Paleogeographic Evolution of Early Campanian to Maastrichtian Supersequences in the Caswell Sub-Basin—Implications for CO2 Storage and Hydrocarbon Entrapment

Abstract

The Caswell Sub-basin is a northeast-trending Paleozoic to Cenozoic depocentre in the Browse Basin, on the Northwest Shelf, offshore Western Australia. As part of a recent study to investigate the CO₂ storage potential of the sub-basin, sequences within the latest Cretaceous section have been high-graded as potential CO₂ storage targets based in part on their sequence stratigraphic and paleogeographic characteristics. Sequence stratigraphic analyses were used to improve the understanding of sequence architecture, facies and palaeogeography during early Campanian-Maastrichtian, a time of relative sea level fall in the basin. This phase is characterised by two supersequences (K60a and K60b), with maximum regression reached by the end of the Maastrichtian Sequence (K60b). During the early Campanian (K60a), a major sea level fall resulted in deep incision and channelling at the base of the supersequence. Wide fluvial systems draining to the north from the Kimberley Craton were captured by east-west channels that fed large submarine fan complexes across the Caswell Sub-basin. This was followed by highstand fluvio-deltaic systems that prograded to the north over the structurally controlled ramp-like inner shelf. Supersequence K60b (near base to late Maastrichtian) was initiated by eustatic sea level fall punctuated by deep incision into K60a. Large EW-trending incised valleys and wide fluvial belts provided sediment pathways from to the basin during lowstand conditions. Outboard, transgressive pro-delta shales of the younger supersequence (K60b) provide a seal for the submarine fans. As for the K60a supersequence and in comparison with sediment transport direction for the lowstand fans, the fluvio-deltaic highstand successions are characterised by rapid northward progradation along the shelf margin. The submarine fan complexes of these two supersequences formed isolated sand-rich bodies sealed by thick transgressive and downlapping highstand shales. Containment risk for the evaluation of these fans as potential CO₂ storage targets considered updip continuity of sand bodies, incision by channels, faulting and top/base seal. The studies identified a variety of CO₂ storage sites and examined the area for potential conflict with active hydrocarbon exploration.

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