淡水湖沼相烃源岩中C31-51长链苯并藿烷系列及相关标志物的分布、组成与成因OA

The saturated fractions and the bitumen removed asphaltene in the extracts of the middle Jurassic source rocks deposited under fresh water lacustrine-swamp environment from the well Yu 25 in the Yuka area of the northern Qaidam basin were systematically analyzed by gas chromatography-mass spectrometry(GC-MS)and gas chromatography-mass spectrometry-mass spectrometry(GC-MS-MS),the distributions and compositions of various benzohopanoids and the related biomarkers were compared in the different source rocks,and their possible origins would be discussed.The analytical results reveal that C31-51 17α(H),21β(H)-extended hopane series(C31H-C51H),C32-51 extended benzohopane series cyclized at the C-20 position(C32BH-C51BH),C31-35 benzohopane series cyclized at the C-16 position(C31bH-C35bH),C34 and C35 isohopanes(iC34H,iC35H)and isobenzohopanes(iC34BH,iC35BH)were detected in the extracts of those source rocks,and their relative retention positions were determined based on the previous literatures.It is found that the contents of C31bH-C35bH are generally less than that of C32BH-C35BH in the studied source rocks,but C31bH-C35bH in the coal and carbonaceous mudstone are much more than in the dark mudstone,suggesting that C31bH-C35bH in the relatively oxic swamp environment are relatively easy to be generated than in the relatively reduced lacustrine environment.Moreover,C32-51BH extended benzohopanes are complete in the coal and carbonaceous mudstone,but the extended hopanes are only up to C36 or C37.However,in the dark mudstones,the extended benzohopanes and extended hopanes are complete and up to C51.The analytical results show that the extended benzohopanes are more abundant than the extended hopanes in the studied source rocks,but in the coal and carbonaceous mudstone their contents are much more than that in the dark mudstones,because they have much higher ratios of benzohopane to hopane compared with the dark mudstones,suggesting that the relatively oxic swamp environment is more suitable for formation of extended benzohoanes than the relatively reduced lacustrine environment.It is noted that iC34H,iC35H and iC34BH,iC35BH are detected in all the studied source rocks,but in the dark mudstones,iC34H and iC35H are relatively abundant,and iC34BH and iC35BH are relatively abundant in the coal and carbonaceous mudstone,suggesting that swamp environment are suitable for formation of isobenzohopanes.Therefore,C34,C35 isohopanes and isobenzohopanes are not a kind of exclusive biomarkers to coal,and should be carefully used.