RECENT ORGANIC GEOCHEMICAL EVALUATION OF THE CENTRAL EROMANGA BASIN

Abstract

Systematic analyses of bulk properties and of molecular composition, by gas chromatography (GC), fluorescence spectroscopy, gas chromatography-mass spectroscopy (GC-MS), reflectance photometry and 'Rock-Eval' pyrolysis of core samples from three stratigraphic bores from the central and northern-central Eromanga Basin, reveal the following:

The organic facies variations in the central Eromanga are considerable; however, the biofacies can be grouped into three major types.

The sediments do not enter the mature zone of oil generation until a reflectance level of 0.65 per cent Rv max. or higher is reached. The Birkhead Formation, with a 'hydrogen-rich' type III kerogen, is only marginally mature and the amounts of hydrocarbons already generated are minor. In the Early Jurassic sedimentary unit (Basal Jurassic Shale Unit), however, there is a distinct increase in the level of S1/Organic Carbon indices and in the maturity of organic matter. While the extracts from younger Jurassic sequences of the basin display a substantial waxy character, the bitumens derived from the Basal Jurassic Unit are highly aromatic in character and contain few saturated hydrocarbons.