Tom Kivior¹, John G. Kaldi², Simon Lang³

(1) University of Adelaide, Adelaide, Australia
(2) National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide, Australia
(3) University of Adelaide, Australia

ABSTRACT: Seal Capacity and Sequence Architecture of Major Seals in the Vulcan Sub-Basin, NW Shelf Australia

The Vulcan Sub-Basin in the Timor Sea has several structural compartments containing major petroleum fields, including Jabiru and Challis. Paleo-oil columns found within the Vulcan Sub-Basin suggest trap breach, either top seal or fault leakage. This study focuses on the potential for top seal leakage and involves regional subsurface mapping of Cretaceous seals within a sequence stratigraphic context.

Sequence stratigraphic correlations based on biostratigraphy, log motifs, cuttings and cores have been used to divide the Late Jurassic to Cretaceous succession into several chronostratigraphic intervals. Three major sequences, bound by the Callovian, Tithonian and Aptian unconformities, have been interpreted. Interpretation of regional seismic was used to define the extent of the major sealing intervals, the most extensive contains the post-Aptian maximum flooding surface in the youngest sequence.

Log motifs, x-ray diffraction data and lithological descriptions of the Cretaceous intervals suggest that the main seals are predominantly calcareous shales and marls. Mercury capillary pressure tests of cuttings and cores show seal capacity ranges from good to excellent.

The regional seal, which is part of the youngest Cretaceous sequence, supports 100m (330ft) of oil column height. Observed hydrocarbon column heights are significantly less than measured seal capacities. This implies that hydrocarbon leakage has occurred by mechanisms other than capillary seal failure, such as fault leakage or recent structural possible tilting.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado