Organic Geochemistry最新文献

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.

Speleothem stable carbon isotopes (δ¹³C_carb) are used to reconstruct past environments, but are a complex signal of karst, soil and plant processes. To help untangle these signals, we used plant waxes, their carbon isotopic values (δ¹³C_wax) and polycyclic aromatic hydrocarbons (PAHs) extracted from stalagmites to evaluate plant photosynthetic pathway (C₃ vs C₄) and biomass burning above a cave. Our test case investigates stalagmites from Anjohibe in Madagascar where at around 1000 CE multiple δ¹³C_carb records increase by ∼ 8–10 ‰. This suggests that humans transformed the local landscape from C₃ vegetation to C₄ grasses through agropastoral practices, which rely on burning to promote grass growth. We evaluated different protocols to remove contamination, finding higher biomarker yields after polishing off the surface of the stalagmite versus ultrasonic pre-cleaning in solvent. Anjohibe stalagmites include n-alkanes from trees and grasses; however, bulk organic δ¹³C and δ¹³C_wax from samples dated to after the transition to the modern C₄ landscape yield values suggesting C₃ vegetation. This is likely due to a disproportionally higher contribution of C₃ waxes to the overall n-alkane signal. PAHs are present in the stalagmite but do not match the types found in overlying soils and further testing is required to determine their source. We find that δ¹³C values of bulk organic carbon, or plant waxes extracted from stalagmites, should be interpreted with caution as the proportion of plant matter on the landscape does not necessarily equate to the proportion of organic molecules produced by those plants or preserved in the sedimentary record.

The formation and evolution of thermogenic gases were investigated using a combination of intermolecular and intramolecular isotope analyses of 32 natural gas samples collected from the Sichuan and Tarim basins (China) and the Arkoma Basin (USA). Three evolution stages (I–III) were identified: In stage I, hydrocarbon gases are produced through thermal decomposition of organic matter, and kinetic isotope effects in C–C bond breakage control their isotopic distributions; In stage II, C₂–C₅ hydrocarbons crack with increasing thermal maturity, with their formation and decomposition tending toward thermodynamic equilibrium, and at the end of this stage, the intermolecular and intramolecular isotopic compositions of gaseous hydrocarbons are in thermodynamic equilibrium; A remarkable feature of stage III is the surface-catalyzed abiotic polymerization of methane, which provides a critical origin of C₂₊ hydrocarbons in this stages and leads to isotopic anomalies in C₂₊ hydrocarbons, including the reversal of δ¹³C distributions of C₁–C₃ and the reverse evolution trend of SP value of propane (i.e., tending to be positive). The contribution of C₂₊ hydrocarbons from the abiotic polymerization of methane can be determined based on a two-end member model. C₂₊ hydrocarbons in the Changning shale gases are all generated from abiotic methane polymerization, and the contribution ratio in the Weiyuan shale gases is about 85 %, while, the contribution of C₂₊ hydrocarbons in dry gases from the Tarim Basin formed by this way is no more than 55 %. High methane abundance, high temperature, and abundant catalyst are beneficial to abiotic methane polymerization.

In a series of recent publications in Organic Geochemistry and elsewhere, investigators from State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry and their collaborators have reported position-specific δ¹³C of propane obtained from natural reservoirs and pyrolysis experiments. Based solely on the very negative Δ_C-T values (δ¹³C_center – δ¹³C_terminal), ranging from −7 to −10 ‰, they advocated for abiogenic origins of the propanes from sedimentary settings (shale and coal deposits). However, they never demonstrated accuracy of the data and it is likely that correct Δ_C-T values for these propanes are close to zero ‰. Therefore, their discussion and interpretation in terms of geochemical processes, including a putative abiogenic origin, are not substantiated.

The Dziani Dzaha (Mayotte Islands, Indian Ocean) is a small, shallow, saline and hyperalkaline maar lake. Its surface waters are characterized by intense primary production, inducing the waters below 2 m depth to remain aphotic and anoxic all year round, at least until 2017, when a ∼5 m-long core was taken from the center of the lake. The recovered sediments were described and sampled at high resolution. They consist of laminated microbial mats, mixed with carbonate lenses or nodular beds, and rare silty detrital facies. The inorganic and organic carbon contents of 160 samples were analyzed using the Rock-Eval® method, including the Rock-Eval® 7S device which quantifies both total organic sulfur and total sulfur content, in addition to conventional Rock-Eval parameters. The Dziani Dzaha sediments are characterized by high TOC content (8.5 wt% on average and up to 27.9 wt%) and variable inorganic carbon content. HI values average 630 mg HC/g TOC and reach up to 834 mg HC/g TOC. TS content varies from 0.3 to 3.6 wt%, with TSorg/TOC ratios close to 0.01 at the top of the core and fluctuating downcore between 0.02 and 0.05. Interestingly, the pyrolysis thermal stability of sulfurized organic matter increases with depth, and the highest HI values are associated with highest Sorg content, sulfurization of organic matter being generally accompanied by reductive processes. The hydrogen- and organic-sulfur-rich sediments of Dziani Dzaha can be considered modern analogues of Type I and I-S petroleum source rock deposits, such as the Eocene Green River shales and Kimmeridgian Orbagnoux laminites, with remarkable facies and geochemical similarities. (258 words)

Gas flushing of oil has been proposed as an essential process for forming some condensate gas reservoirs based on laboratory experiments and modeling but such a process has rarely been documented in subsurface reservoirs. Here, we report a unique type of yellow-ringed oil inclusions (YROIs) discovered from a deep reservoir in the Kuqa Foreland Basin, Western China that recorded the gas flushing of oil process in the subsurface. The occurrence, composition, phase state and PVT conditions of YROIs during trapping were investigated using a suite of fluid inclusion analytical methods including fluid inclusion petrography, microthermometry, fluorescence spectroscopy, PVT modeling and molecular composition analysis.

YROIs found in the reservoir are three-phase (solid–liquid-vapor) fluid inclusions at room temperature with solid rings of 2–3 μm thickness and estimated volume percentages varying between 5 % and 15 %. When heated the solid ring starts to melt at around 46 °C and homogenized with the liquid phase at or above their corresponding homogenization temperatures of coeval aqueous inclusions. YROIs have fluorescence spectral peaks between 530 nm and 550 nm and an elevated spectral shoulder around 650 nm when measured at room temperature; but most spectra show a distinct “blue shift” and an absence of the 650 nm shoulder when measured at their homogenization temperatures. Some YROIs with thick yellow rings show dual spectral peaks at 570 nm and 630 nm at room temperature and their spectral peaks do not change when remeasured at their homogenization temperatures. Molecular compositions derived from synchronous fluorescence spectra and GC-MS analysis suggest that the compositions of YROIs are rich in polyaromatic compounds. PVT modeling indicates that YROIs were trapped under high pressure conditions with a pressure exceeding 50 % of the corresponding hydrostatic pressure.

YROIs are interpreted to have been trapped under an elevated temperature and overpressure condition (around 110 °C and 60 MPa) relating to an intensive (wet) gas flushing of an early-charged wax-rich oil. When light hydrocarbon components in the reservoir oil were partially removed by gas, the wax/resin components rich in polyaromatic hydrocarbon compounds may precipitate due to physio-chemical fractionation. YROIs were entrapped in subsurface from a heterogeneous hydrocarbon fluid containing variable amounts of wax/resin colloids, which subsequently coalesce to form solid rings adhering to the inner wall of oil inclusions at room temperature. The presence of yellow-ringed oil inclusions in a reservoir may be indicative of a rapid gas flushing of waxy oil under high pressure, and their minimum homogenization temperature may approximate their trapping temperature.