Seismic Delineation of Igneous Sheet Complexes on the Exmouth Plateau (NW Australia): Origin, Emplacement Mechanism and Implications for Petroleum Generation
Exploration, Woodside Energy, Perth, WA, Australia.
The Exmouth Plateau is considered to be part of the Gascoyne Plain volcanic margin characterized by anomalously thick oceanic crust and seaward dipping reflector sequences. However in contrast to the margin, the Exmouth Plateau seems to be displaying mainly intrusive sequences consisting of sills and dikes as well as a high velocity body at the base of the crust, imaged on high resolution 2D and 3D seismic reflection data. Igneous activity is related to continental breakup during the late Jurassic/Early Cretaceous derived from the relationship between dated seismic horizons from well penetrations and high impedance igneous intrusions. The high quality data indicates two different types of sill/dike intrusions: 1) an interconnected sill/dike complex that is confined to the relatively deeper levels and 2) Isolated sills with limited areal extend connected to a very deep source, intruding very close to the paleoseabed. The absence of basalt extrusion on the Exmouth Plateau can be explained by hydrostatic calculations constrained by seismic dike thicknesses and horizontal stress. Using these estimates calculated magmatic overpressures at the magma chamber are estimated to be lower than 10 MPa. The results indicate that melt became vertically arrested owing to negative buoyancy generated by an approximately 10 km thick sedimentary sequence. This sequence mainly consists of the Triassic Mungaroo Formation, a more than 5 km thick section of deltaic sediments forming the main reservoir and source rock on the Exmouth Plateau. The influence of sills and dikes on source rocks is limited as a result of relatively thin thermal aureoles of the intrusions as well as the dispersed nature of the coaly source rocks. Since most intrusions are located fairly deep in the basin (> 5 - 6km) they are currently in the overmature window, based on burial alone. However this might have been different in the Cretaceous, just after intrusions were generated at Late Jurassic/Early Cretaceous breakup time. Nonetheless thermal influence is considered to be minor owing to the thickness of the intrusions (several tens of meters) and their limited extent. This is further evidenced by the presence of several TCF-size gas fields in the region apparently unaffected by intrusions at depth. Nevertheless, intrusions could prove to be important by influencing hydrocarbon migration paths.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012