Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin
[Objective]Coal-measure source rocks in Ordos Basin are developed,and the content of tricyclic terpane in saturated hydrocarbons is rich.However,the distribution patterns of tricyclic terpane show important dif-ferences in the study area.Therefore,it is of great significance to explore the main controlling factors for these differ-ences.[Methods]Saturated hydrocarbons of 26 coal-measure source rock samples in the Upper Paleozoic of the Ordos Basin were analyzed in detail by gas chromatography and chromatography-mass spectrometry.[Results and Discussions]According to the distribution characteristics of tricyclic terpanes(TTs),the source rock samples in the study area were divided into three types of distribution patterns.The C19TT is the main peak of the type Ⅰ source rock,and the content of C19TT,C20TT and C21TT gradually decreases.At the same time,the Pr/Ph ratio is high,and the ra-tio distribution range is between 1.19 and 3.84,with an average value of 2.99.The content of C24 tetracyclic terpane is also rich in this type of sample.The ratio distribution range of C24 tetracyclic terpane/C26 tricyclic terpane is between 2.39 and 32.54,with an average value of 9.80.Both C28 tricyclic terpane and C29 tricyclic terpane are absent.This kind of sample is mainly humic coal and carbonaceous mudstone,revealing the characteristics of a lacustrine facies sedimentary environment.By contrast,the tricyclic terpanes of type Ⅱ source rocks are dominated by C23TT,and the content of C19TT,C20TT and C21TT gradually increases,whereas the Pr/Ph ratio is low and the ratio distribution range is mainly within the range of 0.34 and 0.90,with an average value of 0.61.The content of C24 tetracyclic terpane is not abundant,and the ratio distribution range of C24 tetracyclic terpane/C26 tricyclic terpane is mainly between 0.42 and 0.77,with an average value of 0.54.Both the C28 tricyclic terpane and C29 tricyclic terpane are abundant.In addition to coal-measure mudstone,humic coal and carbonaceous mudstone are present in these samples,and their sedimen-tary environment is quite different from that of type Ⅰ source rocks.The distribution of tricyclic terpanes in type Ⅲ source rocks is in the middle of the type Ⅰ and type Ⅱ source rocks.The main peak carbon of tricyclic terpane is C23TT,whereas the relative content of C19TT,C20TT and C21TT decreases in turn with a stepwise distribution.In addi-tion,the Pr/Ph ratio is 2.07 and the C24 tetracyclic terpane/C26 tricyclic terpane is 3.53,both of which are between the type Ⅰ and type Ⅱ source rocks.[Conclusions]The results reveal that with an increase of the thermal evolution degree of source rocks,the total amount of tricyclic terpanes shows a gradually increasing trend,although the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly.To put it another way,the different distribution patterns of tricyclic terpanes in coal-measure source rocks in the study area are less affected by the thermal evolution degree of organic matter.Rather,they are mainly controlled by the sedimentary environment and parent material type.
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