Geochemical Characteristics of Nitrogen in Natural Gases
Du Jianguo; Liu Wenhui; Shao Bo and Eckhard Faber (State Key Laboratory of Gasgeochemistry , Lanzhou Institute of Geology, Chinese Academy of Sciences , Lanzhou 730000) (Bundsanstal Fur Geowisseenschafter .und Rohstoffe, 3000 Hannover 51, Stilleweg 2, F.
The geochemical characteristics and origins of molecular nitrogen in natural gases are discussed on the basis of geology and geochemistry. Most of natural gases from Chinese oil and gas fields contain less than 6 percent of molecular nitrogen. The isotope composition of nitrogen in natural gases has a range of - 57‰ to 95‰, most of δ15N values scatter between 20‰ and +20‰. Natural nitrogen gas in the sedimentary basins adheres to the surface of fragments in rocks as molecule or occurs in pores of rocks as free gas. Natural gases enriched nitrogen are mainly found in the deep-fracture zones or Cenozoic volcanic areas. Nitrogen-rich natural gases usually have higher 3He/4He ratios(several times of the atmospheric value),and some times a higher concentration of carbon dioxide. Nitrogen enriched in natural gases may be mostly derived from the deep-earth, while ntrogen whith Lower concentration in natural gas in the stable platform is mianly generated from sedimentary organic matter. Because nitrogen isotope ratios of natural gases are similar to those of many nitrogen-bearing matters and there are little isotopic-geochemical data of nitrogen available, it is too difficult to ascertain some limits of δ15N value to classify natural gases as carbon isotope data.