Amorphous Organic Matter Research Articles

Current and paleo sources of organic material within fluvial features of the meandering Ruda River, Poland

This study aimed to identify differences in organic matter (OM) composition and origin in the main depositional subenvironments within the floodplain of a meandering river in a temperate climate zone. The study analysed organic-rich sediments found within different facies (side bars, natural levees, crevasse splays, palaeochannels, flood basins, and alluvial fans). Quantitative macrofossil analysis of the OM present in proximal floodplain sediments pointed to a dominance of amorphous OM (AOM), the average proportion of which comprised up to 60% of the total OM. During floods, the most significant phenomenon is the deposition of wood and leaf debris; however, these debris are subject to rapid decomposition in sandy layers and, as a result, do not contribute much to the total OM composition. Root and epidermis remain accumulated in situ and, with an average share of more than 10% of the total OM, are an important part of the more fine-grained alluvial layers. The other components were no more than bioindicators. The proximal floodplain is characterised by sponge spicules in the alluvia and mycelia in the epipedons. In addition, diatom remains are often recorded in the flood layers. The average AOM content in the OM found within the distal floodplain was approximately 40%, although a significantly higher proportion of this component was recorded in flood basin sediments. The accumulation of rhizodermis, epidermis, wood, and periderm (with shares ranging from 5% to 22% of the total OM) plays an important role. Less common components that are of indicative value within the distal zone include the remains of mosses, Cladocera, and sporomorphs. OM within the distal floodplain has high preservation potential, and the sediments in this zone are crucial for carbon sequestration processes in river valleys.

Quantitative Study of the Maceral Groups of Laminae Based on Support Vector Machine

Identifying organic matter in laminae is fundamental to petroleum geology; however, many factors restrict manual quantification. Therefore, computer recognition is an appropriate method for accurately identifying microscopic components. In this study, we used support vector machine (SVM) to classify the preprocessed photomicrographs into seven categories: pyrite, amorphous organic matter, mineral matter, alginite, sporinite, vitrinite, and inertinite. Then, we performed a statistical analysis of the classification results and highlighted spatial aggregation of some categories using the kernel density estimation method. The results showed that the SVM can satisfactorily identify the macerals and minerals of the laminae, and its overall accuracy, kappa, precision, recall, and F1 are 82.86%, 0.80, 85.15%, 82.86%, and 82.75%, respectively. Statistical analyses revealed that pyrite was abundantly distributed in bright laminae; vitrinite and sporinite were abundantly distributed in dark laminae; and alginite and inertinite were equally distributed. Finally, the kernel density maps showed that all classification results, except inertinite, were characterized by aggregated distributions: pyrite with the distribution of multi-core centers, alginite, and sporinite with dotted distribution, and vitrinite with stripe distribution, respectively. This study may provide a new method to quantify the organic matter in laminae.

Palaeoclimate, Palaeoecology and Palaeovegetation in and around Nandira Colliery, Talcher Basin, Odisha, India during Early Permian: Inferences from Typical Karharbari Palynofloral and Palynofacies Analysis

Abstract Palynofloral study along with palynofacies analysis has been undertaken in Nandira colliery, Talcher basin, Odisha, India. Recovered palynoassemblage is characterized by the dominance of monosaccate pollen grains viz., Parasaccites spp. (25%), Plicatipollenites spp. (5%), Potonieisporites sp. (1%) and Diavarisaccus lelei (1%) along with nonstriate bisaccate Scheuringipollenites maximus (35%). The recovered palynoflora shows its affinity towards the Upper Karharbari palynoflora of the early Artinskian age. The organic matter studied from the palynofacies investigation includes palynomorphs (SP), structured phytoclasts/ translucent phytoclasts (ST), opaque phytoclasts/ charcoal (CH), degraded organic matter (DOM), and amorphous organic matter (AOM). The carbonaceous shale (Sample Na) is dominated by SP (34.67%) and sub-dominated by ST (22%) along with CH (13.34%), DOM (19%), and AOM (11%). Coal samples (Nb to Ne) are completely devoid of palynomorphs. They are mainly dominated by CH (24.87-62.34%) and ST (20.20-73.38%) with presence of DOM (0.7-8.7%) and AOM (0.4-8.7%). Deposition of the sediments in a lower carbonaceous shale bed (Na) might have occurred in a dysoxic forest swamp in low-energy settings. The coal beds (Nb to Ne) have been deposited in the oxidized swamps in the proximal setting. The predominant occurrence of the opaque phytoclasts associated with the coal sediments may indicate a regular occurrence of palaeofires during the Artinskian.

Controls on organic matter variation during deposition of the Mahogany Oil Shale Zone of the Parachute Creek Member, Green River Formation, Utah

The Green River Formation of Utah, Colorado, and Wyoming represents a ~10 million-year early-middle Eocene record of an unusually large, productive lacustrine system composed of several interconnected basins. This system gave rise to one of the largest oil shale deposits in the world, which preserves a rich trove of information about the climate and ecosystems that prevailed during the Early Eocene Climatic Optimum. This study uses multiple analytical approaches, including both traditional methods and novel proxies, to determine what brought about the accumulation of the unusually-rich oil shales (>40% TOC) of the Mahogany Zone in the Uinta Basin portion of the Green River Formation. The organic-rich mudstones of the Mahogany Zone exhibit strong micro-scale heterogeneity, indicating complex controls on organic matter production and distribution. Petrographic observations indicate highly variable amounts of terrestrial organic matter (spores, wood and plant debris), with the most abundant being amorphous organic material in thin organic-rich laminae. Biomarker ratios indicate that the organic-rich laminae consist of different types of microbially derived organic matter, primarily bituminite and organo-minerallic aggregates of the fluorescent liptinite group. These largely lacustrine-derived laminated deposits are argued to have been produced by large intrato inter-annual algal blooms and other enhanced microbial productivity events paced by longer-term sub-orbital cycle fluctuations, such as El Niño–Southern Oscillation or sunspot variations. The Mahogany Zone deposits appear to have sequestered enough carbon (~64.25 Gt estimated) to suggest that, in aggregate, they were large enough to cause a draw-down of terrestrial CO2 and exert a significant negative feedback effect on climatic warming. During the climatic cooling that occurred on the declining limb of the Early Eocene Climatic Optimum, the sedimentary provenance in the Uinta Basin shifted from southerly sources dominated by large ephemeral fluvial systems, to northerly sources rich in feldspathic and carbonate detritus derived from northern sites in Wyoming. The latter were delivered to the Uinta Basin via a network of interconnected lacustrine basins. Increasing volcaniclastic material delivered from these northerly basins infilled the Uinta Basin from east to west, and their arrival heralded the end of organic-rich deposition, most notably the prolific oil shales for which the Green River Formation is renowned.

Research on the sedimentary characteristics of organic matter in lacustrine mudrocks and their hydrocarbon generation potential based on palynofacies analysis: Eocene shahejie formation, dongying sag

Lacustrine mudrocks are important hydrocarbon source rocks due to their high contents of organic matter (OM). An accurate determination of the origins of OM in the rocks can contribute greatly to the identification of the sedimentary environment and the evaluation of the hydrocarbon generation capacity of the mudrocks. In this study, the lacustrine mudrocks of the Eocene Shahejie Formation in the Dongying Sag of China were sampled for further discussion of the types and origins of OM and the hydrocarbon generation capacity of the mudrocks. On that basis, the classical palynofacies classification scheme was improved, and five palynofacies types (PF1 to PF5) were classified to accurately reflect different lacustrine environments. Among them, PF1 to PF3 are rich in various amorphous organic matter (AOM), including benthic microbial mats, organic aggregates and algal aggregates, which are part of the lake environment in which aquatic organisms bloom. The hydrocarbon generation abilities of the three palynofacies are different but generally strong. PF5 and PF4 have abundant terrestrial plant fragments and macrophyte fragments, respectively, indicating a shallow, near-provenance lake environment and a shallow, macrophyte-flourishing lake environment. These two types of palynofacies have weaker hydrocarbon generation capacities than the former three. Due to the enrichment of different types of palynofacies, each formation of the Dongying Sag shows distinct hydrocarbon generation potentials. Therefore, it is important to pay special attention to the origin of OM and its influences on the hydrocarbon generation capacity of lacustrine mudrocks for a better understanding of the deposition and hydrocarbon generation potential of source rocks.

Integrated palynology and sedimentology of the Mississippian of the Tisdafine Basin (Eastern Anti-Atlas, Morocco)

The Mississippian (Tournaisian–Visean) of the Jbel Asdaf area in the Tisdafine Basin (Eastern Anti-Atlas, Morocco) has been, for the first time, the subject of a palynological study. This work aims first to describe the lithological and the paleontological composition of the Aït Yalla and Tinerhir Formations, then to refine the age, reconstruct the paleoenvironment and assess the thermal maturity of the organic matter. The studied section is subdivided, from oldest to youngest, into three intervals, according to their lithological and paleontological compositions. Lithologically, the lower interval (lower part of the Aït Yalla Formation) and middle interval (upper part of the Aït Yalla Formation and basal part of the Tinerhir Formation) consist mostly of clay shales and green shales, respectively, both alternating with limestone or sandstone beds. The upper interval (upper part of the Tinerhir Formation) is dominated by sandstones, also alternating with pelitic-sandstone or limestone beds. The kerogen delivered by the analyzed samples is mostly of continental origin, composed mainly of opaque phytoclasts, while translucent phytoclasts and miospores are less represented. Marine fraction, consisting mainly of granular amorphous organic matter and some phytoplankton, are rare. Biostratigraphically, the late Visean Tripartites vetustus–Rotaspora fracta (VF) miospore Zone of the Western Europe biozonal scheme is recognized with some regards in the Tinerhir Formation, based on the first occurrence of the miospore Rotaspora cf. knoxi. Vallatisporites aff. ciliaris is another biostratigraphic marker taxon of the assemblage, whose the last occurrence marks the end of the VF Biozone at the top of the analyzed section. Qualitative and quantitative parameters of the different palynofacies constituents (e.g., shape, size, % of opaque and translucent phytoclasts) show, from the base to the top of the section, a gradual transition from a distal marine environment generally recognized in the Aït Yalla Formation towards an intermediate marine environment in the Tinerhir Formation, suggesting a sea-level fall. Optical analyses performed on the color of palynomorphs, mainly spores, suggest a thermal maturity, ranging from late mature to early post-mature stage for most of the analysed samples, with a burial depth and a temperature of at least 3300 m and 90 °C, respectively.

Optical and geochemical assessment of petroleum source rock potential and palaeodepositional environments of Cretaceous mudstones, Koum Basin, North Cameroon

Limited organic matter (OM) characterisation studies on Cretaceous mudstone successions in the Koum Basin have resulted in a significant knowledge gap in the source rock potential; a petroleum system prerequisite. Field mapping and lithofacies analysis of outcrop sections point to fluvial channel and marginal to profundal lake deposits. Representative mudstone samples were evaluated using multiproxy organo-petrographic and bulk geochemical analyses to assess the kerogen type, thermal maturity, hydrocarbon generation potential, and deduce the paleodepositional environment. Results reveal that ∼90% of the samples are organic-lean and ∼10% organic-rich with TOC content of 2.81–6.91 wt%. Rock-Eval pyrolysis S2 yields of 14.50–54.18 mg HC/g rock and hydrogen index (HI) 784–846 mgHC/gTOC, suggest excellent potential for oil generation. High S1 values of 1.18–1.45 mg HC/g rock and low production indices (PI) of 0.02–0.03 indicate in situ generated free hydrocarbons. PI and Tmax of 438°C–443 °C, consistent with measured vitrinite reflectance (Ro% = 0.28–0.53%), Spore Colour Index (SCI = yellow to golden brown) and Thermal Alteration Index (TAI = 1.5 to 2.6) on the Chevron scale, portraying an immature to marginally mature OM. Oil-prone kerogen Type I-II dominates and to a lesser extent gas-prone Type III, derived from algal-terrestrial organic facies, typified by the predominance of amorphous OM 51.2%, liptinites 24.6%, vitrinite 16.4% and inertinite 7.8%. Based on lithofacies, and palynological evidence, a fluvio-lacustrine depositional setting with limited marine influence, under fluctuating warm humid to hot semi-arid paleoclimates, is inferred. A comparison with surrounding petroleum-producing basins further enhances the understanding of the Koum Basin's hydrocarbon potential within the West Central African rift system context.

Palaeoclimatic and diagenetic controls based on clay mineralogy and organic matter distribution: The continental rift Cuyo Basin (Triassic), west‐central Argentina

AbstractThe Triassic Cuyo rift Basin is one of the renowned petroleum systems in South America that records distinctive alluvial, lacustrine and fluvial settings with volcanic input. The present study focuses on the clay mineral assemblages that represent the syn‐rift to post‐rift fills. Four clay mineral assemblages are defined based on X‐ray diffraction, optical and electron microscopy analyses. The illite/mica‐dominated assemblage is documented in the syn‐rift I (Cerro de las Cabras Formation). Such mineral phases are inherited and derived from the erosion of pre‐rift units, suggesting that physical weathering associated with a warm and semi‐arid climate played a leading role in their occurrence. Authigenic smectite clays are diagnostic in organic‐lean mudstones of the syn‐rift II (lower Potrerillos Formation) which point to alteration of pyroclastic material under a warm and semi‐arid to humid climate. A warm and humid climate prevailed during the deposition of lacustrine hydrocarbon source rocks (upper Potrerillos and Cacheuta formations). Organic‐rich mudstones show the illite/mica–kaolinite–smectite assemblage in conjunction with amorphous organic matter and restricted terrigenous elements. Diagenetic kaolinite booklets and bipyramidal quartz are attributed to the acidification of pore‐water related to kerogen transformation during burial diagenesis. Vitrinite reflectance values (ca 0.63%) confirm that organic‐rich sediments reached the earliest oil window; however, smectite is devoid of illite layers throughout the Triassic succession, probably related to the absence of potassium in the system and the high crystallinity of the smectite, which delayed the transformation of smectite into mixed layers illite–smectite. Red mudstones of the post‐rift II (the fluvial‐lacustrine Rio Blanco Formation) show the kaolinite‐dominated assemblage where the authigenic kaolinite is interpreted to be derived from the alteration of volcaniclastic detritus and indicative of warm climate with markedly wet periods.

Palinofácies de folhelhos do devoniano superior da borda oeste da bacia do parnaíba palynofacies of upper devonian shales from the west edge of the parnaíba basin

An outcrop section of rocks from the Upper Devonian, providing plant and animal microfossils that register a time of global post-anoxia and major marine flooding events, was investigated by applying palynofacies techniques and organic geochemistry. The primary purpose of this work is the palynofacies characterization of a Neo-Devonian section of the Parnaíba Basin, belonging to the Pimenteiras Formation. Thirteen outcrop samples were collected in the western edge of the Parnaíba Basin, Tocantins State - Brazil, where a possible contact between the Pimenteiras and Cabeças formations was identified. Through optical/visual analysis of organic content and organic groups (amorphous organic matter - AOM/Palynomorphs/Phytoclasts), as well as their relative abundance, associated with previous total organic carbon (TOC) data, it was possible to individualize four (4) distinct palynofacies throughout the section. The variation in the proportions of marine and continental palynomorphs suggests sea-level fluctuation during deposition, characterizing a transitional environment, varying from the most distal to the most proximal deposition platform. Based on the predominance of palynomorphs and phytoclasts, the kerogen types II and III were recognized, corroborating with the hydrogen and oxygen indexes obtained through pyrolysis data. The predominance of yellow to chestnut colors exhibited by the prasinophyte and acritarch components under ultraviolet light excitation indicates good organic matter preservation and thermal immaturity following the organic geochemistry data.

Terrestrial and marine organic matter evidence from a Cretaceous deep-sea chert of Japan: Implications for enhanced hydrological cycle during the Aptian OAE 1a

The early Aptian Oceanic Anoxic Event (OAE 1a; ca. 120 Ma) was accompanied by a massive eruption of the Ontong Java Plateau, doubling atmospheric pCO₂, and increasing sea surface temperature by 4–8 °C. In this study, we attempted a palynological analysis of the Hauterivian to Cenomanian deep–sea chert from the Goshikinohama site, and successfully obtained a variety of microremains, mainly represented by amorphous organic matter, aggregations of organic and inorganic particles, and cuticle fragments. One Hauterivian and many Aptian samples unexpectedly yielded terrestrial plant material, such as resins and wood fragments with tracheids, which represent the first record of terrestrial plant microremains from deep-sea pelagic chert. The occurrence of wood fragments within the negative and positive δ13C excursions during the OAE 1a can be correlated with an interval of increased atmospheric CO₂ level. We can also suggest that the occurrence of resins, mainly within the mudstone-rich chert intervals of the early Aptian negative δ13C peak and the middle-upper Aptian interval, are possibly related with periods with sea-level drops. Similar massive resin deposition events have been linked with intensified hydrological cycling during other abrupt climatic disruptions, such as the Carnian Pluvial Event, the Triassic−Jurassic boundary, and the Paleocene−Eocene Thermal Maximum. Therefore, an increase of terrestrial plant burial in deep–sea chert is likely linked with a massive plant discharge related with enhanced hydrological cycles, such as intensified storms and/or some sea–level change during the OAE 1a.

Miocene organic-rich layers of the Saouef Formation (Oriental Tunisia); insights into temporal and spatial variability of environmental conditions during deposition

Miocene organic-rich sediments (coal and carbonaceous clays) from the La Falaise section of the Saouef Formation have been investigated by organic petrographic, palynofacies and organic geochemical approaches. The results are compared with data obtained during a previous study of the nearby Saouef section. The sedimentary succession is built up by repetitive sedimentary sequences (i.e., parasequences) deposited during progradation and retrogradation, respectively. The coal, or carbonaceous layers, developed during the establishment of clay-dominated sedimentation.The organic-rich layers within the samples, containing high amounts of mineral matter, are dark brown, with a stratified texture and are classified as humic coal. Microscopically, the huminite group dominates the maceral composition of the carbonaceous clays and lignite (av. 75% mmf). The variable degrees of the tissue preservation index (TPI) and highly fluctuating vegetation index (VI) values provide evidence for different contributions of vascular plants vs. herbaceous vegetation and macrophytes, to peat formation. Higher contributions of macrophytes (e.g., reed grasses) to peat accumulation at La Falaise are concluded, based on n-alkane distributions. High mineral matter contents and changing gelification index (GI), and ground water index (GWI) values, found in both sections, provide evidence for rheotrophic hydrogeological conditions, and peat formation within wet forest swamp to inundated marsh environment. The low sulphur contents attest to peat formation within a freshwater environment and the relative proportions of ββ to (ββ + αβ)-C31 hopanes argue for slightly acidic conditions. In comparison to the results of the Saouef section (average pH of 5.6), a lower pH (4.2) is revealed from the average hopane isomerization ratio of the La Falaise samples.The palynofacies investigation of the La Falaise deposits indicates the dominance of phytoclasts over amorphous organic matter (AOM) in most of the samples and suggests good preservation of the higher plant remains. In contrast, increased degradation of plant tissues under oxygen-deficient conditions is concluded from the enhanced proportions of AOM found in several of the previously investigated Saouef samples. The molecular analysis of hydrocarbons from all the samples indicated the dominance of n-alkanes and hopanoids. The obtained pristane/phytane ratios argue for sub-oxic conditions during peatification at La Falaise, whereas data from our previous study of the Saouef section provided evidence for a reducing environment. Terpenoid hydrocarbon compositions reveal highly variable contributions of conifers and angiosperms to peat formation, consistent with previous findings at the Saouef sampling site. The presence of polycyclic aromatic hydrocarbons (PAHs), as well as fusinite, provides evidence for mire or forest fires.The comparison of the results from lignite of both sections of the Saouef Formation indicate comparable, highly variable conditions during peat accumulation, probably governed by eustatic sea level changes. The peatlands most likely developed due to the establishment of flood basins situated between alluvial ridges. The obtained regional differences are suggested to reflect slightly different hydrological conditions and associated vegetation cover in the peatlands.

Optimal acceleration voltage for near-atomic resolution imaging of layer-stacked 2D polymer thin films

Despite superb instrumental resolution in modern transmission electron microscopes (TEM), high-resolution imaging of organic two-dimensional (2D) materials is a formidable task. Here, we present that the appropriate selection of the incident electron energy plays a crucial role in reducing the gap between achievable resolution in the image and the instrumental limit. Among a broad range of electron acceleration voltages (300 kV, 200 kV, 120 kV, and 80 kV) tested, we found that the highest resolution in the HRTEM image is achieved at 120 kV, which is 1.9 Å. In two imine-based 2D polymer thin films, unexpected molecular interstitial defects were unraveled. Their structural nature is identified with the aid of quantum mechanical calculations. Furthermore, the increased image resolution and enhanced image contrast at 120 kV enabled the detection of functional groups at the pore interfaces. The experimental setup has also been employed for an amorphous organic 2D material.

Mechanism and Controlling Factors on Methane Yields Catalytically Generated From Low-Mature Source Rocks at Low Temperatures (60–140°C) in Laboratory and Sedimentary Basins

Various studies have shown that geo-catalytically mediated methanogenesis could happen in immature to early-mature source rocks at temperatures ranging from 60 to 140°C based on a series of long-term laboratory heating experimental evidences. The results of those studies show that methane yields at the given temperature are 5–11 orders of magnitude higher than the theoretically predicted yields from early thermogenic methane generation kinetic models. However, different types of source rocks in these laboratory simulation experiments generated varied CH4 and CO2 yields, which suggest that controls on CH4 generation during catalytic methanogenesis are complex. This study summarizes and compares gas yield results from laboratory low-temperature heating simulation experiments. Pre-existing trapped methane in rock chips could mimic newly generated gas during heating. The yields of catalytically generated CH4 from individual source rocks were re-quantified by subtracting the amounts of pre-existing CH4 in the closed pores of the original source rocks from the total methane amounts released from heating experiments and pre-existing CH4 in the closed pores in heated source rocks. The results show that heating temperature and time exert a positive influence on methane catalytic methanogenesis. Mowry and Second White Specks Formation Shale generated approximately ten times more CH4 than New Albany Shale and Mahogany Shale per gram of total organic carbon (TOC). Samples of Springfield Coal #1 and #2 exhibited ten times yield difference from one another at the same heating temperature. Those yield differences are not strongly associated with TOC content, heating time, temperature, metal content, or kerogen type but appear to be more influenced by maceral composition and also maceral–mineral contact area within the source rocks. We conclude that macerals in the liptinite group have a propensity for methanogenesis. Specifically, amorphous organic matter undergoes transformation into hydrocarbons earlier than alginite at low-temperature heating conditions. Sporinite also contributes to higher yields of methane released from the coal source rock. Vitrinite and inertinite show a positive influence on carbon dioxide but no significant effect on increasing methane yields compared to other macerals. The strongest catalytic methanogenesis in the studied sample produced methane yields at 60°C, which amounted to ∼2.5 μmol per gram of organic carbon during one year of heating. We suggest that geocatalytic methanogenesis could generate economically sizeable gas plays from immature to early-mature source rocks over geologic time.

NEOCOMIAN – BARREMIAN LACUSTRINE SHALES IN THE MAYO OULO‐LERE BASIN, NORTH CAMEROON: DEPOSITIONAL ENVIRONMENT AND PETROLEUM POTENTIAL

The synrift Mayo Oulo‐Léré Basin in Northern Cameroon is located in the transition zone between the West and Central African Rift Systems. Structural and stratigraphic elements of the basin resemble those of the Yola Basin in NE Nigeria, an extension of the Upper Benue Trough. The Lower Cretaceous lacustrine shales with source rock potential which occur in nearby rift basins are also present in the Mayo Oulo‐Léré Basin. These shales were investigated at two outcrop locations (Badesi and Tchontchi), and samples collected (n = 60) were subjected to palynofacies and bulk geochemical analyses to evaluate their petroleum generation potential and to interpret their depositional environment. At the studied locations, shales were divided into two lithofacies: grey to black laminated shales containing algal‐bacterial OM together with common woody (phytoclast) debris (“facies Fml”); and cm‐bedded shales which had a higher content of algal‐bacterial OM but a lower phytoclast content (“facies Fmlc”).Palynological and bulk geochemical data indicate that the shales contain well‐preserved organic matter (OM) and locally display good to excellent oil generation potential. Average TOC contents are 2.7% and 1.4% for samples of the Fmlc and Fml facies shales respectively. HI values (94‐889 mg HC/g TOC and 131‐638 mg HC/g TOC respectively) suggest that the shales contain Types I to III kerogen. Organic material in the Fmlc facies shales is dominated by amorphous organic matter (AOM: 90% on average) with a low phytoclast content (6% on average); whereas samples of the Fml facies shales contain less AOM (74% on average) and have a higher phytoclast content (23% on average). AOM in the Fmlc shales is highly fluorescent; these shales are interpreted to have been deposited in dysoxic to anoxic conditions. The AOM in the Fml shales is weakly fluorescent and the shales were deposited under more oxic conditions. The kerogen in the shales ranges from immature to early oil window mature. Average values of the pyrolysis S2 yield are 15.5 mg HC/g of rock and 7 mg HC/g of rock for samples from Fmlc and Fml facies shales respectively. The shales increase in thickness northwards towards the Logone Birni Basin where they may have reached the oil window, as in neighbouring basins. The results of this study of lacustrine shales from the Mayo Oulo‐Léré Basin suggests that there may be potential for oil exploration in northern Cameroon.

Geochemical characteristics and depositional paleoenvironment of source rocks from the Lower Cretaceous Chijinbao Formation in Jiuxi Basin, China

The Lower Cretaceous Chijinbao Formation is one of the primary source rocks in the Jiuxi Basin in Northwest China. However, it is poorly studied due to the lack of a complete stratigraphic sequence exposed by drilling. In this research, an integrated analysis of palynofacies, organic geochemistry, and element geochemistry was systematically conducted on samples with longitudinal continuity collected from the Chijinbao Formation in the Hanxia Section of the Jiuxi Basin to determine the distribution of source rocks and the source and type of organic matter and to reconstruct the depositional paleoenvironment of the Chijinbao Formation. The lower part of the sequence, which is governed by poor source rocks with Type III organic matter, is characterized by palynofacies A (woody tissue–charcoal assemblage) grown in a proximal alluvial fan–shallow lacustrine environment. The excellent source rocks with mostly Type II 1 organic matter are considerable in the middle part of the sequence, which is marked by palynofacies B (amorphous organic matter assemblage), and occur in a distal semi-deep lacustrine–deep lacustrine environment. The upper part of the sequence is largely composed of poor and fair source rocks, represented by palynofacies C (amorphous organic matter–woody tissue assemblage), and a small amount of sporadically distributed good and excellent source rocks (approximately 1/3), which may be deposited in a proximal fan delta and a local semi-deep lacustrine–deep lacustrine environment. A semiarid–semi-humid climate and a freshwater–brackish water environment prevailed during the deposition of the Chijinbao Formation. The lower part of the sequence is characterized by the highest terrestrial detritus influx, strong weathering, humid paleoclimate, oxic water column conditions, and the lowest paleoproductivity. The middle part of the sequence has the lowest terrestrial detritus influx, relatively weak weathering, relatively arid paleoclimate, predominantly anoxic water environment, and the highest paleoproductivity. The climate became humid again during the deposition of the upper part, which is typified by an intermediate terrestrial detritus influx, strong weathering, fluctuations of oxic–anoxic bottom-water conditions, and moderate paleoproductivity. The correlation analysis of element proxies suggests that the increase in salinity during the deposition of the middle part is associated with an arid climate. Amorphous organic matter, which is derived mostly from aquatic organisms and degraded terrestrial higher plants, is the major hydrocarbon generation component in the source rocks of the Chijinbao Formation. It increases in abundance with an improvement in water reducibility and productivity, favoring the formation and preservation of excellent source rocks. • Excellent source rocks distribute in middle part of Chijinbao Formation. • AOM dominate organic components in excellent source rocks of Chijinbao Formation. • Excellent source rocks were deposited under anoxic and relative arid conditions. • High salinity correlates with arid climate during the deposition of middle part.

Hydrocarbon source potential and paleodepositional environment of the (Devonian) Barreirinha formation on the south edge of the Amazonas basin border, Brazil

The present work aims to study the depositional paleoenvironment and the hydrocarbon source potential of the (Devonian) Barreirinha Formation, in the southern portion of the Amazonas basin. To accomplish this task, the geochemical parameters (total organic carbon, Rock-Eval pyrolysis, total sulfur and total nitrogen, whole oil hydrocarbons, and biomarkers) and palynofacies of 22 samples distributed in three outcrops are studied. Total organic carbon (TOC) values in the collected samples vary between 1.42% and 5.56%, indicating great variation in the production and preservation of organic matter. The sulfur content, ranging from 0.05 to 2.90%, indicates deposition in a marine environment. Rock-Eval pyrolysis parameters also reflect variations in the depositional conditions of this formation. The potential for hydrocarbon generation varies from low to excellent, noting that the samples that have the highest values of generation potential (S2) have higher TOC levels. This characteristic is justified by the good quality of the organic matter, since such samples have high hydrogen index (HI) values and low oxygen index (OI) values, indicating type I/II kerogen. The low free hydrocarbon (S1) values present in the samples (<0.5 mg HC/g rock) indicate immaturity, as shown by the Tmax (<440 °C) and by the production index (PI < 0.1). For total oil hydrocarbon chromatographic analysis, the largest contribution of algal organic matter is explicit for terrestrial-aquatic reasons (TAR <1). Furthermore, the pristane and phytane isoprenoid ratios (Pr/Phy) indicate differences between anoxic and oxic environments. Saturated biomarkers allow us to state that the organic matter present in the samples has a marine origin with terrestrial contributions and low thermal maturation. Palynofacies analysis shows that there is a predominance of amorphous organic matter (AOM), with a moderate amount of palynomorphs and subordinate amounts of phytoclasts. In the ternary diagram, these percentages allow us to infer that the depositional paleoenvironment was a distal platform with oxic-anoxic variation.

Mediation of arsenic mobility by organic matter in mining-impacted sediment from sub\u2010Arctic lakes: implications for environmental monitoring in a warming climate

Arsenic (As) is commonly sequestered at the sediment–water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulfides. The results of this study demonstrate that the accumulation of organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories, Canada. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially derived OM (e.g., cutinite, funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulfide minerals (e.g., goethite, orpiment, lepidocrocite, and mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62% As (V), 18% As (III); n = 20) and sediment (median = 80% As (-I) and (III), 20% As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred. Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters.

Microspores, megaspores, palynofacies, and depositional history of the upper Givetian Maywood Formation, Northern Wyoming, USA

Incised valley-fill deposits of the Middle Devonian Maywood Formation at Cottonwood Canyon, Wyoming, USA, contains a significant palynological record. Twenty-six palynotaxa were recovered, of which 21 species have wide paleogeographical distribution. The presence of the microspores Samarisporites triangulatus and Contagisporites optivus, as well as other Givetian index taxa, points to a late Givetian age (C. optivus/S. triangulatus palynozone) of the Maywood Formation. The palynoflora investigated may be correlative with the hermanni conodont Zone or a younger zone, and thus they record the latest Taghanic crisis or the earliest post-Taghanic interval. The playnoflora includes progymnosperms that had a tree-like habit (Aneurophytales and Archaeopteridales) characteristic of proximal fluvio-lacustrine settings of floodplains and paralic environments of internal basins. Spores of the herbaceous lycopsid Selaginellales, shrubby lycopods Protolepidodendrales, and primitive ferns, indicate a proximity of terrestrial plant habitats near the Maywood paleovalley. Additionally, the palynofacies (1) have amorphous organic matter and few algal remains of marine origin (Quadrisporites, Dictyotidium), (2) exist in association with assemblages of monospecific microconchid tubeworms that may have colonized non-biomineralized algae, and (3) are in strata that lack other benthic fossils, all of which indicate that the Maywood paleovalley record fluctuations of salinity. These were generated by mixing of marine and freshwater inputs within the estuarine channel, which was developed along the paleoshorelines of the Maywood Sea.

Characterization of bituminite in Kimmeridge Clay by confocal laser scanning and atomic force microscopy

This work investigates bituminite (amorphous sedimentary organic matter) in Upper Jurassic Kimmeridge Clay source rock via confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM). These petrographic tools were used to provide better understanding of the nature of bituminite, which has been historically difficult to identify and differentiate from similar organic matter types in source rocks. As part of an International Committee for Coal and Organic Petrology (ICCP) working group, an immature (0.42% vitrinite reflectance), organic-rich (44.1 wt% total organic carbon content) sample of Kimmeridge Clay was distributed to multiple laboratories for CLSM characterization. The primary observations from CLSM imaging and spectroscopy include: 1) the interpreted presence of Botryococcus algae as a contributor to bituminite precursors; 2) color red-shift of sulfide reflectance and bituminite auto-fluorescence from below the sample surface; 3) positive alteration of bituminite from laser-induced photo-oxidation of the sample surface, including fluorescence blue-shift; 4) fluorescence blue-shift associated to higher fluorescence intensity regions in bituminite indicative of compositional (fluorophore) differences; 5) the need for fluorescence spectroscopy standardization as applied via CLSM; and 6) the suitability of CLSM fluorescence spectroscopy to predict solid bitumen reflectance from bituminite spectral emission via calibration to an extant dataset. Secondary CLSM observations include detection of reflected laser light from highly reflective inclusions in bituminite, including sulfides and fusinite, and radiolytic alteration of bituminite caused by substitution of U for Fe in sulfides. Findings from AFM include the observation that surface roughening or surface flattening of bituminite are induced by differential broad ion beam (BIB) milling and are dependent on the location and scale of AFM topology measurement. This result highlights our still limited understanding of the effects of BIB milling on sedimentary organic matter and indicates the need for further research before this technique can be advanced as a standard practice in petrographic sample preparation. Collectively, the results of this study illustrate the general applicability and versatility of AFM and CLSM as tools for organic petrology research, specifically for better understanding of the nature and properties of the bituminite maceral.

Palaeoenvironmental reconstruction for the Permian (lower Gondwana) succession of the Godavari Valley Coalfield in southern India based on a combined palynofacies, carbon isotope, and biomarker study

The study presents integrated palynofacies, organic carbon isotope, and biomarker data to reconstruct palaeoenviromental setting for the Permian sediments (borehole MGK-6) of the Kachinapalli block of the Godavari Valley Coalfield, southern India. The palynofacies data reveal three distinct palynofacies (A–C) as follows. Palynofacies A is dominated by abundant arborescent vegetation (glossopterids and conifers) along with structured organic matter, suggesting freshwater forest swamps in proximal settings. Palynofacies B is dominated by abundant degraded organic matter and amorphous organic matter, suggesting lakeshore/flooded palaeomires in slightly dismal settings. It has a high water level in the hinterland, which enables bacterial degradation of organic matter and establishes low-oxygenation conditions within the water column. These conditions would have provided grounds for the accumulation of either degraded or amorphous organic matter. Palynofacies C is dominated by the abundance of charcoal/opaque phytoclasts, indicating prolonged transportation or post-depositional alteration, suggesting a highly-oxidizing condition in distal settings. The bulk organic δ13C range (−25.1‰ to −20.9‰) in the borehole MGK-6 is in close agreement with the global Permian records. This study also shows a weak but statistically significant correlation with the major forms of palynofacies A and C. The interlinked behaviour of bulk organic δ13C and palynofacies suggests that the changes in palaeobiogeography/palaeodepositional settings were most likely driven by the change in mean annual precipitation. The biomarker study shows the presence of n-alkane from C15 to C31 with unimodal and bimodal distribution patterns, revealing the source materials as vascular plants and microbially-altered organic matter. The combined palynofacies, organic carbon isotope, and biomarker data provide vital clue to salient findings for the development of environmental conditions of Godavari Valley Coalfield during lower Gondwana sedimentation.