Tim E. Ruble¹, Dianne S. Edwards², Mark Lisk³, Manzur Ahmed¹, Robinson A. Quezada¹, Simon C. George¹, Roger E. Summons⁴
(1) CSIRO Petroleum, North Ryde, Australia
(2) Australian Geological Survey Organisation, Canberra, Australia
(3) CSIRO Petroleum, Bentley, Australia
(4) Australian Geological Survey, Canberra, Australia

Abstract: Geochemical appraisal of palaeo-oil columns: implications for petroleum systems analysis in the Bonaparte Basin, Australia

Fluid inclusion oils and hydrocarbon stains from two palaeo-oil columns within the Torrens-1 well in the Bonaparte Basin of Western Australia have been analysed by detailed molecular geochemical techniques to determine their origin. Source-related biomarker parameters indicate that all of the samples were derived from marine shales containing mixed inputs from both marine and terrestrial organic material. The distribution of 25-norhopanes and n-alkanes in the samples indicates variable levels of biodegradation and a complex charge history. The inclusion oil from the Permian Fossil Head Formation is similar in composition to the associated oil stains. While these samples cannot be unequivocally matched to any recognised petroleum system, geochemical evidence such as their heavier aromatic carbon isotopic ratios suggest they are most likely derived from intercalated Early Permian source horizons. In contrast, inclusion oil from the Torrens-1 Carboniferous section appears to have a more marine signature, with a higher relative abundance of C₂₇ steranes and lower amounts of rearranged steranes and hopanes. Significantly, this sample contains uncommon tetracyclic biomarkers that have previously been shown to be diagnostic for oils derived from an Early Carboniferous source. The recognition of dual sources for the Torrens-1 palaeo-oil columns is important, as all of the wells drilled to date have targeted plays that rely exclusively on charging from a Permian source. The existence of additional source pods, capable of expelling sufficient volumes to create oil columns, opens a range of new play types and further enhances the prospectivity for liquid hydrocarbons in this lightly explored area.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana