Study On Steranes In Hydrocarbon Source Rocks And Crude Oils Of Saltwater Lacustrine Faces In The Qaidam Basin

Qaidam Basin is located in the western Qinghai province with an area of 12×10⁴km².The most developed area of Tertiary salinied lacustrine hydrocarbon source rocks is in the western and southern Qaidam Basin.The composition,genetic mechanism and geochemical attributes of steranes have attacted increasing attention with more and more foundation of abnormally high abundance of??steranes and short-chain steranes in the salinied lacustrine facies.Taking hydrocarbon source rocks and crude oils from the saltwater lacustrine facies in Qaidam Basin as research objects,the present paper study the geochemical charatersitics of hydrocarbon source rocks and crude oils to systematically discuss the composition of steranes in the saltwater lacustrine facies.On the basis of Gas Chromatography-Mass Sepectrometer?GC-MS?,the short-chain steranes,4-methylsteranes and lanostanes are comprehensively identified and reported with the focus on the distributions and genetic mechanism of??steranes and short-chain steranes in the saltwater lacustrine facies.The hydrocarbon source rocks from the saltwater lacustrine facies in Qaidam Basin have been sytematiclly evaluated to study their organic matter source,thermal maturity.The content of total organic carbon?TOC?is between 0.02%and 2.24%wirh an average of 0.53%.The hydrocarbon potential?S₁+S₂?values are in a range of 0.02mg/g-8.79mg/g with a mean of 1.38mg/g.The bitumen“A”contents range from 0.001%to1.431%with a mean of 0.109%.These index indicate that the abudance of organic matter is medium-good.The degradation rate?D?values is 1.28%-64.09%,the H/C atomic ratio and O/C atomic ratio range from 0.61 to 2.35 and 0.01 to 0.38,respectively.It reveals that the type of organic matter is type?^?,while type?is dominated.The arange of vitrinite reflectance?Ro?is from 0.404%to 1.12%?main region is 0.5-1.3%?,illustrating that most of hrdrocarbon source rocks is in a low-mature and mature stage.The organic geochemistry of hydrocarbon source rocks and crude oils of saltwater lacustrine facies in the Qaidam Basin are described through analyzing the compositions of saturated hydrocarbons and aromatics hydrocarbons.The n-alkanes in hrdrocarbon source rocks and crude oils both display a pronounced even-over-odd preference.The ratio of Pr/Ph in hydrocarbon source rocks ranges from 0.09 to 0.66 witn an average of0.32,while the range of Pr/Ph in crude oils is from 0.23 to 0.40 with a mean of 0.40.Gammacerane/C₃₀ hopane values in hydrocarbon source rocks and crude oils distribute from 0.07 to 188 and 0.40 to 1.88,respectively?mean is 0.52 and 0.86,respectively?.The homohopane series show a distribution of C₃₅ homohopane?C₃₄ homohopane.Furthermore,the relative proportion of fluorene and dibenzothiphene predominates in the hydrocarbon source rocks and crude oils,whereas the content of dibenzofuran is low.It illustrates that hydrocarbon source rocks and crude oils are deposited in a reduced environment with brackish to salt water.The abundances of C₂₇,C₂₈ and C₂₉ steranes of hydrocarbon source rocks and crude oils both have a C₂₇>C₂₉>C₂₈ pattern,while a certain content of C₂₈ and C₂₉9 tricyclic terpenes is detected.The results show that organic matter in Qaidam Basin is from a large amount of aquatic organisms and minor terrigenous higher plants.Through the detailed analysis of the isomerization characteristics of regular steranes in hydrocarbon source rocks and crude oil,the formation mechanism of??steranes in the saltwater lacustrine facies of Qaidam Basin is comprehensively analyzed.The results show that the abudance of different carbon number of??steranes is different in the hrdrocarbon source rocks and crude oils and an excellent correlation exists in the??steranes with different carbon number.The abnormally hjgh abundance of??steranes is unfounded in the conditions of low Pr/Ph ratios,high gammacerane/C₃₀hopane and high contents of dibenzothiphene.In contrast,the abundance of??steranes is lowest.Furthermore,the high abundance of??steranes appears in the conditions of medium Pr/Ph ratios,modderate gammacerane/C₃₀ hopane and medium contents of dibenzothiphene.However,these samples detected high abundance of??steranes are at a more mature stage.The sedimentary environment and thermal maturity parameters suggest that the formation and enrichment of??steranes in the hydrocarbon source rocks and crude oils of saltwater lacustrine facies in Qaidam Basin is related to thermal maturity rather than salined environment.Based on the previous research,combined with GC-MS,except the regular ssteranes detected in the Qaidam Basin,short-chain steranes,4-methylsteranes,lanostanes and long-chain alkylsterans are indetified.The relative proportion of C₂₆short-chain steranes and C₂₁ pregnane predominates in the short-chain steranes,followed by C₂₅ short-chain sterane and C₂₂ homopregnane.While the relative contents of C₁₉,C₂₀,C₂₃ and C₂₄ short-chain steranes is low.The 4-methlysteranes in Qaidam Basin exhibit two different distribution patterens(C₂₈?C₃₀?C₂₉ and C₂₈?C₂₉?C₃₀),but C₂₉ and C₃₀0 methylsteranes in general is dominated.The lanostanes in hydrocarbon source rocks and crude oils both have a pattern of C₃₀?C₃₁?C₃₂.Althogh these special sterane compounds are detected in the study area,the regular steranes still predominates in the sterane compounds,the contents of short-chain steranes and 4-methylsteranes are lower.The relationships between different carbon number of short-chain steranes are uniformity.There is a good linear correlation between the connected short-chain steranes and inconnected short-chain steranes.For the short-chain steranes in hydrocarbon source rocks,the C₂₁ pregnane and C₂₂ homopregnane exhibit a best relationship with other short-chain steranes,while the C₂₅ short-chain steranes display a general correlation with other short-chain sterane,a relatively poor relationship exists between C25 short-chain steranes and other short-chain steranes.On the basis of comprehensive analysis on the biomarkers,the relative abundance of short-chain steranes can be influenced diverse factors.Sedimentary enviorment parameters?Pr/Ph ratos,Gammacerane/C30 hopane and SF/?SF+OF+F??indicate that the strongly reduced and hyperdaline environment is the essential factors for the formation of short-chain steranes and the brackish to salt depositional environment is benefit to the formation of short-chain steranes.Organic matter source parameters(C₂₄ tetracyclic terpane/C₂₆tricyclic terpane,C₂₄ tetracyclic terpane/C₂₃ tricyclic terpane,(C₂₈+C₂₉ tricyclic terpane)/C₃₀hopane and C₂₇/C₂₉9 regular steranes)suggest that the short-chain steranes have a general relationship with aquatic organisms.In addition,maturity indicatiors(C₂₉-20S/?20S+20R?,C₂₉??/???+???,C₂₉diahopane/C₂₉hopane,C₃₀diahopane/C₃₀hopan-e,2,3,6-TMN/?1,4,6+1,3,5-TMN?and 4-MDBT/?4-MDBT+1-MDBT?)reveal that the increasing of thermal maturity is beneificial to the formation and enrichment of short-chain steranes.