Guangzhou Institute of Geochemistry
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The influence of asphaltene matrix on the thermal evolution of polycyclic aromatic hydrocarbons: Experimental evidence and geochemical implications
No full textThe thermal evolution of components trapped in matrix of asphaltenes is supposed to be retarded, as compared to the free ones. Studying the geochemical evolution of trapped polycyclic aromatic hydrocarbons (PAHs) may provide insight for the characterization of high-maturity organic matter. The discrepancies in the thermal evolution between free and trapped (asphaltene-adsorbed/occluded) PAHs (phenanthrene, chrysene, pyrene, and their methylated isomers) were studied by thermal maturation (gold tubes, 300 similar to 400 degrees C) experiments on a low-maturity solid bitumen from the Sichuan Basin, southwestern China. The results show that the thermal evolution of asphaltene-adsorbed PAHs is retarded compared to that of free ones. This is attributed to slight differences in reaction kinetics due to steric hindrance by the asphaltene structure. The asphaltene matrix appears to act as a reaction inhibitor leading to a retardation of the thermal evolution of the methylphenanthrene index (MPI) in asphaltene-adsorbed hydrocarbons. This could provide a possibility for maturity assessment of high-maturity organic matter. The thermal evolution of asphaltene-occluded PAHs in the experiments resulted in thermodynamically controlled isomer distributions of occluded methylphenanthrenes and methylchrysenes. However, the occluded methylpyrene isomers were hardly affected by thermal stress. They probably retained their original distributions and may serve as source indicators
Deciphering the origin and secondary alteration of deep natural gas in the Tarim basin through paired methane clumped isotopes
No full textThe Tarim Basin is one of the most complex, ancient, and petroliferous deep craton basins in the world. Multiple sets of source rocks, stages of tectonism, and episodes of hydrocarbon generation and accumulation complicate the formation and evolutionary processes of hydrocarbons in the basin, especially for the deeply buried petroleum system at >6000 m depth. Paired clumped isotopes (Delta(CH3D)-C-13 and Delta(CH2D2)-C-12) allow the investigation of methane isotopic bond ordering, elucidating the methane generation mechanism and its post-generation processes, which can complement studies of the molecular and conventional stable isotopic compositions of gas hydrocarbons in constraining the origin and source of natural gas. This study focused on methane clumped isotopes, and chemical and conventional stable isotopic compositions of natural gas samples from four typical oil and gas regions of the Tarim Basin. Results indicate that these gases are thermogenic gases with relatively high stable isotopic delta values (i.e., -46.1 parts per thousand 300 degrees C) exceeding the methane "closure temperature". Non-equilibrium gases in the Tabei area yielded a plausible Delta(CH3D)-C-13 apparent temperature (<160 degrees C) with a pronounced deficit in Delta(CH2D2)-C-12 values, possibly owing to irreversible chemical kinetic processes in thermal cracking of organic matter at early thermal maturity (R-o < 1.5 parts per thousand) or mixing between thermogenic gases generated at two distinct temperatures (i.e., similar to 200 degrees C and similar to 120 degrees C). Equilibrium clumped isotope signatures also rule out the possible effects of secondary alteration on natural gases in the Hetianhe area that were previously considered in conventional isotope studies. Such findings demonstrate that methane clumped isotopes provide independent and effective indicators of gas-source correlations in deep basins while also elucidating secondary processes that are often under- or over-estimated in conventional stable isotope studies
Sources and secondary transformation potentials of aromatic hydrocarbons observed in a medium-sized city in yangtze river delta region: Emphasis on intermediate-volatility naphthalene
No full textAromatic hydrocarbons (AHs) play an important role in the formation of ground-level ozone (O3) and secondary organic aerosols (SOA). Although monoaromatics (MAHs) have been extensively measured in previous studies, the heavy AHs such as 2-ring naphthalene (Nap), a key surrogate of intermediate-volatility organic compounds (IVOCs), were overlooked. Additionally, field observations of AHs are mostly restricted to megacities and limited results have been reported in medium/small-sized cities, which could demonstrate different AH pollution characteristics due to the factors like emission source and population. In this study, sixteen MAHs as well as intermediate-volatility Nap were measured concurrently at six urban sites surrounded by traffic lanes, residential and commercial areas in Wuhu, a medium-sized city in Yangtze River Delta (YRD) region of China, in sum-mertime. During the campaign, the mean concentration of seventeen AHs was 2.85 +/- 0.55 ppb (mean +/- 95% C. I.). Compared to megacities, lower MAHs but comparable or even relatively higher Nap levels were observed here during the sampling period. Source apportionment revealed that gasoline exhaust and industrial emission were the major contributors to AHs, accounting for 32.2%-52.6% and 20.3%-46.2%, respectively, followed by biomass/coal burning (8.5%-14.2%), solvent utilization (4.5%-13.1%) and diesel engine emission (3.0%-7.3%). Interestingly, diesel engine emission made the largest contribution to Nap, while industrial emission was less significant, suggesting that the source of IVOC species might be different from that of traditional VOCs. Among the measured AHs, Nap ranked the largest and the third-largest contributor to calculated SOA formation po-tentials (SOAFPs) and O3 formation potentials (OFPs), respectively. Our results highlight the role of intermediate-volatility Nap in the production of SOA and O3 and reveal different source contributions between Nap and MAHs. It is suggested to re-evaluate the contributions of various sources and precursors to urban O3 and SOA after considering the IVOCs
Distribution, occurrence and identification of dibenzofuran, benzo[b] naphthofurans and their alkyl derivatives in Gippsland Basin source rocks
No full textDibenzofuran, 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
Kinetics of Oriented Attachment of Mica Crystals
No full textOriented attachment (OA), that is, the coalescence of crystals through attachment on coaligned crystal faces, is a nonclassical crystal growth process. Before attachment, a mesocrystal consisting of coaligned parallel crystals but with liquid separating them was observed. Fundamental questions such as why OA is kinetically favored and whether a mesocrystal stage is a prerequisite for OA are raised. Through combining brute-force molecular dynamics simulations and path samplings based on extensive umbrella simulations, we address these questions with a case study on the OA of a mica nanocrystal onto a mica crystal substrate in water. Brute-force simulations show that if two mica crystals are attached but largely misaligned, coalignment hardly appears. Thus, if OA is possible, then coalignment must appear before the attachment between crystals. Electrophoresis of the nanocrystal toward the substrate surface is spontaneous, but mesocrystal formation is occasional, also shown by brute-force simulations. Free energies along different pathways show that OA is spontaneous and kinetically favored over non-OA, and a mesocrystal formation is just a bifurcation in the pathway. OA is through a pathway in which the nanocrystal is tilted with respect to the substrate. Part of the nanocrystal is attached to the substrate first, and then, OA is gradually completed. Once a mesocrystal is occasionally formed, then a jump event is needed for the nanocrystal to get back to the OA pathway. The sampling technique here can hopefully guide the design of nanostructured materials facilitated by OA
Geomagnetic paleointensity variations in the northern South China Sea since the Late Pleistocene
No full textThe geomagnetic field plays multiple roles in the investigation of the geodynamic processes of Earth's internal fluids and dating of deposition sequences. However, discrepancies in geomagnetic field intensity on suborbital and millennial time scales across different regions have hindered discussions on these topics. In this study, we constructed a master relative intensity curve (SPIS-150) covering the past similar to 150 kyr by stacking the data from a new core (PC27) with published intensity curves for the northern South China Sea based on the chronology established through the oxygen isotope stratigraphy and relative paleointensity (RPI) correlation. Rock magnetic experiment results reveal that the sediments effectively record geomagnetic field information, although the magnetic minerals are primarily composed of terrestrial and biogenic magnetite. The stacked relative paleointensity (SPIS-150) exhibits a consistent pattern with the global stacked master curve on the orbital time scale, reflecting dipole-dominated changes in the geomagnetic field. However, specific features, such as the profound intensity lows between 75 and 85 kyr and a relatively low peak during 110-120 kyr, display potential differences with the global stacked virtual axial dipole moment (VADM) curves. Additionally, the disagreement in RPI across different regions have emerged significantly since about 25 kyr. Our record appears to align with the western tropical Pacific Ocean and the southern hemisphere but, to some extent, contradicts the stacked VADM from the northern hemisphere. This pattern may indicate the possible influence of reverse flux patches along the magnetic equator and normal flux patches in the north hemisphere
Nitrogen isotopic compositions of organic-rich shales (-560 Ma) in the Chengkou region, South China: Implications for a stable and relatively large nitrate reservoir of the late Ediacaran ocean
No full textThe late Ediacaran period is a critical transitional period in Earth's history, marked by the evolution of the Ediacara biota (-574 to 538.8 Ma). The marine redox state and nutrient level, having a close relationship with bioevolution, can be reflected by sedimentary nitrogen isotopes (815N). Doushantuo Formation Member IV, a widespread organic-rich interval during this critical period of South China, may provide important clues for nitrogen (N) cycling. However, high-resolution 815N research on Member IV is still lacking. In this study, a drill core from the Chengkou sub-basin, with the well-developed Doushantuo Member IV, was carefully studied base on its N isotopic composition. The implications for the N cycling characteristics are discussed using a combination of proxies for bottom water redox conditions and basin openness. The total nitrogen (TN) concentrations (up to 0.79 wt%) of these organic-rich samples are substantially higher than those of other counterpart profiles. The TOC content shows a positive linear relationship with TN concentration, and a significant positive TNintercept (-0.21 wt%) represents the addition of inorganic N produced by the remineralization of sinking organic N. The high TN concentrations may also be due to the flourishing and enhanced burial of primary producers. The 815N value in the Doushantuo Member IV progressively decreases from -8.2%o to -3.2%o (from bottom to top), a result of the changes in basin openness and the enhancement of N2-fixation. Compilation shows the 815N and 813Corg values are approximately covariant, and both have significant temporal and spatial variations at this member of South China, indicating a strong heterogeneity in the water column redox environments and changes in the type of primary producers. A mode 815N value of 4%o indicates a stable and relatively large nitrate reservoir in the late Ediacaran ocean
Shale oil potential and mobility in low- to medium-maturity lacustrine shales: A case study of the Yanchang Formation shale in southeast Ordos Basin, China
No full textOil mobility is essential for the evaluation of shale oil resources, yet there has been limited research conducted on the oil mobility of low-to medium-maturity lacustrine shale. This study integrates multiple methodologies such as organic geochemistry, mineralogy, oil-kerogen adsorption-swelling experiments, and in-situ conversion pro-cess (ICP) models to evaluate the mobility and potential for shale oil in the lacustrine shale of the Yanchang Formation in the Yaoqu 1 (YQ1) well situated in the southeastern region of the Ordos Basin. The Chang 7 shales are categorized as highly favorable source rocks, nonetheless, the oil contained within these layers predomi-nantly exists in an adsorbed-swelling state, resulting in limited mobility as indicated by the oil saturation index, a modified oversaturation index, and production index. Therefore, the utilization of existing conventional tech-niques for shale oil exploitation is not viable for the Chang 7 shale characterized by low to medium maturity in the study area. Nevertheless, during the ICP, the quantity and mobility of oil within the Chang 7 shales exhibit a substantial increase as temperature rises. Moreover, once the Chang 7 type II shales reach a maturity level of around 0.91%Ro, they demonstrate noteworthy prospects for shale oil production. The findings of this study serve as a valuable reference for guiding the advancement of in-situ conversion process technology in the study area, while also offering substantial evidence for addressing the intricate challenge of crude oil mobility in low-to medium-maturity lacustrine shale reservoirs
Distribution and migration of rare earth elements in sediment profile near a decommissioned uranium hydrometallurgical site in South China: Environmental implications
No full textRare earth elements have garnered increasing attention due to their strategic properties and chronic toxicity to humans. To better understand the content, migration, and ecological risk of rare earth elements in a 180 cm depth sediment profile downstream of a decommissioned uranium hydrometallurgical site in South China, X-ray powder diffraction (XRD) and High-resolution transmission electron microscope (HRTEM) were additionally used to quantify and clarify the mineral composition features. The results showed a high enrichment level of total rare earth elements in the sediment depth profile (range: 129.6-1264.3 mg/kg); the concentration variation of light rare earth elements was more dependent on depth than heavy rare earth elements. Overall, there was an obvious enrichment trend of light rare earth elements relative to heavy rare earth elements and negative anomalies of Ce and Eu. The fractionation and anomaly of rare earth elements in sediments were closely related to the formation and weathering of iron-bearing minerals and clay minerals, as confirmed by the correlation analysis of rare earth elements with Fe (r2 = 0.77-0.90) and Al (r2 = 0.50-0.71). The mineralogical composition of sediments mainly consisted of quartz, feldspar, magnetite, goethite, and hematite. Pollution assessment based on the potential ecological risk index, pollution load index (PLI), enrichment factor, and geological accumulation index (Igeo) showed that almost all the sediments had varying degrees of pollution and a high level of ecological risk. This study implied that continued environmental supervision and management are needed to secure the ecological health in terms of rare earth elements enrichment around a decommissioned uranium hydrometallurgical site. The findings may provide valuable insights for other uranium mining and hydrometallurgical areas globally
Microbially mediated sulfur oxidation coupled with arsenate reduction within oligotrophic mining-impacted habitats
No full textArsenate [As(V)] reduction is a major cause of arsenic (As) release from soils, which threatens more than 200 million people worldwide. While heterotrophic As(V) reduction has been investigated extensively, the mechanism of chemolithotrophic As(V) reduction is less studied. Since As is frequently found as a sulfidic mineral in the environment, microbial mediated sulfur oxidation coupled to As(V) reduction (SOAsR), a chemolithotrophic process, may be more favorable in sites impacted by oligotrophic mining (e.g. As-contaminated mine tailings). While SOAsR is thermodynamically favorable, knowledge regarding this biogeochemical process is still limited. The current study suggested that SOAsR was a more prevalent process than heterotrophic As(V) reduction in oligotrophic sites, such as mine tailings. The water-soluble reduced sulfur concentration was predicted to be one of the major geochemical parameters that had a substantial impact on SOAsR potentials. A combination of DNA stable isotope probing and metagenome binning revealed members of the genera Sulfuricella, Ramlibacter, and Sulfuritalea as sulfur oxidizing As(V)-reducing bacteria (SOAsRB) in mine tailings. Genome mining further expanded the list of potential SOAsRB to diverse phylogenetic lineages such as members associated with Burkholderiaceae and Rhodocyclaceae. Metagenome analysis using multiple tailing samples across southern China confirmed that the putative SOAsRB were the dominant As(V) reducers in these sites. Together, the current findings expand our knowledge regarding the chemolithotrophic As(V) reduction process, which may be harnessed to facilitate future remediation practices in mine tailings