Nitrogen isotope compositions and organic matter accumulation in terrestrial hydrocarbon source rocks in China

Abstract

As an important biological element, nitrogen is causally linked with organic matter accumulation, but a systematic and regular understanding has not been developed. This paper discusses this issue by taking the widely developed Permian-Paleogene terrestrial (lacustrine) hydrocarbon source rocks in China as an example. The results show that the terrestrial source rocks in China can be classified into three groups according to the nitrogen isotope (δ15N) compositions and combined with the salinity and evaporative alkali mineral characteristics: the circum-neutral group 1 (average δ15N=4.0‰±1.5‰), the circum-neutral group 2 (average δ15N=7.1‰±1.6‰), and the alkaline group (average δ15N=18.4‰±3.3‰). In the circum-neutral group with δ15N 10‰, the organic matter types are good and shale oil potentials are higher, but the response relationship between δ15N of source rocks and organic matter accumulation is not as good as that of the circum-neutral group, reflecting that the organic matter accumulation in alkaline group is influenced by other comprehensive factors other than δ15N. Accordingly, the organic matter accumulation models of three types of lacustrine source rocks (10‰) classified on the basis of δ15N were established. The δ15N has the potential to trace organic matter accumulation and quality of source rocks. For example, low δ15N type (δ15N 5‰) is of good qualityin lacustrine source rocks. This paper enriches the biogeochemical and hydrocarbon source rock geochemical studies of nitrogen by exploring organic matter accumulation in source rocks from the new perspective of nitrogen isotope composition and nitrogen cycling.

Keywords

• organic matter accumulation
• nitrogen isotopes
• primary productivity
• preservation conditions of organic matter
• continental hydrocarbon source rocks