Petroleum isoprenoid hydrocarbons derived from catagenetic degradation of archaebacterial lipids

The three GC coeluting C₄₀ bis-phytanes with head-to-tail, tail-to-tail and head-to-head linkages considered to be archaebacterial markers, have been recognized in petroleum by enhanced mass-fragmentography. Supporting evidence has been obtained by the simultaneous occurrence of two C₂₁–C₂₄ series of isoprenoid and quasi-isoprenoid hydrocarbons that are supposedly formed during the catagenetic degradation of the isomeric C₄₀ isoprenoids. The C₂₁–C₂₄ quasi-isoprenoid hydrocarbons have been identified conclusively for the first time by comparison with authentic standards.     

Variation of the elastic constants of tourmaline with chemical composition

Elastic wave velocities and lattice parameters of five tourmaline specimens with different chemical compositions have been measured. The piezoelectric effects on the elastic constants have been found to be small and can be neglected. Variations of the elastic wave velocities and elastic constants of the different tourmaline specimens indicate that: (i) partial substitution of Al by Fe in the structure decreases the shear wave velocities, (ii) replacement of Na by Ca increases the resistance of the structure against shear deformation involving C ₆₆, (iii) replacement of Al by Mg seems to decrease the resistance of the structure against longitudinal deformation involving C ₃₃. Elastic constants C ₁₁, C ₃₃, C ₄₄ and C ₆₆ of the different tourmaline specimens used in this study differ individually by 1.7 percent to 6.7 percent, indicating that the large differences (up to 21%) between the values reported by previous authors cannot be explained in terms of the chemical composition alone.     

Lipid geochemistry in a sediment core from Ruoergai Marsh deposit (Eastern Qinghai-Tibet plateau, China)

Peats in a sediment core from Ruoergai bog, which has a cold and moist plateau climate with major source input from herbaceous plants, have been studied by GC–MS in order to understand the composition and diagenetic processes of lipids in this depositional environment. Long chain components (C₂₁–C₃₅) predominate in the n-alkanes, n-alk-1-enes, n-fatty acids, n-alkan-2-ones and n-alkanols with a maximum of C₃₁, C_27, C₂₂ or C_24, C₂₃ or C₂₅ and C₂₂, respectively. A herbaceous origin for these long chain compounds is suggested, and this is supported by their stable carbon isotopic compositions. Diterpenoid hydrocarbons with abietane, pimarane and kaurane skeletons, some of which have not been reported often in modern sediments, are prominent and are derived from higher plants. Several triterpenoid ketones and alcohols with oleanane or lupane skeletons, and a series of des-A-triterpenoid hydrocarbons which have not been reported often in modern sediments are also present, and are assigned to a higher plant source. Hopanoids, including their alkanes, alkenes, ketones, alcohols and esters, are abundant and of bacterial origin. Steroid ketones and alkanols are dominated by C₂₉ homologues. C₂₈ and C₂₉ steroids are derived mainly from higher plants, whereas the C₂₇ component is assigned to a microbial source. The presence of short-chain n-alkanes with no odd-even carbon predominance, bacterially derived fatty acids (C₁₄, C₁₅, iso- and unsaturated acids), n-alkan-2-ones, des-A-triterpenoid hydrocarbons, hopanoids and some steroid ketones indicate that intense microbial reworking of the organic matter has taken place in this depositional environment. The chemical and biochemical conversions of some cyclic alkenes to alkanes, such as tricyclic diterpenoids, tetracyclic terpenoids and steroid ketones, are also evident with depth. The dominance of C₂₀ components in the diterpenoid hydrocarbons may reflect an oxidizing or reducing depositional condition.     

Isoprenoids in a Costa Rican seep oil

The isoprenoid alkanes present in a seep oil from Costa Rica have been examined using gas chromatography and mass spectrometry. In addition to the predominance of the C₁₆ and C₁₈-C₂₀ regular isoprenoid alkanes, the C₂₁ and C₂₃-C₂₅ regular isoprenoid alkanes were identified. The C₂₆, C₂₈ and C₃₀ regular isoprenoid alkanes were tentatively identified. No evidence for the regular C₁₇, C₂₂ or C₂₇ isoprenoid alkanes was found. The compounds 3,7,11- trimethyltetradecane and 3,7,11-trimethylhexadecane were tentatively identified. It is suggested that a higher regular isoprenoid structure (or structures) is required in addition to phytol to account for the distribution of isoprenoid alkanes.     

Typification of oils in the Timan-Pechora province according to the composition of hydrocarbon biomarkers (steranes and terpanes)

Distinctive compositional features of cyclic saturated hydrocarbon biomarkers have been established in oils from the main petroliferous lithostratigraphic complexes of various structural zones in the Timan-Pechora petroliferous province (TPPP). Four geochemical families (types) of oils in TPPP are recognized based on the variations in the geochemical parameters of steranes and terpanes including sterane ratios C₂₇/C₂₉ and C₂₈/C₂₉, K¹ mat and K² mat, diasterane/regular sterane, pregnane (C_21–22)/sterane (C_21–22 + C_27–29), as well as terpane Ts/Tm parameters, adiantane C₂₉/hopane C₃₀, neoadiantane/adiantane, tryciclic terpane/pentacyclic terpane, hopane/sum of C₂₉ steranes, etc. The distribution of various types of oil in the sedimentary sequence of TPPP makes it possible to infer source rocks for each of the four selected types.     

Geochemical characteristics,forming conditions and resource evaluation of oil-cracking gas as exemplified by the platform area of the Tarim Basin

Based on the geochemical characteristics of oil-cracking gas and kerogen-cracking gas revealed by simu-lation experiments and the chemical composition of natural gases in actual gas reservoirs, two kinds of natural gases with different relationships between C2/C3 and C1/C2, C2/C3 and C1/C3, C2/C3 and 100×C1/(C1-C5) were identified in the Tarim Basin, and proposed further by the authors. The relationship charts of C2/C3 and C1/C2, C2/C3 and C1/C3, C2/C3 and 100×C1/(C1-C5) can be used to effectively distinguish oil-cracking gas from kerogen-cracking gas. Petro-leum geological analysis of the oil-cracking gas reservoirs showed that the distribution of oil-cracking gas is mostly related with deep-seated faults or faults with a large fault throw, and the burial depth of paleo-oil reservoir is rela-tively high; crude oil-cracking gas resources have been evaluated by using both forward and inversion methods. The plots of C2/C3 vs. C1/C2, C2/C3 vs. C1/C3, and C2/C3 vs. 100×C1/(C1-C5) were used to distinguish between oil-crack-ing gas and kerogen-cracking gas, and estimate the mixed ratios of the two kinds of natural gases in the main gas reservoirs of the platform area.     

Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau

The carbon isotope composition (δ¹³C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C₃ and C₄ photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ¹³C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C₂₇ and C₂₉) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C₂₉) similar to that of terrestrial C₄ plants. Thus, the use of δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C₄ plants. For sedimentary C₃₁, however, the contribution from submerged plants was minor, so that the δ¹³C values for C₃₁ n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C₄ vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ¹³C values of sedimentary long chain n-alkanes (C₂₇, C₂₉ and C₃₁) may carry different environmental signals. While the δ¹³C values of C₃₁ were a reliable proxy for C₄/C₃ terrestrial vegetation composition, the δ¹³C values of C₂₇ and C₂₉ n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.     

Geochemical characteristics and genetic types of crude oils from Qinjiatun and Qikeshu oilfields in the Lishu Fault Depression,Songliao Basin,northeastern China

The Qinjiatun and Qikeshu oilfields are new Mesozoic petroleum exploration targets in Lishu Fault Depression of Songliao Basin, northeastern China. Currently, researches on geochemistry of crude oils from Qinjiatun and Qikeshu oilfields have not been performed and the genesis of oils is still uncertain. Based on bulk analyses, the crude oils in the Qinjiatun and Qikeshu oilfields of Lishu Fault Depression from the Lower Cretaceous can be classified as three types. TypeⅠoils, from Quantou and Denglouku formations of Qikeshu oilfield, are characterized by high C24tetracyclic terpane/C26tricyclic terpanes ratios, low gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29norhopane ratios and 17α(H)-diahopane/17α(H)-hopane ratios, indicating a brackish lacustrine facies. TypeⅡoils, from Shahezi Formation of Qikeshu oilfield show low C24tetracyclic terpane/C26tricyclic terpanes, high gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29 norhopane and C30diahopane/C30hopane ratios, thus suggesting that they originated from source rocks deposited in a weak reducing brackish lacustrine environment, or clay-rich sediments. Type oilsⅢ, from some wells of Qikeshu oilfield have geochemical characteristics intermediate between those two types and may be mixture of typeⅠand Ⅱoils.     

四川南桐地区二叠系龙潭煤层生物标记物及其地质意义

四川南桐地区二叠系龙潭煤层具有良好的生油潜力。其生物标记物包括正构和异构烷烃,类异戊二烯烷烃,倍半萜烷,二萜烷,三环萜烷,藿烷,一种未知结构的五环三萜烷(C₃₀),甾烷,4-甲基甾烷等。煤层生物标记物特征表明在其形成过程中有藻类和细菌等微生物物质的加入。     

Characteristics of light hydrocarbons (C1–C7) in mesozoic and paleozoic rocks in the Jurong Basin of Jiangsu Province,China

Characteristics have been studied of light hydrocarbons (C₁–C₇) from crude oils and source rocks ranging from Devonian to Triassic in age in the Jurong Basin where carbonate rocks are dominating. The results show that light hydrocarbon compositions (C₁–C₇) can be used to classify organic matter types and maturities as well as to make oil-source rock correlations. It is also an effective method in organic geochemical studies of oils, gases and source rocks in terrains of old carbonate rocks.     

Stable carbon and oxygen isotopic study of carbonate along a loess/paleosol section in Lantian County, Shaanxi Province, on the southeastern edge of the Chinese Loess Plateau

Studies of carbonate carbon isotope of loess/paleosol (δ¹³C_carb) in the Chinese Loess Plateau (CLP) have shown δ¹³C_carb less negative in loess and more negative in paleosol, which is opposite to that of bulk organic matter. Although some mechanisms have been proposed to explain this inconsistency, few studies have been conducted to investigate how carbonate migration could affect the reliability of utilizing δ¹³C_carb as an effective indicator. Here, a loess/paleosol profile with a nodule horizon intercalated in the loess layer, located on the southeastern edge of the CLP, was investigated to understand the influence of carbonate eluviation and reprecipitation on δ¹³C_carb along the section. The mean grain size and magnetic susceptibility generally conform to the field observed loess/paleosol stratigraphy. However, carbonate content shows distinct differences in the two sides of the nodule horizon, clearly indicating eluviation along the section. The variation of carbon and oxygen isotopic compositions of soil carbonate (δ¹³C_SC and δ¹⁸O_SC) and nodule carbonate (δ¹³C_NC and δ¹⁸O_NC) along the profile does not present a clearly meaningful picture. Generally, δ¹³C_SC and δ¹⁸O_SC have a similar change trend along the profile and are positively correlated, but there is no apparent relationship between δ¹³C_NC and δ¹⁸O_NC. More importantly, δ¹⁸O_NC values fall in the range of δ¹⁸O_SC, whereas δ¹³C_NC values are much more positive than δ¹³C_SC. Detailed analyses of the data indicate migration of carbonate along the profile, which is an important factor that determines that loess/paleosol δ¹³C_carb could not be employed as a high-resolution paleovegetational and paleoenvironmental indicator in the CLP, at least on or below the glacial/interglacial scales.     

A homologous series of novel hopane sulfides in petroleum

A homologous series of hexacyclic sulfides ranging from C₃₀ to C₃₅ and containing the 17α(H), 21β (H) hopane carbon skeleton, III, have been identified in the heavy oils of Northern Alberta.     

Geochemical characteristics of Lower Permian coal-measure source rocks in Northeast Ordos Basin

General characteristics and biomarker distributions of Lower Permian coal-measure source rocks in northeastern Ordos Bsain have been analyzed in this paper. The results show that the source rocks are type III kerogen, thermally mature, with high content of total organic carbon. The extracts of the source rock samples with different lithologies from Well Su 27 feature high Pr/Ph ratios, high C19TT and C24Te, high rearranged hopanes, a predominance of C29 sterane in regular steranes, and the absence of C30 4-methylsteranes. These data suggest that the source rocks were deposited in suboxic to oxic conditions with dominantly terrigenous higher plant input. In contrast, the source rocks from wells Shuang 1 and Yu 20, are characterized by low Pr/Ph ratios, low C19TT and C24Te, high C23TT, similar content of C27 sterane and C29 sterane, and the presence of C30 4-methylsteranes, showing that the source rocks were deposited in reducing environment with algae and/or microorganisms and terrigenous higher plant input.     

The effect of source material,rock type and diagenesis on the n-alkane content of sediments

The effects of diagenesis on marine organic material have been compared to those on terrestrial organic material in three Australian oil exploration wells. n-Alkaae distributions obtained for limestones ranged from C₁₅ to C₂₅ with no odd-even predominance and showed little variation with depth. n-Alkane distributions obtained for shales ranged from C₁₅ to C₃₃ and frequently had a pronounced odd-even predominance. Differences in the rates of hydrocarbon diagenesis between different rock types may be due to variations in either clay mineral content or the nature of the parent organic material. The individual n-alkane distributions of the sediments are determined by source material, degree of diagenesis and lithofacies.     

Long-chain unsaturated ketones in recent lacustrine sediments

C₃₇–C₃₉ ketones containing 2–4 double bonds and similarly-unsaturated C₃₈–C₄₀ ethyl ketones have been detected for the first time in lacustrine sediments. Tetra-unsaturated compounds are relatively more abundant than in marine sediments. The similar molecular composition of methyl ketones and cooccurring ethyl ketones containing one additional carbon atom indicates that the two classes are biosynthetically related.No source of these potential biological markers in lacustrine sediments has been identified. The abundance and unsaturation pattern of alkenones in lacustrine sediments may reflect the relative input and species composition, respectively, of algae presumed to belong to the Phylum Chrysophyta, by analogy with the marine source.     

Characterization of high molecular weight biomarkers in crude oils

High-temperature gas chromatography (HTGC) has enhanced our ability to characterize hydrocarbons extending to C₁₂₀ in crude oils. As a result, hydrocarbons in waxes (> C₂₀) have been observed to vary significantly between crude oils, even those presumed to originate from the same source. Prior to this development, microcrystalline waxes containing hydrocarbons above C₄₀ were not characterized on a molecular level due to the analytical limitations of conventional gas chromatography. Routine screenings of high pour-point crude oils by high-temperature gas chromatography has revealed that high molecular weight hydrocarbons (> C₄₀) are very common in most oils and may represent 2% of the crude oil. Precise structures, origins, and significance of these high molecular weight compounds remain elusive. As a preliminary step to expand our knowledge of these compounds their general molecular structures and formulas have been investigated in this study. Initial results suggest that the major high molecular weight compounds include a homologous series of n-alkanes, methylbranched alkanes, alkylcyclopentanes, alkylcyclohexanes, alkylbenzenes and alkylcycloalkanes.     

The petroporphyrins of a Cretaceous oil

The petroporphyrins of a Cretaceous crude oil, La Paz, from western Venezuela are shown to be a mixture of etio and DPEP homologues (C₂₇-C₃₉) maximising at C₃₀ and C₃₁, respectively. Minor amounts of rhodoporphyrins (C₃₀-C₃₉) are also present. Thin-layer chromatography afforded fractions which have been shown by mass spectrometry to contain up to 80% of a single-molecular-weight species. Oxidative degradation of La Paz petroporphyrins to maleimides and mass-spectrometric study of t.l.c. fractions indicate that some of these porphyrins are to a great extent incompletely substituted. Dealkylation reactions have presumably played an important role in their geologic history. Furthermore, the relative simplicity of the alkyl substitution pattern of the La Paz petroporphyrins suggests that transalkylation reactions have not taken place to any significant extent.The fraction of petroporphyrins isolated from the asphaltenes contains a higher proportion of the DPEP homologues than do the total petroporphyrins isolated from the original crude oil.     

Extracts of impact breccia samples from Sudbury, Gardnos, and Ries impact craters and the effects of aggregation on C60 detection

Fullerenes have been detected in carbonaceous chondrite meteorites and in breccia samples from meteorite impact craters, but questions have been raised about contradictory results from similar samples and the sensitivities and accuracies of different analytical methods. We analyzed samples from three impact craters and detected C₆₀ in samples from several locations; we also observed differences in the detection capabilities of various analytical techniques used in the search for fullerenes. The presence of C₆₀ in rocks from the Onaping Formation of the Sudbury impact crater was confirmed. Low levels of C₆₀ were also detected for the first time in samples from the Gardnos (Norway) and Ries (Germany) impact structures.We detected C₆₀ in these samples using surface-enhanced laser desorption/ ionization (SELDI), but the related technique of microprobe laser-desorption, laser-ionization mass spectrometry (μL²MS) did not observe C₆₀ above detection limits. We attribute the absence of μL²MS signal to aggregate formation caused by phthalic acid esters, which appear to easily contaminate samples either during storage or demineralization in plastic containers. The μL²MS technique is incapable of detecting aggregated C₆₀, but aggregation does not suppress detection in SELDI. Phthalate-induced aggregation did, however, enhance SELDI detection of C₆₀ in some cases, and we suggest that this enhancement may help explain previously reported differences in C₆₀ detection from natural samples between laser desorption mass spectrometry (LDMS, a technique analogous to SELDI that has detected fullerenes in meteorite and impact breccia samples) and high-pressure liquid chromatography (HPLC). This work highlights the effects of phthalates and other indigenous compounds or contaminants on certain mass spectrometric techniques and lends support to the idea that several complementary analytical methods should be employed to investigate complex natural samples.     

Hydrothermal hydrocarbon gases: 1, Genesis and geothermometry

Various sources for hydrothermal CH₄ have been proposed over the years. While C isotope studies have narrowed the possibilities, enough higher hydrocarbon gas data now exist both to supplement the isotopic data and to permit additional deductions regarding origins. Comparison of typical C₁–C₆ data for gases of various origins (from sedimentary and crystalline rocks, and hydrothermal systems) reveals certain characteristics. Apart from isotopic differences, hydrothermal hydrocarbons differ from sedimentary hydrocarbons mainly in possessing tendencies towards a relative excess of CH₄, higher normal/iso ratios for butane and pentane, and relatively high amounts of C₆ gases. Despite these differences, consideration of the evidence indicates that hydrothermal hydrocarbon gases in most cases originate like sedimentary basin gases by thermal degradation of organic matter in the relatively shallow subsurface. The principal characteristic of these hydrothermal gases, “excess” CH₄, appears to have a geothermometric function. The following empirical relationship has been derived: t°C=57.8 log(CH₄/C₂H₆)+96.8, which fits moderately well a range of geothermal fields worldwide. This gas geothermometer may be particularly applicable during geothermal exploration in areas where there is little direct knowledge of subsurface conditions.     

Variation and paleoclimatic significance of organic carbon isotopes of Ili loess in arid Central Asia

As an indicator for terrestrial paleovegetation, the stable isotopic composition of total organic matter (δ¹³C_org) in loess sediments has been widely used for paleoclimatic reconstruction in western Europe, the Great Plains of North America and the Chinese Loess Plateau (CLP). However, little is known about the variation and paleoclimatic significance of the loess δ¹³C_org in arid Central Asia (ACA). We report δ¹³C_org data from an Axike (AXK) loess/paleosol profile from the eastern Ili Basin, eastern Central Asia. Along the profile, the δ¹³C_org values were more negative in the paleosol layers observed in the field and were confirmed by environmental magnetic proxies and a higher concentration of total organic carbon (TOC), consistent with results for western Europe and the northwestern CLP. Our results demonstrate that the loess δ¹³C_org in this region documents mainly the response of δ¹³C of locally predominant C₃ plants to paleoclimatic variation, especially paleoprecipitation. Our results also suggest that the loess δ¹³C_org values in the area have the potential for quantitative paleoprecipitation reconstruction on the basis of detailed δ¹³C_org results from modern plants and surface soils in the future.