Using 527 samples of Mesozoic crude from the Middle Ob' area and from northern West Siberia it has been esteblished that the composition of low-boiling hydrocarbons (HC) C5-C8 can be successfully used in diagnosing the its genesis and in studying the effects of secondary processes. The cluster analysis was used as mathematical tool to divide the HC fluids into genetic families. 3 genetic types of studied crude were diagnosed. There are marine, terrestial and mixed. Following criteria were used for genetic classification: cyclopentanes/ cyclohexanes ratio, ethilbenzene/ xylenes ratio, n-heptane/ methilcyclohexane ratio, m-to-o-xylenes ratio. The crude from the Middle Ob' petroleum area are have originated from marine organic matter (OM). The oils and condensates from the northern West Siberia have been sourced from the OM with a considerable admixture of terragene OM. Biodegraded crude were single out as a separate family. There are HC fluids from the Aptian-Albian-Cenomanian deposits and partly from the Hauterivian-Barremian. The parameters of the low-boiling C5-C8 HC composition may be used for identifying the biodegraded oils: n-alkanes/ i-alkanes; n-hexane/ methylcyclopentane; i-hexane/ cyclohexanes etc. The extent of biodegradation may be evaluated from the variation character of hexane and heptane concentration series, in which normal compounds sharply decrease in concentration while isomer homologues become abundant. Higher cyclohexane concentrations, monoethyl-substituted to ethyl-substituted C5-C8 ratio seem to indicate the higher maturity level of some studied crude. Migration is sure to have played an impotent role when oil pools were generated. The pools probably formed at the expense of vertical migration of HC fluids. The conclusion about the preferential vertical component of migration is consistent with the regular areal distribution of different genesis crude. The ratio of C5-6/C7-8 compounds may be informative in analyzing the crude composition during migration. HC may have migrated as a gas phase. In this case oil components might have differentiated in dependence on the phase-retrograde processes, hence the gas solubility of crude components would be of great importance. In phase-retrograde process the mixture of like-molecular-weight cyclopentane and cyclohexane is subject to differentiation with enrichment of a formation gas phase by cyclopentanes.