Pub Date : 2025-11-07DOI: 10.1016/j.orggeochem.2025.105085
Tharika Liyanage , Lennart M. van Maldegem , Janet Hope , Philippe Schaeffer , Pierre Adam , Hans Peter Nytoft , Dianne Edwards , Jochen Brocks
Tricyclic and tetracyclic terpane ratios are used as biomarker proxies for depositional environments and to classify crude oils and source rocks. However, the biological precursors and diagenetic drivers of these proxies are not well understood. This study reports on the discovery of tricyclic and tetracyclic breakdown products of pentacyclic hopanoids that co-elute with 14α-C19 and βα-C20–C22 cheilanthanes and C24 tetracyclic terpane in gas chromatograms. This includes hopanoid breakdown products (HBPs) that are identical to 14α-C19 cheilanthane and C24 tetracyclic terpane, biomarkers that are commonly used as indicators of terrigenous organic matter in source rocks and their derived hydrocarbon products. These HBPs were generated by pyrolysis of C30 diplopterol, a common compound found in the cell membranes of almost all hopanoid-producing bacteria, some lichens and higher plants. HBPs are common in rock-extracted bitumens and crude oils from the Neoproterozoic to the Cenozoic and across a wide range of thermal maturities. Comparison of the relative abundance of HBPs and cheilanthanes against commonly used tricyclic and tetracyclic terpane parameters shows that HBPs can drive or strongly influence terpane-based paleoenvironmental proxies. This study begins to rationalise why some of these parameters indicate specific depositional environments and proposes new ratios that quantify the impact of HBPs on biomarker proxies.
{"title":"Hopanoid breakdown products (HBPs) and their influence on tetracyclic and tricyclic terpane (cheilanthane) oil-source rock parameters","authors":"Tharika Liyanage , Lennart M. van Maldegem , Janet Hope , Philippe Schaeffer , Pierre Adam , Hans Peter Nytoft , Dianne Edwards , Jochen Brocks","doi":"10.1016/j.orggeochem.2025.105085","DOIUrl":"10.1016/j.orggeochem.2025.105085","url":null,"abstract":"<div><div>Tricyclic and tetracyclic terpane ratios are used as biomarker proxies for depositional environments and to classify crude oils and source rocks. However, the biological precursors and diagenetic drivers of these proxies are not well understood. This study reports on the discovery of tricyclic and tetracyclic breakdown products of pentacyclic hopanoids that co-elute with 14α-C<sub>19</sub> and βα-C<sub>20</sub>–C<sub>22</sub> cheilanthanes and C<sub>24</sub> tetracyclic terpane in gas chromatograms. This includes hopanoid breakdown products (HBPs) that are identical to 14α-C<sub>19</sub> cheilanthane and C<sub>24</sub> tetracyclic terpane, biomarkers that are commonly used as indicators of terrigenous organic matter in source rocks and their derived hydrocarbon products. These HBPs were generated by pyrolysis of C<sub>30</sub> diplopterol, a common compound found in the cell membranes of almost all hopanoid-producing bacteria, some lichens and higher plants. HBPs are common in rock-extracted bitumens and crude oils from the Neoproterozoic to the Cenozoic and across a wide range of thermal maturities. Comparison of the relative abundance of HBPs and cheilanthanes against commonly used tricyclic and tetracyclic terpane parameters shows that HBPs can drive or strongly influence terpane-based paleoenvironmental proxies. This study begins to rationalise why some of these parameters indicate specific depositional environments and proposes new ratios that quantify the impact of HBPs on biomarker proxies.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"210 ","pages":"Article 105085"},"PeriodicalIF":2.5,"publicationDate":"2025-11-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145474236","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-11-06DOI: 10.1016/j.orggeochem.2025.105088
Aljasil Chirakkal , Ekaterina Kulakova , Calin C. Steindal , Jago J. Birk , Anton Anoikin , Redzhep Kurbanov , Petr Sosin , Peixian Shu , David K. Wright
This study presents a multi‑proxy investigation of loess–paleosol sequences from the Khovaling Loess Plateau (KLP) in Southern Tajikistan to explore long‑term human-environment interactions during the Middle Pleistocene. This region has been the focus of numerous archaeological projects since Soviet times, as it contains evidence of some of the oldest (∼0.9 Ma) hominin occupations in Central Asia and is one of the key regions connecting eastern Asia to centers of hominin evolution that lie to the west. Previous archaeological expeditions have discovered rich collections of stone tools from alternating non-pedogenically modified loess and paleosol sequences at the archaeological sites of Karatau, Lakhuti, Obi-Mazar, and Kuldura. We analyzed lipid biomarkers such as n‑alkanes, polycyclic aromatic hydrocarbons (PAHs), together with newly generated stable isotopes (δ13C, δ15N), magnetic susceptibility, and archaeological assemblages from three sites: Obi-Mazar/Lakhuti (on‑site), and non-artifact-bearing strata from Kuldara (near‑site) and Khonako‑II (off‑site), focusing on pedocomplexes (PCs) 4, 5, and 6 and their intercalated loess layers. High odd-over-even predominance (OEP) and average chain lengths (ACL) suggest that soil organic matter is predominantly derived from higher-order terrestrial plants, with better preservation in paleosols compared to loess layers. The inverse correlations of δ13C with TOC and C/N reflect organic matter degradation, while elevated δ15N in paleosols suggests enhanced nitrogen cycling under warmer and drier conditions. Magnetic susceptibility and δ13C trends reveal a progressive shift toward open grassland ecosystems since ∼0.8 Ma, with intensified pedogenesis during interglacial stages MIS 11, 13, and 15. The spatial distribution of PAH concentrations and lithic artifact densities further highlights human-environment interactions in the KLP locality. Obi-Mazar exhibits abundant lithic materials and high PAH levels, indicating repeated occupation and frequent burning in a resource‑rich riverine setting. Kuldara exhibits moderate PAHs but minimal artifacts, whereas Khonako-II records only a minimal level of signals, reflecting a decrease in human impact with distance from the water source. Together, these findings demonstrate that Early Paleolithic hominins preferentially occupied ecologically stable, pedogenically developed zones during interglacial phases, contributing to localized fire regimes and shaping their landscapes. This study provides a refined paleoenvironmental framework for understanding hominin adaptations in Central Asia during the Middle Pleistocene.
{"title":"Geochemical records from loess sediments provide insights into early hominin influence on the landscape in the Khovaling region of Southern Tajikistan, Central Asia","authors":"Aljasil Chirakkal , Ekaterina Kulakova , Calin C. Steindal , Jago J. Birk , Anton Anoikin , Redzhep Kurbanov , Petr Sosin , Peixian Shu , David K. Wright","doi":"10.1016/j.orggeochem.2025.105088","DOIUrl":"10.1016/j.orggeochem.2025.105088","url":null,"abstract":"<div><div>This study presents a multi‑proxy investigation of loess–paleosol sequences from the Khovaling Loess Plateau (KLP) in Southern Tajikistan to explore long‑term human-environment interactions during the Middle Pleistocene. This region has been the focus of numerous archaeological projects since Soviet times, as it contains evidence of some of the oldest (∼0.9 Ma) hominin occupations in Central Asia and is one of the key regions connecting eastern Asia to centers of hominin evolution that lie to the west. Previous archaeological expeditions have discovered rich collections of stone tools from alternating non-pedogenically modified loess and paleosol sequences at the archaeological sites of Karatau, Lakhuti, Obi-Mazar, and Kuldura. We analyzed lipid biomarkers such as <em>n</em>‑alkanes, polycyclic aromatic hydrocarbons (PAHs), together with newly generated stable isotopes (δ<sup>13</sup>C, δ<sup>15</sup>N), magnetic susceptibility, and archaeological assemblages from three sites: Obi-Mazar/Lakhuti (on‑site), and non-artifact-bearing strata from Kuldara (near‑site) and Khonako‑II (off‑site), focusing on pedocomplexes (PCs) 4, 5, and 6 and their intercalated loess layers. High odd-over-even predominance (OEP) and average chain lengths (ACL) suggest that soil organic matter is predominantly derived from higher-order terrestrial plants, with better preservation in paleosols compared to loess layers. The inverse correlations of δ<sup>13</sup>C with TOC and C/N reflect organic matter degradation, while elevated δ<sup>15</sup>N in paleosols suggests enhanced nitrogen cycling under warmer and drier conditions. Magnetic susceptibility and δ<sup>13</sup>C trends reveal a progressive shift toward open grassland ecosystems since ∼0.8 Ma, with intensified pedogenesis during interglacial stages MIS 11, 13, and 15. The spatial distribution of PAH concentrations and lithic artifact densities further highlights human-environment interactions in the KLP locality. Obi-Mazar exhibits abundant lithic materials and high PAH levels, indicating repeated occupation and frequent burning in a resource‑rich riverine setting. Kuldara exhibits moderate PAHs but minimal artifacts, whereas Khonako-II records only a minimal level of signals, reflecting a decrease in human impact with distance from the water source. Together, these findings demonstrate that Early Paleolithic hominins preferentially occupied ecologically stable, pedogenically developed zones during interglacial phases, contributing to localized fire regimes and shaping their landscapes. This study provides a refined paleoenvironmental framework for understanding hominin adaptations in Central Asia during the Middle Pleistocene.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"212 ","pages":"Article 105088"},"PeriodicalIF":2.5,"publicationDate":"2025-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145787553","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-27DOI: 10.1016/j.orggeochem.2025.105086
Hatice Kara , Mehmet Ali Ertürk , Leyla Kalender , Cihan Yalçın , Mehmet Deniz Turan , Emine Cicioğlu Sütçü
This study examines the stable carbon (δ13C) and nitrogen (δ15N) isotope compositions of coal samples from the Upper Miocene–Pliocene (Neogene) Elbistan Basin, Turkey, to constrain the sources of organic matter and the depositional conditions of peat formation. A vertical sequence of samples collected from the Kışlaköy open-pit mine was analysed for total carbon and nitrogen contents, C/N ratios, and δ13C–δ15N values. The δ13C values range from −28.09 ‰ to −16.99 ‰, while δ15N values vary between 1.97 ‰ and 4.99 ‰. The carbon isotope data indicate a predominantly C3-type plant origin for the organic matter, with minor C4 contributions inferred from samples exhibiting enriched δ13C signatures. Although some δ13C enrichment may partly reflect microbial alteration of organic matter, the overall values remain consistent with a dominant C3 vegetation signal. Elevated δ15N values suggest fluctuating redox conditions and microbial nitrogen cycling during peat accumulation. In comparison with other Turkish lignite basins and global coal deposits, the Elbistan samples display a broader isotopic range, reflecting a more complex paleoenvironmental evolution. The data suggest that the coal beds formed mainly in freshwater mire settings, where fluctuating water tables, variable redox conditions, and occasional inputs from C4 plants influenced peat accumulation. These results provide new insights into the paleobotanical and geochemical history of the basin and demonstrate the value of stable isotope analysis for reconstructing depositional environments in coal-bearing successions.
{"title":"Isotopic signatures of organic matter in the Upper Miocene–Pliocene Elbistan coal seam (Turkey): Insights from δ13C and δ15N variability","authors":"Hatice Kara , Mehmet Ali Ertürk , Leyla Kalender , Cihan Yalçın , Mehmet Deniz Turan , Emine Cicioğlu Sütçü","doi":"10.1016/j.orggeochem.2025.105086","DOIUrl":"10.1016/j.orggeochem.2025.105086","url":null,"abstract":"<div><div>This study examines the stable carbon (δ<sup>13</sup>C) and nitrogen (δ<sup>15</sup>N) isotope compositions of coal samples from the Upper Miocene–Pliocene (Neogene) Elbistan Basin, Turkey, to constrain the sources of organic matter and the depositional conditions of peat formation. A vertical sequence of samples collected from the Kışlaköy open-pit mine was analysed for total carbon and nitrogen contents, C/N ratios, and δ<sup>13</sup>C–δ<sup>15</sup>N values. The δ<sup>13</sup>C values range from −28.09 ‰ to −16.99 ‰, while δ<sup>15</sup>N values vary between 1.97 ‰ and 4.99 ‰. The carbon isotope data indicate a predominantly C<sub>3</sub>-type plant origin for the organic matter, with minor C<sub>4</sub> contributions inferred from samples exhibiting enriched δ<sup>13</sup>C signatures. Although some δ<sup>13</sup>C enrichment may partly reflect microbial alteration of organic matter, the overall values remain consistent with a dominant C<sub>3</sub> vegetation signal. Elevated δ<sup>15</sup>N values suggest fluctuating redox conditions and microbial nitrogen cycling during peat accumulation. In comparison with other Turkish lignite basins and global coal deposits, the Elbistan samples display a broader isotopic range, reflecting a more complex paleoenvironmental evolution. The data suggest that the coal beds formed mainly in freshwater mire settings, where fluctuating water tables, variable redox conditions, and occasional inputs from C<sub>4</sub> plants influenced peat accumulation. These results provide new insights into the paleobotanical and geochemical history of the basin and demonstrate the value of stable isotope analysis for reconstructing depositional environments in coal-bearing successions.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"210 ","pages":"Article 105086"},"PeriodicalIF":2.5,"publicationDate":"2025-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145413424","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-14DOI: 10.1016/j.orggeochem.2025.105084
Prakhin Assavapanuvat , Joshua L. Breithaupt , Ding He , Ralph N. Mead , Thomas S. Bianchi
The poleward expansion of mangroves into native salt marshes, because of global warming, has the potential to alter coastal soil organic carbon (SOC) stocks and composition. However, its effects on organic carbon decomposition remain largely unknown. This study compared concentrations and degradation indices of lignin, sterol, triterpenoid, and n-alkane biomarkers between live-and-dead leaf-and-root tissues and SOC down to 45-cm depth across habitats dominated by two mangrove and two salt marsh species in Apalachicola coastal wetlands, Florida, USA. A significant increase in n-alkanes and lignin oxidation products was observed following the senescence of Avicennia germinans mangrove tissues. Dead leaves of all four plant species, especially Spartina alterniflora and Juncus roemerianus salt marshes, became hotspots of fungal colonization as indicated by elevated ergosterol content. Interpretation of biomarker degradation ratios was complicated by their large difference between living root and leaf endmembers. In soil cores from all plant habitats, the predominance of tetracyclic triterpenoid degradation products (tetracylic:pentacylic ratio = 0.6–518) indicated prevalent anaerobic decomposition, agreeing with higher cholestanol/cholesterol and stigmastanol/stigmasterol ratios in SOC (0.53 ± 0.29 and 1.14 ± 0.83), relative to plant tissues (0.16 ± 0.17 and 0.57 ± 0.44, respectively). In contrast, the presence of pentacyclic degradation products in deep mangrove and salt marsh soils suggested localized aerobic degradation, likely due to subsurface oxygen release from plant roots. In conclusion, salt marsh-to-A. germinans mangrove replacement modified degradability of surface litter, yet degradation of deep SOC was more dependent on biogeochemical characteristics of the soils.
{"title":"Transformation of lignin and lipid biomarkers through degradation of plant tissues and soil organic carbon in a salt marsh-mangrove ecotone","authors":"Prakhin Assavapanuvat , Joshua L. Breithaupt , Ding He , Ralph N. Mead , Thomas S. Bianchi","doi":"10.1016/j.orggeochem.2025.105084","DOIUrl":"10.1016/j.orggeochem.2025.105084","url":null,"abstract":"<div><div>The poleward expansion of mangroves into native salt marshes, because of global warming, has the potential to alter coastal soil organic carbon (SOC) stocks and composition. However, its effects on organic carbon decomposition remain largely unknown. This study compared concentrations and degradation indices of lignin, sterol, triterpenoid, and <em>n</em>-alkane biomarkers between live-and-dead leaf-and-root tissues and SOC down to 45-cm depth across habitats dominated by two mangrove and two salt marsh species in Apalachicola coastal wetlands, Florida, USA. A significant increase in <em>n</em>-alkanes and lignin oxidation products was observed following the senescence of <em>Avicennia germinans</em> mangrove tissues. Dead leaves of all four plant species, especially <em>Spartina alterniflora</em> and <em>Juncus roemerianus</em> salt marshes, became hotspots of fungal colonization as indicated by elevated ergosterol content. Interpretation of biomarker degradation ratios was complicated by their large difference between living root and leaf endmembers. In soil cores from all plant habitats, the predominance of tetracyclic triterpenoid degradation products (tetracylic:pentacylic ratio = 0.6–518) indicated prevalent anaerobic decomposition, agreeing with higher cholestanol/cholesterol and stigmastanol/stigmasterol ratios in SOC (0.53 ± 0.29 and 1.14 ± 0.83), relative to plant tissues (0.16 ± 0.17 and 0.57 ± 0.44, respectively). In contrast, the presence of pentacyclic degradation products in deep mangrove and salt marsh soils suggested localized aerobic degradation, likely due to subsurface oxygen release from plant roots. In conclusion, salt marsh-to-<em>A. germinans</em> mangrove replacement modified degradability of surface litter, yet degradation of deep SOC was more dependent on biogeochemical characteristics of the soils.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"210 ","pages":"Article 105084"},"PeriodicalIF":2.5,"publicationDate":"2025-10-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145358841","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-07DOI: 10.1016/j.orggeochem.2025.105062
Alexis Gilbert , Jakub Surma , Naizhong Zhang , Sebastian Danielache , Qi Liu
The measurement of doubly-substituted (‘clumped’) isotopologues provides important information regarding the source, sink, and potentially the temperature of formation of a given molecule. While currently limited to small molecules such as methane or carbonate, recent technological developments are expected to expand the range of molecules studied, paving the way for clumped isotope measurements in various organic compounds. Theoretical calculations, including those based on density functional theory (DFT), can serve as guidelines for future methodological developments. However, these calculations have so far been reported for a small number of organic molecules and clumping types.
Here, we report DFT calculations for DD, 13CD, 13C13C, 13C15N, and 13C18O clumping in organic molecules. Using 32 model molecules, we calculate a total of 143 Δ values (Δ representing the deviation from the stochastic distribution due to equilibrium isotope effects) at temperatures ranging from 300 K to 1000 K. The overall trend follows: ΔDD > Δ13CD > Δ13C18O > Δ13C13C ≈ Δ13C15N, with values decreasing as temperature increases. Through multiple correlation analysis, we demonstrate that the reduced mass of the atoms, bond multiplicity, and hybridization collectively explain 80 % of the differences observed between bond types. Given current analytical uncertainties, the potential for a geothermometer is primarily limited to DD and 13CD clumping, while 13C13C, 13C15N, and 13C18O clumping could be applicable at low temperatures (typically 100 K) or with instrumental precision one order of magnitude higher.
The calculations presented here provide a framework to assess the instrumental precision required for utilizing clumped isotopes in organic molecules as geothermometers. Future improvements in analytical techniques and computational methodologies could further refine these predictions and broaden the applicability of clumped isotope thermometry in organic geochemistry.
{"title":"Exploring clumped isotope fractionation of organic molecules using density functional theory","authors":"Alexis Gilbert , Jakub Surma , Naizhong Zhang , Sebastian Danielache , Qi Liu","doi":"10.1016/j.orggeochem.2025.105062","DOIUrl":"10.1016/j.orggeochem.2025.105062","url":null,"abstract":"<div><div>The measurement of doubly-substituted (‘clumped’) isotopologues provides important information regarding the source, sink, and potentially the temperature of formation of a given molecule. While currently limited to small molecules such as methane or carbonate, recent technological developments are expected to expand the range of molecules studied, paving the way for clumped isotope measurements in various organic compounds. Theoretical calculations, including those based on density functional theory (DFT), can serve as guidelines for future methodological developments. However, these calculations have so far been reported for a small number of organic molecules and clumping types.</div><div>Here, we report DFT calculations for DD, <sup>13</sup>CD, <sup>13</sup>C<sup>13</sup>C, <sup>13</sup>C<sup>15</sup>N, and <sup>13</sup>C<sup>18</sup>O clumping in organic molecules. Using 32 model molecules, we calculate a total of 143 Δ values (Δ representing the deviation from the stochastic distribution due to equilibrium isotope effects) at temperatures ranging from 300 K to 1000 K. The overall trend follows: ΔDD > Δ<sup>13</sup>CD > Δ<sup>13</sup>C<sup>18</sup>O > Δ<sup>13</sup>C<sup>13</sup>C ≈ Δ<sup>13</sup>C<sup>15</sup>N, with values decreasing as temperature increases. Through multiple correlation analysis, we demonstrate that the reduced mass of the atoms, bond multiplicity, and hybridization collectively explain 80 % of the differences observed between bond types. Given current analytical uncertainties, the potential for a geothermometer is primarily limited to DD and <sup>13</sup>CD clumping, while <sup>13</sup>C<sup>13</sup>C, <sup>13</sup>C<sup>15</sup>N, and <sup>13</sup>C<sup>18</sup>O clumping could be applicable at low temperatures (typically 100 K) or with instrumental precision one order of magnitude higher.</div><div>The calculations presented here provide a framework to assess the instrumental precision required for utilizing clumped isotopes in organic molecules as geothermometers. Future improvements in analytical techniques and computational methodologies could further refine these predictions and broaden the applicability of clumped isotope thermometry in organic geochemistry.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"210 ","pages":"Article 105062"},"PeriodicalIF":2.5,"publicationDate":"2025-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145264125","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-25DOI: 10.1016/j.orggeochem.2025.105083
Lucas Pinto Heckert Bastos , René Rodrigues , Carmen Lucia Ferreira Alferes , Debora Bonfim Neves da Silva , Danielle da Costa Cavalcante , Guilherme Brugger Lemos , Sergio Bergamaschi , Egberto Pereira
Several studies have examined how high heating rates influence the molecular composition of organic matter, particularly in geological settings where magma interacts with organic-rich rocks. In Brazil, such interactions have been extensively documented in the Paraná Basin. However, the behavior of geochemical proxies under intense thermal stress, as well as the natural chemical products generated through the thermal cracking of organic matter, are not yet fully understood. In this study, a combination of bulk and molecular geochemical data from 233 m of Devonian strata in the Paraná Basin—intersected by two dolerite sills measuring 3.0 m and 0.86 m in thickness—was used to assess the extent and effects of magmatic heating on originally immature organic matter. The work includes a large dataset of total organic carbon and Rock-Eval pyrolysis along with thermal-maturity-related molecular geochemical proxies. The thermal evolution of organic matter was better assessed using methylated aromatic hydrocarbons, as saturate hydrocarbons are less resistant to high temperatures and may not respond well to rapid heating. The borehole profile was subdivided into four stages (I–IV) based on the response of geochemical proxies to thermal stress. The intermediate zone (Stages II and III) showed elevated MPI-1 (methylphenanthrene index) values and distorted saturate hydrocarbon ratios. Additionally, samples from this high thermal stress zone of the sills showed a significantly higher abundance of dimethyl alkanes—compared to less altered samples dominated by monomethyl alkanes. Multiple alkyl substitutions have been naturally favored in high-temperature zones by acidic clay mineral catalysis and sustained thermal stress, potentially from percolating hydrothermal fluids. The thermal alteration zone exceeds the expected halo from the two sills intersecting the borehole, which is interpreted as resulting from the percolation of hydrothermal fluids and/or a complex igneous body geometry.
{"title":"Magma-induced thermal alteration in Devonian organic-rich rocks (Paraná Basin, Brazil): molecular transformations and clay-catalyzed maturation","authors":"Lucas Pinto Heckert Bastos , René Rodrigues , Carmen Lucia Ferreira Alferes , Debora Bonfim Neves da Silva , Danielle da Costa Cavalcante , Guilherme Brugger Lemos , Sergio Bergamaschi , Egberto Pereira","doi":"10.1016/j.orggeochem.2025.105083","DOIUrl":"10.1016/j.orggeochem.2025.105083","url":null,"abstract":"<div><div>Several studies have examined how high heating rates influence the molecular composition of organic matter, particularly in geological settings where magma interacts with organic-rich rocks. In Brazil, such interactions have been extensively documented in the Paraná Basin. However, the behavior of geochemical proxies under intense thermal stress, as well as the natural chemical products generated through the thermal cracking of organic matter, are not yet fully understood. In this study, a combination of bulk and molecular geochemical data from 233 m of Devonian strata in the Paraná Basin—intersected by two dolerite sills measuring 3.0 m and 0.86 m in thickness—was used to assess the extent and effects of magmatic heating on originally immature organic matter. The work includes a large dataset of total organic carbon and Rock-Eval pyrolysis along with thermal-maturity-related molecular geochemical proxies. The thermal evolution of organic matter was better assessed using methylated aromatic hydrocarbons, as saturate hydrocarbons are less resistant to high temperatures and may not respond well to rapid heating. The borehole profile was subdivided into four stages (I–IV) based on the response of geochemical proxies to thermal stress. The intermediate zone (Stages II and III) showed elevated MPI-1 (methylphenanthrene index) values and distorted saturate hydrocarbon ratios. Additionally, samples from this high thermal stress zone of the sills showed a significantly higher abundance of dimethyl alkanes—compared to less altered samples dominated by monomethyl alkanes. Multiple alkyl substitutions have been naturally favored in high-temperature zones by acidic clay mineral catalysis and sustained thermal stress, potentially from percolating hydrothermal fluids. The thermal alteration zone exceeds the expected halo from the two sills intersecting the borehole, which is interpreted as resulting from the percolation of hydrothermal fluids and/or a complex igneous body geometry.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"210 ","pages":"Article 105083"},"PeriodicalIF":2.5,"publicationDate":"2025-09-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145128188","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Diamondoid generation and maturation behaviors in natural system remain unresolved. Oils/condensates from the Tarim Basin have elevated diamondoid concentrations. Diamondoid concentrations and maturity ratios for 167 oils/condensates from the Tarim Basin were used to document diamondoid generation and maturation in combination with the maturity frameworks of the deep source rocks. These samples include 29 Kuqa condensates from the Kuqa Depression, and 45 Tabei oils/condensates from the Tabei (Northern Tarim) Uplift and neighboring slope area and 93 Tazhong oils/condensates from the Tazhong (Central Tarim) Uplift and neighboring slope area of the cratonic region. The main observations are: (1) Concentration distributions of 4- + 3-methyldiamantane (4 + 3MD) for the Kuqa condensates and the Tabei and Tazhong oils/condensates are consistent with the maturity frameworks of the Triassic source rocks in the Kuqa Depression and the Cambrian source rocks in the cratonic region of the Tarim Basin from basin modeling, respectively. Concentrations of total adamantanes (As), total diamantanes (Ds) and 4 + 3MD have a close linear positive correlation with each other for these studied oils/condensates. These results suggest that diamondoids were generated at increasing rate with increasing maturity of these deep source rocks and did not decompose in these source rocks at maturity up to %Ro 4.0–4.5 in the Tarim Basin. (2) The effectiveness of the nine diamondoid maturity ratios (MAI, MDI, DMAI-1, DMAI-2, TMAI-1, TMAI-2, DMDI-1, DMDI-2 and EAI) increases, while the influence of source facies on these maturity ratios, in particular on the last three ratios (DMDI-1, DMDI-2 and EAI) decreases with increasing maturity of the source rocks based on Pearson correlation coefficients (r) between each other among the nine maturity ratios and concentrations of As, Ds and 4 + 3MD and As/Ds ratio. (3) The maturities of the deep source rocks for the studied samples decrease in the sequence of the Kuqa condensates > the Tazhong oils/condensates > the Tabei oils/condensates based on the maximum values and ranges of the nine diamondoid maturity ratios and the r values between each other among the nine maturity ratios, concentrations of As, Ds and 4 + 3MD. and the As/Ds ratio. This sequence provides a constraint in basin modeling on the relative maturities of the Triassic source rocks in the Kuqa Depression and Cambrian source rocks in the cratonic region.
{"title":"Generation and maturation of diamondoids for petroleum reservoirs in the Tarim Basin, NW China","authors":"Shuang Yu , Yuanyuan Bian , Chenxi Zhou , Wenyu Huang , Haizu Zhang , Zhongyiao Xiao , Changchun Pan","doi":"10.1016/j.orggeochem.2025.105080","DOIUrl":"10.1016/j.orggeochem.2025.105080","url":null,"abstract":"<div><div>Diamondoid generation and maturation behaviors in natural system remain unresolved. Oils/condensates from the Tarim Basin have elevated diamondoid concentrations. Diamondoid concentrations and maturity ratios for 167 oils/condensates from the Tarim Basin were used to document diamondoid generation and maturation in combination with the maturity frameworks of the deep source rocks. These samples include 29 Kuqa condensates from the Kuqa Depression, and 45 Tabei oils/condensates from the Tabei (Northern Tarim) Uplift and neighboring slope area and 93 Tazhong oils/condensates from the Tazhong (Central Tarim) Uplift and neighboring slope area of the cratonic region. The main observations are: (1) Concentration distributions of 4- + 3-methyldiamantane (4 + 3MD) for the Kuqa condensates and the Tabei and Tazhong oils/condensates are consistent with the maturity frameworks of the Triassic source rocks in the Kuqa Depression and the Cambrian source rocks in the cratonic region of the Tarim Basin from basin modeling, respectively. Concentrations of total adamantanes (As), total diamantanes (Ds) and 4 + 3MD have a close linear positive correlation with each other for these studied oils/condensates. These results suggest that diamondoids were generated at increasing rate with increasing maturity of these deep source rocks and did not decompose in these source rocks at maturity up to %Ro 4.0–4.5 in the Tarim Basin. (2) The effectiveness of the nine diamondoid maturity ratios (MAI, MDI, DMAI-1, DMAI-2, TMAI-1, TMAI-2, DMDI-1, DMDI-2 and EAI) increases, while the influence of source facies on these maturity ratios, in particular on the last three ratios (DMDI-1, DMDI-2 and EAI) decreases with increasing maturity of the source rocks based on Pearson correlation coefficients (<em>r</em>) between each other among the nine maturity ratios and concentrations of As, Ds and 4 + 3MD and As/Ds ratio. (3) The maturities of the deep source rocks for the studied samples decrease in the sequence of the Kuqa condensates > the Tazhong oils/condensates > the Tabei oils/condensates based on the maximum values and ranges of the nine diamondoid maturity ratios and the <em>r</em> values between each other among the nine maturity ratios, concentrations of As, Ds and 4 + 3MD. and the As/Ds ratio. This sequence provides a constraint in basin modeling on the relative maturities of the Triassic source rocks in the Kuqa Depression and Cambrian source rocks in the cratonic region.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"211 ","pages":"Article 105080"},"PeriodicalIF":2.5,"publicationDate":"2025-09-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145098726","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-01DOI: 10.1016/j.orggeochem.2025.105059
Weiqing Liu , Haiyang Li , Yu Qiao , Wei Wu , Jienan Pan , Chuanlong Mou , Jianxin Yao , Yao Chen , Changsong Lin
Shale gas geochemical characteristics are critical for genetic identification and enrichment mechanism analysis. Current research focuses predominantly on marine shale gas, whereas studies on transitional shale gas (particularly the Permian Longtan Formation in South China) remain relatively limited, which constrains our understanding of the geochemical characteristics and genesis of transitional shale gases. The geochemical characteristics of transitional shale gas from the upper Permian Longtan Formation in western Hubei Province indicate that the organic matter is predominantly sapropelic, with some humic organic matter. The gas composition of the Longtan Formation consists primarily of CH4, with a low content of C2H6. The δ13CCH4 values range from −25.40 ‰ to −21.70 ‰, the δ13CC2H6 values range from –32.00 ‰ to −27.02 ‰, and the δ2HCH4 values range from −124.01 ‰ to −119.46 ‰. These findings imply that shale gas is composed mainly of oil-type gas of thermal origin, with a potentially minor presence of mixed gas. Isotope analysis of the shale gas reveals that it has undergone reversal (δ13CCH4 > δ13CC2H6), which is attributed to the cracking of liquid hydrocarbons under overmature conditions. The CO2 content ranges from 0.21 % to 2.33 % and the δ13CCO2 values range from −21.80 ‰ to −19.00 ‰, suggesting that the CO2 in the study area is of organic thermal origin. Additionally, a geochemical evolution pattern suggests that the gas composition of different sedimentary phases is controlled by the type of kerogen and the degree of thermal evolution. The multistage cracking of organic matter thermal evolution products caused the dynamic changes in carbon isotopes.
{"title":"Geochemical characteristics of Longtan transitional shale gas in the western Hubei Basin, northwest Middle Yangtze Block: Implications for the origin and carbon isotopes","authors":"Weiqing Liu , Haiyang Li , Yu Qiao , Wei Wu , Jienan Pan , Chuanlong Mou , Jianxin Yao , Yao Chen , Changsong Lin","doi":"10.1016/j.orggeochem.2025.105059","DOIUrl":"10.1016/j.orggeochem.2025.105059","url":null,"abstract":"<div><div>Shale gas geochemical characteristics are critical for genetic identification and enrichment mechanism analysis. Current research focuses predominantly on marine shale gas, whereas studies on transitional shale gas (particularly the Permian Longtan Formation in South China) remain relatively limited, which constrains our understanding of the geochemical characteristics and genesis of transitional shale gases. The geochemical characteristics of transitional shale gas from the upper Permian Longtan Formation in western Hubei Province indicate that the organic matter is predominantly sapropelic, with some humic organic matter. The gas composition of the Longtan Formation consists primarily of CH<sub>4</sub>, with a low content of C<sub>2</sub>H<sub>6</sub>. The δ<sup>13</sup>C<sub>CH4</sub> values range from −25.40 ‰ to −21.70 ‰, the δ<sup>13</sup>C<sub>C2H6</sub> values range from –32.00 ‰ to −27.02 ‰, and the δ<sup>2</sup>H<sub>CH4</sub> values range from −124.01 ‰ to −119.46 ‰. These findings imply that shale gas is composed mainly of oil-type gas of thermal origin, with a potentially minor presence of mixed gas. Isotope analysis of the shale gas reveals that it has undergone reversal (δ<sup>13</sup>C<sub>CH4</sub> > δ<sup>13</sup>C<sub>C2H6</sub>), which is attributed to the cracking of liquid hydrocarbons under overmature conditions. The CO<sub>2</sub> content ranges from 0.21 % to 2.33 % and the δ<sup>13</sup>C<sub>CO2</sub> values range from −21.80 ‰ to −19.00 ‰, suggesting that the CO<sub>2</sub> in the study area is of organic thermal origin. Additionally, a geochemical evolution pattern suggests that the gas composition of different sedimentary phases is controlled by the type of kerogen and the degree of thermal evolution. The multistage cracking of organic matter thermal evolution products caused the dynamic changes in carbon isotopes.</div></div>","PeriodicalId":400,"journal":{"name":"Organic Geochemistry","volume":"208 ","pages":"Article 105059"},"PeriodicalIF":2.5,"publicationDate":"2025-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144923054","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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