Palaeo and in-situ stress analysis of the Perth Basin: implications for subsurface fluid flow.

Abstract

Understanding the orientation, stress conditions, and fluid-flow history of natural fracture networks can provide tighter constraints on the fracture genesis and tectonic evolution of the Perth Basin, Western Australia. To date, combined data from King et al. (2008) and Bailey et al. (2012) indicate a present-day transpressional stress regime, and a maximum horizontal stress orientation of 076°E. However, palaeo stress and the timing of fracture generation are poorly constrained. A comparison and analysis of present-day stress, and palaeo stress orientations and magnitudes, is integrated with an analysis of natural fracture network orientations, using data from wellbore image logs, well tests (leak-off tests) and sample analysis across eight petroleum drill holes in the Perth Basin. Stress indicators in wellbore image logs are analysed to further constrain present-day stress, and Etchecopar’s (1984) computerised Calcite Stress Inversion Technique is used to constrain the palaeo stress conditions in the basin. This data is then used to establish a four-dimensional understanding of the tectonic history of the Perth Basin, and the potential structural permeability of its tectonic features. From analysis of stress indicators, the mean regional maximum horizontal stress orientation in the study area is 089.9°N. Through fracture analysis, orientations comply with past studies (Bailey et al., 2012; King et al., 2008), as defined by a clear set of resistive fractures dipping 157°-337° (NNE-SSW), reflecting palaeo-basinal stresses and present-day in-situ stresses. Magnitudes highlighted in the Perth Basin closely match previous works defining the basin as a transpressional (σHmax> σV≈σHmin) stress regime. This includes a vertical stress profile established in seven wells at 22 MPa km-1. The minimum horizontal stress was measured using two leak-off tests and formation integrity tests at 14 MPa km-1. The maximum horizontal stress magnitude was measured using frictional limits (Kirsch, 1898) at 30 MPa km-1. Calculated palaeo stress tensors indicate that polyphase deformation has occurred in the Perth Basin. These tensors include; a NNWSSE compressional event attributed to Early to Middle Triassic N-S compression; Late Triassic to earliest Jurassic E-W extensional phase; and a NW-SE extensional regime in the Jurassic concluding with Post-Neocomian, N-S compression.