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/R_a 1, herein R and R_a 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/R_a 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/R_a 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.