Carbon and Hydrogen Isotopic Characteristics of Hydrocarbons in Coal Type Gas from China
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摘要: 我国主要含油气盆地中的煤型气和油型气烃类气体的稳定碳、氢同位素组成的对比研究表明：煤型气甲烷的碳同位素组成在未成熟到低成熟阶段（Ｒ０<０.７%）与油型气无明显差别。从低成熟开始，煤型气甲烷随热演化程度的增高相对油型气逐渐富集重碳同位素。在热演化程度达到１.０左右时，二者δ１３Ｃ１的差别最大（达７‰左右）。之后随热演化程度的增加，二者δ１３Ｃ１的差值逐渐减小，当热演化程度达到过成熟阶段之后，二者又基本相近。煤型气和油型气重烃的碳同位素组成，主要受气短岩母质碳同位素组成的控制。重烃的碳同位素组成是鉴别煤型气和油型气的较有效的指标，通常煤型气的δ１３Ｃ２>－２８‰，而油型气的δ１３Ｃ２<－２８‰。热演化程度对煤型气重烃的碳同位素组成影响不够明显，而对油型气重烃碳同位素组成有一定的影响。煤型气（甲烷及重烃）的氢同位素组成主要与源岩沉积时的水介质有关，由陆相淡水环境沉积的源岩形成的煤型气相对富集氢同位素气。煤型气从甲烷到了烷随着碳数的增加，氢同位素组成依次增重，即δＤ１<δＤ２<δＤ３<δＤ４.Abstract: The thermogenic gas can be divided into coal type gas originated from humic organic matter and oil type gas formed by sapropel organic matter according to the type of organic matter. The stable isotopic compositions of hydrocarbons of coal type gas and oil type gas from main oil - gas- bearing basins in Chins are relatively systematically studied in this paper. The results show that it is not certain that the methane of coal type gas always enriches the heavy carbon isotope in comparison with oil type gas. At the stage from immature to low mature, there is no obvious difference in carbon isotopic compositions of methane between coal type gas and oil type gas. From low mature stage, in comparison with oil type gas, the methane of coal type gas gradually enriches heavy carbon isotope with the increasing of thermal evolution. When vitrinite renectance (Ro) reaches about 1. 0, the difference in δ13C1 between coal type gas and oil type gas is the largest,about 7～8‰, then gradually decreases with increasing thermal evolution. After thermal evolution reaches the stage of over - maturity(Ro2. 2% ), the carbon isotope composition of coal type gas is basically similar to that of oil type gas. The carbon isotope compositions of heavy gaseous hydrocarbons (ethane to butane) of coal type gas and oil type gas are mainly controlled by the carbon isotopic composition of organic matter in source rock. Therefore, the carbon isotopic composition of heavy gaseous hydrocarbon is an effective indicator for the identification of coal and oil type gas. In general, the δ13C2 of coal type gas is more than -28‰,while the δ13C2 of oil type gas is less than - 28‰. Thermal evolution has no obvious innuence on carbon isotopic composition of heavy hydrocarbons of coal type gas. However, it has certain influence on carbon isotopic composition of heavy hydrocarbons of oil type gas. With the increase of thermal evolution, the ethane of oil type gas gradually enriches heavy carbon isotopes. The hydrogen isotopic composition of methane and heavy gaseous hydrocarbons of coal type gas is mainly related to the salinity of water under which the source rock was deposited. The coal type gas formed by the source rock deposited in terrestrial freshwater environment relatively enriches protium of hydrogen isotope. However, the coal type gas derived from the marine source rock relatively enriches deuterium of hydrogen isotope. In addition, with the increase of carbon number from methane to butane, the hydrogen isotopic composition of hydrocarbons of coal type gas successively becomes heavier, i. e.,δD1<δＤ2<δＤ3<δＤ4.
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