• journal_title：AAPG Bulletin
• Contributor：Tongwei Zhang ; Mingjie Zhang ; Baojun Bai ; Xianbin Wang ; Liwu Li
• Publisher：American Association of Petroleum Geologists (AAPG)
• Date：2008-03-01
• Format：text/html
• Language：en
• Identifier：10.1306/10230706141
• journal_abbrev：AAPG Bulletin
• issn：0149-1423
• volume：92
• issue：3
• firstpage：341
• section：REGULAR ARTICLES

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, line-graphic" alt="Graphic" src="341/embed/inline-graphic-1.gif" />, 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 line-graphic" alt="Graphic" src="341/embed/inline-graphic-2.gif" /> 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 line-graphic" alt="Graphic" src="341/embed/inline-graphic-3.gif" /> (−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 line-graphic" alt="Graphic" src="341/embed/inline-graphic-4.gif" /> 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.