Twenty-five monoterpanes from six types of essential oils and hydrogenated turpentine oil have been identified and their stable carbon isotope composition determined.Monoterpanes in essential oils sourced from terrestrial higher plants display a δ13C value in the range of-34‰-26‰,and mostly between-29‰ and-27‰.The δ13C value of any single monoterpane is very consistent in different essential oils.Acyclic monoterpanes show closer isotope composition between-28.6‰ and-26.2‰,with an average value of-27.7‰.In contrast,the isotope composition of cyclic monoterpanes is more scattered with an average value of-28.6‰.Isotopic fractionation with 13C enrichment has been observed during both artificial and geological hydrogenation of monoterpenoids to monoterpanes,and this is more obvious for the acyclic monoterpenoids.In addition to higher plants,acyclic monoterpane 2,6-dimethylheptane in crude oil can also be originated from other organic inputs.
Oils,condensates and natural gases in the Kekeya Field,southeast depression of the Tarim Basin were studied for their geochemical characteristics. According to the distribution analysis of the C2/C3 values with C1/C2 values,C2/C3 values with C1/C3 values,as well as C2/C3 values with dryness index,there are two different types of natural gases in the studied field,which are spatially regularly distributed. One is the oil cracking gas,located on shallow reservoirs over X52 reservoir,namely Upper oil legs;the other is kerogen cracking gas,located on X72 reservoirs,X8 reservoirs and E2k reservoirs,namely Lower oil legs. In addition,the distribution patterns of molar concentration of oils and condensates with different carbon numbers of the n-alkanes in the Kekeya Field indicate that the crude oils have experienced several kinds of secondary alterations,which were closely related to the charging of gaseous hydrocarbons after petroleum accumulation. These results indicate that,based on the research of δ13C values of individual hydrocarbons,heptane values and isoheptane values of light hydrocarbons and aromatic maturity parameters for oils,condensates and natural gases,oils and gases were charged at different geological time in the Kekeya Field.
ZHANG Min1,2,HUANG GuangHui1,2,ZHAO HongJing1,2 & WANG Xiang1,2 1 Department of Geochemistry,Yangtze University,Jingzhou 434023,China
The composition characteristics of light hydrocarbons from crude oil,chloroform bitumen A,saturated hydrocarbon fraction,aromatic hydrocarbon fraction,and asphaltene fraction during cracking have been studied systematically. The results revealed that the content of n-alkanes,branched alkanes and cycloalkanes in light hydrocarbons from the samples gradually decreased as the simulation temperature increased,and finally almost depleted completely,while the abundance of methane,benzene and its homologues increased obviously and became the main products. The ratios of benzene/n-hexane and toluene/n-heptane can be used as measures for oil cracking levels. Variation characteristics of maturity parameters of light hydrocarbons,for example,iC4/nC4,iC5/nC5,isoheptane value,2,2-DMC4/nC6,and 2-MC6+3-MC6/nC7 for different samples with increasing pyrolysis temperature,are consistent with those in petroleum reservoirs,indicating that these parameters may be efficient maturity index.
CHEN XiaoHui1,2,ZHANG Min1,2,HUANG GuangHui1,2,HU GuoYi3,WANG Xiang1 & XU GuanJun3 1 Department of Geochemistry,Yangtze University,Jingzhou 434023,China
