Geochemical characteristics of Palaeozoic petroleum systems in Northwestern Australia

Isotopic and biomarker analyses carried out on Cambrian to Permian oils and source rocks in the Arafura, Bonaparte (Petrel Sub-basin) and Canning Basins have been used to geochemically characterise five distinct petroleum systems within the Larapintine and Gondwanan Petroleum Supersystems. The Larapintine 1 Petroleum System is characterised by isotopically light, free hydrocarbons in the Arafura Basin (δ13Csat = -32 ‰ Arafura-1) which have been correlated to kerogens of similar isotopic signature within the Middle Cambrian Jigaimara Formation. The richness and maturity of these source rocks indicate that an effective Larapintine 1 Petroleum System may exist in the northern parts of the Arafura Basin. Larapintine 2 oils, with Gloeocapsomorpha prisca-type signatures, are found on the Barbwire-Dampier Terraces and Admiral Bay Fault Zone in the Canning Basin. These oils can be correlated to source rocks in the Early Ordovician Goldwyer Formation on the Barbwire Terrace and the Bongabinni Formation in the Admiral Bay Fault Zone by their diagnostic odd-carbon-number preference in the C15–C19 n-alkanes. Larapintine 3 oils are derived from Late Devonian marine carbonates in the Canning Basin and Petrel Sub-basin and have a diagnostic biomarker signature which includes a predominance of steranes relative to diasteranes and abundant gammacerane and 30-norhopanes, similar to those observed in the Late Devonian Gogo and Pillara Formations. Larapintine 4 oils are derived from Early Carboniferous marine, clay-rich mudstones in both the Petrel Sub-basin and Canning Basin. They are isotopically light (mean δ13Csat = -28 ‰) and have a unique terpane signature which has been identified within the Milligans Formation. Gondwanan 1 Petroleum System hydrocarbons, represented here by the Petrel-4 condensate, have a heavy isotopic signature (δ13Csat = -24 ‰) which, coupled with an abundance of the diasterane and diahopane biomarkers, indicates derivation from Permian deltaic source facies. Recognition of the diagnostic geochemical components of each Palaeozoic petroleum system has led to the identification of Permian-like isotopic signatures in some hydrocarbon accumulations in the Timor Sea that were previously attributed to Mesozoic sources.

Citaton: Edwards D, Summons R, Kennard J, Nicoll R, Bradshaw J, Bradshaw M, Foster C, O'Brien G, Zumberge J. (1997) GEOCHEMICAL CHARACTERISTICS OF PALAEOZOIC PETROLEUM SYSTEMS IN NORTHWESTERN AUSTRALIA. The APPEA Journal 37, 351–379