Organic Geochemistry最新文献

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.

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.

Glycerol dialkyl diethers (GDDs) are membrane lipids and a variation of the more commonly known glycerol dialkyl glycerol tetraethers (GDGTs). GDGTs include both archaeal and bacterial membrane lipids that are both frequently used for paleoclimate reconstruction in a range of terrestrial and aquatic environments. In contrast to GDGTs, GDDs lack one of the terminal glycerol moieties. Although both isoprenoidal (iso) and branched (br) GDDs have been found, this study focuses on isoGDDs. These lipids have been found in a few terrestrial and aquatic environments. However, the origin of isoGDDs is debated and the extent of their distribution across the surface of the Earth is poorly constrained. Based on a few single site studies, previous authors hypothesised that isoGDDs are degradation products of isoGDGTs, but more recent studies that isolated isoGDDs from cultured nitrososphaerota (formerly thaumarchaeota) proposed a biological source through direct archaeal biosynthesis. Here we compiled a global dataset of isoGDD and isoGDGT abundance in environmental samples to thoroughly investigate the distribution of isoGDDs and the correlation with isoGDGTs on a global scale and across a variety of environments (peat, mineral soils, lake sediments, and marine sediments). We find that isoGDDs are present in most samples that we analysed. Their abundance is strongly proportional to isoGDGT abundance (r² = 0.85), dominated by the GDGT-crenarchaeol/GDD-crenarchaeol ratio (r² = 0.94) and supported by individual compound isoGDGT/isoGDD ratios (r² = 0.56–0.94). In addition, the degree of cyclisation of isoGDDs, reflected in the ring index, is positively correlated (r² = 0.84) with that of isoGDGTs across all environments. We conclude that isoGDDs are abundant on the surface of the Earth and predominantly originate from the degradation of isoGDGTs.

In this study, the geochemical significance of the δ¹³C and δ²H values of selected C₇ light hydrocarbons (nC₇ = n-heptane; MCH = methylcyclohexane; Tol = toluene) is investigated in 58 light oil/condensate (LOC) samples from the Kuqa Depression of the Tarim Basin, NW China. For most of the LOCs in the western region, the nC₇, MCH, and Tol exhibit systematic differences where δ¹³C_nC7 < δ¹³C_MCH < δ¹³C_Tol and δ²H_Tol < δ²H_nC7 < δ²H_MCH. LOCs from the eastern area have higher δ¹³C_oil, δ¹³C_Tol, and δ¹³C_MCH values than those from the western area and their δ¹³C values for C₇ hydrocarbons are similar and δ²H_nC7 < δ²H_MCH < δ²H_Tol. These organic geochemical differences indicate that the LOCs from the western and eastern areas of the Kuqa Depression were mainly derived from lacustrine and coaly parent materials, respectively. The LOC reservoirs generally received a late hydrocarbon charge of the high-maturity natural gas, and the LOCs from the western Kela–Keshen areas subsequently experienced gas leakage. Hierarchical cluster analysis indicates that the most important influencing factor on the LOCs of the Kuqa Depression is the sedimentary facies (coaly and lacustrine), followed by gas leakage and then parent organic type (lacustrine sapropel and humic organic matter). A cross-plot of the Δδ²H_Tol–MCH versus Tol/MCH can distinguish the origins of continental facies parent materials and gas leakage of the LOCs.

The Lower Cretaceous Jiufotang Formation, situated in western Liaoning Province, China, has attracted attention for abundant Jehol Biota fossils and the potential to improve understanding of the relationships among climate change, evolution of terrigenous organisms, and organic matter (OM) enrichment in lake systems during the Early Cretaceous. Based on organic petrology and biomarker analyses, this study investigated the depositional environment, OM sources, and enrichment mechanism of Jiufotang mudstone in the Kazuo Basin. Maceral composition shows that the OM source of Jiufotang mudstone shifted from mainly aquatic to terrigenous, and biomarkers such as abundant diterpenoids and retene indicate that terrigenous higher plants were mainly conifers during deposition of the Jiufotang Formation. Pr/Ph ratios (0.07–0.65) show that the studied section was deposited in anoxic bottom water that may be necessary for OM enrichment in lake systems. Gravity flows were accompanied by input of many clastic minerals and oxygenation of bottom waters, which is unfavorable for OM enrichment. Therefore, the best OM enrichment model for lake systems includes anoxic bottom water, stable water column stratification, low sedimentation rates, and high phytoplankton contributions. The climate changed from relatively dry to humid, which drove changes in the OM enrichment model in the lake. More importantly, the climate and terrigenous plant information obtained by biomarkers is useful to reconstruct the ecological environment of the Jehol Biota.