Temporal resolution of an oil charging history – A case study of residual oil benzocarbazoles from the Gidgealpa Field

Reservoired oil is usually a mixture of multiple discrete petroleum charges. Unravelling the contribution and character of each charge is amongst the most daring challenges in petroleum geochemistry. This paper describes an approach that involves the evaluation of both free and residual oils to reconstruct the history of petroleum accumulation within the multi-reservoir Gidgealpa Field, located in the Cooper and Eromanga Basins of central Australia. Porewall-coating residual oils form by reservoir wetting processes and grow continuously by adsorption of polar compounds, retaining molecular information on the first oil that was present in the pore system. Such sequentially recovered residual oils proved useful in reconstructing the record of oil charges to the Gidgealpa Ridge. Using Araucariacaean markers for source discrimination and the benzocarbazole a/(a + c) ratio (BCR) for identification of the source region, five discrete charging events were distinguished. In contrast to most previous studies, benzocarbazole distributions were found to be independent of lithofacies, organic facies and maturity. However, alkylated carbazoles did exhibit a pronounced facies dependency. The application of alkylcarbazoles as source markers in the study area provides the first evidence of the expulsion of petroleum from Cambrian marine source rocks in the underlying Warburton Basin.