Distribution and Significance of Methyl Steranes in Bohai Bay Basin, East China

31 samples of shales and mudstones developed in reducing hypersaline andfreshwater setting and 20 oil samples of Bamianhe oilfield, Bohai Bay basin were collected for adetailed study of petroleum system. Composition and distribution, especially significance ofsteroids, are discussed as a part of those. Abundant steroidal biomarkers, including C_(27-29)regular Steranes, C_(28-30) 4-methyl steranes, di-nosteranes and aromatic steranes, were detected.Results show that the composition and distribution of the steroids in samples studied are functionsof thermal maturity, organic source, paleoenvironment and lithology of potential source rocks.Alga-rich Es_4 shales (brackish water) developed in the south slope of depocenter (Niuzhuang sag)were found particularly rich in steroidal biomarkers including C_(30) 4-methyl steranes anddinosteranes. Es_3 mudstones (fresh water) were found devoid of dinosteranes. Distribution patternsof regular steranes are completely different from methyl steranes indicating different origins ofthe specific compounds of the fractioa Diagnostic distribution of steranes in rock extracts ofdiverse intervals makes the compounds to be essential indictors of source-rock tracing. Methylsteranes prove to be much more useful in oil-source rock correlation than regular steranes. Resultsalso show that alga-rich Es_4 shales located in the south slope are not likely the primary sourcerock responsible for the oils discovered based on the composition and distribution of steroids.

Maturity effects on steranes of source rocks in Lishu fault depression,Songliao Basin

Geochemical analysis results of 30 source rock samples from Lishu fault depression in Songliao Basin show that samples are compositionally similar and represent a uniform organic facies.Differential distribution of steranes among the samples is due to rank,with vitrinite reflectances(Ro)ranging from 0.65 to 1.61%.In the maturity stage(Ro=0.65-1.20%),the absolute concentrations of steranes increase,which is affected by kerogen degradation,whereas the pyrolysis of high mature source(Ro=1.20-1.61%)show a decrease in the abundance of steranes.Simultaneously,the parameters of steranes vary greatly with maturity.Decrease of prognane/regular sterane value with the increase of maturity suggests that pregnane is not a product of regular steranes pyrolysis.The ratios of C27/C29regular sterane gradually increase,while the value of C29-20S/(20S+20R)and αββ/(αββ+ααα)decrease at high maturity stage,which is associated with the difference in the thermostability of steranes involved.This reversed trend can be used to determine the high-maturity stage of source rocks.Whereas the values of rearranged steranes/regular steranes in source rocks show an apparent positive correlation with maturity,it,therefore,appears to be particularly useful for maturity assessment at elevated levels.

DISCOVERY OF C_(30)-STERANES IN TERTIARY SEDIMENTS OF SALINE LAKE FACIES

Moldowan first reported the identification of C30-steranes. He is of opinion that C30-steranes are novel markers for marine petroleum and sedimentary rocks. Subsequently, the same author, in another paper, further suggested that as a reduced product of early diagenesis, the C30-steranes may be principally derived from propylidene-cholesterol widespread in marine organisms such as diatom,

The Diagenesis of Plant Lipids during the Formation of the Krepoljin Coal Basin(Serbia)—Using Multivariate Statistical Analysis in the Saturated Biomarkers

The composition of the saturated hydrocarbon fraction of soluble organic matter （bitumen） from the Krepoljin brown coal basin （East Serbia） of Miocene age was investigated. The Krepoljin basin is especially interesting from a geochemical point of view. Namely, by the end of Tertiary Period, the basin was covered by powerful Mesozoic formations of great possibilities, the bedding became hermetically enclosed and protected from subsequent external influences. The nature of early diagenetic processes in the environment with abundant accumulation of organic substance is defined by the molecular content of bitumen through the connections of molecular structures to potential precursors in paleoplant phylla, as well as by the degree of their diagenetic transformations. The hierarchy of parameters based on molecules of saturated hydrocarbons was determined by applying the principal component analysis to the soluble organic matter. The most significant, most ＂loadings＂ values of component CI, are the plant types such as precursors or participants in early diagenetic transformations of steranes Cs27-algal precursor material, Cs28-moulds, Cs29-higher land and water plants; resinous from the group of higher plant gymnosperms （G）; also triterpanes a and β-amirindicotyledonous angiosperms （A） and bacterial and/or terrestrial plants population represented by hopanes/moretanes; as well as the parameters which describe the degree of diagenetic/maturation transformation of precursor biomass, and based on next reactions： （1） shifting of methyl group, influenced by the inorganic sediment constitutents, sterane-rearranged steranes, （2） isomerization in the ring system, ααα→βββ C29 sterane as well as （3） isomerization on chiral center of the side chain sequence R→S C29 sterane S/（S＋R）. Additionally, a pyrite-derived inhibitory effect on the rearrangment of C29 steranes must be taken into account, but not on triterpanes reaction. Non- appearance of statistically important correlations, before all with maturational depended parameters, and after all also with source indicators contributies to the affirmation of the newly-suggested parameter α-Phyllocladane/S27 as the real source biomarker of coal. Indicators derived from the distribution and abundance of n-alkanes and isoprenoid alkanes are of less significance in hierarchy of parameters.

石油样品GC-MS分析中存在的问题及解决方法

Some problems often occur in the analysis for petroleum samples by GC-MS.When high content wax composes oil samples,the n-alkanes in saturated fraction would disturb the mass chromatograph distribution and result of biomarkers.The n-alkanes can be removed from the saturated fraction by complexation,which n-alkanes react with carbamide supersaturated in methanol solution.Nevertheless,levels of the complexation can also affect on the mass chromatograph distribution and result of biomarkers.This paper discusses these problems and measures.

GEOCHEMISTRY OF PETROLEUM MIGRATION DURING THE FORMATION OF KARAMAY OIL FIELD

The phenomena of geochemical migration during the formation of the Karamay Oil Field have been revealed by correlating the compositions of non-hydrocarbon, asphaltene, steranes and terpanes from between oils and rocks. The study on abnormal compaction observed in a deep well, which is located between the source area and the oil field, and the corresponding depletion of hydrocarbons further clarifies the conditions of the migration of oil and gas. Results show that the necessary condition for the formation of this giant oil field is a long distance of lateral migration of oil and gas followed by a redistribution along Ka.-Ur. large fault before accumulation in a large uplift.

GEOCHEMISTY OF BICYCLIC ALKANES IN SEDIMENTS, COAL AND CRUDE OILS

By using GC-MS to analyze the extracts of source rocks, coal and crude oil samplesformed in different sedimentary environments in China, New Zealand and Australia, twenty-two compounds of five types of bicyclic alkanes have been dis covered in the samples. Thispaper makes an exposition of stereochemical structure, abundance and distributive features ofthese compounds and their relation to sedimentary environments and thermal evolution. Furt-thermore, it demonstrates how the abundance ratios of bicyclic alkanes to steranes and ter-panes, drimane to homodramane and the relative abundance of C15,C16,C12-C14 series of bicyclic-alkane compounds can be used to judge and determine types of oil and gas forming environ-ments （the depositive environments of source rocks from which the oil and gas wereformed）, characteristics of source rocks and degree of maturation.

The effects of biodegradation on biomarker maturity indicators in sequentially biodegraded oils from Liaohe Basin,China

By aid of gas chromatogram/mass spectrometry(GC-MS) ,the distributions and the compositions of biomarkers in a set of sequentially biodegraded oils from Liaohe Basin,China,have been quantitatively analyzed,and it has been found that during the biodegradation process of crude oils,the molecular maturity parameters such as Ts/Tm,homohopane C31 22S/(22S+22R) and sterane C29 20S/(20S+20R) ratios will be affected to different extent. The results show that except homohopane C31 22S/(22S+22R) ratio,Ts/Tm ratio will decrease with increasing biodegradation,but for C29 20S/(20S+20R) ratio,it will almost remain constant in slightly and moderately biodegraded oils,and then will increase quickly in severely biodegraded oils. The main reason is that there are some differences in the ability of resistant biodegradation for different isomer of biomarkers with different stereo configuration,resulting in the fact that destroying rate by bacteria for those biomarkers with weak ability will be higher than those with strong ability in resistant biodegradation. For example,18α(H) -22,29,30-trisnorhopanes(Ts) will be destroyed more quickly than 17α(H) -22,29,30-trisnorshopanres(Tm) ,and 20R isomer is more quickly than 20S isomer for C29 sterane,resulting in the relative ratios changed with increasing biodegradation. Therefore,much more attention should be paid to the biodegradation extent of crude oils and the type of biomarker maturity indicators,when the distributions and the compositions of biomarkers in biodegraded oils are used to determine the maturity of biodegraded oils.

Distribution and characterization of C_(31) sterane from cretaceous sediments and oils,Songliao Basin,China

Relatively high abundance of C31 steranes has been identified from cretaceous source rocks and crude oils in the Songliao Basin, China. C31 steranes which occur as ten isomers were tentatively identified as 4, 22, 23, 24-tetrametyhlcholestane based on their retention time and mass spectrum.They bear a lower thermal stability and occur as important components in the immature oils of Chaochang region because they have three-methyl groups in their side chain. The relatively high abundance of C31 steranes in the samples can indicate the source input of some special algae in the brackish and saline environment. Therefore they can be used as very useful biomarkers for organic facies analysis and oil correlation. Their biological source perhaps related to the dinoflagellate such as G. foliaceum and P. foliaceum. The gorgostanol are considered as possible precursors for the putative C314methyl sterane which formed in the stage of diagenesis.