Establishing Hydrocarbon Charge to the Ashmore Platform, Browse Basin, Australia: A Natural Seeps Study

Abstract

The Browse Basin is located offshore, along Australia's North West Shelf, a proven hydrocarbon province. To date there is no evidence to support the accumulation of hydrocarbons on the Ashmore Platform, with the migration of hydrocarbon charge remaining an exploration risk. Elsewhere in the basin there is proven lateral migration of generated hydrocarbons to the basin bounding highs, as evidenced by oil and gas accumulations (e.g., Cornea, Gwydion) and hydrocarbon seeps located on the Yampi Shelf. In order to establish if migrating subsurface hydrocarbons reach the flanks of the Ashmore Platform basement high, a natural hydrocarbon seeps survey was designed to investigate sites of potential natural hydrocarbon seepage. The integrated study, undertaken in December 2013 and January 2014, utilised remote sensing, geophysical, acoustic and photographic techniques to characterise each site. A combined total of 17 piston cores and vibrocores were recovered from 8 sites, with sediments subject to headspace gas and organic geochemical analysis. Three areas showed potential indications of natural hydrocarbon seepage: one area characteristic of persistent seepage, with two areas of observed and interpreted episodic leakage. Although headspace gas compositional data showed concentrations consistent with background levels, geochemical data collected at one of the sites provided strong evidence of thermogenic liquid hydrocarbon migration, with isotopic compositions falling within the range of values exhibited by oils sourced from the Lower Cretaceous Echuca Shoals Formation. The identification of active natural seepage along the southern flank of the Ashmore Platform provides strong evidence that hydrocarbons generated within the Caswell Sub-basin laterally migrate onto the flanks of the Ashmore Platform structural high. As such, these findings reduce the hydrocarbon charge risk assessment for the Ashmore Platform and as a corollary reduce regional exploration risks.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015