The Vesta oil and gas field, Vulcan Sub-basin, Timor Sea, Australia

Abstract

The Vesta oil and gas field is located in the Swan Graben of the Vulcan Sub-basin. The structure consists of a number of separate tilted fault blocks located on a northwest-trending accommodation zone that forms a high, separating the southeast-dipping half-graben of the Swan Graben North from the northwest-dipping half-graben of the Swan Graben South. Drilled in 2005, Vesta–1 intersected a 17 m thick oil-bearing slope-fan sandstone of the Late Jurassic Elm Sandstone in the Lower Vulcan Formation. Drill-stem testing flowed oil and gas and indicated that the reservoir is normally pressured surrounded above and below by over-pressured claystone. In 2007, Vesta–2 intersected gas-bearing sandstone in a separate fault compartment. Understanding the geometry of the hydrocarbon-bearing Elm Sandstone reservoir has proved a challenge due to the very poor 3D seismic imaging, the variable sandstone thickness and quality, and abundant evidence of thin steeply-dipping injected sandstones above and below the main reservoir sandstone. The Lower Vulcan and Upper Vulcan Formation claystone provides the vertical and lateral seal for the Elm Sandstone. This thick seal has protected the Vesta oil and gas accumulations from the effects of the Late Tertiary tectonism, which had a significant effect on the integrity of the palaeo-oil filled closures on the adjacent Eclipse Trend. Three phases of hydrocarbon charge of the Vesta structure have been identified with oil-source correlation indicating a Lower Vulcan Formation marine source. The source interval intersected in Vesta–1 is presently post-mature, with oil and gas generation associated with high heat flow in the Late Jurassic. Expulsion of hydrocarbons from the source was most likely compaction-driven, with gas expulsion in the Early Cretaceous, and oil expulsion much later with increasing hydrocarbon saturation.