Variation of vertical stress in the McArthur and Beetaloo basins

Abstract

The McArthur and Beetaloo basins have been the focus of unconventional hydrocarbon exploration, specifically targeting shale gas and tight gas resources. Vertical stress (SV), which is one of the three principal stresses, is a critical parameter in geomechanical analysis of sedimentary basins and has implications in fracture gradient calculation, pore pressure prediction, assessing the present-day stress regime, and hydraulic fracturing design. In addition, variations in vertical stresses can yield additional insights into the tectonic history of an area. In this study, we examine the variability of vertical stress magnitude and vertical stress gradients by employing a systematic analysis of density log and check-shot velocity data obtained from 31 wells located within the McArthur and Beetaloo basins. Our analysis revealed that vertical stress gradients range from 23 to 27 MPa/km (equivalent to ~1.0–1.2 psi/ft), with an average gradient of 25 MPa/km ± 1.0 (1.1 psi/ft). This variability in SV gradients underscores the inadequacy of the commonly assumed 1.0 psi/ft approximation for geomechanical analyses within these basins. The origins of such diverse SV gradients can be attributed to several factors; however, the likely cause of the SV gradient within this region is the presence, thickness, and depth of the shallow high-density volcanic rocks and a complex history of uplift and erosion.