西豪州ガス田における重合前深度マイグレーションを用いた速度モデリングの高精度化

抄録

Methods used to reduce depth uncertainty and improve image quality for the reservoir section of the Ichthys Field, Browse Basin, offshore Australia, were examined. The methods implemented were detailed tomographic velocity modelling combined with 3D prestack depth migration (PSDM) and subsequent well calibration.
Velocity analysis for 3D prestack time migration (PSTM) presented geologically implausible and undulating RMS velocity patterns at reservoir depths across the main section of the field and the reservoir depth accuracy of the result was low. Further investigation revealed that the velocity distortions were primarily induced by shallow Tertiary sequences, containing highly contrasting, narrow, elongate velocity anomalies. To solve these problems, Kirchhoff PSDM was applied.
A hybrid velocity modelling method combining layer-based tomography with grid-based tomography was applied to obtain more accurate velocities. Firstly, 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 finally derived velocity field was systematically correlated with that observed at the wells, and the corresponding depth structure produced by PSDM appeared to contain less distortion and to be more geologically realistic. The resultant velocity model is currently being incorporated into the structural evaluation and subsurface characterisation for the Ichthys Field.