Constraining the distribution and relationship between overpressure, natural fracture density and temperature in the Cooper Basin, Australia

Abstract

To effectively and safely extract hydrocarbon from low permeability and overpressured reservoirs in the Cooper Basin, a thorough understanding of the regional and field scale distribution of overpressure, temperature and fracture density is essential. Previous research omitted the effect of fluid expansion and hydrocarbon generation mechanisms for overpressure generation in the basin, albeit reservoir temperatures have sharply increased in the past five million years. The authors collate pressure (>8,000 samples) and temperature (>6,000 samples) data from 1,095 wells across the SA portion of the Cooper Basin and incorporate natural fracture densities from 28 interpreted borehole image logs to investigate the spatial variation, and potential relationship, between pressure, temperature and natural fracture density. Results show significantly lower geothermal gradients within the Patchawarra Trough, likely attributed to a lack of shallow volcanics, blanketing coals or low uranium content. Shallow volcanics are common in high-temperature areas such as the Moomba/Big Lake and Gidgealpa fields and deeper portions of the Nappamerri Trough, with overpressured wells (>0.45 psi/ft) appearing to cluster in these areas, particularly south of the Gidgealpa-Merrimelia-Innamincka Ridge. Fracture density shows no obvious relationship to pressure, inferring a dominant structural origin for natural fracture development. Although the authors cannot exclusively attribute fluid expansion and hydrocarbon expansion mechanisms to overpressure, they likely have a profound effect. Future work should investigate the feasibility of integrating pressure, vertical stress and sonic velocity to constrain the overpressure generation mechanism within the basin while incorporating field scale seismic attribute analysis for natural fracture detection and overpressure analysis.