f Formation of Co2 in Sedimentary Basins and Assessment of Co2 Risk in Gas Prospects

The unexpected occurrence of CO2 in gas reservoirs is a major risk in exploration for natural gas, particularly in SE Asia. The fundamental processes of CO2 formation in sedimentary basins are still a matter of dispute. Many explorationists assume that “carbonates in the basement” at high temperatures are the major source of CO2 in Tertiary basins although solid carbonates could only produce any CO2 at contact metamorphic temperatures above 700oC. However, basin-fill sediments in Tertiary basins contain abundant carbonate and silicate species that react at temperatures above 320oC to form CO2 (Figure 1). Based on this concept we have developed a complete model of CO2 generation, migration in which high mole fraction CO2 gas is generated by the breakdown of siderite (FeCO3) and magnesite (MgCO3) only where, parts of the basin are being heated above ~330°C (CO2 Kitchen). CO2 reacts with Fe-, Mg-, and Ca-silicates as it migrates upward and away from its source kitchen. Near the kitchen, where all the above silicates have been destroyed by previous packets of migrating CO2, gas moves upward without lowering its CO2 mole fraction. Further out, where Fe- and Mg silicates are still present in the sediments, the fugacity of CO2 is lowered to buffed levels described by Smith and Ehrenberg (1989). In this zone reservoirs will have mole fractions CO2 of a few percent at 200°C. Still further from the source (or at the same location but earlier in the basin's history) where Ca-silicates are encountered, CO2 concentrations fall nearly an order of magnitude below the Fe- and Mgsilicate buffered levels. The lowering of CO2 mole fraction by reaction with and titration by Fe-, Mg-, and Casilicates must occur along a "regional" migration path rather than "locally" within a reservoir because local reduction of high CO2 gas concentrations would result in more intense carbonate precipitation than is generally observed. A model of CO2 generation and "regional" CO2 titration has been constructed and integrated into a conventional basin model. Application of the model is illustrated by computing CO2 generation in three transects in the Malay Basin.