Eocene Seismic velocity Anomalies From the Timor Sea

Several wells in the Timor Sea have encountered Eocene sections with anomalously high velocities. This paper presents case history examples of wells drilled for field appraisal or for up-dip exploration tests. Quantitative analysis of the seismic anomalies and the velocity effects on the time images of deeper targets is presented for each example.

The seismic anomalies observed in the Timor Sea region range in width from 200 m to 600 m and occur over strike lengths of up to 3000 m. The degree of seismic two-way time ‘pull-up’ on deeper seismic events is proportional to the width of the anomaly, to the relative depths of the anomaly and the underlying events. At Jabiru-2 the time image of the Jurassic reservoir zone is unaffected by the anomaly which is undershot by most of the CDP ray paths. Other wells, such as Avocet-2, exhibit considerable time pull-up due to broader high velocity zones. At Keeling-1 the target horizon was only partially affected by the overlying velocity anomaly.

The seismic anomalies can be recognised by a narrow zone of noise below the Base Miocene seismic marker, varying degrees of time pull-up below the high velocity limit, and local thinning of the Eocene time interval. A common attribute of the anomalies is the confinement of the high velocity sediments to the Pre-Miocene section. This suggests that the anomalies are not due to diagenetic changes associated with fluids moving along fault planes. If this were the case, then the Lower Miocene section would be similarly affected. It is proposed that the anomalies are caused by diagenetic changes related to sub-aerial exposure of topographic highs during the Oligocene.