RECENT ADVANCES IN THE SEQUENCE STRATIGRAPHY OF THE TRIASSIC TO LOWER CRETACEOUS SUCCESSION IN THE NORTHERN CARNARVON BASIN, AUSTRALIA

Abstract

Application of sequence stratigraphy to well and seismic data has resulted in major revisions to the stratigraphic subdivision and understanding of the hydrocarbon system in the Northern Carnarvon Basin of Australia.

A sequence stratigraphic nomenclature which integrates the biostratigraphic control and sequence terminology has been developed. This has proven to be an invaluable tool in enabling effective communication between geologists, biostratigraphers, and seismic interpreters. This nomenclature reduces the need to refer to absolute time scales and Epoch or Stage names, all of which are subject to change. The revised subdivision and new terminology can be used to reliably correlate across wide areas on the North West Shelf and possibly beyond.

Six first and second order megasequences are recognised within the Triassic to Lower Cretaceous succession. These megasequences are mostly bounded by transgressive surfaces that are expressed on well logs as abrupt facies changes and on seismic as major downlap surfaces, indicating significant acceleration in subsidence rates and an increase in accommodation space in the basin. Each megasequence consists of an initial transgressive section of mainly fine-grained clastics overlain by a regressive and usually extensive succession of coarse-grained deposits. Recognition of the significance of these megasequence boundaries, and the stages of extension, has resulted in a vastly improved understanding of surfaces, some of which have previously been mis-interpreted as 'break-up unconformities'. Two distinct stratigraphic events, a Callovian transgression and an Oxfordian Iowstand, have been recognised as separate megasequence boundaries, which has a significant impact on the prospectivity of the Northern Carnarvon Basin.