Martin Schoell¹ and L. M. Cathles²
(1) Chevron Research and Technology Company, c/o Chevron Overseas Inc. B2320, 6001 Bollinger Canyon Rd, San Ramon, CA 94583, phone: 925-842-5204, fax: 925-842-3442, [email protected] 
(2) Cornell University, Ithaca, NY

The unexpected occurrence of CO2 in gas reservoirs is a major risk in exploration for natural gas, particularly in SE Asia. Any approach that can forecast the occurrence of CO2 in the subsurface reduces risk in gas exploration. We extended existing models of CO2 formation in hydrothermal systems to sedimentary basins in general and infer that CO2 forms in sedimentary basins in specific geologic environments from decomposition of carbonates that react with silicates at temperatures > 300°C. Deeply buried clastic sediments in areas of high heat flow can be termed CO2 kitchens. CO2 from these kitchens can migrate with other fluids to shallower depths and can be removed from the gas/fluid phase in reversible reactions with silicates that buffer the CO2 content of the fluid phases. If Ca-silicates are present CO2 is buffered to very low values. If Ca-silicates are not present but Mg or Fe-silicates are, the CO2 content of the gas phase will lie between 3 and 15 mole percent, depending on temperature. The CO2 content of a gas phase can rise above 70 mole percent if silicates with Ca, Mg and Fe are not present. These silicates are removed by early venting of CO2 , especially along faults. We have developed a capability to model this titration and assess CO2 risk in sedimentary basins by incorporating the above chemical principles into the Cornell/GBRN/GeoGroup BasinLAB 2D basin modeling computer code. Examples from SE Asia demonstrate that CO2 risk is closely related to heatflow in an area as well as regional faults and mud-diapirs that tap into the deep CO2 kitchens.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia