THE STRUCTURAL AND STRATIGRAPHIC FRAMEWORK OF THE BEAGLE SUB-BASIN, IMPLICATIONS FOR HYDROCARBON MIGRATION AND TRAP DEVELOPMENT

Abstract

The Beagle Sub-basin is situated in the northernmost Carnarvon Basin, northeast of Dampier, West Australia. Twenty years of exploration in the sub-basin has yielded disappointing results, and despite the drilling of ten wells, the area is considered an under-explored region of the Northern Carnarvon Basin. A regional study of open-file geophysical and geological data from the Beagle Sub-basin was undertaken in anticipation of gazettal of the area in late 1992.Regional mapping of structural elements in the basin included:a complex, E/W to NE/SW-trending basin margin fault system;N/S-trending intra-basin horsts (Sable, Ronsard and Picard Blocks);an E/W to NE/SW-trending trough system (Cossigny and Beagle Troughs); and,Outer Basin PlatformFive unconformity-bounded megasequences of Triassic to Cretaceous age were identified and evaluated with regard to depositional setting, reservoir, source and seal potential. Palaeogeographic maps were produced for four time intervals and relate basin development and stratigraphy to regional eustatic cycles.Traps in the basin were largely developed during the Triassic to Middle Jurassic period of rifting, and range from pre-breakup fault blocks to post-breakup sands that developed along the margins of emergent structural highs. Potential source intervals range in age from Early Triassic (Locker Shale) to Late Jurassic. Migration and entrapment of hydrocarbons from the principal source areas (the Cossigny and Beagle Troughs) is largely dependent on effective pathways and the presence of a regional Aptian age seal. Extensive wrench reactivation in the Miocene has occurred along the basin margin fault system, and may breach existing traps as well as create new plays.