Delineation of Shallow Velocity Anomalies in the Timor Sea by 3D Checkshot Velocity Survey

Abstract

The occurrence of localised high velocity zones due to cementation within the Tertiary section is a well documented phenomenon in the Timor Sea. The presence of such anomalies over the Skua Oil field in Licence area AC/L4 has complicated the depth conversion of the top porosity time structure. These velocity anomalies are small in relation to the spread length of a common depth point (CDP) gather, and in the vicinity of an anomaly only a fraction of the traces in a CDP gather are affected. In order to compensate for the effect of the anomaly it is necessary to know exactly which traces are affected by velocity pull-up and by how much. This can be calculated only if the anomaly can be accurately delineated. The areal extent of the anomalies can be estimated from areas of velocity 'pull-up' on the Base Eocene time structure map. However, they cannot be delineated precisely enough from this map, as the relationship between the shape of the velocity pull-up and the anomalies is very complex. In order to delineate an anomaly in the vicinity of the Skua-7a well, a radial pattern of walkaway checkshot surveys was acquired. Some 28 equally spaced profiles, with offset ranges varying from 2000 to 3000 m, were acquired between azimuths 90° and 225° east of north. Single shots were fired at approximately 35-m intervals along each line, and the first- break P-wave seismic energy travel times to a three-component geophone at a depth of 2516 m subsea (ss) was measured for every location. The velocity pull-up effect of the anomaly was clearly seen in the variation of first-break time with offset along each profile. A two-dimensional image of the anomaly along each profile was generated through inverse modelling of the travel times, and a three-dimensional model of the anomaly was created by combining the 28 two-dimensional images. The resultant anomaly model was significantly smaller in areal extent than indicated by the areas of pull-up on the Base Eocene time structure map, although the measured reduction in travel time, the velocity pull-up, through the anomaly was much larger than was indicated from the same map. This depth model is a more accurate representation of the velocity anomaly in the vicinity of the Skua-7a well and highlights the shortcomings of delineating velocity anomalies from their expression on surface seismic data. This model represents a starting point for more sophisticated inversion techniques and more accurate depth conversions.