The origin of fossil fuels

Sand and mud deposited in low-lying basins, such as seas or lakes, build up in layers that can eventually form sedimentary rocks like sandstone. Microscopic plants that are trapped, crushed and heated in these sedimentary deposits can become layers of coal, oil or gas.

Commercial natural gas from the Otway Basin in the South East of South Australia had its source in the organic material trapped within cracks in folded rocks and sandstones such as the Pretty Hill Formation. The gas has been trapped below the shales of the Laira Formation and is extracted from wells north west of Mount Gambier.

Other oil and gas reserves in South Australia come from the Cooper Basin. They are found in the 250 million year old rocks deep in the Great Artesian Basin which lie below the Eromanga Basin. The gas comes from coal deposits that formed in cool, high-latitude forests and swamps in the early Permian Period. Coal formation ended with an ice age that covered much of southern Australia with glaciers. The evidence of this Permian ice can be seen at Hallett Cove, south of Adelaide, where glacial pavements and glacial sediment is exposed on the coast.

Layers of Earth History in South Australia

This cross-section shows a simplified record of the sedimentary formations deposited in South Australia over hundreds of millions of years. These layered sedimentary rocks record the history of changes in sea-level and climate through time. Geologists study these rocks using surface outcrops, drill holes and seismic records to locate the likely sources of petroleum and mineral resources.

Source Rock for Natural Gas

A core sample of the Pretty Hill Formation from the bore hole, Banyula 1, in the Otway Basin from SE South Australia, about 50 km north of Mount Gambier. The Pretty Hill Formation is a sandstone reservoir for natural gas. It was formed as rivers valley and delta deposits on the edge of Australia, 120 million years ago.

Fossil Pollen

Fossil pollen from the Cretaceous sedimentary rocks of the Otway Basin, show that there were many kinds of trees, shrubs and ferns that live in cold climates and moist climates today. The pollen of extinct vegetation can be use to correlate (match up) sedimentary rocks of the same age in different places.

Fossil Logs

Fossils of sunken conifer logs, like the modern Bunya pine, are found in the Bulldog Shale near Coober Pedy. They are clues to the nature of the forests in South Australia, in the Cretaceous Period. Plant material from this time produced the natural gas that we use today.

Forests of conifers such as Bunya pines, were common in southern Australia in the Cretaceous, 120 million years ago.

Dinoflagellates

Of the wide range of single-celled phytoplankton found as microfossils in the Eromanga sediments, dinoflagellates are the most common. They are the microplankton responsible for 'red tides' in modern oceans.

Phytoplankton formed the base of the food chain in the Eromanga Sea, by using sunlight to convert carbon dioxide to organic chemicals. Their buried remains are a major source of the oil and gas that is the fossil fuel we use in Southern Australia.