Characteristics of molecular fossils in source rocks of Lower Carboniferous Baishan Formation in Kushuigou area of Ejin Banner,western Inner Mongolia

Based on gas chromatography and gas chromatography mass spectrometry techniques,the authors studied molecular fossils characteristics of Baishan Formation source rocks in Kushuigou,Ejin Banner,and analyzed information and petroleum geological significance indicated by sources of organic matter,sedimentary environment,maturity and some other factors.Gas chromatography peaks of saturated hydrocarbons from Baishan Formation source rocks showed the former of single peak distribution and main peak of carbon composed mainly of C18.This indicates that main sources of organic matter were marine organic matter,and Pr/Ph ranged from 0.4 to 0.6,suggesting a strongly reducing sedimentary environment.Molecular fossil assemblages of Baishan Formation source rocks were abundant in tricyclic terpane with long side chains,had high level of gammacerane,and showed dominant distribution of C27 steranes.High content of gammacerane in organic matter suggested that organic facies of source rocks was in a saltwater sedimentary environment.The hopanes/steranes ratio was 1.06 on average,implying that bacteria and algae had almost same contribution to organic matter.And high level of C27 steranes showed that alga was main source of hydrocarbon precursors.The methylphenanthrene indexes of Baishan Formation,which were effective molecular parameter indexes for measuring thermal evolution of organic matter,ranged from 0.36 to 0.67,and their conversions into vitrinite reflectance Rc ranged 1.9 to 2.1,corresponding well to actually measured data,indicating that source rocks of Baishan Formation are at over-mature stage of thermal evolution.