Composition Characteristics And Genesis Of Tricyclic Terpanes In Upper Paleozoic Coal Measure Source Rocks In Ordos Basin

Key words: 
• Tricyclic terpanes /
• main controlling factors /
• sedimentary environment /
• coal measure source rock /
• Ordos Basin

Abstract: The saturated hydrocarbons of 26 coal measure source rock samples in the Upper Paleozoic of Ordos Basin were analyzed by gas chromatography and chromatography-mass spectrometry. According to the distribution characteristics of tricyclic terpanes (TT), the source rock samples in the study area were divided into three different 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 content of C24 tetracyclic terpane is also rich in this type of sample. This kind of samples are mainly humic coal and carbonaceous mudstone, revealing the characteristics of lacustrine facies sedimentary environment. On the contrary, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the contents of C19TT, C20TT and C21TT are gradually increasing, while the Pr/Ph ratio is low, and the content of C24 tetracyclic terpane is not abundant. In addition to coal-bearing mudstone, there are also humic coal and carbonaceous mudstone in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is between type I and type II source rocks. On the one hand, the main peak carbon of tricyclic terpane is C23TT, on the other hand, the relative content of C19TT, C20TT and C21TT decreases in turn and shows the characteristics of stepwise distribution. In addition, the Pr/Ph ratio and the content of C24 tetracyclic terpane are between type I and type II source rocks. The results also reveal that with the increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows an gradually increasing trend, but the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly, in other words, the different distribution patterns of tricyclic terpanes in coal-bearing source rocks in the study area are less affected by the thermal evolution degree of organic matter, but mainly controlled by sedimentary environment and parent material type.