Low Total Organic Carbon Content Research Articles

Geochemical insights of arsenic mobilization into the aquifers of Punjab, Pakistan

It is well known that groundwater arsenic (As) contamination affects million(s) of people throughout the Indus flood plain, Pakistan. In this study, groundwater (n = 96) and drilled borehole samples (n = 87 sediments of 12 boreholes) were collected to investigate geochemical proxy-indicators for As release into groundwater across floodplains of the Indus Basin. The mean dissolved (μg/L) and sedimentary As concentrations (mg/kg) showed significant association in all studied areas viz.; lower reaches of Indus flood plain area (71 and 12.7), upper flood plain areas (33.7 and 7.2), and Thal desert areas (5.3 and 4.7) and are indicative of Basin-scale geogenic As contamination. As contamination in aquifer sediments is dependent on various geochemical factors including particle size (3–4-fold higher As levels in fine clay particles than in fine-coarse sand), sediment types (3-fold higher As in Holocene sediments of floodplain areas vs Pleistocene/Quaternary sediments in the Thal desert) with varying proportion of Al-Fe-Mn oxides/hydroxides. The total organic carbon (TOC) of cored aquifer sediments yielded low TOC content (mean = 0.13 %), which indicates that organic carbon is not a major driver (with a few exceptions) of As mobilization in the Indus Basin. Alkaline pH, high dissolved sulfate and other water quality parameters indicate pH-induced As leaching and the dominance of oxidizing conditions in the aquifers of upper flood plain areas of Punjab, Pakistan while at the lower reaches of the Indus flood plain and alluvial pockets along the rivers with elevated flood-driven dissolved organic carbon (exhibiting high dissolved Mn and Fe and a wide range of redox conditions). Furthermore, we also identified that paired dissolved AsMn values (instead of AsFe) may serve as a geochemical marker of a range of redox conditions throughout Indus flood plains.

Microscopic pore structure characteristics and controlling factors of marine shale: a case study of Lower Cambrian shales in the Southeastern Guizhou, Upper Yangtze Platform, South China

A systematic study of the pore structure characteristics of Lower Cambrian shales in the southeastern Upper Yangtze Platform, was conducted using organic geochemistry, mineralogy, nitrogen adsorption, physical property analysis, and scanning electron microscopy. The results indicate that: 1) The Total organic carbon (TOC) content shows a strong correlation with quartz and clay minerals. Shales with low TOC content and rich in clay minerals primarily exhibit slit-shaped and narrow slit-like inter-clay particle pores with pore size distribution is dominated by mesopores and macropores. Shales with high TOC content predominantly feature narrow slit-like and ink bottle-shaped pores with pore size distribution dominated by micropores and mesopores. 2) Shale pore structures vary significantly under different gas content and preservation conditions. Shales under favorable preservation conditions exhibit a relatively “high porosity, low permeability, and high gas content” pattern, with well-developed organic pores and a strong pore-permeability correlation. In contrast, shales under unfavorable preservation conditions appear dense, with excessively developed fractures increasing both average pore size and local permeability. The pore-permeability correlation is weak, presenting a relatively “low porosity, high permeability, and low gas content” pattern. 3) TOC content plays a crucial role in controlling pore structure, showing overall positive correlations with pore volume, specific surface area, and porosity, and negative correlations with pore size. High TOC content enhances shale plasticity, resulting in lower pore diameters. Factors such as compaction and unfavorable preservation conditions lead to the shrinkage, collapse, and closure of some narrow pore throats, negatively impacting pore volume, specific surface area, brittleness, and fractal dimension, exhibiting a negative correlation with TOC content. 4) The pore structure of Lower Cambrian shales is complex, with fractal dimensions D1 and D2 exhibiting negative correlations with average pore size and positive correlations with TOC, specific surface area, and total pore volume. A high D1 value indicates well-preserved nanoscale pore surface structures with low complexity, suggesting minimal alteration by external fluids and better shale gas preservation. D1 serves as an indicator for shale gas content and preservation conditions. D2 shows better correlations with various pore structure parameters, making it suitable for characterizing pore structures.

Stabilization of PFAS-contaminated soil with sewage sludge- and wood-based biochar sorbents

Sustainable and effective remediation technologies for the treatment of soil contaminated with per- and polyfluoroalkyl substances (PFAS) are greatly needed. This study investigated the effects of waste-based biochars on the leaching of PFAS from a sandy soil with a low total organic carbon content (TOC) of 0.57 ± 0.04 % impacted by PFAS from aqueous film forming foam (AFFF) dispersed at a former fire-fighting facility. Six different biochars (pyrolyzed at 700–900 °C) were tested, made from clean wood chips (CWC), waste timber (WT), activated waste timber (aWT), two digested sewage sludges (DSS-1 and DSS-2) and de-watered raw sewage sludge (DWSS). Up-flow column percolation tests (15 days and 16 pore volume replacements) with 1 % biochar indicated that the dominant congener in the soil, perfluorooctane sulphonic acid (PFOS) was retained best by the aWT biochar with a 99.9 % reduction in the leachate concentration, followed by sludge-based DWSS (98.9 %) and DSS-2 and DSS-1 (97.8 % and 91.6 %, respectively). The non-activated wood-based biochars (CWC and WT) on the other hand, reduced leaching by <42.4 %. Extrapolating this to field conditions, 90 % leaching of PFOS would occur after 15 y for unamended soil, and after 1200 y and 12,000 y, respectively, for soil amended with 1 % DWSS-amended and aWT biochar. The high effectiveness of aWT and the three sludge-based biochars in reducing PFAS leaching from the soil was attributed largely to high porosity in a pore size range (>1.5 nm) that can accommodate the large PFAS molecules (>1.02–2.20 nm) combined with a high affinity to the biochar matrix. Other factors like anionic exchange capacity could play a contributing role. Sorbent effectiveness was better for long-chain than for short-chain PFAS, due to weaker, apolar interactions between the biochar and the latter's shorter hydrophobic CF2-tails. The findings were the first to demonstrate that locally sourced activated wood-waste biochars and non-activated sewage sludge biochars could be suitable sorbents for the ex situ stabilization and in situ remediation of PFAS-contaminated soil, bringing this technology one step closer to full-scale field testing.

Paleoenvironmental reconstruction of the Berriasian organic-rich interval of the Vaca Muerta Formation (Neuquén Basin, Argentina): Insights for the characterization of unconventional hydrocarbon shale reservoirs

The 50 m-thick Berriasian organic-rich interval (BORI) of the Vaca Muerta Formation (Neuquén Basin, Argentina) represents one of the most attractive stratigraphic intervals for unconventional hydrocarbon exploration in the unit. Nevertheless, little is known about the environmental conditions prevailing during the accumulation of the sediments. To unravel the causes that favored the preservation of the organic matter (OM) in the BORI, a detailed multiproxy sedimentological approach was conducted. The study included the analyses of microfacies, palynological content, calpionellid-calcisphere dinocysts, mineralogy, spectral gamma-ray, inorganic geochemistry, Rock-Eval pyrolysis and stable carbon isotopes of whole rock carbonate (δ13Ccarb), and associated OM (δ13Corg). The BORI presents a late Early to early Late Berriasian age and is subdivided into a lower and upper interval. The lower interval (25.6 m-thick) is primarily formed by radiolaritic wackestones and presents higher total organic carbon (TOC) content (4.2 wt% on average), mainly in the form of amorphous OM (AOM), with variable contribution of phytoclasts. The geochemical analyses (Si, Ni, Cu, Mo, U, V) indicate high productivity of the water column and overall sea bottom anoxia. Oppositely, the upper interval (24.4 m-thick) is mostly constituted by peloidal packstones/grainstones and presents a lower TOC content (1.5 wt% on average) mainly constituted by phytoclasts, with variable contribution of AOM. A decrease of the productivity and an increase of sea bottom oxygenation is recorded based on the geochemical analyses. The high OM content of the BORI responds to the combination of high productivity of the water column and sea bottom anoxia in response to the Berriasian transgression and a worldwide paleoclimatic change towards more humid conditions. The results of our study position the BORI as an attractive interval for unconventional hydrocarbon production due to its high TOC, low bulk clay mineral and high biogenic quartz content, granting an adequate geomechanical behavior.

Organic carbon burial and their implication on sea surface primary productivity in the middle Okinawa Trough over the past 200 ka

The long-term burial of organic carbon in marginal seas plays a critical role in Earth’s carbon cycle and climate change. However, the mechanism of organic carbon (OC) burial in the Okinawa Trough (OT) during glacial-interglacial timescales remains unclear. In this study, we analyzed the foraminiferal carbon isotopes, total organic carbon (TOC), and δ13C-TOC over the past 200 ka in core Z1 collected in the central OT. We aimed to reveal the history of OC burial in the middle Okinawa Trough during the past 200 ka, and we combined our findings with relevant paleoenvironmental indices to reveal underlying mechanisms. We found reduced surface primary productivity during MIS 6, which may indicate changes in the pathways of the Kuroshio Current (KC). Furthermore, we observed decoupling between high TOC flux and low OC burial during glacial periods. We proposed that the dilution effect caused by the high sedimentation rate and poor OC preservation during the glacial period resulted in the low TOC content. Ventilation of the North Pacific Intermediate Water (NPIW) regulated the redox conditions of the intermediate water in the Okinawa Trough. Additionally, the intensified Kuroshio Current during interglacial phases led to water column stratification, creating reducing conditions in the bottom water and facilitating improved OC preservation. Subsequently, the enhanced water column oxygenation resulting from the oxygen carried by the intensified glacial NPIW weakened the burial of OC. This study sheds new light on our understanding of the carbon cycle in marginal seas on a glacial-interglacial timescale.

Controls on organic matter enrichment of the early-middle Miocene lacustrine oil shale, central Tibet (SW China): New insights from clay minerals and geochemistry

The Lunpola oil shale is the largest lacustrine oil shale deposit in Tibet (SW China), which deposited in a Cenozoic lacustrine rift basin and contains abundant organic matter (OM), whereas the crucial factors that controlled the OM enrichment remain elusive. Here, we present new datasets of total organic carbon (TOC) contents, clay minerals and whole-rock geochemistry of the early-middle Miocene Middle Dingqinghu Formation oil shale from the Lunpola basin, central Tibet, in order to elucidate the paleoclimate, paleo-environment condition and the controlling factors on the OM enrichment of the lacustrine oil shale.Oil shales from the Lunpola East section have high TOC contents (avg. 9.60%), while the contiguous mudstones contain low TOC contents (avg. 0.44%). During the oil shale deposition, high smectite and kaolinite contents, low illite contents and low Sr/Cu ratios suggest strong chemical weathering and hence warm and humid climate, which may probably have close relationship with the mid-Miocene climatic optimum (MMCO). However, during the mudstone deposition, the relatively high illite and low smectite contents and higher Sr/Cu ratios suggest weak chemical weathering, indicating relatively arid climatic condition. The paleo-environment indexes (Corg/P, MoEF and UEF) and MoEF-UEF covariation reflect an anoxic bottom water condition during the oil shale deposition, while the mudstone formed in oxic-dysoxic water environment. The weak and moderate positive relationship between bio-productivity proxies (P/Ti and Cu/Ti ratios) and TOC contents reveal that the bio-productivity condition is not the crucial factor for the OM enrichment. Moreover, the water column environment was brackish and stratified during the oil shale deposition, and the sedimentation rate was moderate, which were also conducive to the OM preservation. The positive correlations between the TOC and the paleoclimate and paleoenvironment proxies demonstrate that warm and humid climate during the MMCO and anoxic water conditions play predominant roles in OM enrichment of the Lunpola lacustrine oil shale. Besides, modest sedimentation rate can prevent the dilution of OM, which can also promote the OM enrichment. This study can not only deepen the understanding of OM enrichment mechanism in lacustrine rift basin but also provide important information for predicting the distribution of the oil shale in the Lunpola Basin.

Geochemical evaluation and source rock zonation by multi-layer perceptron neural network technique: a case study for Pabdeh and Gurpi Formations-North Dezful Embayment (SW Iran)

In this study, using a multi-layer perceptron neural network (MLPNN) model, total organic carbon (TOC) and hydrogen index (HI) values for Pabdeh and Gurpi Formations in the oil fields of Naft Sefid (NS-13), Kupal (KL-36, KL-38, and KL-48) and Palangan (PL-2) were calculated in the North Dezful Embayment located in the southwest of Iran. To build the MLPNN model, the geochemical data calculated by the Rock–Eval pyrolysis method (TOC and HI) and the conventional petrophysical well log data, including sonic transit time log (DT), formation density log (RHOB), total resistivity log (RT), spectral gamma-ray log, computed gamma-ray log and neutron porosity log from the NS-13 well were used. The log data were the input layer, and the geochemical data were the output layer of the model. Twenty-four datasets were used for MLPNN training, and seven datasets were used for MLPNN testing. Two hidden layers were considered in this technique. Each hidden layer has an activation function (tanh) and a solver parameter (lbfgs). The accuracy of measurement of TOC and HI indices of Pabdeh and Gurpi Formations in terms of R2 was 0.93 and 0.90, respectively. This model has higher accuracy than the ΔlogR technique (R2: 0.28). Considering the relationships between the input data and other wireline logs is an advantage of this technique. These two formations have five source rock zones. Pabdeh Formation has three zones. The middle zone of the Pabdeh Formation (Pz. II) has the highest TOC (2.6 wt%) and source rock potential. Pabdeh Formation has kerogen type II. Gurpi Formation has a weaker source rock potential than Pabdeh Formation due to its low TOC content (< 1%). Both source rock zones of this formation have low TOC, but in some layers of the lower zone of the Gurpi Formation (Gz. II), high values for TOC were predicted. Gurpi Formation has Kerogen types II and III.

Sedimentary Environment Interpretation and Organic Matter Enrichment of the Lower Cambrian Shuijingtuo Shale in the Yichang Slope, South China: Insight from Sedimentary Geochemical Proxies with Major/Trace Elements

The vertical geochemical variations in total organic carbon (TOC) content and major and trace elements of the Lower Cambrian Shuijingtuo Formation from the Yichang Slope in the Upper Yangtze were investigated to assess the environmental conditions (redox conditions, water mass restriction, terrigenous input, relative sea-level terrestrial, and paleoproductivity) and to determine the primary controlling factors for organic matter enrichment. The Shuijingtuo Shale is divided into three intervals. The lithofacies of interval I are mainly black siliceous shale with high TOC content, and interval II is mainly black siliceous shale with moderate TOC content. Interval III consists of black, clay-rich siliceous shale and dark-gray calcareous shale and is characterized by a low TOC content. The effects of diagenesis and hydrothermal activity on the elements were evaluated prior to analyzing the environmental condition. There are good positive correlations between TOC and U/Al or Mo/Al ratios, suggesting that major/trace elements still retain the geochemical signature of the sedimentary environment. Meanwhile, the geochemical proxies consisting of Al, Fe, Mn, and Ti indicate that the study area did not experience hydrothermal deposits. The redox proxies (U/Th, Corg:Ptot, and MoEF–UEF) indicate that the interval I samples were formed under a strong reducing condition. The diagram of TOC vs. Mo indicates that the water mass was moderately restricted during the deposition of interval I shales. Proxies of terrigenous input and relative sea-level (Zr/Al and Zr/Rb) suggest that the organic-rich shales at the bottom were deposited under a relatively high sea-level and experienced minimal input of terrigenous debris from the source area. Additionally, the paleoproductivity indicated by Sibio and (Ni + Cu + Zn)/Al was high for interval I samples. During the interval II period, the relative sea-level began to decline, the seawater still remained in reducing conditions, and there was no change in the productivity, but the input of terrigenous debris increased significantly. In the interval III depositional period, the relative sea-level continued to decrease, the seawater shifted to a dysoxic condition, and the paleoproductivity was also at a lower level. The evolution of the sedimentary environment indicates that the high TOC content in the interval I samples is mainly attributed to the strong reducing condition, the preservation condition and debris dilution together control the organic matter content within the interval II samples, and the low TOC content within the interval III samples is constrained by a combination of the poor preservation conditions and lower paleoproductivity.

Variable depositional environments and organic matter enrichment of Early Cambrian shales in the Middle Yangtze region, South China

Organic-rich shales of the Lower Cambrian Shuijingtuo Formation (SJT) in the Middle Yangtze region (South China) represent an important target for shale gas exploration and development in China. The lower section of the SJT mainly consisting of organic-rich black shale deposits has been regarded to be a favorable target for shale gas exploration and received substantial attentions, while the upper section of the SJT primarily consisting of limestone, silty shale and black shale deposits has been rarely studied. Most recently, a major breakthrough of shale gas exploration and development has been made in the upper section shale deposits. In order to better understand the redox evolution of sedimentary environment throughout the whole Early Cambrian time and organic matter (OM) enrichment mechanism of the SJT shale deposits, geochemical investigations, including analyses of total organic carbon (TOC), as well as major and trace element compositions, were conducted on the upper section of the SJT shales from Well EYY1. Our results showed that the upper section of the SJT shale deposits was accumulated during the Cambrian Stage 3. The TOC values of the studied SJT shale samples ranged from 0.34 to 3.65 wt%. The lower part of the upper section of the SJT shale, which exhibited relatively high TOC contents (∼2.0 wt%), was deposited under anoxic bottom-water conditions during the early Cambrian Stage 3, while the upper part, showing relatively low TOC content (1.0 wt%), was deposited under suboxic bottom-water conditions with intermittent episodes of anoxic bottom-water conditions during the middle–late Cambrian Stage 3. The palaeo-productivity of the SJT shale was estimated to be generally low, and displayed little variation. The OM enrichment mechanism of the SJT shale deposits underwent dynamic changes due to relative sea-level changes in the Yangtze Ocean. The OM enrichment of the early and late Cambrian Stage 3 shale deposits was primarily controlled by reducing conditions, while that of the middle Cambrian Stage 3 shale deposits may have been influenced by episodic upwelling activities.

The short-term response of soil microbial communities to digestate application depends on the characteristics of the digestate and soil type

Anaerobic digestion of organic waste is a key process to produce renewable energy and meet the growing demand for sustainable energy. The residues of anaerobic digestion – called digestates – can be used as soil amendments to improve crop yields. However, the effect of digestates on the soil biota, especially on microorganisms, needs to be better documented before a large scale use of digestates in agriculture. In addition, how the quality and composition of the digestate may affect soil microbial communities has not been properly addressed yet. We designed a microcosm experiment under controlled experimental conditions to compare effects (42 days) of four digestates produced from varying intakes (cattle manure and/or energy crop and/or food residues and/or slurry) on soil microbial communities; a control microcosm made of undigested cattle manure was also used. Each digestate was applied on three contrasting soils representing contrasted pedo-climatic conditions (especially soil type and climate). These three soils presented different prokaryotic and fungal communities structures. The effect of digestate inputs on the soil microbial biomass and diversity was assessed using molecular DNA-based tools (quantification of extracted soil DNA and high-throughput sequencing, respectively) in comparison to the untreated cattle manure control condition. Our results show that 42 days after digestate application, significant differences of soil microbial communities were observed according to the digestate characteristics; these differences were soil-dependent. Thus, in the silty clay loam soil, no effect of digestates was observed on soil microbial biomass or diversity (P > 0.05), as compared to the undigested cattle manure. In the two other soil types (loam and sandy loam), soil microbial biomass decreased (around −40 %, P < 0.001) when digestates having a low total organic carbon content (from 0.61 to 3.3 g.100 g−1) were applied. None of the digestates affected the soil prokaryotic diversity whatever the soil type (P > 0.05). Digestate application resulted in higher fungal diversity (around +35 %; P < 0.001) in soils with low C/N ratio (9.14 in average). The microbial community structure of coarse-textured soil appeared more impacted by organic inputs than fine-textured soils. To conclude, our results show that different soil types, harboring distinct microbial community structures, responded differently to different digestates application. This response was also digestate-dependent.

Pore Structure Characterizations of Shale Oil Reservoirs with Heat Treatment: A Case Study from Dongying Sag, Bohai Bay Basin, China.

Heat treatment plays a significant role in determining the petrophysical properties of shale reservoirs; however, the existing studies on the evolution of pore structures are still insufficient. This study conducts a series of tests, including Rock-Eval, low-temperature nitrogen adsorption-desorption, nuclear magnetic resonance (NMR) T2, and T1-T2 tests on samples from Shahejie Formation, Dongying Sag, Bohai Bay Basin. The tests aim to determine the changes in the shale pore structures under increasing heat treatments (ranging from 110 to 500 °C) and identify the factors that control pore structures. The results show that the gradual decomposition of organic matter leads to an eventual decrease in the total organic carbon (TOC) content. The decrease in TOC is more prominent when the temperature exceeds 300 °C. For shales with lower TOC contents (<2%), the Brunauer-Emmett-Teller specific surface area (BET SSA) first decreases, then increases, but eventually decreases again. However, the average pore diameter demonstrates an opposite trend when the temperature increases. In contrast, for organic-rich shales (TOC > 2%), the BET SSA increases at temperatures above 200 °C. The similarity between the D1 values implies that the complexity and heterogeneity of shale pore surface only undergo minor changes during heat treatment. Porosity shows an increasing trend, and the higher the contents of clay minerals and organic matter in shales are, the greater the change in porosity is. The NMR T2 spectra suggest that micropores (<0.1 μm) in shales first decrease and then increase, whereas the contents of meso- (0.1-1 μm) and macropores (>1 μm) increase, corresponding to the increase in free shale oil. Moreover, shale pore structures are primarily controlled by clay minerals and organic matter contents during heat treatments, with higher contents resulting in better pore structures. Overall, this study contributes to detailing the shale pore structure characteristics during the in situ conversion process (ICP).

Sedimentary response to the early Silurian (Rhuddanian-Aeronian) post-glacial transgression and Kwangsian Orogeny in the Upper Yangtze region, South China

Unlike the lowermost Silurian ‘hot’ shales deposited in many other regions of the world, the early Silurian (Rhuddanian-Aeronian) succession in South China exhibits a distinctive lithofacies transition from organic-rich black siliceous mudstone in the early Rhuddanian, to interlaminated siltstone and mudstone with relatively low total organic carbon content in the late Rhuddanian, and finally to organic-lean argillaceous mudstone or calcareous mudstone in the late Aeronian. The interlaminated siltstone and mudstone interval has been the subject of a long-standing dispute over its nature, whether it belongs to turbidite or contourite. In this study, we employed a range of approaches, including petrographic analyses, UPb dating of detrital zircons, and measurements of paleocurrent directions to generate a dataset that reveals: (1) four typical fine-grained facies developed within the interlaminated siltstone and mudstone interval, (2) detrital zircon UPb ages yielding a wide age range from Archean (∼2.50 Ga) to early Paleozoic (0.42 Ga), with a predominant peak of ca. 447 Ma based on 516 detrital zircon UPb analyses, and (3) paleocurrent directions indicate the presence of current circulation in the Upper Yangtze region. The results of this study demonstrated that the detritus of the interlaminated siltstone and mudstone interval originated from concurrent explosive volcanic eruptions along the southeastern margin of the South China Craton. It was deposited and reworked by bottom currents under the background of the early Silurian post-glacial transgression. We suggest that the occurrence of the interlaminated siltstone and mudstone interval is indicative of a far-field sedimentary response to the onset and progradation of the Kwangsian Orogeny in the Upper Yangtze region.

Callovian source rock modelling as encountered in the Chotts Basin and the Gulf of Gabes: Generation and maturity distribution

The Callovian series are considered as proven to have sourced the reservoirs of the Ezzaouia and El Bibene fields of south-eastern Tunisia. However, its potential is still subject of debate associated with its paleogeographic distribution along with its burial and thermal histories. In this study, 17 wells drilled in the southern Chotts Basin and the Gulf of Gabes were used to model the burial and maturity histories of the Callovian series and estimate volumes of generated and expelled hydrocarbons.The Callovian organic material is generally marked by type II-III with low to moderate contents (up to 1%). The Callovian successions located in the north-eastern part of the Chotts Basin bear low total organic carbon (TOC) contents averaging 0.25%.TOC content and petroleum potential maps were generated with the Callovian series showing a source potential, southwards of the study area.Modelling results show variable burial and thermal histories depending on the location of selected wells. The series with source rock potential in wells located south of the Chotts Basin are in a an early to mid-mature stage with substantial amounts of generated oil. The Callovian series in the Gulf of Gabes are marked by advanced thermal maturity with gas generation being initiated since 10My. A maturity map is proposed for the Callovian series underlying four different stages of maturity.

Low content of highly reactive iron in sediments from Prydz Bay and the adjacent Southern Ocean: Controlling factors and implications for sedimentary organic carbon preservation

Examining iron (Fe) speciation in marine sediments is critical to understand Fe and carbon biogeochemical cycling in polar regions. In this study, we investigated the speciation of Fe in sediments from Prydz Bay and the adjacent Southern Ocean, and examined the factors controlling Fe speciation and its relationship with total organic carbon (TOC). Our results reveal that unreactive silicate Fe (FeU) is the dominant pool of total Fe (FeT), followed by poorly reactive sheet silicate Fe (FePRS), reducible crystalline Fe oxides (Feox2), easily reducible amorphous/poorly crystalline Fe oxides (Feox1), and magnetite (Femag), with carbonate-associated ferrous Fe (Fecarb) being the smallest pool. The highly reactive Fe (FeHR)/FeT ratios (0.13 ± 0.06) in our study area are among the lowest end-member globally, primarily due to weak bedrock weathering and slow glacier melting. The Feox1/FeT ratios are similar to those in continental shelf and marginal seas containing highly weathered materials, while the Feox2/FeT ratios are significantly lower. This result implicates that low temperature inhibits the aging of iceberg melting-sourced Feox1 potentially, and accordingly the regulation of weathering on the FeHR/FeT ratio is mainly reflected in Feox2/FeT ratio. There are no significant correlations between TOC and FeHR, Fecarb, Feox1 or Feox2 in the research region. Four distinct patterns of TOC/FeHR ratio can be discerned by summarizing the global data set: (a) high TOC/FeHR ratios (&amp;gt; 2.5) are likely the result of high marine primary productivity and low chemically weathered source materials; (b) low TOC/FeHR ratios (&amp;lt; 0.6) are caused by high rates of FeHR inputs and OC remineralization; (c) mid-range TOC/FeHR ratios (0.6 – 2.5) typical of most river particulates and marginal sea sediments indicate the same FeHR and OC sources and/or interactions between each other; (d) both low TOC and FeHR content is the result of low marine primary productivity and weak chemical weathering. Our findings provide new insights into the relationship between FeHR and TOC in polar sediments.

Organic matter accumulation mechanisms in the Wufeng-Longmaxi shales in western Hubei Province, China and paleogeographic implications for the uplift of the Hunan-Hubei Submarine high

Organic matter accumulation exhibits different distribution patterns in time and space and tends to be controlled by many factors. This study investigated the mechanisms that control the accumulation of organic matter in the Xianfeng area of western Hubei Province, South China through sedimentological and geochemical analyses of wellbore samples. The results showed that the total organic carbon (TOC) content, mineral content, and elemental geochemical data of the Wufeng-Longmaxi Formations varied significantly with depth. Based on the sharp changes in the vertical profiles of the TOC content, Gamma-ray, and numerous geochemical parameters, as well as regional stratigraphy, this study divided the section from the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation into six units (W1, W2, GYQ, L1, L2, and L3). The TOC content was positively correlated with redox proxies (DOPT, VEF, and MoEF) and primary productivity proxies (CuEF and NiEF), and negatively correlated with terrigenous input proxies (Al, K, and Ti). The correlation coefficients suggest that the primary factors affecting organic matter accumulation are redox conditions and paleoproductivity, while terrigenous input is a secondary controlling factor. This study proposed an organic matter accumulation model based on the organic matter characteristics, lithofacies, sedimentary environment, and geological setting of the study area. During the deposition of Unit W1, organic matter was well-preserved owing to the anoxic bottom water environment resulting from strong water mass restriction. During the deposition of Unit W2, the rising sea level led to the development of anoxic-euxinic bottom water conditions, and the joint influence of nutrients from upwelling in the open sea and volcanic eruptions enhanced productivity, resulting in the most intense accumulation of organic matter. Unit GYQ had low TOC content due to low productivity despite anoxic conditions. During the deposition of Unit L1, the uplift of the Hunan-Hubei Submarine High caused the depocenter to shift westward and the sea level in this area to drop, resulting in suboxic bottom water and relatively low productivity in the surface water layer. During the deposition of Unit L2, the regional sea level drop resulted in increased input and dilution of terrigenous clasts, thus leading to low TOC content. During the deposition of Unit L3, continuously enhanced terrigenous input due to climate warming and sea level drop became the major factor controlling the organic matter accumulation. In summary, the main factors controlling the organic matter accumulation differed significantly at different evolutionary stages. Compared with the Jiaoshiba area, the Xianfeng area in western Hubei experienced continuously increased terrigenous input, a less reductive water mass, a significant decrease TOC content, and a westward shift of the depocenter since the early stage of the Longmaxi Formation deposition. These changes may be related to the intense uplift of the Hunan-Hubei Submarine High triggered by the Kwangsian Orogeny. These results provide important implications for the understanding of the accumulation of organic matter in the Wufeng-Longmaxi Formations and paleogeographic changes.

Geochemistry and petrology of petroleum source rocks in the Upper Cretaceous Qingshankou Formation, Songliao Basin, NE China

The Upper Cretaceous Qingshankou Formation in the Songliao Basin, NE China is known to contain excellent petroleum source rocks. In addition, it serves as a valuable record to study depositional conditions within the water column and climatic change. In this study, a large set of organic petrography, major and trace element geochemistry, as well as bulk and molecular organic geochemistry data obtained on 38 core samples of the first Member of the Qingshankou Formation (K2qn1) in the south central depression is reported. All rocks are thermally oil-mature, reaching the early to peak oil generation stage, indicated by the vitrinite reflectance (VRr) of approximately 0.8%. Two different types of lacustrine source rocks (units I and II) and two OM-rich layers within unit I are identified in the studied well. Basically, there is a lower unit 1, characterized by lower total organic carbon (TOC) content and an upper unit 2 characterized by higher TOC content. Furthermore, within unit 1, thin marine transgressive layers of approximately 1–2 m were identified. Generally, unit 1 is characterized by the lowest TOC and sulfur contents with average values of 1.31 wt% and 0.56 wt%, respectively. By contrast, unit 2 samples have significantly higher TOC (avg. 2.39 wt%) and sulfur contents (avg. 1.24 wt%). Samples, which represent the postulated marine transgressive events show the highest TOC (avg. 2.82 wt%) and sulfur contents (avg. 2.01 wt%). Unit 1 was mainly deposited in oxic-dysoxic and freshwater-brackish water column conditions based on multiple organic and inorganic geochemical proxies. By contrast, unit 2 and marine transgressive layers were deposited in less oxygenated and more saline environments. Source rocks from the K2qn1 generally show only minor terrestrial organic matter input. However, unit 1, containing type II-III kerogen, shows indications for a slightly more pronounced contribution from terrigenous higher plants than unit 2 and the marine transgressive layers, where the kerogen is almost exclusively derived from planktonic/algal and microbial organic matter. The low TOC content in unit 1 is attributed to multiple adverse factors for organic matter accumulation, including a relatively shallow, oxic-dysoxic, and weakly stratified water column and high dilution by terrigenous mineral matter. By contrast, more favorable conditions persisted during the deposition of marine transgressive layers and unit 2, e.g., a more reducing and stratified water column and lower dilution by terrigenous mineral matter.

Near equatorial paleoclimatic evolution and control on organic matter accumulation during the Cenomanian in the Abu Gharadig Basin, southern Tethys: Insights from palynology, organic petrography, and geochemistry

The beginning of the Late Cretaceous (Cenomanian) attested to a major marine transgression interrupted by sea level falls in north Africa, which led to significant environmental perturbation and widespread accumulation of organic matter-rich sediments. During this period, northern Egypt was located at the southern edge of the Tethys Ocean and deposition of thick siliciclastic alternations between shales, siltstones, and sandstones of the Bahariya Formation took place in the BED2 gas field of the Abu Gharadig Basin, north Western Desert. An integrated approach of palynofacies analysis and palynomorph composition, organic petrography, and geochemical analysis of total organic carbon (TOC)/Rock-Eval pyrolysis, carbonate content, XRD analysis, and clay mineralogy of thirty-nine cutting samples obtained from the lower-middle Cenomanian Bahariya Formation was conducted. This approach was used to characterize organic matter and possible occurrence of source rock intervals, paleoenvironmental and paleoclimatic conditions and their role in organic matter accumulation. Moderate to high proportions of pteridophyte and water fern spores and pollen grains of Araucariaceae (Araucariacites), Cheirolepidiaceae (mainly Classopollis), and Gnetalean (Elaterate pollen) and Afropollis versus the sparsely recorded xerophilous ephedroids (Ephedripites) indicate prevalent conditions of a warm and humid climate. This is consistent with clay mineral composition that is dominated mainly by kaolinite and smectite compared to low illite and the lack of chlorite, which indicated a predominantly warm-humid climate state. Palynofacies analysis showed two assemblages; the first one is dominated by phytoclasts and indicates gas-prone Type III kerogen and deposition in a fluvio-deltaic environment. The second palynofacies assemblage is characterized by moderate abundance of phytoclasts and AOM mostly of terrestrial origin, suggesting a mixed Type III/II kerogen and deposition in a marginal to shallow neritic shelf environment. The organic geochemical characterization showed relatively low average values of TOC, S2, and HI, indicating that the Bahariya Formation is characterized by Type III gas-prone kerogen. Thermal maturity of the Bahariya Formation ranges from immature to the early stage of the oil window. Additionally, organic matter characteristics of the Bahariya Formation are qualitatively and quantitatively similar to their equivalents from nearby basins. Low TOC contents are attributed to a high sedimentation rate and an increased terrestrial input of sand- and silt-sized fractions at shallow water column settings that controlled strong dilution and decomposition versus low burial efficiency of the organic matter.

Evaluation of Lacustrine Shale Brittleness and Its Controlling Factors: A Case Study from the Jurassic Lianggaoshan Formation, Sichuan Basin

To investigate the brittleness of shale and its influencing factors, triaxial rock mechanics experiments, combined with X-ray diffraction, total organic carbon (TOC) measurement, rock pyrolysis, and scanning electron microscopy, were conducted on shales from the Jurassic Lianggaoshan Formation in the Sichuan Basin. BI1, based on the elastic modulus and hardness, BI2, based on mineral composition, BI3, based on strength parameters, and BI4, based on the post-peak energy of shale, were calculated. Additionally, the effects of mineral composition, density, hardness, and organic matter on the brittleness of shales were analyzed. The results show that the shale mineral compositions were dominated by quartz (mean of 45.21%) and clay minerals (mean of 45.04%), with low carbonate mineral contents and high TOC contents. The stress–strain curve showed strong brittleness characteristics. When comparing different evaluation methods, the brittleness evaluation method based on the stress–strain curve (damage energy) was found to be more effective than the mineral fraction and strength parameter methods. The higher the density and hardness, the more brittle the shale. The higher the organic matter and quartz content, the less brittle the shale. The brittleness of sub-member I of the Lianggaosan Formation in Well XQ1 was higher than that of sub-members II and III. This study investigated the brittleness of lacustrine shale and its influencing factors, which is beneficial for the development of shale oil in the Sichuan Basin.

Potential analytical interferences when measuring Tmax during temperature programmed pyrolysis of hydrothermally altered volcanoclastic sediment

AbstractThe temperature of maximum pyrolysis yield (known as Tmax) can be used to determine the level of thermal alteration in sedimentary organic matter; higher Tmax values represent higher thermal alteration. Tmax is commonly measured on petroleum source rocks or similar sediments with high organic carbon contents. It would be desirable to measure the Tmax of volcanic sediments because they can have complex patterns of thermal alteration. However, volcanic sediments often have low total organic carbon contents and consequently are susceptible to analytical interferences. Despite this, it can be shown that meaningful Tmax measurements can still be made in sediment with organic carbon contents as low as 0.2% and that interference caused by bitumen or ionizable salts can be mitigated by solvent extraction and rinsing with water. Thus, it is reasonable to use temperature programmed pyrolysis to assess levels of thermal alteration in even low total organic carbon volcanoclastic sediments.

Thermal effect of igneous intrusions on organic-rich Irati Formation and the implications for petroleum systems: a case study in the Paraná Basin, Brazil

The organic geochemistry of organic-rich facies including shales, marls, and carbonates (Assistência Member) of the Irati Formation, Paraná Basin, Brazil, were analyzed to evaluate the thermal effects of igneous intrusions upon the kerogen present in these facies. Total organic carbon (TOC) content and hydrocarbon source potential (S2) of the Irati source rocks range from 0.03 to 20.4% and 0.01 to 112.1 mg HC/g rock, respectively, indicating excellent potential as a source for hydrocarbon generation. Hydrogen index (HI) values reveal that the kerogen is predominantly type I (HI: up to 892.6 mg HC/g TOC) and, therefore, an oil source, except for samples having low TOC content due to severe maturation caused by the heat from diabase intrusions. The thickness of igneous intrusions in the 64 wells investigated in this study ranged from 2 to 231 m. They clearly had a major impact on TOC, HI, and S2 values, which decrease in the vicinity of intrusions, indicating a gradual increase in maturation toward the igneous body. In wells without the influence of igneous intrusions, Tmax values of Rock-Eval pyrolysis and %Ro indicate that the organic matter is immature for the generation of hydrocarbons.