The success of pre-stack depth migration over the Anama structure in the Papuan Foreland Basin, PNG: a case history

Abstract

The successful application of Pre-Stack Depth Migration (PreSDM) over the Anama structure has been demonstrated by the recent drilling of the Anama-1 well. The effectiveness of traditional depth conversion methodologies is limited in the Papuan Gulf Area due to significant lateral velocity variations introduced by thick Tertiary carbonates, channelling and shallow gas accumulations in Pliocene-Recent sediments. As a result the validity of Mesozoic structural closures previously drilled in the offshore Papuan Foreland Basin is highly questionable. Sparse well control, and stacking velocities significantly affected by raypath distortions, hinders the definition of accurate velocity fields. In an attempt to resolve these limitations and improve the accuracy of the interpreted velocity fields, PreSDM was applied to a modern 2D seismic grid acquired in 1998. Detailed velocity interpretation was conducted using coherency inversion across the seismic grid over the entire Anama structure. The resultant velocity model was tied across the seismic data set prior to generating the PreSDM sections. The final depth interpretation was completed on a workstation using the PreSDM sections. The PreSDM based depth map was used to locate the Anama-1 well. A depth closure was identified with the interpretation of the PreSDM sections where no valid closure was present in the time domain. Drilling results from Anama-1 showed the maximum error for predicted tops based on the PreSDM processing was within 1.2% while the error for the predicted top target sand was less than 0.5%. PreSDM processing is principally aimed at improving imaging and lateral continuity. However by tying the velocity model used in the PreSDM processing across a seismic grid it was possible to improve the accuracy of absolute depths without detrimentally affecting both imaging and lateral continuity. In this case history it was possible to use PreSDM as a predictive tool for absolute depths.