Carbon Dioxide (CO2) Distribution in the Sarawak Basin, and Its Relationship with Entrapment

Carbon dioxide content in both associated and non-associated gases in Sarawak Basin fields varies up to a maximum of 90%. High CO2 content in natural gas reduces the economic value by lowering the saleable gas volume, as well as reducing the BTU content. In addition, special infrastructures are required to develop and process gas accumulations containing high CO2. Understanding the likely geological parameters that control CO2 regional distribution patterns will assist explorationist in targeting prospects with a lower CO2 content. General current understanding on the CO2 distribution in a basin are, CO2 percentage increases with depth and high percentage CO2 accumulation are of inorganic origin and tend to be associated with structures with deep seated faults to facilitate CO2 migration up dip from basement. However, we observe that CO2 percentage varies vertically in a field and does not necessarily increases with depth and could also decreases with depth. CO2 of same inorganic origin are present in several reservoirs of a field; and yet one reservoir may have very low CO2 compared to the other reservoirs. Field observations in the Sarawak Basin CO2 distribution are: 1) Depth of accumulation and origin of CO2 does not influence the percentage distribution.<br>2) Geometry of traps and seal effectiveness dictates how much CO2 the reservoir can hold. These scenarios are also observed in Sarawak Basin. Major marine transgressive shale provides good and effective top seal. Thus reefal carbonate terminated by drowning can support higher gas column with low CO2 content.