Geochemical characteristics and its influencing factors of light hydrocarbon in coal-derived gas: A case study of Sichuan Basin.
煤成气轻烃地球化学特征及其影响因素——以四川盆地须家河组为例

Light hydrocarbon in natural gas has been concerned widely in the study of the origin of natural gas due to its abundant geochemical information,although the content of light hydrocarbon in natural gas is very low.Authors considered coal-derived gas of Xujiahe Formation and oil-associated gas of Leisan Formation in Sichuan Basin as an example,and analyze the geochemical characteristics and influencing factors of light hydrocarbon in coal-derived gas by means of GC and GC-IR-MS technologies systematically.The content of methylcyclohexane among C7 in coal-derived gas is of the maximum.Among the relative contents of methylcyclohexane,n-heptane and dimethylcyclopentane,methylcyclohexane has a content ranging from 48%～73%with an average of 63.9%.Carbon isotope values of monomer hydrocarbon in light hydrocarbon in coal-derived gas are large with δ13C ranging from-25.1‰～-20.0‰,and most of them range from-23‰～-21.0‰.The distribution of light hydrocarbon is influenced by various factors.The influence of maturity of natural gas on the content variation of aromatics is complex.When in the stage of maturity and high maturity,the content of aromatics in light hydrocarbon is low,and the maturity has little influence on the content of aromatics;but in the over maturity stage,the content of aromatics is high and the influence is obvious.Based on the comparison of compositions of light hydrocarbon in natural gas and condensate oil,evaporative fractionation affects the thermal evolution parameters,such as Ctemp,the heptane value and toluene/n-heptane value obviously,but slightly on genetic type parameters such as(2-MH+2,3-DMP)/C7,(3-MH+2,4-DMP) /C7,P2,N2/P3,n-heptane value and the relative content of methylcyclohexane and dimethylcyclopentane.The genetic type of natural gas has great influence on Mango parameters of light hydrocarbon such as K1 value,n-heptane and the relative content of methylcyclohexane and dimethylcyclopentane,which can be used to discriminate the genetic types of natural gas.