GC profiles of oils reflect organic maturity rather than depositional environment of parent source rock: examples from Central Australia

Abstract

Review and analysis of 1332 gas chromatography (GC) n-alkane traces of oils from the Cooper and Eromanga basins indicate the shape of any GC trace profile is primarily controlled by the degree of organic maturity (early, peak or late) at which the oils were expelled from the parent source rock, rather than indicating the depositional environment, and hence organic composition, of that source rock. The depositional environment of a source rock may still be inferred, however, from the position of the n-alkane maximum on the GC traces of early expulsion oils in association with the pour point of the oil. Departures of GC trace profiles from the standard early, peak or late expulsion profiles can indicate mixing of oils of different maturities, while variations in the GC trace profiles of oils within adjacent reservoir units may indicate phase separation of the parent liquid, or possible seal breach by an accumulation that exceeds the capacity of its overlying seal.

1. KEY POINTS

2. GC trace profiles of 1332 oils from across the Cooper and Eromanga basins of central Australia have been reviewed.

3. Organic maturity, rather than organic composition, of the parent source rock controls the shape of any GC trace profile.

4. All early maturity oils display a consistent GC trace profile shape that is different from all peak maturity oils and different again from all late maturity oils.

5. Depositional environment of the source rocks within a basin can be inferred from the relative pour points of the resultant oils.