In situ stresses of the West Tuna area, Gippsland Basin

Abstract

The in situ stress tensor has been evaluated in the West Tuna area of the Gippsland Basin based on petroleum well data. Borehole breakouts and drilling-induced tensile fractures interpreted on image logs from six wells constrain the maximum horizontal stress orientation to 138°N. Four leak-off pressures and one closure pressure suggest the upper bound to the minimum horizontal stress in the West Tuna area is 20 MPa/km. The vertical stress was derived from checkshot velocity, density and sonic log data and the average value from sea-level is 20 MPa/km to 1km and 22 MPa/km to 3km depth. Formation test pressures indicate that pore pressure in the West Tuna area is hydrostatic above 2800 m. The maximum horizontal stress magnitude was constrained to 39 - 42 MPa/km based on the occurrence of drilling-induced tensile fractures on the West Tuna image logs. The in situ stress regime in the West Tuna area is therefore interpreted to lie on the boundary of strike-slip and reverse faulting (SHmax > Sv Shmin). The maximum horizontal stress orientation determined herein is broadly consistent with previous orientations derived from 4-arm caliper logs from nine other fields across the Gippsland Basin. The consistent northeast - southwest orientation suggests that large-scale tectonic forces are the primary control on the in situ stress tensor in the Gippsland Basin and indeed elsewhere in southeastern Australia. The horizontal stress magnitude in the Gippsland Basin with the minimum horizontal stress approximately equal to the vertical stress, are significantly higher than in other Australian basins including the Otway Basin. The (oblique compressional) plate boundary at New Zealand may be primarily responsible for the horizontal stress orientation and high horizontal stress magnitude in the Gippsland Basin and is discussed herein.