Stable carbon isotopes in source-bed organic matter of West and East Siberia

Abstract

13C/ 12C ratios were measured for 157 insoluble organic matter samples and for 47 kerogenbitumoid (chloroform extract) syngenetic pairs from facially different sediments of main oil-bearing complexes in West and East Siberia. The δ 13C of organic matter of marine and continental sediments ranges from −32.2 to −27.5% and from −28.0 to −23.5% respectively. The carbon isotope composition of sopropelic bitumoids is insignificantly lighter or equal to δ 13C of related kerogens while the difference in δ 13C between humic kerogens and syngenetic bitumoids is about 2–3%. For kerogens of both types the linear dependence of δ 13C vs H/C atomic ration is observed. Sapropelic kerogens H/C atomic ratio is 1.5–2 times greater than that in humic insoluble organic matter. The organic matter of recent lacustrine sediments, the main precursor of which was algal plankton, has δ 13C values between −27.1 for −26.3 0 for both the insoluble and dissolved parts of organic matter. There is considerable difference in δ 13C between insoluble (δ 13C from −28.8 to −26.3% 0) and dissolved (δ 13C from −35.3 to −28.5% 0) organic matter in sediments of those lakes, in which higher aquatic plants accumulated and the contribution of terrestrial organic debris to sediments was significant. In present-day biota the 12C concentration is lower in marine plankton and higher aquatic plants as compared to higher land plants. The results showed that the isotopic differentiation similar to that found in living organisms is observed in recent sediments. This differentiation is not applicable to sedimentary organic matter. Evaluation of the data suggests that the different types of kerogen forming reactions, different pathways of organic matter transformation during decay, burial and protocatagenesis in marine and continental environments result in different δ 13C distributions in sedimentary organic matter, than in living organisms and recent sediments. The relationship between carbon isotopic composition of Jurassic oils from West Siberia and the different types of OM from source-beds of one and the same age in this region is established.