NEW LIGHT ON THE ORIGIN OF COOPER BASIN OIL

The Cooper Basin in central Australia is a major producer of gas and oil. It is generally accepted that the organic matter in the Permian terrestrial sediments of the Basin was the source of the oil and gas. However, both the coals and the dispersed organic matter (DOM) are rich in inertinite and both inertinite itself and inertinite-rich organic matter have been widely discounted as a possible source for oil. Recent co-ordinated transmission electron microscope and light microscope work on the inertiniterich coals of the Cooper Basin has shown that up to several per cent of some coal samples are composed of microscopic and sub-microscopic alginite. This includes material that had previously been identified with the light microscope alone as degraded sporinite, liptodetrinite or resinite, as well as algal-derived matter, which is too fine to observe with light microscopy. Much of this material of algal origin was selectively degraded at about the time of its deposition, and this degradation appears likely to have had the effect of further enhancing its potential to yield hydrocarbons. Thus, such material should be ranked among the richest potential sources of hydrocarbons when appropriate diagenetic conditions have been attained. Since inertinite and this kind of alginite occur in particularly close association, the presence of inertinite-rich coals and DOM within potential source rocks should be regarded as a highly favourable rather than an unfavourable, indication (as in the past). The quantity of alginite in the very large volumes of inertinite-rich coal in the Basin is more than adequate to account for the oil accumulations. In the Cooper Basin the coals, rather than the DOM, had the better potential for oil generation.