Compositional Variations and Geochemical Significances of Stable Carbon Isotope for Organic Matters from Marine and Terrestrial Source Rocks in Sichuan Basin
ZHU YangMing1, LI Ying1, HAO Fang2, ZOU HuaYao3, GUO XuSheng4
1. School of Earth Sciences, Zhejiang University, Hangzhou 310027, China;
2. China University of Geosciences, Wuhan 430074, China;
3. College of Earth Sciences, China University of Petroleum, Beijing 102249, China;
4. Southern Exploration Company, SINOPEC, Chengdu 610000, China
Over 500 kerogens as well selected saturate and aromatic fractions and n-alkanes from various age source rocks in the Sichuan basin were analyzed using MS and GC-ir-MS for δ13C ratios, to characterize the age-trend in organic carbon isotopic composition in marine and terrestrial sediments and their thermal evolution during over-mature stage. Moreover, different kind of marine and terrestrial source rocks with respect to their biological organic sources were distinguished using carbon isotopes in combination with other analytical data. The results indicate the kerogens from marine sediments in Sinian Dengying to middle Triassic Leikoupo Formations in this basin show a trend toward isotopically heavy values with decreasing age, possibly due to biological evolution. Whereas, a reversal isotopic-age trend for the terrigenous organic carbon is observed in upper Triassic Xujiahe to middle Jurassic Qingfoyan Formations and is considered to be dependent of biological source and sedimentary environment. The organic inputs of marine and terrestrial source rocks cannot be differentiated using the carbon isotopic ratios of kerogen, however, it can be distinguished by δ13C values of saturated and aromatic fractions and CV values. The marine and lacustrine source rocks with different facies are of obviously varying δ13C values of kerogens, thus the isotopic ratio, combined with additional relate-source data, can be acted as an indicator for their organic source. Coal and mudstone in coal-bearing strata is not distinguishable for their kerogen carbon isotopes, but has different profiles of n-alkane isotopes with more negative values in mudstone related to coal. The marine (type-Ⅱ)and terrigenous (type-Ⅲ) kerogens both continuously become isotopically enriched in 13C by 1‰~2‰ with maturation during high-over maturity. The negatively sloping curve of carbon isotope for individual n-alkanes from coaly source rock at mature stage is transferred into a flat one at high maturity. These variations in carbon isotope compositions are helpful for oil & gas-rock correlation and hydrocarbon source identification.
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