Variation of vertical stress in the Carnarvon Basin, NW Shelf, Australia

Abstract

Variations in vertical stress magnitudes can be attributed to lateral facies variations, diagenesis, localized uplift and disequilibrium compaction overpressure. Vertical stress magnitudes have been calculated using density and checkshot data from 47 wells in the Carnarvon Basin, on the NW Shelf of Australia. The vertical stress ranges from 20.2 MPa km- 1 to 23.8 MPa km- 1 below seabed; demonstrating a significant variation across the basin. The largest vertical stress magnitudes were calculated for the Dampier Sub-Basin, the eastern Barrow Sub-Basin and on the Peedamullah Shelf. The smallest vertical stress magnitudes were calculated in the Kangaroo Syncline, the western Barrow Sub-Basin and offshore across the Exmouth Plateau. High rock densities, thus, high vertical stress magnitudes, can be due to uplifted rock that is over-compacted relative to its current depth. In the Carnarvon Basin, areas of elevated vertical stress magnitudes correspond to areas of identified uplift. Uplift at the Bambra Anticline has previously been estimated to be approximately 900 m. Calculations within the Bambra-4 well demonstrate that a 900 m increase in burial depth of a regionally consistent lithostratigraphic unit (the Muderong Shale) equates to a 0.22 g cm- 3 increase in rock density. Thus, the difference between uplifted, over-compacted rock and normally-compacted rock (no uplift) is equivalent to a difference in the vertical stress magnitude of 2.1 MPa km- 1 at the Bambra Anticline. This value, totalling the difference between over-compacted (uplifted) and normally-compacted sedimentary rock (2.1 MPa km- 1), accounts for some part of the 3.6 MPa km- 1 variation in vertical stress magnitude observed across the Carnarvon Basin. Therefore, recent tectonic uplift has had a significant impact on the present-day magnitude of the vertical stress and is interpreted to be responsible for the majority of observed variation of vertical stress in the Carnarvon Basin. © 2009 Elsevier B.V. All rights reserved.