin covers an area of about 330,000 km2 and is currently the only oil and gas-producing basin in New Zealand. Although the reservoir rocks are mostly sandstones, one of them is fractured Oligocene limestone similar to that found in the Waitomo region of the mid-west North Island. This block of limestone is located in the Tikorangi Formation, and the onshore Waihapa-Ngaere oil field taps into this hydrocarbon-bearing 170-metre thick limestone deposit.
Waihapa Production Station
The Waihapa Production Station consists of the Waihapa Oil Plant and the Tariki Ahuroa Gas Plant. Daily production is around 15,000 barrels of oil and condensate and 40 million cubic feet of natural gas.
This oil field was discovered in 1988 and is part of what is commonly known as the TAWN field complex. The TAWN fields currently produce an estimated 2400 barrels per day of oil and condensate and 21 million cubic feet per day of gas. (The oil industry most often uses non-SI units to express volumes of gases and liquids: 1 barrel ~ 159 L; 1 cubic foot ~ 28.3 L.)
Tikorangi limestone compared to Ōtorohanga limestone
The Tikorangi limestone lies 2 km below the surface just to the east of the Taranaki town of Stratford. It is muddier, finer and compositionally slightly different from the Ōtorohanga limestone, some of which is being extracted from the Ōparure Road quarry at Te Kūiti situated 150 km north-east of Stratford. However, it is of the same age, so it was forming at the same time as the quarry limestone.
Core sample from the Tikorangi limestone
The tiny cracks and fissures present in the Tikorangi limestone have been filled with hydrocarbons. These were formed in sediments lying below the limestone and have migrated upwards.
Given that the Tikorangi limestone is hydrocarbon bearing indicates that its underlying source rocks have a different structure and composition from those associated with the Ōtorohanga limestone. The sediments that developed into these source rocks contained an accumulation of once-living plant and animal remains. Over many millions of years, exposure to high pressures, high temperatures and a changing chemical environment has converted these remains into hydrocarbons. These have then migrated upwards, filling in the tiny cracks and fissures present in the fractured limestone sitting above.
Researchers from the Earth and Ocean Sciences Department at the University of Waikato have worked alongside oil industry and New Zealand Government Ministries to unravel the geologic transformations that have led to the establishment of the Tikorangi Formation.
