SOURCE POTENTIAL OF UPPER TRIASSIC FLUVIO-DELTAIC SYSTEMS OF THE EXMOUTH PLATEAU

Abstract

The fluvio-deltaic rocks of the Upper Triassic Mungaroo Formation of the Exmouth Plateau contain abundant organic material in the form of both coal seams and dispersed fragments. Several thousand metres of these sediments lie within the oil generation window (0.5 to 1.3 per cent R̅O max vitrinite), but they have commonly not been considered as source rocks for oil because the organic matter is of terrestrial origin. Increasing acceptance of plant material as a source for oil warrants an assessment of the oil generating potential of the Mungaroo Formation.

The maceral composition of organic matter is dependent on the depositional environment. The large-scale uniformity of environments (mostly lower delta plain) in the Mungaroo Formation leads to a similarity in the organic matter, with an approximate composition of 90 per cent vitrinite in the coals and 50 per cent vitrinite, 20 per cent exinite and 30 per cent inertinite for the dispersed organic matter (DOM). The exception to this is organic material from the alluvial plain environment, where the coal has a vitrinite content of 60 per cent and the dispersed organic matter has a composition of 20 per cent vitrinite, 20 per cent exinite and 60 per cent inertinite.

A quantitative assessment of the oil-generating potential of the organic matter based on maceral composition, vitrinite reflectance and volume of coal and dispersed organic matter was made for a range of time-rock units. On this basis, the Carnian-Ladinian and Carnian sequences have the best source potentials.

On the basis of vitrinite reflectance measurements, it appears that the Mungaroo Formation has been generating oil since late in the Triassic or early in the Jurassic, through to the Tertiary. Lack of suitable seals during the early generative phase may have permitted some oil to escape. However, significant oil generation has occurred subsequent to the deposition of the younger Mesozoic sequences, when trapping and sealing were more effective.