f 3D Avo Volume Visualization in Identifying Hc Leads in Offshore Sarawak

A qualitative 3D Amplitude Variation vs Offset (AVO) analysis was applied in a 500 kmsq area offshore West Baram Delta. The primary objective was to predict presence of deep prospects within or below the overpressure regime in the area. The scope of work basically involves PreStack Depth Migrated velocity modeling where the velocity model was converted to time to be utilized as input velocity in generating the 3D AVO (Amplitude Variation v Offset) cubes. The velocity model was produced via a tomographic velocity modelling process, while standard 3D seismic reprocessing was applied to generate selected AVO volumes/cubes. Several leads were identified from the 3D seismic AVO analysis where proven reservoirs/fields were used for validation of the method. Previous rock physics study shows that velocity (Vp) and density of hydrocarbons can be discriminated from wet sand and shale. Vp/Vs ratios of oil and gas sands were also significantly lower than the background. Thus, we predict a Class III AVO response for the hydrocarbon sands in this area. Based on geophysical overpressure study done at a well location, we infer either a Class II “normal” (Small negative impedance with large positive gradient) or Class II “positive” (Small positive impedance with large positive gradient and polarity change) for overpressured reservoirs. The leads/prospects were identified based on strong amplitude anomalies correlated between the Far Angle, PxG and Difference AVO attributes volumes following extensive analysis and utilization of the 3D volume visualization capabilities in the Petronas Visualization Center. The strong AVO anomalies observed on the PxG and Difference attributes conform with the fault delineations and structure. Classic Class III AVO responses are observed from the Petronas Research processed 3D AVO cubes. As a result, several new leads/prospects have now been identified for further analysis and/or validation by PCSB including several deep prospects in the overpressure regime.