Abstract

Large quantities of natural gas has been discovered in South Sumatra since the early 1990's. The search for natural gas has resulted in significant discoveries at Gelam, Dayung, Sumpal, Betara, Gemah, Geragai, Rayun, Singa, and – of major importance – the multi-TCF Suban field. Markets in Central Sumatra, Singapore and, in the future, West Java are using natural gas to replace more costly oil.

Along with hydrocarbon gas, very high percentages of CO₂ have been encountered. A predictive model of CO₂ content is important in the search for future reserves. A review of the existing gas composition and stable carbon isotope data has given a basis for understanding the origins of South Sumatra CO₂.

The data can be interpreted in terms of a mixing model of CO₂ generated from organic sources (coals and shales) and CO₂ generated from inorganic sources. The data are consistent with worldwide observations of low CO₂ concentrations of gases derived from organic sources. Higher concentrations are more consistent with reservoir temperatures, and isotope values, which would be expected from carbonate dissociation or magmatic sources.

In general, CO₂ is primarily controlled by the source and present day reservoir temperature/depth. No simple model can explain high CO₂ (> 40%) gases in low temperature reservoirs. Secondary controls are also recognized as playing a role in the ultimate CO₂ content of a reservoir. Tectonic history including lateral and vertical migration and paleo temperatures, reservoir lithology and mineral buffering also may come into play during the creation of a gas field.