The Vesta Structural Complex and the Elm Sandstone—Tectono-Stratigraphic Evolution in the Vulcan Sub-basin, Timor Sea, Australia

Abstract

The Vesta structural complex encompassing the Vesta oil and gas field is located in the Swan Graben of the Vulcan sub-basin on a northwest trending accommodation zone high. Vesta is an arrangement of separate to partially-separate fault blocks within the Late Jurassic Elm Sandstone of the Lower Vulcan Formation, bound by mainly northeast-trending faults, the result of extension during the late Middle Jurassic to Early Cretaceous. The structure is also influenced by the older Permian-Triassic northwest-trending Swan accommodation zone. Late Jurassic (Oxfordian) slope-fan Elm Sandstone was intersected within the Vesta structural complex by exploration wells Vesta-1 (oil and gas discovery) and Vesta-2 (gas discovery). The Oxfordian interval in the Swan Graben and on the Eclipse high to the east is found to contain steeply-dipping injected sandstones both within and below the main Elm Sandstone. Seismic interpretation of the Elm Sandstone has been problematic in the past due to very poor seismic imaging and structural complexity. The latest multi-azimuthal reprocessed 3D seismic volume is much improved at the Jurassic level (for which it was designed) and has provided for the first time an opportunity to identify and interpret the Elm Sandstone package within the Swan Graben. This has led to the development of a depositional model guided by seismic data that can be linked from the Swan Graben around to the adjacent Eclipse high over the main basin-bounding faults. The interpretation of Vesta's structural evolution has been aided by the improved seismic data with on-lap onto the top of the Elm Sandstone package, previously unidentifiable, indicating that Late Jurassic sedimentation was coincident with a period of significant change in tectonic activity. The Elm package thins onto the Vesta high where a pinch-out edge is interpreted on seismic. Syn-depositional tectonic uplift of Vesta, possibly associated with the growth of the nearby Swan diapir, created accommodation space for thick Elm package development into the graben to the northeast, and in-front of the basin-bounding faults to the southeast. It is likely that this thick Elm section provided hydrocarbon carrier beds aiding charge from the Late Jurassic Lower Vulcan source rocks in the Swan Graben to oil fields to the northeast of Vesta (Jabiru, Challis etc.). It also serves as a prospective reservoir target in the Swan Graben and Eclipse high areas and as potential feeder beds for the sandstone injectites.

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