Series Of Steranes Research Articles

Structural characterization of steranes and hopanes in petroleum by ion mobility mass spectrometry

The structural characterization of hydrocarbon biomarkers with high molecular weight still remains a challenge even using the ultrahigh resolution mass spectrometry (UHRMS). In this work, a homologue series identification strategy was proposed based on the “sequentiality” of crude oil, where “sequentiality” revealed a compositional continuum of the naturally existing components in crude oil. With the steady increase of mass to charge ratio (m/z), the collision cross-section (CCS) value was measured by trapped ion mobility mass spectrometry (TIMS-MS) and showed a linear relationship with m/z. The structure of C20-C34 steranes and C27-C38 hopanes was determined using linear correlation. The accuracy of molecular formula was confirmed by UHRMS, and the structural determination was verified by MS/MS. The identification of sterane series and hopane series demonstrated the application of the homologue series identification strategy in the structural determination of hydrocarbon biomarkers with high molecular weight. Compared with gas chromatography-mass spectrometry (GC–MS) within the GC-able reach, the new analytical strategy showed better qualitative results and comparable quantitative results, which made it promising to explore novel hydrocarbon biomarkers in crude oil.

Free and kerogen-bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid-Neoproterozoic marine communities.

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C30 sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C27 sterane (cholestane) predominance among total C26 -C30 steranes is a common feature found for all samples investigated, with lower amounts of C28 steranes (ergostane and crysotane) also present. No traces of known ancient C30 sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.

Uncommon steranes in Brazilian marginal crude oils: Dinoflagellate molecular fossils in the Sergipe-Alagoas Basin, Brazil

Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC–TOFMS) was used to identify unusual steranes in branched-cyclic hydrocarbon fractions of crude oils from Sergipe-Alagoas Basin, Brazil and infer the geochemical settings of their Cretaceous source rocks. Hopane/sterane and sterane ratios, and the presence of gammacerane and β-carotane in the oil samples indicates a saline source rock depositional environment and predominantly marine organic matter (OM) input. Steroidal hydrocarbons were represented by alkyl steranes and several sterane series, including abundant short-chain steranes (C21 and C22), regular steranes (C27–C29), and diasteranes (C21–C23, C27–C29). The ring-A methyl sterane series contained 3β-methyl steranes (C22 and C23, C28–C30), abundant 3β-methyldiginane (C22), and 3β-methyldiacholestanes (C28), including 4-methyl steranes with abundant C30 4α,23,24-trimethylcholestanes in the branched-cyclic fraction of these crude oils. The C29 sterane isomers, 23,24-dimethylcholestane and 24-ethylcholestanes, were resolved by GC×GC–TOFMS, and a C29 dinosterane derivative was identified for the first time in Brazilian crude oils. The 23,24-dimethyl and 4,23,24-trimethyl steroids can be related to dinoflagellate OM and abundant dinosteranes suggests dinoflagellate blooms during source rock deposition in this Brazilian marginal basin. Additionally, the detection of C28–C32 3β-alkyl cholestanes, C29–C33 3β-alkyl 24-methylcholestanes, and C30–C34 3β-alkyl 24-ethylcholestane with ethyl to pentyl groups at C-3 resulted in a complex sterane fingerprint, as expected for petroleum generated from source rocks deposited under predominantly marine saline conditions with high microalgal input. These biomarker results provide additional insight into the character and geologic processes responsible for the source rocks of crude oils in the Brazilian marginal basins. This information will be a valuable exploration tool that can be used to constrain the distribution of different oil types in these basins.

Analysis of crude oils using gas purge microsyringe extraction coupled to comprehensive two dimensional gas chromatography-time-of-flight mass spectrometry

As a complex mixture, the analysis of individual compounds in crude oil is a great challenge. The traditional sample preparation method such as column chromatography is time-consuming, solvent-wasting, and very possible to cause the loss of light hydrocarbons. In addition, coelution of compounds with similar boiling points or polarities often occurs in one dimensional gas chromatography–mass spectrometry (1D GC–MS). To overcome these disadvantages, a homemade gas purge microsyringe extraction (GP-MSE) device was prepared and coupled to comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC×GC-TOFMS) for the analysis of crude oils from different regions. The extraction conditions of GP-MSE were optimized in detail. The whole process of GP-MSE extraction only takes 7min, only 20μL of organic solvent is needed, and almost complete extraction of analytes is achieved. Compared with the common used methods, light hydrocarbons in all the samples were well retained and more compounds were separated and identified, including diamondoid series, pyrrolic nitrogen compounds and other important biomarkers such as terpane and sterane series. Coelution of compounds in 1D GC–MS was eliminated in GC×GC-TOFMS. Based on the developed GP-MSE-GC×GC-TOFMS, the distribution of trace compounds in crude oils and effect of biodegradation were discussed in detail.

Identification and significance of 3β-alkyl steranes in the Eogene lacustrine sediments and petroleum of China

Several homologous series of steranes with alkyl side chains (C1 to C4) at the 3β position have been identified in the Jiyang Eogene lacustrine deposition. It is postulated that its precursors represent a new class of steroids, alkylated at the C-3 position with a polyhydroxy n-alkane. These precursors may have been formed by the bacterial addition of a ribose sugar to Δ2-sterenes, diagenetic alteration products of steroids synthesized by eukaryotes. 3-alkyl steroids might substitute for hopanols in bacterial membranes. When they are present in a sample, the patterns of the isomer distributions of 3-alkyl steranes are similar to desmethyl steranes except for lower rearranged ones. It is shown that the configurational isomerization of 3-alkyl steranes is trending in line with that of desmethyl steranes with increasing of maturity. The abundance of 3-alkyl steranes may be controlled by the depositional environments; they are primary in saline or near shore lacustrine, moderate in shallow lacustrine, poor in sub-deep to deep lacustrine relative to the 4-methyl steranes.

Geochemical characteristics of crude oils from Zao-V oil measures in Shenjiapu oilfield

The geochemical characteristics of crude oils from Zao-V oil measures in the Shenjiapu oilfield are systematically described in terms of the fractional composition of crude oils, GC characteristics of saturated hydrocarbon fraction of crude oils and the characteristics of their bio-markers. The depositional environment, type and evolution of the biological source are also discussed. All pieces of evidence such as low saturated hydrocarbon fraction, high resin and asphalt, high isoprenoid alkane, weak odd-carbon number predominance ( CPI ranging from 1.23 to 1.29, OEP ranging from 1.14 to 1.16) and low sterane and terpane maturity parameters show these crude oils are immature oils. Low Pr/Ph ratios (0.66–0.88) and high gammacerance/C31 hopane ratios (0.59–0. 86) indicate the source rocks were formed in a slightly saline to brackish reducing lake depositional environment. Gas Chromatographic characteristics of the saturated hydrocarbon fraction and the predominance of C30 hopane in terpane series and C29 sterane in sterane series indicate the biological source of the crude oils is composed mainly of bacterial and algal organic matter, and some algae are perhaps the main contributor of organic matter to the source rocks.

Distribution and Comparison of Steranes From Blue-Green Algae Under Different Simulated Conditions

Two remarkably opposite distribution patterns of sterane series are revealed from living blue-green alga (or cyanobacterium) under different simulated conditions. The organic matter derived from hydrous-pressure pyrolysis of the blue-green alga is characterised by the predominance of C_(27) sterane over C_(29) sterane; this is coincident with traditional thought. On the contrary, the organic matter derived from artificially silicified microfossils of the blue-green alga is characterised by the predominance of C_(29) sterane over C_(27) sterane; this appears to be an unusual distribution pattern. Such kind of unusual sterane distribution also has been found in the samples of Precambrian cherts with abundant well-preserved microfossil algae. All these imply that the special source of organisms and the medium, procedure or conditions of simulated pyrolysis may result in the different distribution patterns of sterane biomarkers.

A new class of natural products revealed by 3β-alkyI steranes in petroleum

THE polycyclic hydrocarbons steranes and hopanes are nearly ubiquitous in petroleum, and their relative resistance to degradation makes them useful as 'biomarkers' for assessing the maturity and history of the oil. Steranes represent the reduced form of sterols, which serve as membrane rigidifiers in eukaryotic organisms; similarly, hopanes derive from hopanols, which serve the same function in prokaryotes. We have identified several homologous series of steranes with alkyl side chains (Cl to C6) at the β position. These compounds, when liberated from the polar fractions of the oils by deuterated Raney nickel desulphurization, give fragmentation mass spectra indicating that the 3-alkyl side chains of the precursor steroids contained several functional groups. 3β-pentyl steranes are anomalously abundant in many samples. These data suggest that the precursors represent a new class of steroids, alkylated at the C–;3 position with a polyhydroxy n-alkane. These precursors may have been formed by the bacterial addition of a ribose sugar to Δ-sterenes, diagenetic alteration products of steroids synthesized by eukaryotes. 3-alkyl steroids might substitute for hopanols (of prokaryotic origin) in bacterial membranes1.

Bitumen from Coalport tar tunnel

The Coalport (Shropshire, U.K.) Tar Tunnel bitumen has been known since 1787 and the first geochemical data are reported here. The bitumen was analyzed for molecular markers useful for correlational studies. Gas chromatographic analysis of aliphatic and aromatic hydrocarbons failed to detect any specific major components normally used for genetic correlational and maturational studies. A search for minor and trace components by gas chromatographic-mass fragmentographic analysis showed the presence of triterpenoid hydrocarbons primarily of the 17α (H)-hopane series (C 27 to C 35, ex. C 28) and a C 26 to C 31 series of ring A/B demethylated hopanes. Two homologous sterane series (C 27 to C 29) of the 5α, 14β, 17β (H)-sterane and 13β, 17α (H)-diasterane type were also detected. Pophyrins of the DPEP and etio series (C 27 to C 41, DPEP/etio > 1) were also found. Characterization of their alkyl substitution pattern demonstrated C 1, C 2 and C 3 substituents on the pyrole moieties of the parent petroporphyrins. The molecular markers detected in this bitumen indicate its biogenic origin and show evidence of diagenetic and geothermal maturation processes. The overall geochemical characteristics of the Coalport Tar Tunnel bitumen suggest that it corresponds to a well matured crude oil, which was heavily altered by in-reservoir biodegradation or close to surface exposure.

Relict tetracyclic and pentacyclic hydrocarbons in ancient oils of the Siberian Platform

Oils from the southern part of the Siberian Platform are among the most ancient oils in the Soviet Union. The relative abundance of relict polycyclic hydrocarbons were determined by gas-liquid chromatography. It was found that the total relict polycyclane content decreases with depth of burial. A decrease in the sterane and hopane content of oils from deep-lying strata was also noted. These changes in the auantities of relict hydrocarbons in the oils as a function of their degree of catagenetic alteration is in good agreement with the changes in the content of different stereoisomers of the sterane series hydrocarbons. (JMT)

A-nor-steranes, a novel class of sedimentary hydrocarbons

As part of our investigations of Cretaceous black shales1 we have analysed samples from a location near Aqualagna in the north central Apennines, Italy (43°39′ N, 12°43′ E). The saturated hydrocarbon fraction of one sample, a dark-grey shale of Upper Barremian age (115 Myr BP), contains novel series of steranes, which have been identified as C26–C28 5β(H)- and 5α(H)-A-nor-steranes. Because these compounds are structurally related to 3β-hydroxymethyl-A-nor-steranes which have been found in some sponges, we suggest here that these steranes might be biological markers for certain types of sponges.