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The CO2 content in natural gas in the Huanghua depression, Bohai Bay Basin, China, is highly variable, ranging from 0.003 to 99.6%. Understanding the origin and distribution of the CO2 is important to assess risk prior to drilling. This study uses gas geochemistry to identify the origins of CO2 in the sedimentary basin and places these findings within a geologic context.
Chemical compositions, , 3He/4He, and 40Ar/36Ar were measured for 50 gas samples collected from gas- and oil-producing wells located in different tectonic regions in the depression. From these analyses, we determined that the CO2 in the Huanghua depression originated from three sources: thermal decomposition of organic matter, thermal decomposition of carbonate minerals, and mantle degassing. Gases with low amounts (<3%) of CO2 tend to be organogenic. This organogenic CO2 occurs in hydrocarbon accumulations and is characterized by values ranging from −20 to −10‰ and low 3He/4He (R/Ra < 1, herein R and Ra represent the 3He/4He ratio of sample and air, respectively). Carbon dioxide originating from thermal carbonate decomposition occurs as a minor component (<10%) in hydrocarbon gas accumulations and is characterized by a narrow range of (−2 to +2‰) and R/Ra < 1. Huanghua depression natural gases with CO2 content in excess of 15% resulted from mantle degassing and mainly occur at the intersection of faults. These gases have 3He/4He ratios in excess of atmospheric value (R/Ra > 1) and ranging from −5 to −3‰. Volatiles from mantle degassing during the postmagmatic stage are the most likely major source for CO2 in these high-CO2-content reservoirs. Basement faults likely provide pathways for the upward migration of CO2-rich mantle fluids. Consequently, CO2-rich gas pools are locally concentrated in the Gangxi and Dazhongwang fault zones within the depression.
T. Zhang is a research geochemist and laboratory manager at the California Institute of Technology (Caltech). He was a postdoctoral scholar in chemistry at the Caltech. He holds a B.S. degree in geology and a Ph.D. in isotope geochemistry. His research focuses on petroleum and natural gas geology and geochemistry and CO2 sequestration.
M. Zhang is currently a professor and director of the Institute of Geological Science at Lanzhou University. He holds a B.S. degree in geology, an M.S. degree in mineralogy and petrology, and a Ph.D. in geochemistry. His research interest mainly focuses on stable and noble-gas isotope geochemistry in natural gas and the Earth's mantle.
B. Bai is an assistant professor of petroleum engineering at the University of Missouri–Rolla. He was a postdoctoral scholar at the California Institute of Technology. He holds a B.S. degree in reservoir engineering and Ph.D.s in both petroleum engineering and geology. His research focuses on enhanced oil recovery using chemicals and CO2/CO2 foam.
X. Wang is currently a professor of gas geochemistry at the Institute of Geology and Geophysics, Chinese Academy of Sciences. He holds a B.S. degree in geochemistry from the Chinese University of Sciences and Technology. He has worked on petroleum and natural gas geochemistry, noble-gas isotope geochemistry, and deep earth interiors for more than 40 years.
L. Li is currently a professor of geochemistry at the Institute of Geology and Geophysics, Chinese Academy of Sciences. He has worked on gas composition and stable isotope analysis since 1992. He holds an M.S. degree and a Ph.D. in nuclear physics from the Institute of Modern Physics, Chinese Academy of Sciences.