Geochemical Analysis of Oils and Condensates from the Gippsland Basin: Implications for Future Prospectivity
Grant, Cameron W.
Honours Degree, 2004
University of Adelaide
This study represents the results of the first regional geochemical investigation of the light components (C5-C8 range) of oils from the offshore Gippsland Basin, Australia. Gas chromatography (GC) traces for 114 oil/condensate samples from 28 fields were analysed and interpreted. The oils/condensates display considerable variations in chemical composition due to the effects of secondary alteration processes, namely biodegradation and water-washing.
A large freshwater wedge is present along the northwestern flank of the Gippsland Basin. The GC traces of samples from wells located in the freshwater wedge showed deficiencies in aromatics, indicating that invasion of the aquifer system had modified the petroleum system. However, analysis of the GC traces identified phase fractionation as the first-order alteration process affecting the hydrocarbon composition and distribution in the basin.
The 114 samples from the Gippsland Basin were interpreted as pristine oils, evaporative products, and residual products. Evolving residual and evolving evaporative phases were also recognised. The lighter evaporative products typically have a high abundance of normal alkanes with a low waxy component and depleted aromatics. The residual products have a high waxy component with a low abundance of normal alkanes and an enrichment of aromatics.
The hydrocarbons in wells along the northern flank are dominantly residual products of phase fractionation. Residual products are usually reservoired at deeper depths (2,000–3,500 m) and have API gravities between 30o and 40o.
Evaporative products dominate the northwestern flank of the basin, although some wells on the northwestern flank also contain residual products remaining from the fractionation event. Evaporative products typically occur in reservoirs at shallow depths (<2,000 m) and have high API gravities (typically 45–60o).
Pristine oils are largely confined to the central part of the basin and are characterised by API gravities between 40o and 50o. Such oils are capable of sourcing both the evaporative and the residual products.
There are four wells in the Gippsland Basin from which samples of both phases were collected and thus providing diagnostic evidence for phase fractionation. The samples from Whiting-2, on the northwestern flank of the basin, show an evaporative product that was collected from a depth of 1409m, and a deeper residual product that was collected from 2615m. Recombination of their GC traces resulted in a trace similar to a pristine oil, such as those observed in the deep central part of the Gippsland Basin.
The interpretation of phase fractionation as a primary process within the Gippsland Basin has significant implications for future exploration strategies. Identification of phase fractionation suggests that, depending on the trap style and the drive mechanism, potential residual phase reservoirs may exist either below or ‘inboard’ from the evaporative phase fields along the northwestern flank of the Gippsland Basin.