We performed a chromato-mass-spectrometric study of the distribution of hopane hydrocarbons in the organic matter of Jurassic-Lower Cretaceous mudstones from wells drilled in the western Yenisei-Khatanga regional trough. A series of 25-norhopanes has been first identified; these hydrocarbons, often found in naphthides, are commonly related to biodegradation processes. We consider other hypotheses of formation of 25-norhopanes. Based on the examined distribution of rare rearranged hopanes, new biomarker parameters have been introduced, which ensure a more reliable estimation of the conditions of accumulation of organic matter in rocks and of its thermal maturity
