不同沉积环境及成熟度干酪根的碳氢同位素地球化学特征

The Geochemical Characteristics of Carbon and Hydrogen Isotopes of Kerogens of Various Maturity and Depositional Environments

干酪根的碳同位素组成与源岩类型有无相关性存在不同认识,此次研究表明,在陆相沉积条件下,腐泥型干酪根相对富集轻碳同位素^(12)C(δ^(13)C一般小于-28‰),腐殖型干酪根相对富集重碳同位素^(13)C(δ^(13)C一般大于-26‰)。热模拟实验表明,热演化过程对干酪根碳同位素不产生明显影响,但其H/C原子比随热演化程度增高而减小。已有资料对干酪根氢同位素研究较少,本文研究表明,干酪根的氢同位素组成主要与沉积介质的盐度有关。笔作者将干酪根氢同位素组成δD=-130‰作为源岩沉积介质为淡水和咸水一半咸水的界限,前者小于-130‰,后者大小-130‰。

40 kerogen samples were collected from 12 regions(basins) in China.Three of them have been heated for 40 hours at 8 different temperatures from 250℃to 700℃,respectively.And 24 heated kerogen samples have been obtained.All these 64 kerogen samples were analysed for their carbon isotopic compositions and 50 of them were analysed for their hydrogen isotopic compositions. A few of the samples were analysed for the C,H,O,N elemental compositions.The carbon isotope compositions of the analysed kerogens range from -23.0 to -31.2%o(PDB),the hydrogen isotopic compositions of the kerogens range from -94 to -159‰(SMOW).The research indicated that:1.in the terrestrial depositional environments,the carbon isotope composition of kerogen is mainly controlled by the types of kerogen,i.e.the sapropelic type of kerogen has lighter carbon isotope composition(δ^(13)C<-28. 0‰),and the humic type of kerogen has heavier carbon isotope composition(δ^(13)C>-26.0‰),and the sapropelic-humic or humic -sapropelic types of kerogens have the carbon isotope compositions ranging from -26.0 to -28.0%o.2.The effect of the thermal maturation on the carbon isotope composition of kerogen is not obvious in the oil and gas generation maturation stage.On the contrary, the H/C atomic ratio of kerogen decreases with the increasing maturity of source rocks.Consequently,in the higher thermal maturation stage,the carbon isotope composition of kerogen is more effective than the H/C atomic ratio of kerogen to differentiate the types of kerogens.3.The hydrogen isotope composition of kerogen mainly reflects the salinity of the depositional medium in which the source rocks were deposited.The kerogens of the source rocks deposited in the fresh water environments have the hydrogen isotope compositions lighter than -130‰,and the kerogens of the source rocks deposited in the brackish to salt water environments have the hydrogen isotope compositions heavier than -130‰.