In situ stress and natural fractures in the Carnarvon Basin, North West Shelf, Australia

Abstract

Fractures are important with regards to permeability within the basin subsurface; thus, understanding their generation within a given stress regime is crucial to the extraction of petroleum resources. A total of 517 naturally occurring fractures are identified on 12 resistivity image logs from the Exmouth and Beagle sub-basins, the Rankin Platform and the Exmouth Plateau in the Carnarvon Basin on the North West Shelf of Australia. The fractures have been identified on 12 image logs and can be divided into two sets: (1) electrically resistive and conductive fractures striking northeast–southwest; and (2) electrically resistive and conductive fractures striking east–west. There were 235 electrically resistive fractures identified that dominantly strike northeast–southwest, and 282 conductive fractures identified that dominantly strike east–west. The latter are considered to be open for fluid flow. The in situ stress field (orientations and magnitudes) is a major control on the ability of fractures to transmit fluid. This study identified 123 drilling-induced tensile fractures and 175 borehole breakouts present in 12 image logs, and a mean maximum horizontal stress orientation of 110°. Density logs and leak-off tests were used to calculate the in situ stress magnitudes with a vertical stress gradient (Sv) of 21.7 MPa km⁻¹, a minimum horizontal stress gradient (Shmin) of 16.8 MPa km⁻¹ and a maximum horizontal stress gradient (SHmax) of 23.4 MPa km⁻¹. This defines a strike-slip faulting stress regime (SHmax>Sv>Shmin) for wells in this study in the Carnarvon Basin. The in situ stress and natural factures determined and identified in this study provide further clarity to the exploration and production processes occurring in the Carnarvon Basin.