Improving the accuracy of vertical stress magnitude determinations in sedimentary basins

Summary

This study aims to highlight sources of uncertainty in the determination of vertical stress estimates using density log data and examines the compounding impacts of various workflows for calculating the vertical stress. To achieve this, we use petroleum data sets from the Moomba Gas field in the Cooper Basin, South Australia, Australia. The datasets encompass density, gamma and sonic ‘check-shot’ logs that enable stepwise analysis of overburden lithologies. The approach employed in this study helps to determine how the accuracy of vertical stress estimates can be influenced by (1), The calibration of sonic to density transforms; (2) Check-shot survey data and preparation (3) The application of contrasting methods for sonic-density transforms (e.g. the Gardner and Nafe-Drake methods) and (4) The use of filters to remove data in zones with poor borehole conditions (e.g. coals) and interpolation between voids in data. We find that the largest source of uncertainty in vertical stress determinations is the calibration of sonic to density transforms. These transforms are used to estimate the density of rock mass above which the density logging tools start recording. with the other factors contributing to uncertainty in decreasing magnitudes. Our analyses suggest that at 3 km (i.e. reservoir) depths, workflow and data processing decisions can introduce uncertainty in vertical stress magnitude determination equivalent to 3.5 MPa.

• stress
• uncertainty
• geomechanics