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.