DEPOSITIONAL SEQUENCE MODEL FOR THE POST-BARROW GROUP NEOCOMIAN SUCCESSION, BARROW AND EXMOUTH SUB-BASINS, WESTERN AUSTRALIA

Abstract

Structural traps at the top 'Barrow Group' are the most successful oil exploration targets in the Barrow/Exmouth Sub-basins. However, a reinterpretation of recent exploration activities undertaken by BHP Petroleum Pty Ltd, combined with regional investigations on the Exmouth Plateau, has cast doubt on the validity of accepted stratigraphic nomenclature for the Neocomian succession. A more geologically rational subdivision of the upper part of the Neocomian succession into two discrete sequence stratigraphic units is proposed.

Key seismic data from the Exmouth Plateau, tied into wells with good age control, have enabled precise recognition of the Intra-Valanginian Unconformity within the currently-defined Barrow Group. The Barrow Group (sensu stricto) is redefined in this paper as the Barrow Megasequence (restricted to a Berriasian age succession), comprising a rapid progradational phase, which was abruptly terminated by the Intra-Valanginian event.

Local erosion of the Barrow Megasequence along the Novara Arch through an Early Valanginian uplift, during the ensuing Valanginian regional transgression, contributed to the development of a parasitic clastic wedge, previously referred to as the Upper Barrow Delta on the Exmouth Plateau and here named the Zeepaard Sequence, with a nominated section in Zeepaard-1. The Zeepaard sequence is terminated by a Top Valanginian unconformity, upon which a final deltaic clastic pulse was deposited as the Birdrong Sequence. Each successive clastic wedge had a more limited development, with the basinward progradation terminating well short of the underlying stratigraphic unit. The Birdrong Sequence was terminated by an Intra-Hauterivian unconformity upon which the highly glauconitic, thin, Mardie Greensand Member of the Muderong Sequence was developed.

The Zeepaard Sequence and overlying Birdrong Sequence can be characterised using both seismic and well log character. Well data in particular has enabled detailed stratigraphic mapping of the Birdrong Sequence which is thin and generally not seismically resolvable across the Barrow Sub-basin. This paper presents a detailed sequence stratigraphic analysis of the Birdrong Sequence using well log data.