Prestack Time Migration as a Crucial Step in Resolving AVO Anomalies in the Synrift Sandstones of the Bredasdorp Basin, Offshore South Africa: A Case History

Amplitude versus offset methods have been demonstrated as highly useful tools to<br>suggest the presence of hydrocarbons in the sandstones of the Bredasdorp Basin, both<br>within the drift sequence (e.g. Oribi Field) and within the synrift sequence on the<br>northern flank of the basin (e.g. F-A Field). The E-OI well, drilled near the southern<br>flank of the basin in Block 9, found a 15 m oil column in the shallow marine<br>sandstones at the top of the preserved synrift section. The sand lies immediately<br>below the lAti major unconformity and the sand itself is eroded up dip forming a<br>wedge trap, with overlying shale providing the seal Since this truncation follows a<br>regional structural trend, a sizable trap could exist containing several hundred<br>MMBOIP. With only 2D seismic available, one dip and one strike line through the<br>discovery well were reprocessed to evaluate the potential for using AVO in the synrift<br>section on the southern flank of the basin. Initially, the data were not migrated. Only<br>the strike line indicated a weak anomaly within the sand unit and misties between the<br>two lines and with the well created an interpretation dilemma. To resolve the matter,<br>prestack time migration was performed in the offset domain. Results were improved<br>with anomalies evident on both lines and ties more acceptable. With reflection energy<br>better imaged, AVO responses from both the dip and strike lines clearly show the<br>possible limits of the oil-bearing sand and even suggest the ability to interpret the oil<br>water contact. The method will be used in reprocessing additional 2D lines to map<br>the extent of the hydrocarbon-bearing sand within this subcropping wedge as well as<br>several other similar synrift discoveries in the central and southern portions of the basin.