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Origins of CO2 and Its Accumulation in Sedimentary Basins

Speaker:
Tongwei Zhang, Ph.D.
Power, Energy and Environmental Research Center Caltech
Covina, CA Abstract: The content of CO2 in natural gas is in a wide range from less than 0.1% to as high as 99.6%, and high CO2 concentrations (>20%) in natural gas are encountered in many sedimentary basins. The presence of CO2 would reduce the resource value by lowering methane content and increase the infrastructure to remove and dispose it from natural gas to prevent the corrosion. For the risk assessment of encountering high concentration of CO2 gases in natural gas accumulation in a specific sedimentary basin, gas geochemistry provides an effective tool to discriminate the different origins of CO2 in natural gas.
In general, there are four major sources for CO2 generation in sedimentary basins. They are organic matter decomposition (oil, kerogen ), thermal decomposition of carbonate minerals (calcite, dolomite and so on), mantle degassing/ exsolution from magmas and hydrocarbon oxidation by thermochemical sulfate reduction.
A successful model to identify different origins of CO2 in natural gas by means of gas geochemistry approach will be presented from the geological case study of Huanghua depression, China. CO2 content in natural gas has a so wide range from 30ppm to 99.6% in the depression that understanding high-content CO2 origin is of very significant importance both in economic and academic aspects. According to the characterization of gas geochemistry and the fault development, we have determined that CO2 in Huanghua depression has three kinds of origins, i.e. organic matter decomposition, carbonate decomposition and mantle degassing.
Also, it is the first time to successfully simulate petroleum-anhydrite reactions to generate H2S and CO2 in the laboratory and found that the presence of magnesium ions in solution is crucial for sulfate reduction. The complex formation of Mg(OH)2:MgSO4 in the co-existence of magnesium and sulfate ions results in increasing [H+] concentration of the solution at high temperature of laboratory's conditions. With Talc-silica as mineral buffering, we experimentally determined that the yield ratio of CO2/H2S in mole is about 1.33 while CaSO4 is almost totally reduced by octane in the presence of magnesium ion catalyst.

Location: Kaprielian Hall (KAP) - 156

Audiences: Everyone Is Invited

Contact: Evangeline Reyes