Application of prestack depth migration across the Ichthys field, Browse basin

Abstract

The following study examines methods used to reduce depth uncertainty and improve image quality for the reservoir section of the Ichthys field, Browse Basin. The methods implemented were detailed tomographic velocity modelling combined with isotropic Kirchhoff PSDM and subsequent well calibration. Initial stacking velocity analysis for 3D prestack time migration (PSTM) revealed geologically implausible and undulating RMS velocity patterns at reservoir depths across the main section of the field, reducing image quality and reservoir depth accuracy. Further investigation with forward modelling revealed that the velocity distortions may be primarily induced by shallow Tertiary sequences, containing highly contrasting, narrow, elongate velocity anomalies. Layered/blocky modelling combined with dense residual moveout picking and 3D finite-offset tomography enabled the construction of a complex velocity model in the shallow section. Subsequent grid-based global tomography with constraints was then used for updating the entire velocity field. The final derived velocity field was more systematically correlated with that observed at the wells and the corresponding depth structure produced from PSDM appeared to contain less distortion and to be more geologically realistic. The resultant velocity model is currently being incorporated into the structural evaluation for the Ichthys Field.