Biomarkers of Middle to Late Jurassic marine sediments from a canonical section: New records from the Yanshiping area, northern Tibet
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Sterane
Dinoflagellate
Dominance (genetics)
Terrigenous sediment
Biomarkers and their genesis were studied.The data of(w(Pr))/(w(Ph)),(w(Pr))/(w(nC_(17))) and(w(Ph))/(w(nC_(18))) indicated that oils formed from sapropel organic matters of reducing environment.Abundance of gammaceraneC_(30) hopane indicated that the oils formed from saltwater environment.The abundance of C_(27) sterane,tricyclic terpane,tetracyclic terpane and higher tricyclic terpane/hopane parameters showed that original source materials were homonemeaes.The abundance of rearranged sterane was higher and higher(w(Ts))/(w(Ts+Tm)) and (w(C_(29)ββ))/(w(ββ+αα)) sterane showed that the maturity of the oils was higher.The detection of 25-norhopane indicated that the oils met with biodegradation and,together with integral distribution of n-alkane,it showed that oil pool underwent no less than two oil accumulating processes.
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Based on the analytical results of basic geochemical features on the source rocks from Oligocene Liushagang Formation from Well Xuwen-1 in MaIchen Sag of Beibuwan Basin,it was found that the abundance and the type of organic matter in the 2nd member of the source rocks in Liushagang Formaton were better than that in the 3rd member,it was implied that the hydrocarbon potential in the 2nd Member was higher than that in the 3rd Member.There was a similarity in the distribution of triterpane series of source rocks in the 2nd Member and the 3 Member,such as with dominant hopane series,moderate oleanane and almost absent gammacerane.It was indicated that those source rocks are deposited in fresh-water environment with abundant terrestrial materials input.In C27-29starane series,the relative content of C27 sterane from algae input in the source rocks of 2nd member is more that in the 3rd Member,and vice versa in C29 sterane from terrestrial matter input.For the crude oil from Well Xuwen-1,high Pr/Ph ratio and almost absent gammacerane are characterized,it shows that it is derived from the source rocks deposited under fresh-water and weakly reduced environment.By comparison,the distributions of triterpanes and steranes in the crude oils are between the source rocks of 2nd and 3rd Members,it is a typically mixed oil.Based on the quantitative analytical results of diasterane C27/C29 ratio,it is found that in this mixed oil,the contribution of the 2nd member and 3rd member 3 is about 77% and 23%,respectively.
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Phytane
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The biomarker assemblage characteristics and origin of crude oils in Well Tian 2 in Tiancao Sag were studied by using quantitative gas chromatography-mass spectrometry analysis.The geochemical characteristics were described as follows: abundance of hopanes from C31 hopane to C35 hopane decline gradually, gammacerane with higher abundance, the absence of rearranged hopane and rearranged sterane, reverses shape of L in the distribution of C27~C29 sterane and higher abundance of C28 sterane.Sterane isomerization parameters and alkyphenanthrene index l indicate that the maturity of crude oil is low.On the basis of comprehensive analysis of the biomarker assemblage characteristics of possible source rocks in this region, and from the characteristics of biomarker fingerprints and their absolute concentrations, it is affirmed that there is a genetic relationship between crude oil of Well Tian 2 and the lower part of source rocks in Bayingobi Formation of lower Cretaceous.It is pointed out that source rocks of lower part of Bayingebi Formation are the main source rocks of Tiancao Sag.
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Sterane
Organic geochemistry
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Phytane
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ABSTRACT Sterane and triterpane biomarkers are becoming increasingly important in the application of geochemistry in exploration programs. These compounds are often used to help describe organic facies, determine maturity of oils and extracts, and correlate oils with each other or with rock extracts. Steranes are derived from precursor steroid molecules present in both higher plants and algae. Regular steranes can be useful in evaluating the types of photosynthetic material in the sediments. Diasterane concentrations are governed mainly by maturity and lithology, and thus provide additional facies information. Unusual compounds such as 4-methylsteranes may occasionally be useful as facies indicators. Triterpanes, derived primarily from bacteria, can indicate diagenetic conditions, and thus complement the information derived from sterane distribution. Hopanes are the dominant triterpanes. The presence of unusual compounds like 28,30-bisnorhopane, or unusual concentrations of common compounds such as the homohopanes, gammacerane, and oleanane can indicate certain depositional environments. Changes in certain sterane and triterpane ratios have been used as maturity indicators for extracts and crude oils. However, many biomarker ratios depend on facies as well as on maturity. Modern correlations, which rely heavily on biomarkers, are usually highly reliable. Failures in correlations are usually due to low biomarker concentrations, lack of distinguishing characteristics in the biomarker distributions, or extreme biodegradation. The relative stability of steranes and triterpanes during biodegradation makes them useful in correlating somewhat biodegraded samples, but where biodegradation is extreme even these biomarkers will experience some transformation. Geologists who understand the fundamentals of biomarker interpretation can greatly increase their understanding of a basin: its geological history, migration patterns, and future potential. This increased knowledge will translate into greater exploration efficiency. However, biomarker data should always be fully integrated with all other available geochemical and geological information to make the most reasonable interpretation possible. RESUME Les biomarqueurs de sterane et de triterpane deviennent de plus en plus importants dans l'application de la geochimie dans les programmes d'exploration. Ces composes sont souvent utilises pour decrire les facies organiques, determiner le niveau de maturite des huiles et des extraits, et correler les huiles entre elles ou avec les extraits de la roche. Les steranes sont derivees de molecules de steroides precursives presentes tant dans les especes vegetales plus avancees que dans les algues. Les steranes ordinaires peuvent etre utiles pour determiner les types de materiaux photosynthetiques dans les sediments. Les teneurs en diasteranes sont gouvernees surtout par le niveau de maturite et la lithologie, et donc fournissent des informations supplementaires sur les facies. Des composes inhabituels tels que les 4-methylsteranes peuvent a l'occasion etre utilises comme indicateurs de facies. Les triterpanes, derivees surtout a partir de bacteries, peuvent indiquer les conditions diagenetiques, et donc ajouter a l'information obtenue de la distribution des steranes. Les hopanes sont les triterpanes dominantes. La presence de composes inhabituels tels que 28,30-bisnorhopane, ou des concentrations inhabituelles de composes communs tels que les homohopanes, les gammaceranes, et les oleananes peuvent indiquer certains milieux sedimentaires. Des changements dans certains rapport de steranes et de triterpanes ont ete utilises comme indicateurs de niveau de maturite des extraits et des petroles bruts. Par contres, plusieurs rapports de biomarqueurs dependent du facies ainsi que du niveau de maturite. End_Page 357------------------------ Les correlations modernes, qui dependent grandement des biomarqueurs, sont d'habitude tres sures. L'echec des correlations est d'habitude du a de trop faibles concentrations des biomarqueurs, au manque de caracteristiques distinctives des repartitions des marqueurs, ou a un degre extreme de biodegradation. La stabilite relative des steranes et des triterpanes durant la biodegradation les rendent utiles dans la correlation des echantillons quelque peu biodegrades, mais par contre lorsque la biodegradation est extreme meme ces biomarqueurs subiront des transformations. Les geologues qui comprennent les principes fondamentaux de l'interpretation des biomarqueurs peuvent elargir grandement leur niveau de comprehension d'un bassin: son histoire geologique, ses reseaux de migration, et son potentiel futur. Ce niveau de connaissances plus eleve se traduira par une plus grande efficacite des efforts d'exploration. Par contre, les donnees de biomarqueurs doivent toujours etre pleinement integrees avec toutes les autres donnees geochimiques et geologiques disponibles, afin d'obtenir une interpretation qui soit la plus raisonnable possible. Traduit par Marc Charest.
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Oleanane
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Thirteen crude oil samples from fractured basement reservoir rocks in the Bayoot oilfield, Masila Basin were studied to describe oil characteristics and to provide information on the source of organic matter input and the genetic link between oils and their potential source rock in the basin. The bulk geochemical results of whole oil and gasoline hydrocarbons indicate that the Bayoot oils are normal crude oil, with high hydrocarbons of more than 60%. The hydrocarbons are dominated by normal, branched and cyclic alkanes a substantial of the light aromatic compounds, suggesting aliphatic oil-prone kerogen. The high abundant of normal, branched and cyclic alkanes also indicate that the Bayoot oils are not biodegradation oils. The biomarker distributions of isoprenoid, hopane, aromatic and sterane and their cross and triangular plots suggest that the Bayoot oils are grouped into one genetic family and were generated from marine clay-rich source rock that received mixed organic matter and deposited under suboxic conditions. The biomarker distributions of the Bayoot oils are consistent with those of the Late Jurassic Madbi source rock in the basin. Biomarker maturity and oil compositions data also indicate that the Bayoot oils were generated from mature source rock with peak oil-window maturity.
Sterane
Organic geochemistry
Petroleum geochemistry
Light crude oil
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Sterane
Terrigenous sediment
Phytane
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Sterane
Cenomanian
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This paper presents the varied characteristics of biomarkers of saturated hydrocarbon and aromatic hydrocarbon in process of deep thermal fluid.Under the impact of thermal energy of deep fluid,the values of m(Σ_nC_(21-))/m(Σ_nC_(22+)) and m(Pr)/m(Ph) increase while the n-alkane's odd-even carbon number predominance gets weaken;the values of m(C_(31) hopane)/m(C_(30) hopane) and m(C_(35) hopane)/m(C_(34) hopane) decrease,while those of m(C_(27)-C_(29) sterane 20S)/m[C_(27)-C_(29) sterane(20S+20R)]and m(C_(27)-C_(29) steranββ)/m[C_(27)-C_(29) steran(ββ+αα)] increase,aromatic hydrocarbons are enriched.If the deep thermal fluid is rich in hydrogen,the biomarkers like m(Σ_nC_(21-))/m(Σ_nC_(22+)), m(Pr)/m(Ph),m(Ts)/m(Ts+Tm),etc.will fail in indicating maturity and sedimentary environment.Based on these,the identified biomarkers of organic matter in crude oil or hydrocarbon source rock that influenced by the deep thermal fluid are proposed preliminarily in this paper.
Sterane
Carbon number
Alkane
Sedimentary organic matter
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