High Total Organic Carbon Content Research Articles

Environmental controls on organic matter enrichment of the lower Cambrian source rocks in the Tarim Basin, Northwest China

In the Tarim Basin, the largest petroliferous basin in China, a set of organic-rich deposition hosted in the lower Cambrian Yuertusi Formation is considered to be the main hydrocarbon source rocks. However, the limited and scattered distribution of the lower Cambrian constrains present studies mostly on some certain sections where samples are accessible, inhibiting a systematic understanding of characteristics of these source rocks and controls on organic matter enrichment. Here, we conduct a systematic study on a drillcore in northwestern margin of the Tarim Basin (Taxibei), integrated with previously published data from other available sections. These data show the Yuertusi Formation was mainly composed of mudstone/shale, carbonate and siliceous rocks, and distributed dominantly in the Taxibei, northern (Tabei) and eastern-northeastern areas of the Tarim Basin (Tadong-Tadongbei) with an increased thickness (mean 30 m to mean 60 m) towards the Tadong-Tadongbei areas. High total organic carbon (TOC) contents (up to 30%) were found in the mudstone/shale dominated intervals in both the Taxibei and Tabei areas, characterized by I-II kerogen and highly to over-matured thermal maturity. Paleo-redox reconstruction based on high V/(V + Ni) ratios (>0.6) and FeHR/FeT ratios (>0.38) suggests anoxic watermass conditions occurred during the deposition of the organic-rich intervals along with moderate sedimentation rate, providing consistently good preservation for organic matter accumulation. Based on a combined analysis, we infer primary productivity in surface waters exerted a key control on TOC contents of the organic-rich intervals, which was driven by nutrient supplies from hydrothermal activities with partial upwelling effects in the Taxibei area, and from upwelling waters in the Tabei area.

Sowing carbon solutions: Decoding soil characteristics and carbon fluxes in maize-dominated cropping systems of Tamil Nadu, India

This study on soil carbon dynamics provides valuable insights for sustainable agricultural practices, optimizing crop productivity and environmental sustainability in maize-based cropping systems. The present study aimed to find out the soil characteristics and carbon dynamics in maize-based cropping systems in the Western zone of Tamil Nadu, India. Soil samples from six cropping systems were analyzed for bulk density, sand, silt, clay content, pH, available nutrients (N, P, K, Zn), total organic carbon (TOC), oxidizable organic carbon fractions, microbial biomass carbon (MBC), and carbon pools. The distribution of oxidizable organic carbon fractions varied among cropping systems and soil depths. The easily decomposable and moderately labile fractions were highest in the maize-black gram system, while the recalcitrant fraction showed variations across cropping systems. The active carbon pool (Cf1 + Cf2) was highest at 2.53 g kg-1 in the maize-blackgram system, while the passive carbon pool (Cf3 + Cf4) was also highest at 3.79 g kg-1 in this system. The study also assessed the carbon stock and microbial biomass carbon. TOC content decreased with depth, with the highest values observed in the topsoil. The maize-black gram system had the highest TOC content at all depths. MBC content followed a similar pattern, with the highest values in the topsoil and the maize-black gram system. These findings provided insights into the soil characteristics and carbon dynamics in maize-based cropping systems in the study area. The long-term integration of maize cultivation with blackgram demonstrated significant enhancements in organic carbon levels, TOC content, microbial biomass carbon (MBC), and both passive and active carbon pools characterized by rapid turnover rates.

Constraining the origin of sedimentary organic matter in the eastern Guangdong coast of China using δ13C and δ15N

The source apportionment of organic carbon is of great scientific significance for understanding the carbon sink pattern in the coastal ocean. The concentrations of total organic carbon (TOC) and total nitrogen (TN) and their isotopic compositions (δ13CTOC and δ15NTN) were determined in surface sediments and sediment cores (Y3 and Y5) collected in the eastern Guangdong coast (EGDC) during spring, summer and winter cruises in 2021, in order to determine the distribution and source of sedimentary organic matter. Spatially, TOC and δ13CTOC distributions in the EGDC have contrasting patterns; TOC concentrations decreased and δ13CTOC values increased with distance offshore. Higher TOC concentrations and enriched δ13CTOC were observed in the marine aquaculture area off EGDC. The measured values of δ13CTOC, δ15NTN and the C/N ratio constrained the two dominant sources of organic matter, marine and terrestrial material, in the EGDC. Using a two end−member mixing model, we estimated the proportion of marine organic matter to be 74 ± 20% in spring, 74 ± 16% in summer and 75 ± 12% in winter, respectively. This suggests in situ production of marine phytoplankton dominated the organic carbon pool (average=~74%) in the EGDC sediment. These consistent marine organic matter contributions are also observed in sediment cores, with proportions averaging 59 ± 5% in core Y3 and 66 ± 4% in core Y5. The observed variations in biogeochemical proxies in sediments are thus mainly caused by differing relative contributions of marine and terrestrial organic matter. Additionally, the high TOC inventory in the marine aquaculture area indicates increased carbon storage and carbon fixation capacity compared to the other areas investigated. Overall, this study establishes a background carbon database in the coastal environment that can be used in future studies of blue carbon to achieve carbon neutrality in China.

Quantitative effect of kerogen type on the hydrocarbon generation potential of Paleogene lacustrine source rocks, Liaohe Western Depression, China

Kerogen types exert a decisive effect on the onset and capacity of hydrocarbon generation of source rocks. Lacustrine source rocks in the Liaohe Western Depression are characterized by thick deposition, high total organic carbon (TOC) content, various kerogen types, and a wide range of thermal maturity. Consequently, their hydrocarbon generation potential and resource estimation can be misinterpreted. In this study, geochemical tests, numerical analysis, hydrocarbon generation kinetics, and basin modeling were integrated to investigate the differential effects of kerogen types on the hydrocarbon generation potential of lacustrine source rocks. Optimized hydrocarbon generation and expulsion (HGE) models of different kerogen types were established quantitatively upon abundant Rock-Eval/TOC/vitrinite reflectance (Ro) datasets. Three sets of good–excellent source rocks deposited in the fourth (Es4), third (Es3), and first (Es1) members of Paleogene Shahejie Formation, are predominantly types I–II1, II1–II2, and II–III, respectively. The activation energy of types I–II2 kerogen is concentrated (180–230 kcal/mol), whereas that of type III kerogen is widely distributed (150–280 kcal/mol). The original hydrocarbon generation potentials of types I, II1, II2, and III kerogens are 790, 510, 270, and 85 mg/g TOC, respectively. The Ro values of the hydrocarbon generation threshold for type I–III source rocks gradually increase from 0.42% to 0.74%, and Ro values of the hydrocarbon expulsion threshold increase from 0.49% to 0.87%. Types I and II1 source rocks are characterized by earlier hydrocarbon generation, more rapid hydrocarbon expulsion, and narrower hydrocarbon generation windows than types II2 and III source rocks. The kerogen types also affect the HGE history and resource potential. Three types (conventional, tight, and shale oil/gas) and three levels (realistic, expected, and prospective) of hydrocarbon resources of different members in the Liaohe Western Depression are evaluated. Findings suggest that the Es3 member has considerable conventional and unconventional hydrocarbon resources. This study can quantitatively characterize the hydrocarbon generation potential of source rocks with different kerogen types, and facilitate a quick and accurate assessment of hydrocarbon resources, providing strategies for future oil and gas exploration.

Sustainable approach to the control of airborne phytopathogenic fungi by application of compost extracts

The search for new sustainable alternatives for plant disease control has gained interest in the last decades. Compost extracts are nowadays considered a remarkable alternative to agrochemicals due to their biopesticidal properties. However, these properties could be affected by the different variables of extraction protocols and by starting compost. This work focused on the physicochemical and biological characterization of compost extracts obtained from a wide range of composted materials and different extraction protocols (CEP). CEP-1 and CEP-4 involved incubation at 20 °C for 48 h and 14 days, respectively; CEP-2 incubation for 24 h at 40 °C; while CEP-3 were incubated for 12 h at 70 °C. Electrical conductivity, pH, total organic carbon (TOC) and phenolic content were determined as well as the actinobacterial count and enzyme profiles related to plant pathogen suppression. Additionally, the influence of the different materials and protocols on the in vitro growth inhibition of Alternaria alternata and Botrytis cinerea was determined. The starting materials and extraction protocols significantly influenced the physicochemical and biological characteristics of extracts. Treatments based on long incubation times at 20 °C, as well as those based on short incubation times at 40 °C, resulted in extracts with increased suppressive properties. However, extracts derived from CEP-3 protocol were characterized by high phenolic and TOC content, low functional biodiversity, and a more discreet antagonistic capacity. Therefore, the development and optimization of suitable extraction protocols could lead to compost extracts with increased phytoprotective capacities, thus becoming an effective and sustainable alternative to chemical inputs.

Paleoenvironmental characterization of the Ponta Grossa formation (Devonian) at the northwest border of the Paraná Basin, Brazil

Previously uncharacterized Devonian shale samples from the Ponta Grossa Formation (Paraná Basin) were collected from three outcrops in Mato Grosso (DA01 and DA02) and Mato Grosso do Sul (PG02) along the northwest of the basin. The depositional paleoenvironment and hydrocarbon source potential were assessed using Rock-Eval pyrolysis, saturate and aromatic biomarkers, and carbon isotope data. For the DA01 shales, poor total organic carbon (TOC) contents and low hydrogen index (HI) indicate a high predominance of type III kerogen. The biomarker analysis results suggest a transitional depositional paleoenvironment with a greater contribution of continental than marine organic matter, pointing to a regressive marine event. In DA02, the TOC contents ranged from 0.24 to 4.46%, with kerogen types II and III, S2 ranging from poor to good, HI ranging from 78 to 410 mg HC/g TOC, and biomarkers indicating a marine transitional environment and transgressive marine event. Shales from outcrop PG02 had the highest TOC contents, particularly at the top, with predominantly kerogen types II and III, S2 varying from low to excellent, and HI ranging from 100 to 350 mg HC/g TOC. Two types of paleoenvironments were identified based on the biomarkers of PG02: regressive at the base and transgressive in the middle and top of the outcrop. The DA01 outcrop was located at a more proximal position to the coast, the DA02 were at more distal position, while PG02 was located in a transitional position between DA01 and DA02. Ratios of C29αββ/(αββ+ααα) and 20S/(20S + 20R) steranes, the presence of unresolved complex mixture, and a higher proportion of polar compounds to the detriment of saturated and aromatic compounds indicated that the shales were immature for the generation of hydrocarbons. This study represents the first broad organic geochemical evaluation of the northwest Ponta Grossa Formation.

Depositional settings of the Eocene Suhostrel bituminous coal, SW Bulgaria, inferred from organic petrology and molecular proxies

The paper reports results from organic petrological and geochemical characterization of Suhostrel bituminous coal deposit, SW Bulgaria. Three high ash (21–47 wt%) and thin (up to 0.6 m) coal seams, representing splits from a locally compact coal bed (∼1.8 m thick), were formed around the southern and south-western rim of the Padesh Graben during its Eocene evolution. High total organic carbon contents (avg. ∼86 wt%, daf), vitrinite reflectance (avg. Ro = 1.05%) and Tmax (avg. 457 °C) argue for high vol. A/medium vol. bituminous coalification rank. Maceral composition is dominated by vitrinite (avg. ∼ 95 vol%), accompanied by small amounts of liptinite (avg. ∼ 5 vol%). Apart from few occurrences of funginite, inertinite is not present. The organic matter is mostly represented by detrovitrinite and collotelinite, the latter generally occurring in the form of gelified leaf-derived tissues. Liptinite is mainly represented by cutinite and microsporinite and display no evidence for organic matter contribution from algae. Low TPI and VI indices indicate peat formation from vegetation with poor preservation potential. Insignificant contents of gelovitrinite, coupled with high GI indices denote greater importance of the geochemical processes for the gelification of the organic matter. Striking predominance of mid- and long-chain n-alkanes argues for peat formation from a mixture of aquatic macrophytes and terrestrial vegetational communities. Presence of oleanane, albeit in low concentration, provides robust argument for contribution from angiosperm plants, whereas absence of resin-derived sesqui- and diterpenoid hydrocarbons denotes insignificant role of conifers during peat accumulation. Reconstructed depositional settings indicate organic matter accumulation under marginal aquatic, marsh/fen-type peatland with meso- to rheotrophic characteristics (GWI > 0.5). High mineral matter contents, coupled with the abundance of siliciclastic grains, imply hydrologically active environment. Plant remains deposited under oxidizing conditions (Pr/Ph = 1.5–2.5), but because of high water table were likely subjected to frequent (perhaps seasonal) changes in Eh settings. The resultant limited aerobic biodegradation of the plant remains is evidenced by low concentrations of hopanoid biomarkers and their diagenetic degradation products – drimane-type sesquiterpenoids. Post-depositional marine influence and downward infiltrating sulfate-rich waters is considered responsible for the presence of dibenzothiophene derivatives in Suhostrel coal.All molecular maturity-related ratios (e.g. CPI, Ts/(Ts + Tm), βα/(αβ + βα) C30 hopanes, ββ/(ββ + αα) C29 steranes, MPI-1, etc.) are fully compatible with the bulk geochemical parameters (e.g., TOC, Tmax, Ro) and confirm the bituminous coalification rank of the coal. Based on this data, temperatures of ∼130–140 °C were likely reached during burial of the sediments in a zone, characterized by high thermal influx (geothermal gradient ∼43–47 °C/km).

Sheepfolds induce significant increase of seasonal CO2, CH4 and N2O emissions in temperate steppes of Inner Mongolia

IntroductionThe changes in grassland management and grassland types are strongly linked with dynamics in soil physico-chemical properties and vegetation attributes, with important implications for carbon/nitrogen cycling and greenhouse gas (GHG) fluxes. However, the seasonal variations of GHG emissions from sheepfolds, and the underlying biotic and abiotic drivers affecting GHG exchanges across different steppe and management types remain largely unclear.MethodsTaking the Inner Mongolian grassland as a model system, we measured the fluxes of CO2, CH4 and N2O, as well as soil and vegetation variables, in three contrasting grassland management areas (grazing, sheepfold, enclosure) and in three representative (wet typical, dry typical, desert) grassland ecosystems in July, September and November 2016.ResultsOur results showed that: (1) GHG fluxes were mostly higher in the plant growing season (July and September) than in the nongrowing season (November); sheepfold area had significantly higher GHG emissions (in July and mean over the season) than enclosed and grazing areas, with the effects being most pronounced in dry typical steppe. (2) The high GHG emissions in dry typical steppe were closely associated with the interactions among favorable soil temperature and moisture, high total organic carbon (TOC) content, and high aboveground biomass. The important predictors for CO2 emission were soil TOC and pH, whereas that for CH4 and N2O emissions were soil temperature and moisture content, in sheepfold areas. (3) Three GHG emissions were negatively affected by species richness across all steppe and management types, which might be a consequence of indirect effects through the changes in soil TOC and total nitrogen (TN).DiscussionThese results indicate that sheepfold areas are intensive hotspot sources of GHGs in the steppes, and it is of great importance to help to account GHG emissions and develop mitigation strategies for sheepfold areas for sustainable grassland management in the natural steppe based pastoral production ecosystems.

Geochemical characteristics of coal measure source rocks of Upper Carboniferous Benxi Formation in Daning-Jixian area, eastern margin of Ordos Basin

The Carboniferous-Permian in Daning-Jixian area of Ordos Basin is a typical marine-continental transitional facies deposit, with many sets of organic-rich shales, and abundant shale gas resources. Through unremitting exploration, the shale gas exploration of Shanxi Formation has achieved a major breakthrough, but the related research of Benxi Formation remains to be further studied. Therefore, based on the test and analysis of total organic carbon (TOC) content, rock pyrolysis, kerogen stable carbon isotope (δ13CPDB) and vitrinite reflectance (Ro), this paper systematically discusses the geochemical characteristics and hydrocarbon generation potential of Benxi Formation coal-bearing source rocks, in order to grasp their exploration prospect. The results show that the shale of Benxi Formation has high TOC content (averaging 5.13%) and high Ro (averaging 2.75%), showing good hydrocarbon generation potential. In addition, shale, coal and sandstone are interbedded and superimposed, which can realize the combined production of three gases, improve the yield and benefit, and further promote the exploration process of marine-continental transitional shale gas.

Burn Severity and Its Impact on Soil Properties: A Study of the 2016 Erskine Fire in the Southern Sierra Nevada, California

Wildfires can cause debris flow events in affected areas due to changes in the physical properties of burned soils, which are linked to burn severity. A study in California’s Sierra Nevada explored the impact of burn severity on soil physical properties using various tests. Results showed that higher burn-severity soils had higher total organic carbon content and liquid limit, and the plastic limit was also higher. The plasticity index was highest among low burn-severity soils, and high burn-severity soils had lower smectite and kaolinite/chlorite abundances compared to lower burn-severity soils. Grain size distribution and shear strength were not significantly impacted by burn severity. The study suggests that total organic carbon content is the most significant factor affecting the physical and mechanical properties of soil. These findings may help assess debris flow hazards in burned areas and highlight the need for further research on the effects of wildfires on soil properties and their contribution to debris flow events.

The Environment Effect on Differential Organic Matter Enrichment of the Lower Cambrian Shale in the Tarim Basin, NW China

AbstractThe early Cambrian shale is of an important petroleum source rock on the Earth. Due to few drilling data and poor seismic data, the organic matter enrichment of the Lower Cambrian shale is still an enigma in the Tarim Basin. Total organic carbon (TOC), major and trace element data of shale samples from five boreholes were experimented to decipher the mechanism of the organic matter enrichment in the early Cambrian shales. The results show that the shales deposited in the western restricted intraplatform have much higher TOC contents (3.2%–19.8%, on average 11.0%) than those from the eastern basin (2.2%–10.2%, on average 4.5%). The paleoproductivity proxies (Ba, Ba/Al, P/Al) in the western restricted platform are much higher than those in the eastern basin. The trace element indicators such as V/Cr, Ni/Co, Mo‐TOC and MoEF‐UEF suggest an anoxic environment across the basin, but a more restricted environment in the western intraplatform. It is concluded that the paleoproductivity rather than anoxic condition and hydrothermal activity have resulted in the differentiation of the organic matter enrichment from the western intraplatform to eastern basin in the early Cambrian shales, and restricted environment is favorable for paleoproductivity and preservation of organic matter.

High-quality source rocks in an underexplored basin: The upper Carboniferous–Permian succession in the Zaysan Basin (Kazakhstan)

The Zaysan Basin is a petroliferous basin in eastern Kazakhstan. Its upper Carboniferous–Permian sedimentary succession outcrops in the Kenderlyk Trough and includes, from base to top, the Kenderlyk, Karaungur, Tarancha, and Maychat formations, which contain 5 to 65 m-thick oil shale deposits—the principal subject of this study. Results from 49 outcrop samples show high total organic carbon (TOC) contents (1.2–21%, mean 7.8%), extract yields (1.2–15 mg HC/g rock, mean 6.8 mg HC/g rock), and ultimate expulsion potential (UEP; up to 23 mmboe/km2), making these oil shale deposits among the best source rocks in the world. Sedimentological, organic geochemical and organic petrographic analyses suggest an overall evolution of the basin from a deep/semi-deep lake during deposition of the Kenderlyk, Karaungur, and Tarancha formations to a deltaic setting during deposition of the Maychat Formation. The organic material in the Kenderlyk, Karaungur, and Tarancha formations is predominately composed of Type I and mixed I/III kerogens with high hydrocarbon generation potential (S1 + S2; up to 172.8 mg HC/g rock), high hydrogen index (HI; up to 838 mg HC/g TOC), and low oxygen index (OI; < 60 mg CO2/g TOC). The sporadic influx of terrigenous organic matter resulted in layers with lower HI and higher OI index, and increased inertinite and vitrinite contents. The Maychat Formation includes hydrogen-poor Type III kerogen with low HI (< 200 mg HC/g TOC) and high OI index (> 60 mg CO2/g TOC) in the Kenderlyk Trough. While a highly oxidizing environment during deposition of the Maychat Formation is postulated for the Kenderlyk Trough, it is likely that oxygen-depleted conditions in the depocenter of the Zaysan Basin favored accumulation of Type I kerogen. The oil-source correlation shows that the produced oils are chemically distinct from the source rock extracts of the Kenderlyk, Karaungur, and Tarancha formations. We propose that these source rocks are “massive”, where the retention of generated oil is too high, causing the hydrocarbons to “bleed” from the source rock's edge only. The expelled oil has probably charged the lower Permian deposits, which have not yet been explored. Regional geological cross-sections and seismic lines allow for selecting sweet spots, characterized by high TOC, yields, and temperatures needed for oil generation and unconventional hydrocarbon resource development.This study provides the play concept and a risk assessment analogue for tectonically and magmatically dynamic settings and basins with multiple organic rich strata. Furthermore, the results and proposed concepts may play a significant role in future petroleum exploration and development activities in the Zaysan Basin. In addition, this highly multidisciplinary study emphasizes the significance of integrating several data sources and weighing contradictory information to get the most reasonable conclusion.

Multi-analytical Approach (FTIR, XRF, XRD, GC-MS) Characterization of Source Rocks from the Anza Basin, Kenya

Potential source rocks from three wells (Chalbi-3, Sirius-1, and Ndovu-1) in the Anza Basin of northeastern Kenya were analyzed using elemental carbon-hydrogen-nitrogen analysis, and mineralogical characterization by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM-EDX).)&lt;strong&gt;. &lt;/strong&gt;A Gas Chromatography-Mass Spectrometry (GC-MS) analysis of the saturated aliphatic biomarkers was carried out to illuminate the organic matter source. The analytical results revealed that the source rocks have moderate to high total organic carbon (%TOC) content, suggesting conditions in the Basin favor organic production and preservation. The Hydrogen Indices (HI: 0.19–0.60 atomic ratios) typify a predominance of mixed type II/III (oil/gas-prone) with more type III (gas-prone) and less II (oil-prone) kerogens. The FTIR, XRF, and XRD results reveal that the studied source rock samples comprised mainly of quartz, followed by silicate-clay minerals and calcite minerals. The solvent-extractable organic matter investigation revealed biomarker distributions of n-alkanes and isoprenoids (pristane/ phytane ratios), suggesting that source rocks were derived from algae and bacteria deposited weakly anoxic and low toxic environmental conditions, with minimal contribution from terrestrial organic matter sources. Consequently, all three wells have hydrocarbon generation potential, particularly Ndovu-1, which displays good organic matter content to produce oil and gas. The hydrocarbon potential is excellent and capable of making expulsions of oil and/or gas from the wells at sufficient depths.

Pore Fractal Characteristics of Lacustrine Shale of Upper Cretaceous Nenjiang Formation from the Songliao Basin, NE China

The evaluation of pore characteristics in lacustrine shales is of significance for shale oil and gas exploration in terrestrial sedimentary basins. However, due to the complex depositional environments, there have been few reports on the quantitative evaluation of lacustrine pores. In this study, we carry out experiments, e.g., scanning electron microscopy (SEM), nitrogen adsorption, X-ray diffraction (XRD), and rock pyrolysis, on sixteen lacustrine shale core samples of the Late Cretaceous Nenjiang Formation (K2n1+2) in the Songliao Basin. At the same time, we bring in the fractal dimensions to quantitatively characterize the pores of shale samples in K2n1+2. The kerogen type of shales in the Nenjiang Formation is mainly type Ⅰ. The shales of K2n1+2 develop a large number of mineral intergranular pores and a small number of organic pores and mineral intragranular pores. The primary minerals of the samples are clay minerals and quartz. The total organic carbon (TOC) has a correlation with the pore physical parameters, whereas the content of minerals does not correlate significantly with pore development. The average pore fractal dimensions obtained from the analysis of nitrogen adsorption data are D1 = 2.5308 and D2 = 2.6557. There is a significant negative correlation between the fractal dimensions (D1 and D2) and the TOC content: the lower the fractal dimensions, the higher the TOC, and the more the free oil. In low maturity shales with higher TOC content, due to hydrocarbon generation, larger pores with a regular shape (small fractal parameters) can be produced, which can store more free oil. The shale reservoir evaluation of K2n1+2 in the Songliao Basin should be focused on shales with higher TOC and lower pore fractal parameters. Our work provides a reference for the shale oil evaluation of K2n1+2 in the Songliao Basin and complements studies on lacustrine pore characteristics.

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.

Microstructures of continental organic-rich shale and its adjacent siltstone and carbonate rocks—An example from the Lucaogou Formation, Jimusar Sag, Junggar Basin, NW China

To investigate pore structure of a continental shale and its adjacent carbonate rock and siltstone, 93 shales, 30 carbonate rocks and 10 siltstone samples from the Lucaogou Formation, Junggar Basin were examined by using low-pressure gas (N2 and CO2) adsorption, mercury intrusion and FE-SEM. Total organic carbon (TOC) evaluation, Rock-Eval pyrolysis, and X-ray diffraction (XRD) were used to characterize the organic geochemical parameters and mineralogical parameters of studied samples. The results showed that the free hydrocarbon S1 decreased in the order of siltstone, shale and carbonate rock, which is in accordance with the macropore surface area results. The carbonate rock has the highest TOC content and pyrolytic hydrocarbon S2, followed by shale and siltstone. The inter-crystalline pores of clays and intergranular pores exist widely in shale and siltstone, and a large number of dissolution pores were developed within shale, carbonate rock and siltstone. The smectite, illite and illite/smectite together control the micropores and mesopores of shale, more macropores and mesopores are present in the low TOC shale, carbonate rock and siltstone, however, the promotion effect of TOC on micropores of carbonate rock is stronger than clay. The more macropores of shale and siltstone, causes higher amounts free hydrocarbon S1. In addition, the free hydrocarbon S1 is higher at low fractal dimension D1 (D1＜2.3) and high fractal dimension D2 (D2＞2.6) of mesopore in shale. The free hydrocarbon S1 of the massive siltstone and thin layered shale are significantly higher than that of other types, which may be related to that high macropore volume and low mesopore fractal dimension D1 of the thin layered shale and the high macropore surface area of massive siltstone.

Geochemical conditions regulating chromium preservation in marine sediments

In the marine sediment record, concentrations and isotope ratios of chromium (Cr) can be used to reconstruct ocean biogeochemical conditions. These reconstructions rely on a detailed understanding of the chemical pathways that Cr undergoes as it is transferred from the water column to the sediment record. We examined Cr concentrations in marine pore fluids and sediments from six continental margin sites, which can be grouped into two basic environments: (1) sites where sediments are oxygenated and rich in solid phase Mn (herein termed oxic), and (2) sites where sediments are organic C (Corg)-rich and oxygen is depleted (anoxic). We found Cr concentrations to be lower (maximum of 12 nM in pore fluids and 124 ppm sediment solid phase) at oxic sites compared with anoxic sites (maximum of 77 nM and 184 ppm). Our findings confirm previously published interpretations of dissolved Cr in pore fluids (Brumsack and Gieskes, 1983; Shaw et al., 1990). In oxic surface sediments, particulate Cr(III) can be oxidised by Mn oxides, which leads to elevated concentrations of dissolved Cr co-occurring at the same depth as elevated Mn concentrations in the sediment. Under these oxidising conditions, down-core sediments contain relatively low solid-phase Cr concentrations. In oxic sediments, Cr speciation reveals that most of the pore fluid Cr is in the Cr(VI) state. At the site where Mn oxide-rich sediments rest below an oxic water column, oxidative loss of Cr from the sediment to the bottom water leads to the lowest estimated Cr burial efficiency of the sites examined here. Under anoxic Corg-rich conditions, both pore fluids and sediment solid phases contain high Cr concentrations, with 40–80% of dissolved pore fluid Cr present as Cr(III). This enrichment of Cr appears to be tightly linked to the presence of high total organic carbon (TOC) content and scavenging of Cr by (organic) particles in the water column. Combined, these data highlight the strong dependence of Cr on both sedimentary redox conditions as well as biological productivity. Based on the data from modern continental margin sediments, we propose that Cr concentrations and isotope compositions of the authigenic sediment fraction may record a combination of redox conditions and biological productivity in the water column. If confirmed by Cr isotope analyses, these findings will add support for the notion that Cr may serve as a proxy for ocean biological and chemical sedimentological conditions. Thus, careful assessment of the impact of organic matter on Cr is required for reconstructions of redox conditions with sedimentary records.

Lithofacies Characteristics and Their Effects on Shale Oil Enrichment: A Case Study from Shahejie Formation of the Qibei Sag, Bohai Bay Basin, China

The lithology and lithofacies assemblage of shale are highly complex and heterogeneous in the continental shale formations due to rapid changes in the sedimentary environment and source material, complicating the evaluation of shale oil enrichment areas, such as the member 3 of the Shahejie Formation in the Qikou sag, Bohaibay Basin, China. We used core observations and descriptions of well F39 × 1 and performed X-ray diffraction, scanning electron microscopy, nitrogen adsorption analysis, and nuclear magnetic resonance analysis to investigate the shale lithofacies characteristics and types in member 3 of the Shahejie Formation, and their effects on shale oil enrichment. The results showed the following. (1) The lithofacies are divided into four types according to the shale’s laminar structure, lithological characteristics, mineral composition, and organic matter content: thin laminar shale, thick laminar shale, massive mudstone, and argillaceous siltstone. These are divided into six subcategories. Each lithofacies has thin vertical layers. (2) The thin and thick laminar shale layers have favorable conditions for shale oil enrichment, such as a high total organic carbon content (TOC) (1.1–1.6%), many micropores (with the diameter of 0.5–2 µm) and fissures, a high residual hydrocarbon content (1.0–2.3 mg/g), and a good source-reservoir relationship, making them suitable for shale oil exploration. (3) The degree of lamina development influences the organic matter and residual hydrocarbon contents, the number of micropores, and the degree of shale oil enrichment. The semi-deep and deep lake facies are favorable areas for shale oil development.

Sedimentological observations and geochemical characteristics of paleo-tsunami deposits along the east coast of Sri Lanka in the Indian Ocean

The 2004 Indian Ocean tsunami caused 230,000 fatalities and massive physical damage along the shorelines of the Indian Ocean. Holocene sedimentary archives along the coastline of Sri Lanka can provide evidence for similar past events. The objective of our current study is to investigate the sedimentological and geochemical characteristics of Sri Lankan paleo-tsunami deposits, and their chronology. Sediment samples were collected down to 10 m depth using core drilling at a sheltered site (low-lying swale depression) in Koddiyar Bay (Trincomalee) on the east coast of Sri Lanka. Visual stratigraphic observations and grain size analyses were carried out to identify lithological changes. Bulk sediments, shells, and wood fragments were used for 14C age dating. Seven sand layers were identified throughout the core in this marshy wetland. Sedimentological observations and geomorphological features show that sheltered/protected swales of the inner bay have a high potential for preservation of paleo-tsunami sediments. Three possible paleo-tsunami deposits were identified earlier than 700 years, ca. 2000 years, and ca. 2700 years. These paleo-tsunami sand layers exhibited relatively high total organic carbon (TOC) contents due to admixture of continental shelf and lagoon sediments. Analysis of paleo-tsunami records suggests that the Andaman Sea is vulnerable to both ‘low-risk and high-frequency’ and ‘high-risk and low-frequency’ type tsunamis, whereas other areas in the Indian Ocean are more prone to destructive and low-frequency type tsunamis. This study contributes to improving tsunami risk assessment and mitigation strategies along the east coast of Sri Lanka.

Metric, kilometric and large-scale coherence of metamorphic conditions from graphitic phyllite in the Upper Lesser Himalaya of Nepal: Contribution to the estimation of carbon stored during Himalayan orogeny

Besides carbonates, carbonaceous material in meta-sediments represents an important carbon reservoir on Earth. In orogenic belts, carbon can be stored as graphite, a contribution to the global carbon cycle that remains insufficiently constrained. Here, we focus on the Himalayan belt where moderately crystalline graphite is found at the footwall of the Main Central Thrust. Using various analytical methods, we investigate metamorphic conditions of graphite at increasing spatial scales in the Upper Lesser Himalayan Sequence (upper-LHS) phyllites of Nepal. First, we develop and validate a new SEM-BSE technique to measure total organic carbon (TOC) content. Graphite shows mostly inter- and intra-granular flaky crystals associated with carbonates. Elemental carbon analyses yield large range of TOC content in phyllites with a mean of 3.7 ± 0.4 vol.%. Inferred from micro-Raman spectroscopy, peak metamorphic temperature (peak-T) ranges from 519 ± 9 to 590 ± 8 °C. We observe a significant coherence of metamorphic conditions of graphitic phyllites at metric, decametric and kilometric scales using extensive data obtained in the Upper Trisuli valley, Central Nepal (n = 77), and, at larger spatial scales, using data from Far-Western to Eastern Nepal (n = 67). Mean carbon isotopic composition (δ13C) of −26.3 ± 0.1 ‰ indicates biogenic source. Less negative δ13C values, also reported in other orogenic belts, suggest interaction with pre-existing sedimentary carbonates, probably related to metamorphic isotopic re-equilibration. First-order extrapolations at the upper-LHS scale suggest meta-sediment organic carbon may represent up to 20 % of the total amount of carbon stored in orogens. Central Nepal segment, with higher TOC content and peak-T, is also characterized by the largest current metamorphic carbon dioxide (CO2) emissions of the chain. This hardly coincidental association, at the scale of the Himalayan belt, between organic carbon stored as graphite and mobile inorganic carbon released as CO2, represents an important clue to investigate further for global carbon budget estimations.