Abstract

Seismic interval velocities have been used to produce an average porosity map of the gas-bearing carbonate reservoir of the L–Structure, located in the Natuna D–Alpha Block, South China Sea. The maximum gross gas thickness of the L–Structure is 5250 feet, and the field covers approximately 110 square miles. Interpretation of logs, cores and samples from five wells, together with the interpretation of a modern seismic grid has established a simple, two-component geological model for the carbonate facies. The two facies are platform carbonates and deep water detrital carbonates. The distribution of each facies throughout the reservoir has been mapped using a seismic stratigraphic approach.

From log data, crossplots were prepared of porosity versus interval velocity for both carbonate facies. These show a simple linear relationship between porosity and interval velocity which can be used throughout the field for each facies.

Comparisons of seismic interval velocities near each well with check shot and sonic log interval velocities have indicated that the seismic velocities are adequate for porosity mapping purposes. One problem was the estimation of the water zone velocity. This was needed to derive velocity in the gas zone from the total carbonate velocity. An equation for this was determined from the limited relevant well and seismic data available. It was found to give geologically reasonable results in the peripheral areas of the field where the water zone is much thicker than the gas zone. Over most of the field, however, the gas zone is much thicker than the water zone, so possible inaccuracies in the water zone velocity determination will not materially affect the porosity calculation. The map prepared using this technique is considered more reliable for reserves estimation purposes than a porosity map from the well data alone.