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Correcting a false assumption—offshore Brunei

Emerald Geoscience Research, Tulsa, Oklahoma, U.S.

R. K. Foster

Unocal Indonesia, Balikpapan, Indonesia

Corresponding author: ROLindsay@Emeraldgrc.com

The first 20% of the full text of this article appears below.

Seismic amplitude, classical (two-parameter) AVO analysis, and impedance inversion are exploration tools used by geoscientists with varying degrees of success in basins all over the world. It is only natural to apply these technologies to offshore Brunei. Yet the exploration challenge in the more distal sediments of offshore Brunei is finding clean, porous, reservoir sand. If successful in this quest, the hydrocarbons tend to follow. Then what is needed is an exploration technique to find sand in the distal portions of a hydrocarbon-saturated basin. The classical seismic-analysis tools are not the best solution. This article discusses why classical procedures have failed in Brunei, suggests alternative methods, and describes why these new techniques should work.

Classical AVO is a tool used to identify the presence of gaseous-state hydrocarbons. It does so primarily by responding to a drop in V_P/V_S ratio across the interface from the seal into the reservoir. And although much emphasis is placed on the importance of V_S, it is the V_P that responds to the gas. It is the fact that V_P is highly sensitive to gas and V_S is insensitive to gas that allows classical AVO analysis to identify hydrocarbons. This process does not work reliably in Brunei. There is just too much gas in the system. When compared to other basins around the world, everything in offshore Brunei is an AVO anomaly.

Three-parameter AVO inversion can potentially segregate the V_P, V_S, and density contributions to the amplitude and AVO signatures. V_P is very sensitive to gas, even in minute amounts. V_S is insensitive to gas in any amount, leaving the V_P/V_S ratio a good gas-presence indicator. However complex the implementation, this simple dependence is the core of classical AVO analysis. Bulk density is proportionally related to porosity and gas . . . [Full Text of this Article]