CHARACTERISTICS OF BIOMARKER COMPOUNDS IN THE MIDDLE JURASSIC CARBONATE SEQUENCE OF THE BUQU FORMATION AND THEIR IMPLICATION IN THE QIANGTANG BASIN, THE NORTHERN TIBET

Characteristics of biomarker compounds from Middle Jurassic marine carbonate sequence in Well QZ2 are reported for the first time.Abundant biomarkers,including n-alkanes,isoprenoid,steranes and terpanes,have been detected by GC-MS in this stratum.Chromatogram figure shows that n-alkanes contain nC₁₂-nC₃₅ with a relatively wide range,nC₁₇,nC₂₀,nC₁₈,nC₁₅ or nC₁₆ can be as the highest peaks.Examination of the relative abundance also shows an absolute dominance of low carbon molecular components in the n-alkane.OEP ratio ranges from 0.37～1.14 with the average of 0.89.This data shows little dominance of odd-even carbon number in the alkane.Pr/Ph ratio ranges from 0.56 to 1.03 with an average of 0.75,which shows an obvious dominance of phytane in isoprenoids.The relative abundance is pentacyclic triterpane>tricyclic terpane>quartcyclic terpane.Steranes also contain a little gammacerane.Sterpane ∑C₂₇/∑C₂₉ ratio ranges from 0.67 to 1.22 and this data fluctuates in the vertical carbonate sequence.Sterpane ∑(C₂₇+C₂₈)>∑C₂₉ and this parameter also indicates that the organic matter is originate from algae and the contribution from terrestrial plant sources cannot be confirmed yet.Different kinds of parameters indicate that the organic matter of the whole carbonate sequence is in the mature stage.Full sedimentary environment of carbonate sequence is in the reduction condition and sea water salinity is normal.