Publications

The North Madura Platform is a stable platform made of a simple Tertiary geology that ironically possesses severe seismic velocity anomalies. There are four velocity anomaly phenomena in the North Madura Platform: vertical velocity variation, lateral velocity variation, pull-up effect, and push-down effect. These phenomena have affected the interpretation of the structure and stratigraphic architecture of the main reservoirs of Kujung and CD carbonates. This paper provides a methodology to solve the problem: recognizing the anomalies, building velocity model, and revealing the correct structure and platform architecture of the reservoirs. Vertically varied velocity in the North Madura Platform is largely due to facies contrast between clastic formations (slow velocity) and carbonate formations (fast velocity). Lateral velocity variation is interpreted to be caused by north-south variation in sediment content in the overburden Kawengan Formation. The pull-up effect occurs locally where there are fast velocity pockets. The push-down effect is caused by the Wonocolo carbonate platform containing intra-reef tidal channels, resulting in a push-down imprint effect on the underlying formations due to their lower velocity values compared to the surrounding carbonate platform. A 3D velocity model was constructed to correct velocity anomalies by integrating all velocity data (seismic and well check shots) and geological information (well picks, horizons, and facies interpretations). The push-down effect was corrected by applying replacement velocity values within Wonocolo intra-reef tidal channel bodies to vertically correct imaging of the underlying formations. Velocity push-down imprints varied from one channel to another as well as laterally within each channel, which caused variation in the replacement velocity values ranging from about 1800 m/s in the northern part to 2200 m/s in the southern part, compared to 2500 m/s average value of Wonocolo carbonate build-ups. The method has successfully revealed the true structure and stratigraphic architecture of the Kujung and CD carbonate reservoirs. This paper concludes that a collaboration between holistic geological understanding and good velocity procedure were the key to successfully solve the problem.