Abstract: The Origin of Nonhydrocarbon Gas in Petroleum Deposits

Ian Hutcheon

Most petroleum deposits, particularly natural gas, contain nonhydrocarbon gases of which carbon dioxide (CO₂), hydrogen sulfide (H₂S), nitrogen (N₂), and helium (He) are the most abundant. Unless present in high enough concentrations to be commercially valuable, these gases decrease the value of natural gas deposits and increase the costs of handling produced fluids. To predict the amounts of nonhydrocarbon gas it is necessary to understand their origin and possible sources and sinks. Nonhydrocarbon gases can be divided into the reactive gases, CO₂ and H₂S, and the inert gases N₂ and He.

CO₂ and H₂S can be produced from organic and inorganic source material by organic and inorganic processes. Below about 80°C, organic processes tend to dominate and both gases are produced by various bacterial processes. In the presence of iron, H₂S is precipitated as pyrite, generally preventing very high concentrations. Feldspathic or lithic rocks usually have minerals that will buffer the CO₂ content to predictable levels, but high concentrations are observed in carbonates or quartz sandstones. High CO₂ and H₂S caused by bacterial sulfate reduction as a result of hydrodynamic flow causing mixing of fluids can be documented in oil fields of the Western Canada sedimentary basin (WCSB). Above 100°C, inorganic p ocesses dominate and H₂S is produced by thermal sulfate reduction (TSR) in which hydrocarbons are abiogenically oxidized to produce CO₂, as is observed in carbonates of the WCSB. CO₂ content is predictable at high temperatures because reactions between carbonate and silicate minerals in siliciclastics buffer the concentration. N₂ and He are present in the atmosphere and can be concentrated by regional hydrodynamic flow. Although both gases are "inert" they can be produced by subsidiary chemical processes, N₂ from hydrocarbon maturation and He from radioactive decay in rocks of broadly granitic composition. In the WCSB both processes can be recognized on the basis of geological and isotopic data. The diversity of sources and sinks, pa ticularly for CO₂ and H₂S, makes prediction of nonhydrocarbon gas content challenging, but recognition of the hydrodynamic setting, burial history, and lithology can allow the origins of these gases to be understood.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers