Organic Geochemistry最新文献

The application of routine geochemical analyses for identification of dry natural gas sources and post generation processes poses a challenge as its hydrocarbon composition is typically dominated by CH₄ (∼≥99 %) while the presence of non-hydrocarbons may be limited. In particular, the possibility of dry gas interaction with H₂S at reservoir conditions (low temperatures, high pressures, millions of years residence time) is not established as previous studies of this reaction focused on conditions typical for industrial reactors (high temperature, ambient pressure, residence time of seconds). To address these issues, we studied the molecular and isotopic (δ³⁴S) composition of volatile organic sulfur compounds (VOSC) formed during pyrolysis experiments between CH₄ and H₂S at 360 °C (4–96 h), reaching %Ro equivalent of 0.80–1.25 which represents thermally mature oil and gas reservoirs. The results demonstrated that methanethiol (MeSH) is the main product of the reaction, while its δ³⁴S value suggest equilibrium isotopic effect with its parent H₂S. Subsequent analysis of the molecular and isotopic (δ¹³C, δ²H, δ³⁴S) composition of thermogenic, H₂S containing, dry natural gases from the Jiannan gas field in China revealed the presence of short thiols and sulfides dominated by MeSH. The δ³⁴S of the VOSC identified suggests they all formed by gas-phase reaction of alkanes (mainly CH₄) with the associated H₂S.

This study demonstrates the applicability of VOSC as a proxy for identification of interaction between H₂S and dry gas and identification of H₂S sources within a gas reservoir or a basin.

Steroid acids are unique molecular fossils composed of a steroid carbon skeleton linked to a carboxyl group. These biomarkers contain special geological and biological clues in their structures. However, the origin of steroid acids remains a subject of debate, and there is a lack of direct evidence in previous studies. Here, we present experimental evidence for the origin of steroid acids. The origin of steroid acids during biodegradation in crude oil was observed for the first time. Molecular structures of these steroid acids reveal that they arose from steranes via bacterial oxidation of sidechain terminal methyl groups. Our results suggest that microbial activities indeed contribute to the formation of carboxyl groups on the eukaryotic steroid skeletons, and we believe steroid acids are a promising biomarker for both geomicrobial processes and ancient eukaryotic life in the rock record.

High-resolution mass spectrometry can be utilized to select specific proxies for the quantitative assessment of crude oil biodegradation degree, offering higher accuracy and convenience compared to conventional GC-MS methods. However, the current evaluation proxies are invalid for severely biodegraded crude oil. In this study, freshwater and saltwater lacustrine crude oils from the Liaohe Western Depression (Bohai Bay Basin) with varying degrees of degradation, were characterized using negative ion electrospray ionization [ESI (−)] Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results show that seven heteroatom classes were identified including N₁, N₁O₁, N₁O₂, O₁, O₂, O₃ and O₄. Certain differences exist in the abundance of heteroatom compounds in the nondegraded crude oils from the two origins, but both are dominated by N₁. The relative abundance of O₂ class species significantly increases, while the relative abundance of O₁ and N₁ class species decreases with an increase in the degree of biodegradation, reflecting the increase in the content of acid compounds as biodegradation products. O₂ class species become the predominant compound in the severe degradation stage. Biodegradation results in the enrichment of compounds with greater condensation, while the abundance of highly alkyl-substituted compounds decreases. The nitrogen-containing compound pairing proxies (DBE_12,13,15/DBE_9∼11-N₁) can be employed to assess the degree of biodegradation in crude oil under conditions of similar maturity. As the degree of biodegradation increases, the content of 2 ∼ 5-cyclic naphthenic acids increase, while the content of acyclic acids with weaker resistance to degradation decreases. The ratio of acyclic acids to 2 ∼ 5-cyclic naphthenic acids (Modified A/C Ratio 2) can effectively assess the biodegradation level of crude oils ranging from nondegraded to severe degradation. The O₂/(N₁ + O₁) Ratio reflects the formation of acids during the biodegradation process and exhibits a robust positive correlation with crude oil density and degradation degree. The new proxies provide higher precision and broader applicability compared to conventional methods, enabling quantitative evaluation of biodegradation levels. The application of ESI FT-ICR MS technology holds significant importance in the assessment of heavy oil and the exploration of its genetic mechanisms.

Abnormal stable carbon isotopic (δ¹³C) compositions, deviating from the conventional order of δ¹³C₁ < δ¹³C₂ < δ¹³C₃, are frequently observed in natural gas reservoirs. For thermogenic gas, these anomalies, such as δ¹³C₁ < δ¹³C₃ < δ¹³C₂ or δ¹³C₁ > δ¹³C₂ > δ¹³C₃, have multiple formation mechanisms including gas mixing in conventional systems and desorption processes of gaseous hydrocarbons in unconventional shale gas systems due to their inconsistency with Rayleigh fractionation processes. Considering distinct reaction pathways (e.g., C₁ polymerized to C₂), these aberrant δ¹³C signatures are often construed as intrinsic hallmarks of extensively evolved natural gas. However, on the basis of gas generation simulation, not all findings exhibit abnormal δ¹³C values, hinting at multifaceted and intricate mechanisms governing the isotopic fractionation of alkane gas components. This study conducted gold tube pyrolysis of an Australian torbanite, revealing four distinct types of δ¹³C anomalies in hydrocarbon classes. Polycyclic aromatic hydrocarbons (PAHs) exhibited δ¹³C values more negative than co-occurring n-alkanes. δ¹³C₃ displayed a negative trend shift from EasyRo = 3.5 %, resulting in a partially δ¹³C-reversed gas (δ¹³C₁ < δ¹³C₃ < δ₁₃C₂) formed at EasyRo ≈ 4.1 %. Moreover, intramolecular δ¹³C₃ (both terminal and central carbons, termed δ¹³Ca and δ¹³Cb, respectively) reversed alongside the overall δ¹³C₃ trend. Additionally, the evolution of site preference in δ¹³C₃ (termed ‰SP = δ¹³Ca – δ¹³Cb) transitioned from progressively negative to positive. The results of this study demonstrate that the potential conversion between saturated hydrocarbons, aromatic hydrocarbons, and alkane gases is at least partially responsible for δ¹³C anomalies, but also that gaseous hydrocarbons formed from other fractions at high maturity cannot be ruled out, such as kerogen and pyrobitumen.

The lacustrine Lucaogou Formation was deposited during the middle Permian in the Junggar Basin, northwestern China. The depositional environment was an alkaline, brackish to saline lake. The biological diversity of the Lucaogou Formation was reconstructed using biologically informative molecules such as n-alkanes, steranes, and hopanes, and the δ¹³C composition of organic matter. C₃₀ 4α-methyl-24-ethylsterane, C₃₁ lanostane, and abundant C₃₀ − C₃₁ 3β-methylhopanes were detected in the organic matter, suggesting the presence of type Ⅰ aerobic methanotrophs including the Methylococcaceae. C₃₀ − C₃₁ 2α-methylhopanes and 7- and 8-methylheptadecanes are suggestive of the presence of cyanobacteria. Cyanobacteria and green algae are proposed to have made the dominant contribution to the organic matter. The presence of more ¹³C-enriched hopanoids in the lower unit compared to the upper unit of the Lucaogou Formation suggest a greater contribution of cyanobacteria to the organic matter in the lower unit. The high abundances of C₂₈ and C₂₉ steranes relative to C₂₇ steranes are likely due to green algae from the Chlorophyta, including the class Chlorophyceae. Dinosteranes that are diagnostic for dinoflagellates were not detected. The low concentrations of C₄₀ isorenieratane, renieratane, β-isorenieratane, renierapurpurane, and β-renierapurpurane that are indicative of photosynthetic green and purple sulfur bacteria indicate a minor contribution from Chlorobiaceae and Chromatiaceae to the organic matter. It is inferred that low to moderate amounts of ciliates grew at or below the relatively unstable chemocline in the lake. The presence of C₁₉-norisopimarane, 8β(H)-labdane, and 4β(H)-19-norisopimarane suggests the presence of coniferous plants growing around the lake, probably including Pinaceae, Araucariaceae, Cupressaceae.

In this study, we conducted swelling experiments on eleven macromolecular geological organic materials using five types of organic solvents. Our primary objective was to investigate the absorption capacity of different solid organic materials in organic fluids and analyze the influence of their chemical structures. The samples utilized in this research included solid bitumen, kerogen, and coal samples from various basins. The chemical structure of the samples was assessed using solid-state ¹³C nuclear magnetic resonance (NMR), while X-ray diffraction (XRD) was employed to monitor any detect the structural changes during the swelling process. Our findings reveal that macromolecular geological organic compounds demonstrate a preferential expulsion of saturated hydrocarbons, followed by polycyclic aromatic hydrocarbons, upon interaction with liquid organic matter. The sulfur-containing compounds in solid organic matter demonstrated higher solubility than hydrocarbon compounds, while the solubility of oxygen-containing compounds varied based on the structure of the aliphatic chain and the proportion of oxygen atoms. This research, introduces LA [= L_ac × A_ac] as a new parameter to assess the combination of aliphatic chain length [L_ac] and [A_ac] abundance in solid organic matter. Furthermore, XRD testing revealed that the chemical structure unit of heteroatom compounds in solid organic matter consists of amorphous carbon, primarily composed of aliphatic chains. In this study, we evaluated the retention capacity of various macromolecular geological organic matter for both hydrocarbons and heteroatomic compounds. Additionally, the extent of swelling was investigated, providing theoretical support to diverse fields including organic petrology, petroleum geology, coal geology, and organic geochemistry.

Reservoir bitumens in the northwestern Sichuan basin are significant for elucidating the sources and charging history of oil and gas, given their widespread occurrence in multiple strata in the area and abundant biomarkers. The debate regarding the origin and genesis of these bitumens has persisted for a long time, due to severe biodegradation and the development of multiple sets of high maturity source rocks with few biomarkers for oil-source correlation. To resolve these questions, asphaltenes, which are more resistant to biodegradation than free hydrocarbons, were systematically analyzed with the bulk multi-isotopes (C-S-N-H), bound molecules and the carbon isotopic compositions of bound individual n-alkanes. These characteristics were then compared to those of bulk bitumen and free hydrocarbons, leading to three main conclusions. In most samples, relatively abundant 25-norhopanes and 17-nortricylic terpanes were identified along with n-alkanes in free hydrocarbons, suggesting at least two oil charging events. In the samples having free n-alkanes, the carbon isotopic compositions of asphaltene-bound n-alkanes closely resemble those of corresponding free n-alkanes. Moreover, the bulk C-S-N isotopic compositions of asphaltene also approach those of corresponding bulk bitumen. These results suggest that the source of the oil charges occuring at different times are mostly the same for an individual sample, though different bitumen samples may have distinct sources. Free hydrocarbons, including n-alkanes and biomarkers, may have been produced by the secondary cracking of asphaltenes. Second, the integration of bulk C-S-N isotopic compositions of the asphaltenes and bitumens has enabled the studied samples to be classified into four groups. Source facies is the primary control of the distinct isotopic compositions with other factors like biodegradation, thermal maturity and migration having only minor influence. In combination with biomarkers, the organic matter and sedimentary environment of source rocks could be characterized for each group. A careful comparison of the bulk C-S-N isotopic compositions of asphaltenes and bitumens with those previously reported for source rocks (organic C and S and bulk N isotopes) suggests the main source rocks in the Upper Ediacarian-Lower Cambrian Formations, supporting the conclusions of previous studies. Furthermore, those source rocks in much younger formations, such as the Middle Permian as well as the Middle Devonian Formations, may also have contributed significantly to the widespread reservoir bitumens in the region. These findings highlight the usefulness of bulk C-S-N isotopic composition of asphaltenes for distinguishing oils with complex genesis and large gas exploration potential of the Upper Paleozoic source rocks in the region.

Methodologies based on benzene polycarboxylic acids (BPCA) selectively target the polymeric aromatic fraction of black carbon (BC) and are considered adequate to quantify pyrogenic inputs in environmental samples such as soils, lakes, and marine dissolved organic carbon. However, the usefulness of these methodologies to quantify BPCA-derived BC in deep-sea sediments has not been fully evaluated. In this manuscript we describe and validate a procedure to quantify BPCAs in deep oceanic sediments with very low organic carbon content. The resulting analytical procedure has produced reproducible quantitative data for BPCAs over a period of 10 months (coefficient of variation, CV = 6.4 − 6.6%). The stable carbon isotopes (δ¹³C) of BC_BPCA have been characterized using an LC Isolink™-irMS system with an accuracy better than 0.5‰. The quantitative and isotopic composition of several marine sediments has been characterized to investigate the relative contributions of marine/diagenetic and continental/pyrogenic sources to the BC accumulated in oceanic sediments from different contexts, ranging from upwelling systems to remote oceanic locations. Overall, a significant fraction of the sedimentary BC is of marine origin and should be considered in inventories of pyrogenic materials accumulated in the world oceans. However, the continental/pyrogenic sources can be largely dominant in marine settings with large inputs of pyrogenic materials.