Roger E. Summons¹, Graham A. Logan¹, Dianne S. Edwards², Christopher J. Boreham³, Marita T. Bradshaw¹, Jane E. Blevin¹, Jennifer M. Totterdell², John E. Zumberge⁴

(1) Australian Geological Survey Organisation, Canberra, Australia
(2) Australian Geological Survey Organisation, Canberra, ACT, Australia
(3) Australian geological Survey Organisation, Canberra, Australia
(4) GeoMark Research, Inc, Houston, TX

ABSTRACT: Geochemical Analogs for Australian Coastal Asphaltites - Search for the Source Rock

Heavy asphaltic crude oil commonly strands on beaches on Australia's Southern Margin. Conventional biomarker approaches indicate that the likely source rock is devoid of terrestrial organic matter and is a deep-water, marine shale or calcareous shale of Late Jurassic or Cretaceous age. The history and locations of the strandings suggest that they arise from the local offshore basin. However, drilling in this region is sparse and confined to shallow water and the specific source rock unit has not been identified1.

In this study, we measured a standard set of geochemical attributes known as OilMod parameters for a suite of asphaltite samples. We then compared these data directly and using statistical methods to a large global data set comprising approximately 4000 crude oils, which represent all the important and productive petroleum systems presently known. No exact analog was found although the asphaltites have many features in common with oils sourced from the Late Jurassic Bazhenov-Neocomian petroleum system of the West Siberian basin and some Cretaceous oils from the Middle East. The closest local analogs are bitumens from the Albian Toolebuc Formation from the Eromanga Basin. This marine organic rich source rock is immature to marginally mature throughout its known onshore distribution but offshore counterparts with similar organic facies may be developed in deep-water Cretaceous successions of the Bight Basin. Seismic data is being used to map the location and extent of likely depot centres. We also report on the geochemical methodology and the value of large, internally consistent geochemical data sets.

1. Edwards D.S., McKirdy D.M. and Summons R.E., 1998, Enigmatic asphaltites from the Southern Australian Margin: Molecular and carbon isotopic composition. Petroleum Exploration Society of Australia Journal 26, 106-129.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado