The Genetic Type and Geochemical Characters and Their Significance of the Condensates in China

During recent decades exploration, it has been proved that a cerium amount of condensate (or light oil) exists in many natural gas reservoires from most of the oil/gas bearing bssins.The condensate (or light oil)can be divided into three types according to their organic matter in source rock, i.e.,the condensate formed by sapropel organic matter of marine factes, the condensate originated from humic organic matter in coal-bearing strain, and the condensate generated by mixed organic matter of terrestrial sedimentary rock.Detailed discussion is presented in this Paper on the foraging mechanic characteristics of the three types of condensates which originated from different ways.The condensate generated from marine sapropel organic matter is generally formed by the splitting of kerogcn or the high molecular liquid hydrocarbon formed formerly, only the thermal evolution reaches high maturation to over maturation stage (the range of R0 is about 1.3% to 2.0% ).In coal-bearing strata, from immaturation (R00.6%) to over mauration stages, the liquid hydrocarbons formed by the humic organic matter generally are in the from of condensates (ot lightoil) and accompany with natural gases.The condensate might also be formed low to high maturation stage by the mixed organic matter of terrestrial source rock.In order to understand the maturation of the condensates from different bssins in China.The paraffin index and heptane values are calculated by the compositions of condensates.The maturation of the condensate which are estimated by paraffin indexI and heptan values are almost in line with the real geological settings, which shows that the thermal evolution of source rock for the condensates in China ranges from immature to over mature.Some differences are found among the three types of condensates on the aspect of geochemicalcharacteristics.As for their composition,the condensate formed by the humic organic matter of coalbearing strata is relatively rich in aromatic hydrocarbons which is generally 16.2% to 23.5%,and saturation/aromatic ratios range from 3.2 to 5.2, morever, its Pr/Ph ratios are comparatively high (generally higher than 3).The condensalte formed by the marine sapropel organic matter is relatively lower than 1.The mentioned ratios of condensate formed by terrestrial mixed organic matter range between the former two types.The composition of aromatic fraction (naphthanten series, bi-benzene series, and fluoreneseries),low molecular bimarkers (terpane), carbon isotopic composition of alkane and aromatic hydrocarbons are obviously different among the three type of condensates.Based on the above geochemical characteristics the maturity and genetic type of condensate can be identified effectively,and finally to confirm the origin and source of the natural gase associating with them.