#gask

Are Dinos fuelling your car?

Judging by the internet many people believe that when they fill up at the pumps they are filling their cars with long dead and decayed dinosaurs. Even the most boring family hatchback sounds exciting when you think it’s being powered by liquefied T-rex. However, I have some very sad and disappointing news; it simply isn’t true. Oil does not form from dinosaurs, it forms from organisms far more boring and widespread.

So what are you filling your car with? Well predominantly a concoction of algae, plant remains and some woody material. This may sound boring, but the composition of the kerogen (defined as insoluble organic matter) can tell us a lot about the type of hydrocarbons the rock is producing. Kerogen is split into five groups; woody, herbaceous, amorphous (mainly algae), vitrinite and inertinite. Plant material is indicative of being either terrestrial or lacustrine in origin whereas algal matter tells us we’re dealing with a marine deposit.

Some of these groups contain more hydrogen in their structure than others and are known as ‘oil prone’ due to their predisposition to produce oil upon heating. More hydrogen is required to convert kerogen to oil than to gas and therefore rocks containing amorphous and herbaceous kerogen are known as oil prone. Woody and vitrinite kerogens contain far less hydrogen and therefore are gas prone. Inertinite, as the name suggests, produces neither oil nor gas and is usually disregarded.

Source rocks usually contain a mix of kerogen types meaning they will produce both oil and gas. However, kerogen isn’t the only control on the type of hydrocarbon produced, temperature also plays an important role.

This is because a rock has to be heated to a certain cut off before it starts breaking down the kerogen. Much like cooking you can either heat the rock at a very high temperature for a short period of time or at a lower temperature for much longer.

For example, there are source rocks producing hydrocarbons that are Ordovician (485 – 444 million years ago) in age where a low geothermal gradient (18oC/Km) (the increase in temperature with depth, averages at 27oC/Km) exists. In other areas, where the crust in thinner and therefore the geothermal gradient can be much higher (up to 90oC/km), Miocene (23 – 5.3 million years ago) source rocks are producing.

The temperature the rocks reach can help dictate the hydrocarbons produced. At 70 – 100 degrees Celsius (158 – 212 F) the rocks reach the oil window and the kerogen begins to transform into oil. At 100 – 120 (212 – 248 F) degrees both liquids and gas are produced in a stage known as ‘wet gas’. Beyond 120 (248 F) degrees only gas is produced and the rock has reach the dry gas window.

So by knowing the kerogen type and the temperature the rock reached, geochemists can tell the likely proportions of oil to gas that may be held in a reservoir. So even if algae are nowhere near as exciting to study as dinosaurs, they can still tell us a great deal about the rocks around us and help us to unravel the past.

Watson

References: http://bit.ly/1I7mkzE http://bit.ly/1JJyEXa

Further Reading: http://bit.ly/1dExr6p http://bit.ly/1KDZQEE http://on.doi.gov/1E6fMtT