Subsidence and thermal history modelling:new insights into hydrocarbon expulsion from multiple petroleum systems in the Browse Basin

Subsidence and thermal history analysis of 31wells and 25 depocentre sites has been undertaken to examine the generation and expulsion history of Jurassic and Early Cretaceous petroleum systems in the Browse Basin. The models incorporate new palaeo-bathymetric estimates of the deepwater post-Valanginian to Early Tertiary succession based on seismic geometries and detailed benthic foraminiferal data, as well as new kerogen kinetic data for multiple potential source units.

The models suggest multiple effective source units for gas expulsion in the basin, whereas effective oil charge is largely restricted to the Heywood Graben in the northeast, and the rift section in the deepwater Serringapatam Sub-basin. Significant quantities of oil are modelled to have been expelled from Jurassic sediments (Plover and Lower Vulcan formations) in the Heywood Graben during the Tertiary and Late Tertiary, respectively. These charges are likely to have sourced the thick palaeo-oil columns interpreted at Heywood-1 and Crux-1.

Modelled oil expulsion in the deepwater Seringapatam Sub-basin is highly sensitive to the organic richness and quality of interpreted Jurassic rift sediments. Provided these units contain good quality source rocks comparable to those within Jurassic rifts elsewhere on the North West Shelf, then significant quantities of oil and gas are modelled to have been expelled from this graben during the Late Tertiary. Evidence of such an oil charge may be provided by thin zones of elevated GOI data in Jurassic reservoirs at North Scott Reef-1, Scott Reef-2A and Brecknock-1, all located on a major anticlinal trend inboard of the Seringapatam graben.

Regional models of the recently identified, oil-prone, Early Cretaceous (Echuca Shoals Formation) petroleum system suggest that these rocks have not expelled significant volumes of oil. However, organic-rich sediments within this succession occur as thin transgressive sheets deposited in response to fluctuating sea level on a gently inclined ramp margin, and thus higher-resolution sequence stratigraphic models are required to more accurately assess their local expulsion history. Local effective oil charge from these sources is confirmed by the Cornea, Gwydion-1 and Caswell-2 accumulations.

The expulsion models highlight the importance of Late Tertiary carbonate clinoforms in controlling the generation and expulsion history of effective source rocks in the Browse Basin. The rapid progradation of these carbonates is likely to cause an outward-migrating compaction wave that forces expelled fluids and hydrocarbons to outboard areas. This mechanism may apply elsewhere on the North West Shelf where carbonate clinoforms are similarly well developed, and could provide new exploration opportunities in outboard areas.