Geochemical interpretation of partially filled hydrocarbon traps in the nearshore Gippsland Basin

Abstract

This study aims to analyse the relative importance of competing processes in determining the distribution of liquid and gaseous hydrocarbon accumulations in the nearshore Gippsland Basin. These processes include primary source/charge factors such as kerogen type and maturity at the time of expulsion, fractionation processes during migration, and secondary alteration effects such as biodegradation and/or water-washing in the presence of a strong low-salinity water-wedge that has pushed out 20–40 km from the coastline during Quaternary time. The authors find that small accumulations of liquid petroleum predominate west of Emperor, Snapper, and Bream. These small accumulations show evidence of primary charges of wet-gas, and various stages of in-reservoir alteration. Compared to liquids in the Central Deep and in the basal oil-legs of the giant wet-gas fields at Snapper and Marlin, wax contents are low, and API gravities are high, close to the Emperor-Bream line, but then decrease westwards towards the coastal region. Oil bubble-point pressures decrease westwards (despite shallower burial than the Bream, Snapper, and Marlin fields, further offshore), due to increasing under-saturation with respect to dissolved gas. The authors note that the dominance of gas in the Barracouta Field is probably due to strong recharge with unaltered wet gas but that dry gas in the Golden Beach Field suggests biogenic methane, as proven for the Baleen gasfield by carbon isotopes. The authors infer that the presence or absence of thin oil-legs in the Barracouta, Snapper, Marlin and Bream wet gas fields reflects a balance of water-washing and fresh charge. Residual oil shows below the Dolphin and Perch OWCs, and a lost column at Amberjack–1, demonstrate volume reduction following peak charge. The authors, therefore, conclude that observed underfill of structures is generally a result of in-reservoir processes post-charge, rather than a deficiency of charge or seal.