Organic Geochemistry最新文献

The distribution of bicyclic sesquiterpanes in crude oils was studied to assess the applicability of the sesquiterpane ratios as maturity parameters. GC–MS analysis was conducted on ten oil samples that were generated in Miocene-Paleogene deposits located within the Kamchatka Peninsula and southeast Chukotka, the Russian Far East. During catagenesis, there is a shift in the distribution of sesquiterpanes with increasing thermal maturity, e.g., a decrease of the relative content of C₁₆ homodrimanes towards an increase of C₁₅ drimanes and C₁₄ sesquiterpanes. New maturity indices, based on sesquiterpanes, correlate well with commonly used maturity indicators, such as a methylphenanthrene index (MPI-1) and a methyldibenzothiophene ratio (MDR) (the R² value greater than 0.746). Since C₁₄ sesquiterpanes can arise as a product of thermolysis of higher homologues or may be derived from other organic sources, it is not included in one of the new indices.

The responses of angiosperms and gymnosperms to environmental variables (e.g., rainfall [MAP], temperature [MAT] and pCO₂) remain ambiguous and require systematic investigation. This study monitored the plants' responses to changing environments and measured variations in molecular n-alkane indices, δ¹³C_bulk, δ¹³C_n-alkanes, and δ²H_n-alkanes values in woody gymnosperms (n = 36) and angiosperms (n = 17) from an altitudinal gradient (0.9 to 3.8 km) in the Himalayas The results indicate that δ¹³C_bulk values in angiosperms show a moderate correlation with altitude, MAP, and pCO₂, while gymnosperms show no response in their δ¹³C_bulk. The δ¹³C_n-alkanes values in angiosperms remain unaffected by MAP, MAT, and pCO₂, whereas gymnosperms exhibit a moderate negative correlation. The fractionations between δ¹³C_n-alkanes and δ¹³C_bulk values (ɛ_alk/leaf) are approximately –4.0 ± 1.6 ‰ and –5.6 ± 1.5 ‰ for gymnosperms and angiosperms, respectively. In gymnosperms, the fractionation between δ²H_n-alkane and δ²H_rain (ɛ_alk/rain) is –133.2 ± 39.8 ‰ and –122.8 ± 38.0 ‰ (C₃₁), while in angiosperms, it is –88.5 ± 44.6 ‰ (C₂₉) and –62.4 ± 22.9 ‰ (C₃₁). The δ²H_n-alkane values in gymnosperms and angiosperms are weakly and positively influenced by MAT and MAP, respectively. We found that gymnosperms are enriched in ¹³C (∼1.5–3.0 ‰) and depleted in ²H (∼56–60 ‰) compared to angiosperms; this pattern is consistent throughout the latitudes sampled in this study. The species-specific isotopic response to environmental factors is primarily driven by lower stomatal conductance, smaller leaf-size, and presence of complex tracheids in gymnosperms compared to angiosperms. The statistical K-means algorithm of dual isotope analysis (δ¹³C_n-alkanes and δ²H_n-alkane) provided two distinct clusters with an accuracy of 70 % for angiosperms and gymnosperms, which has implications for studying past vegetation transitions.

It has long been believed that the region on the north of the Dongsheng gas field has only limited exploration potential due to the lack of effective source rocks. However, several areas of Cretaceous oil seepage have been discovered in the area and their sources are unclear. In this study, the organic geochemical characteristics of these oil seepages and potential source rocks (coals and carbonaceous mudstones of different formations) are examined using TOC analysis, pyrolysis, gas chromatography-mass spectrometry, and carbon isotopes. The results show that the oil seepages are low-to-moderately mature (0.63 %-0.78 % R_c) and slightly biodegraded. The molecular biomarker compositions of the oil seepages are characterized by C₂₉≫C₂₈>C₂₇ normalized relative abundance of regular steranes, high values of Pr/Ph, (C₁₉TT + C₂₀TT)/C₂₃TT and C₂₄TeT/C₂₆TT ratios, low values of Ga/C₃₀H, and low abundance of dibenzothiophenes. The biomarker compositions, as well as the stable carbon isotopes, indicate that the source of the oil seepages was deposited in a lacustrine fluvial/deltaic environment under oxic to sub-oxic conditions, with organic matter input predominantly from terrigenous higher plants. Oil-source correlation revealed that the oil seepages were closely related to the coals in the Shanxi formation in the south source kitchen, implying that the hydrocarbons generated in the south source kitchen have been able to migrate to the north edge of the Ordos Basin. The oil seepages are accompanied by significant gas leakage. The zone between the Dongsheng gas field and the Wulangar Uplift is therefore likely to have very good accumulation and preservation conditions and may offer a promising exploration prospect for natural gas.

Dibenzofuran, benzo[b]naphthofurans and a series of their alkylated isomers were identified in coal, coaly shale, and shale extracts from the Gippsland Basin (Victoria, Australia). The dibenzofuran series include C₀-C₄ dibenzofurans, and the benzo[b]naphthofuran series are composed of benzo[b]naphthofuran, methylbenzo[b]naphthofurans, and possible dimethylbenzo[b]naphthofurans. Seventeen C₂ dibenzofurans isomers (including six new isomers) were tentatively identified in rock extracts from the Gippsland Basin. The distribution and contents of C₀-C₄ dibenzofurans are different for Eocene and Paleocene rocks, providing a source rock age diagnostic correlation tool. Biological origin is the main factor that controls the abundance and distribution of dibenzofuran, benzo[b]naphthofuran and their alkylated isomers, based on investigation of source-, environment- and maturity-derived parameters. Variations in the C₁ dibenzofurans/dibenzofuran ratio and the benzo[b]naphtho[2,1-d]furan/benzo[b]naphtho[1,2-d]furan ratio may reflect local paleoclimate changes in the Gippsland Basin.

Forest canopy structure is a fundamental ecosystem attribute affecting regional and global climate through primary production by CO₂ drawdown and evapotranspirative feedback. Environmental changes in temperature and light affect leaf physiology and thus canopy functioning. Leaf physiological changes may be reflected in expressed chemical compounds (e.g., leaf lipid biomarkers), that offer opportunities to characterize and quantify climatic effects on plant canopies in the present and the past. To assess this possibility, we systematically investigated the lipids from leaves of deciduous angiosperm (Quercus buckleyi, April–October 2019) and evergreen gymnosperm (Juniperus ashei, April–October 2019 and January 2020) tree species at monthly sampling intervals over one growing season in a natural sub-humid ecosystem of central Texas. Fatty acid unsaturation in Q. buckleyi and J. ashei leaves was negatively correlated with air temperature. The average chain lengths of leaf wax n-alkanols of Q. buckleyi were strongly correlated with leaf area index (LAI) and absorbed photosynthetically active radiation (APAR) (r² > 0.5). The stigmasterol/β-sitosterol ratio was correlated with light transmittance in the canopy of Q. buckleyi, with values of the sterol ratio three-fold higher in shaded leaves than in sparse canopies. The observed seasonal changes in leaf lipid molecular composition and chain-lengths might be related to their biosynthetic responses to temperature and light stresses. Finally, we developed multi-lipid regression models resolving seasonal differences in temperature, LAI, and APAR. We posit that the specific lipid biosynthetic responses to variations in temperature and light are a basis for reconstructing terrestrial paleoenvironmental changes.

A comprehensive geochemical program was carried out on rock samples collected in the NDI Carrara 1 drill hole, the first stratigraphic test of the newly discovered Carrara Sub-basin located in the South Nicholson region of northern Australia. The drill hole recovered continuous core from 284 m to total depth at 1750 m and intersected approximately 1120 m of Proterozoic sedimentary rocks, unconformably overlain by 630 m of Cambrian Georgina Basin carbonate-rich rocks. Total organic carbon (TOC) contents from Rock-Eval pyrolysis highlight the potential for several thick black shales to be a source of petroleum for conventional and unconventional plays. Cambrian rocks contain an organic-rich section with TOC contents of up to 4.7 wt.% and excellent oil-generating potential. The Proterozoic section is overmature for oil generation but mature for gas generation, with potential for generating gas in carbonaceous mudstones showing TOC contents up to 5.5 wt.% between 680 and 725 m depth. A sustained release of methane (up to 2%) recorded during drilling from 1150 to 1500 m suggests potential for an unconventional gas system in the Proterozoic rocks from 950 to 1415 m depth, which exhibit favourable organic richness and thermal maturity. The Proterozoic rocks, which are comparable in age to the sediment-hosted deposits of the Century Mine, contain local occurrences of lead, zinc and copper sulfide minerals providing hints of mineralisation. The combined geochemical results offer the promise of a potential new resource province in northern Australia.

Glycerol dialkyl glycerol tetraethers (GDGTs) are microbial membrane-spanning lipids that are produced in a variety of environments. To better understand the potentially confounding effect of soil chemistry on the temperature relationship of branched GDGTs (brGDGTs), isoprenoid GDGTs (isoGDGTs) and GDGT-based proxies MBT’_5ME and TEX₈₆, soils from 6 elevation transects (mean annual air temperature 0 – 26 ℃, n = 74) were analyzed. Corroborating earlier work, the MBT’_5ME index correlates well with mean annual air temperature in the low pH (pH < 7), non-arid soils under study (r = 0.87, p < 0.001). However, a clear over-estimation of reconstructed temperature in the lowest pH (<3.5) soils is observed, explained by the correlation between brGDGT Ia and free acidity. TEX₈₆ also shows a significant correlation with mean annual air temperature (r = 0.45, p < 0.001), driven by temperature dependent concentration changes of isoGDGTs 3 and cren’. However, an overarching correlation with P/E values dominates concentration changes of all supposed Thaumarchaeotal isoGDGTs lipids (GDGT1-3, cren and cren’), implying a potential impact of soil moisture on TEX₈₆ values. In addition to identifying the impact of these confounding factors on the temperature proxy, GDGT ratios that can be used to constrain changes in soil chemistry, specifically exchangeable Ca²⁺, sum of basic cations, exchangeable Fe³⁺ and sum of soil metals are proposed (0.53 < r² < 0.68), while existing ratios for soil moisture availability are tested for the first time in a dataset of non-arid soils. While the impact of soil chemistry on GDGTs may complicate the interpretation of their temperature proxies, our proposed GDGT ratios can potentially be used to constrain a subset of soil chemistry changes through time.

Highly mature organic-rich source rocks in the Early Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwestern China, were deposited in an alkaline lacustrine environment. Analysis of extracts of eight core samples by gas chromatography–mass spectrometry detected a series of short-chain lanostanes (C₂₃–C₂₅) lanostanes. The relative abundance of short-chain lanostanes has a strong positive correlation with those of long chain (C₃₀–C₃₂) homologues, indicating a possible common biological origin and genesis. Compound specific isotopic analyses of phytane, steranes, hopanes and C₃₀ lanostane from the core extracts reflect input from primary photosynthetic producers including cyanobacteria. Abundant lamalginite was observed in the samples from the study area. A well-defined positive relationship is also evident between the relative abundance of lanostanes and the ratios of 7-+8-methyl heptadecanes/C_max and 2-methyl hopanes/C₃₀-hopanes. Given that 7-+8-methyl heptadecanes and 2-methyl hopanes are considered to be diagnostic molecular biomarkers for organic matter derived from cyanobacteria, it is possible that cyanobacteria may serve as a possible biological precursor for lanostanes in the Fengcheng Formation. The influence of maturity on the lanostanes is considered to be minor, as no significant relationship is found between lanostanes and thermal maturity parameters, although this may be due to a limited maturity range of the samples. The co-existence of high contents of phytane, β-carotanes, and gammacerane, and their correlations in abundance with lanostanes, indicates a strongly reducing sedimentary environment.