Low Total Organic Carbon Content Research Articles

Organic matter-driven electrical resistivity of immature lacustrine oil-prone shales

Organic matter (OM) and minerals are major particle components in lacustrine organic-rich shales. Their association and distribution control the development of the primary pore space. The response of OM-driven conduction by modifying the pore-space volume and structure in organic-rich shales of the virgin zone is still unclear. Based on a detailed study of geochemical, mineralogical, and geophysical properties from immature lacustrine oil-prone shales of the Songliao Basin (Northeast China), we have observed a novel continuous variation of electrical resistivity driven by large ranges of total-organic-carbon (TOC) content (0.64–24.51 wt%). The reduced resistivity at low TOC content ([Formula: see text]) and then enhanced resistivity at high TOC content ([Formula: see text]) are present in our immature shales. These variations in electrical resistivity are confirmed by fluid (S1) and solid organic compounds (S2). Furthermore, clay and detrital minerals in shales contribute to the variation of electrical resistivity as well as OMs at low and high TOC content. The electrical resistivity of shales is closely related to the pore-space volume and structure for the electrical flow pathway. Two resistivity trends are highlighted by pore parameters such as the pore volume, throat/pore ratio, pore diameter, and bulk density. Although reduced amounts and the arrangement of large pores for low TOC content cannot decrease the conduction, the enhanced additional clay conduction and low OM concentration reduce the resistivity of shales. Moreover, increased amounts of nonconductive fluid and solid organic compounds and the effect of OM filling on pore space during high TOC content enhance the resistivity of shales. Thus, modified minerals and pore space driven by various OMs affect the electrical resistivity of immature shales. These results improve the understanding of OM-driven conduction in shales and contribute to the evaluation of source rocks using a well-log method.

Signature of modern glacial lake outburst floods in fjord sediments (Baker River, southern Chile)

AbstractGlacial Lake Outburst Floods (GLOFs) constitute a major hazard in glacierized regions. They are particularly pronounced in the Baker River watershed (Chilean Patagonia, 48°S), where 23 events occurred between 2008 and 2020. Although GLOF deposits have previously been studied in lake settings, how modern GLOFs are recorded in fjord sediments remains mostly unknown. To address this issue, ten sediment cores collected in the fjord immediately downstream of the Baker River (Martínez Channel) were investigated and compared to the recent GLOF history of the river. Results show that sediments accumulate at 2.0 to 3.4 cm year−1 and that GLOF deposits can be distinguished from background sediments by their finer grain size (5.98 ± 0.82 μm) and lower organic carbon content (0.31 ± 0.06%), reflecting the release and transport in suspension of high amounts of glacial rock flour during GLOFs. Although 21 GLOFs from Cachet 2 Lake occurred between 2008 and 2017, the first events left a stronger imprint in the sediment, suggesting that more sediment of glacial origin was released during those initial events, possibly due to lake‐bed erosion. An older GLOF deposit was tentatively linked to the outburst of Las Lengas Lake in 1988. The sediment cores also contain fine‐grained turbidites, especially in the prodelta area. These turbidites confirm recent channel activity, but most of them seem to have been triggered by processes other than GLOFs. Overall, the results of this study suggest that GLOF deposits are distinct from typical flood turbidites. They are best identified by their low grain size and total organic carbon content, and best archived on the delta slope, away from any submarine channel influence. Finally, these results highlight the potential of fjord sediment archives to establish pre‐historical GLOF records and ultimately improve GLOF hazard assessments.

Preparation of a rice straw-based green separation layer for efficient and persistent oil-in-water emulsion separation

Inefficiency, high cost, and complex operation have emerged as shackles for large-scale separate oil-in-water emulsion. Herein, a low-cost and eco-friendly separation layer with a rough structure and rich anionic groups was fabricated from rice straw (RS) via a simple acid-base treatment and slight squeeze process. The separation layer’s morphology, composition, and wettability were investigated. It was then employed to separate oil-in-water emulsion. The RS after acid and alkali treatment (A1A2-RS) exhibited a clear fiber structure and abundant humps, which made the separation layer superwettable and highly electronegative (−26.55 mV). The overlapped and intertwined A1A2-RS layer structure owned a superior performance for hexadecyl-trimethyl-ammonium-bromide (CTAB) adsorption and tiny oil interception. As a result, the separation layer had stable fluxes (>500 LMH) for multiple CTAB-stabilized emulsions and the obtained filtrates performed low total organic carbon (TOC) contents (<30 mg/L). In addition, the A1A2-RS layer had excellent renewability (10 cycles/ 200 mL) and the flux could be substantially recovered merely by aqueous wash. Moreover, filtrate analysis showed that the A1A2-RS layer had a good effect on actual emulsion treatment with a TOC removal rate of 89.56%.

Influence of a chromium reduction treatment in the analysis of organic carbon isotopes in Quaternary coastal plain incised valley sediments

This paper reports bulk organic carbon isotope data from a laboratory study designed to evaluate whether acid chromium reduction (ACR), commonly used to prepare rock/sediment samples for sulfur isotope analysis, significantly fractionates bulk organic carbon isotopes compared with the standard decarbonation (SD) method in Quaternary coastal plain incised valley sediments. In this study, 36 sediment samples obtained from a Quaternary sediment core from the Qiantang River incised valley on the East China coastal plain were tested. The results show differences in the δ13Corg values between the ACR and SD methods for Quaternary incised valley sediments ranging from – 0.97‰ to +0.34‰, with most values (86%) showing a negative bias. Approximately half of the samples yield δ13Corg values that differ by ≥ ± 0.3‰. The carbonate and total organic carbon (TOC) contents of the sediment samples have no significant effects on the bulk organic carbon isotopes derived by ACR. There is also no obvious correlation between the organic carbon loss by ACR and the difference of the δ13Corg values. We believe that ACR loses more 13C-enriched labile organic carbon and/or volatile organic compounds (VOCs) than SD, which leads to a negative bias in the δ13Corg values. The mineral composition was speculated to be related to a significant positive bias in the δ13Corg values by ACR observed in two particular samples and requires further study. Consequently, ACR should be used with caution for sediments containing primarily young labile organic carbon with medium to high TOC contents (TOC ≥ 0.5 wt %), as it may cause a large offset of the δ13Corg values compared with SD. However, ACR may be used for sediments containing organic matter primarily from terrestrial plants with relatively low TOC contents (TOC < 0.5 wt %).

Novel watermass reconstruction in the Early Mississippian Appalachian Seaway based on integrated proxy records of redox and salinity

Redox state and salinity are fundamental properties of watermasses, and in modern environments, detailed analysis of spatial variability in redox and salinity is possible through direct measurement. Watermass reconstruction is difficult in deep-time systems, however, because the sedimentary record of ancient watermasses is often incomplete or difficult to access on spatial scales large enough to permit basin-scale reconstruction. During the Middle Devonian to Early Mississippian periods, the Appalachian Basin of eastern North America was characterized by a variably restricted watermass, leading to extensive deposition of organic-rich black shale associated with the regional development of bottom-water anoxia. Watermass reconstruction during deposition of these units has previously been attempted, but only on small spatial scales, with most studies focused on trace element proxies applied to individual study sections. Here, we focus instead on a broader geographic reconstruction of watermasses in the Lower Mississippian (Tournaisian) Sunbury Shale and present a new geochemical dataset that spans five drill cores across a basin transect from northeastern Ohio to southern Kentucky (USA). Iron speciation and trace metal abundances reveal a strong paleoredox gradient that can be related to water depth and basin hydrography. In the northeastern-most core of Ohio, iron speciation reveals largely ambiguous redox conditions; however, oxic conditions are potentially indicated by very low total organic carbon and trace metal contents. These oxygenated conditions were likely related to proximity to the Catskill deltaic wedge complex that fed freshwater into the basin and lowered the position of the regional pycnocline. In contrast, in central and southern Ohio, paleoredox indicators reveal the development of ferruginous conditions associated with rapid deepening to the south into the central axis of the Appalachian Basin. Further to the south in Kentucky, our data reveal dominantly euxinic conditions, which developed as environments shoaled towards the basin-bounding Cumberland Sill. We also investigated paleosalinity across the Appalachian Basin using strontium/barium (Sr/Ba) ratios. Our data reveal a subtle paleosalinity gradient from low-brackish (i.e., close to freshwater) conditions close to the Catskill Delta complex to increasingly brackish conditions to the southwest. This trend was disrupted by large paleosalinity variation across a shallow mixing zone in the southwestern-most core. Lastly, strong enrichments of redox-sensitive trace metals in both ferruginous and euxinic environments are parsimoniously attributed to the operation of an iron-manganese particulate shuttle that continuously supplied oxide-sensitive metals to the Appalachian Basin. Ultimately, this study is among the first to present detailed watermass reconstruction in an ancient epeiric sea using both redox and salinity proxies applied over a large lateral transect. We suggest that a similar approach may be useful in other deep-time systems of geobiological, paleoclimatic, and paleoceanographic significance.

Earthworm biomass and population structure are negatively associated with changes in organic residue nitrogen concentration during vermicomposting

Vermicomposting is an efficient and environmentally friendly technology to dispose of agricultural organic residues. The efficiency of organic residue decomposition during vermicomposting is directly affected by the biomass and population structure of earthworms. In this study, we investigated how the earthworm biomass and population structure responded to changes in the physicochemical properties of six types of organic residue (cattle dung, herbal waste, rice straw, soybean straw, garden waste, and tea residues) during vermicomposting. Each type of organic residues was placed in a pot with earthworms Eisenia fetida, and the physicochemical properties of the organic residues and earthworm growth dynamics were recorded at 0, 30, 60, and 90 d of vermicomposting. The biomass and population structure of earthworms were stable or increased in rice straw, garden waste, and cattle dung within 60 d of vermicomposting, whereas in tea residues and herb waste, very little earthworm activity (3 adults and 2 cocoons) was recorded on day 30. Among the physicochemical parameters, the substrate C/N ratio was negatively correlated with earthworm growth dynamics. Decomposing organic residues showed higher NH4+ -N and NO3– -N concentrations but a lower total organic carbon content, which negatively affected earthworm growth and reproduction. We recommend that chemical properties of vermicomposting systems should be monitored regularly. At the threshold levels of decomposing organic residue NH4+ -N and NO3– -N concentrations, earthworms should be removed and the vermicompost can be harvested. Small- and large-scale farmers thus need to monitor the physicochemical properties of vermicompost to sustain active earthworm populations.

Different accumulation mechanisms of organic matter in Cambrian sedimentary successions in the western and northeastern margins of the Tarim Basin, NW China

Lower Cambrian black shales are widely distributed in the Tarim Basin, but those in the Keping area in the west have a high total organic carbon (TOC) content different from those in the Kuluketage area in the northeast of the basin. The Cambrian Yuertusi Formation in the Shiairike section (Keping) and the Xishanbulake Formation in the Yaerdangshan section (Kuluketage) are time-equivalent and can be correlated. Black shales of the Yuertusi Formation have lower δ13CKer values (<−34‰) than the Xishanbulake Formations (δ13CKer values > − 4‰). These values suggest that black shales in the Yuertusi Formation were dominated by benthic algae, whereas rocks in the Xishanbulake Formation contain substantial amounts of planktonic algae. Analyses of trace elements demonstrate an anoxic depositional environment, with high productivity in both the Yuertusi and Xishanbulake Formations. The enhanced productivity was promoted by upwelling fluids and hydrothermal events, leading to the enrichment of nutrient elements such as barium, cadmium, copper, phosphorus, nickel, vanadium, and zinc. More terrigenous inorganic detritus input to the Xishanbulake Formation, which led to less amounts of organic matter and degradation of planktonic algae during sedimentation, resulted in a lower TOC content than the Yuertusi Formation.

Evaluation of hydrocarbon source rock potential: Deep marine shales of Belaga Formation of Late Cretaceous-Late Eocene, Sarawak, Malaysia

In the Sibu-Tatau region of Sarawak, the Belaga Formation (Late Cretaceous- Late Eocene turbidites) is interpreted in relation to a submarine fan model deposited in the extinct ocean basin of the proto-South China Sea. A number of clastic rocks, which include sandstone, siltstone, mudstone, and black shale, define the formation. While it is considered one of the Rajang group's extensive and well-studied formations, yet no comprehensive studies have been carried out on the shales in Sarawak, Malaysia, to interpret their importance as a source rock to oil and gas fields. The present study highlights the prominent lithofacies in addition to the detailed source rock characterization of each member of Belaga Formation to discuss the potential source rock intervals present in these Cretaceous to Eocene rock units. Four major types of shales and mudstones facies were observed in the field, i.e. (i) Variegated colored shale facies (VSF), (ii) Black “Papery” shale facies (BSF), (iii) Gray to black mudstone facies (GMF), (iv) Silty/sandy shale facies (SSF). They show hemipelagic settling in a calm environment to turbiditic deposition at levees as well as distal lobe depositional environment. In this study primary geochemical studies such as Rock-Eval pyrolysis and Total Organic Carbon (TOC) analysis were used to classify the thermal maturity, organic matter richness, and kerogen type of the studied shales. Forty-six selected shale samples were studied from the formation, including samples from members of the Layar (Upper Cretaceous), Kapit (Paleocene to Early Eocene), Pelagus (Early to Middle Eocene), Metah (Middle to Late Eocene) and Bawang (Eocene). The shales of Belaga Formation typically display low to moderate source rock potential, which is obvious by low TOC contents between 0.14 wt% and 7.22 wt% (average of about 2.97 wt%) and show a wide range of maturity (immature to over mature) evident from very low to very high Tmax values ranging from 293 °C to 610 °C. The results show that the kerogen is of type IV, with some units can be potential sources of gas.

Perylene preservation in an oxidizing paleoenvironment and its limitation as a redox proxy

Sedimentary rocks from the Eocene Shahejie Formation including the third (Es3) and fourth (Es4) members, in the Dongying Depression, Bohai Bay Basin, NE China, have been analyzed to determine bulk and molecular geochemical characteristics. Sedimentary rocks from the Es4 Member contained no or trace amounts of perylene, but C32–C35 17α(H),21β(H) homohopanes were well preserved along with concomitant high concentrations of β-carotane and phytane, suggesting reducing depositional conditions. However, some samples, mostly from the Es3 Member contained abnormally high concentrations of perylene with high dibenzofuran and 9H-fluoren-9-one consistent with oxidizing environments. On the other hand, indications from the relatively high pristane/phytane (Pr/Ph) values, the absence of C32–C35 homohopanes and β-carotane are not consistent with reducing conditions in these sedimentary rocks, indicating the perylene of the investigated samples was deposited in oxidizing conditions. In combination with the sample locations in the depression, bulk geochemical characterization including low total organic carbon (TOC) contents and poor kerogen quality, and a series of molecular geochemical records, perylene has a high probability of being preserved by rapid deposition under oxidizing environments. As the conversion of quinones to perylene must ultimately involve a reduction stage, perylene preservation under oxidizing conditions seems to be formed by alternative precursors during oxidation, such as by cyclodehydrogenation of 1,1′-binaphthyl. Hence, caution should be taken when perylene is used as an indicator of strongly reducing environments.

P.354 Bloomed again: seasonality and clinical correlates in bipolar disorder

Important depositional characteristics indicative of climate change has been discovered in 50-m sedimentary succession consisting of oil shales with a marlstone and carbonate matrix of the Viséan shales from the Lower Carboniferous Emma Fiord Formation on Devon Island, Canada's Arctic Archipelago. Older Carboniferous (Tournaisian) shales, are however absent from Devon Island, but elsewhere such as Big Marsh, Nova Scotia, Tournaisian shales show no comparable evidence for climate change.The techniques used include reflected light microscopy, RockEval analyses, instrumental neutron activation analysis (INAA) for elemental concentration.Indeed, Viséan shales from the Lower part the Emma Fiord Formation from Devon Island have a marlstone matrix with abundant algal matter. The matrix also has low carbonate (32%) and total organic carbon (TOC) content between 33.3%and 55.6%. Moreover, the TOC show a positive correlation with authigenic uranium, and Mo, which suggests a high productivity due to the high rain, deeper water lacustrine depositional environment. Other Viséan oil shales from the Emma Fiord Formation on Devon Island have somewhat different characteristics from those with marlstone matrix. Indeed, they have a high (75%) carbonate content with calcified algal matter and low TOC (1.14–17.4%). A very low correlation between TOC and authigenic U, and Mo. Most importantly very high oil generation (98–413 L/t) potential of marlstones as compared to low yield of carbonate mudstones (4.1–17.0 L/t) shows a dramatic climatic shift to low precipitation and much dryer conditions. Such a drastic climatic condition has not been detected for the Tournaisian oil shales.

Geochemistry, origin and accumulation of natural gas hydrates in the Qiongdongnan Basin, South China Sea: Implications from site GMGS5-W08

The recent discovery of natural gas hydrates in the Qiongdongnan Basin (QDNB) during the fifth “China National Gas Hydrate Drilling Expedition” (GMGS5) confirms the great potential of gas hydrate resources within this basin. Based on the comprehensive geochemical investigation of the hydrate-related gas and hydrate-bearing sediments collected from the drilled site GMGS5-W08, this study determines the source and accumulation mechanism of gas hydrates. The hydrate-related gases from the site GMGS5-W08 consist of abundant methane (81.21%–97.69%) and some C2+ hydrocarbon gases (2.31%–18.79%). The stable carbon isotopic compositions of C1–C3 hydrocarbon gas components indicate that the methane has a mixed microbial and thermogenic origin (−59.5‰<δ13C1<−48.8‰), while the C2+ hydrocarbon gases are of thermogenic origin derived from coaly-type source rocks (−27.5‰<δ13C2<−22.4‰, −23.2‰<δ13C3<−20.2‰). The sediment samples collected at site GMGS5-W08 are characterized by low total organic carbon (TOC) contents (mostly < 0.4%), low hydrocarbon indices (mostly < 50 mg HC/g TOC), and low in situ temperatures (3.5°C-21°C), suggesting a poor microbial gas generation potential for the in situ uncompacted sediments. The extracts of the hydrate-bearing sediments are composed of in situ immature terrigenous organic matters and allochthonous mature hydrocarbons from the deeply buried strata. The allochthonous hydrocarbons (C14+ components) approximately originate from the low mature to mature marine mudstones (0.75%<calculated vitrinite reflectance (%Rc)<0.87%) of the Miocene Sanya and Meishan formations. The accumulation process of hydrate gas at site GMGS5-W08 can be subdivided into three stages: migration of gas and liquid hydrocarbons (stage I), gas accumulation, pressure increase, and microbial alteration (stage II), and pressure release, gas chimney and gas hydrate occurrence (stage III). The microbial methane within the gas hydrates is mainly derived from the secondary microbial methane generated in stage II and primary microbial methane generated from sediments within the zone of optimal temperature (35°C–75°C).

Scrutinizing the anthropogenic metals in the Brunei Bay sediment cores off the Malaysian waters

Brunei Bay, Borneo is a unique water ecosystem which has been conserved a wide range of wildlife. Hitherto, the rapidly increase of human activities has been denoted surround and within the bay ecosystem. Four sediment cores has been collected within Brunei Bay with the purpose of assessing metal’s pollution level and geochemical characteristics of the cores. Metals total concentration, sediment physico-chemical characteristic, and sediment sequential extraction analysis were determined in each layer of the sediment cores. Generally, all metals showed different pattern of enrichment or depletion in each core. Sediment core of station LB recorded the highest average of metals compared to other sediment cores. Brunei Bay sediment cores are dominant by silt with low total organic carbon (TOC) content. However, both parameters are not affecting the concentration of metals in the bay sediment. Largely, all metals in all sediment cores are classiffied as uncontaminated-to-moderately contaminated except for lead (Pb). During the period of 1980’s, Pb was moderately-to-strongly contaminated at station LB. It is most probably caused by the high usage of local boat and Labuan Port activities. The risk assessment code analysis has exhibited that only copper (Cu) has low risk on the benthic health in Brunei Bay sediment especially for station B5 and B7. In overall, Brunei Bay sediment shows no significant pollution caused by the metals; however, metal enrichment behavior in the sediment will increased the pollution risk on the bay ecosystem in the future.

Environmental response to volcanic activity and its effect on organic matter enrichment in the Permian Lucaogou Formation of the Malang Sag, Santanghu Basin, Northwest China

Lacustrine organic-rich fine-grained rocks are widespread in the second member of the Permian Lucaogou Formation (P2l2) in the Malang Sag, Santanghu Basin, with complex lithology and significant changes of lithofacies due to frequent volcanic activity. In this study, the mineralogical and petrological characteristics, types of organic matter (OM), and effect of volcanism on the sedimentary environment and OM accumulation were analyzed using organic, carbon, and oxygen isotopes, as well as major and trace element data. The results revealed that the P2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals, with four main rock types: tuff, dolomitic tuff, tuffaceous dolomite, and dolomite. The fine-grained rocks dominantly contain type I and II1 kerogens, which were in a low-mature stage and exhibited excellent original hydrocarbon generative potential. When volcanic activity was strong, large amounts of nutrients were released by the volcanic ash, which then entered the lake, in turn promoting the blooming of phytoplankton, and causing the δ13C values in the tuff to decrease. In addition, the paleotemperature decreased for a short time, and the suboxic-euxinic environment was conducive to the preservation of OM, resulting in high total organic carbon (TOC) content in the tuff. In contrast, during intermittent or weak periods of volcanic activity, the climate was hotter and drier, and evaporation resulted in a sustained increase of salinity. Consequently, the growth of algae was restricted, which brought about an increase in the δ13C values and relatively low TOC content in the dolomite. The relationships among primary productivity, redox conditions, clastic influx proxies, and TOC content illustrate that primary productivity was the most vital control on the accumulation of OM. High primary productivity was mainly driven by volcanic ash falls. Meanwhile, high salinity and suboxic-euxinic conditions were beneficial to the preservation of OM, giving rise to a high degree of OM enrichment.

Sedimentary environments and mechanism of organic matter enrichment of dark shales with low TOC in the Mesoproterozoic Cuizhuang Formation of the Ordos Basin: Evidence from petrology, organic geochemistry, and major and trace elements

Proterozoic strata have become a new target for hydrocarbon exploration worldwide. They exhibit the potential to become a resource replacement target. To provide a reference for hydrocarbon source rock evaluation, the sedimentary environment and mechanism of organic matter enrichment of dark shales in the Mesoproterozoic Cuizhuang Fm. were investigated for the first time by combining organic and inorganic research concepts. Although the total organic carbon (TOC) content in the Cuizhuang Fm. is relatively low (mean value of 0.59%), it still can be considered as a potential source rock. Thin-section observations, main and trace element contents, and biomarker compounds revealed the following: (1) the Cuizhuang Fm. was deposited in a neritic shelf sedimentary environment at a passive continental margin, and the climate was warm and humid. The hydrocarbon-generating organisms mainly comprised prokaryotes such as cyanobacteria and other bacteria. All these characteristics meet the conditions of source rock formation. (2) The low oxygen level, felsic source provenance, and absence of submarine hydrothermal activity led to low productivity, and the low productivity and low-grade hydrocarbon-generating organisms led to low TOC content in the Cuizhuang Fm. (3) The enrichment of V in the Cuizhuang Fm. is apparent, but does not reach the hyper-enrichment threshold of 500 ppm associated with euxinic conditions, revealing anoxic ferruginous conditions. Moreover, the V content is positively correlated with TOC content, indicating that organic matter enrichment was mainly controlled by the redox conditions of the bottom water. Therefore, the sedimentary areas with high productivity were related to basic source provenance and submarine hydrothermal activity, and anoxic bottom water may have promoted the development of better source rocks. This findings may represent the main direction for identifying effective source rocks in subsequent work.

Distribution and geodynamic setting of the Late Neoproterozoic– Early Cambrian hydrocarbon source rocks in the South China and Tarim Blocks

Abstract The Late Proterozoic–Early Cambrian, one of the most critical intervals in Earth’s history, is characterized by dramatic tectonic, climatic, oceanic and biological changes. In China, thick black mudstones and shales formed in intracontinental rifts during this period, associated with interglacial and postglacial phases of the Neoproterozoic global-scale glaciations. Previous studies have shown these black mudstones and shales were potential hydrocarbon source rocks that contributed to the Proterzoic-Early Paleozoic oil and gas discoveries. However, less work has been done to systematically characterize these important source rocks. In the South China Block, these excellent source rocks (~30–200 m) were discovered in the Nanhua (Cryogenian), Sinian (Ediacaran), and Lower Cambrian Systems, with total organic carbon (TOC) contents of 2–10%. In contrast, black mudstones in the Nanhua and Sinian Systems in the Tarim Basin had low TOC contents (0.3–1.5%) perhaps affected by intense weathering, whereas excellent source rocks (TOC ~ 6–18%) existed in the Cambrian Yuertusi Formation (12–80 m). Our study indicates the formation and distribution of excellent source rocks were mainly controlled by the Late Proterozoic–Early Cambrian rifts, which was supported by a decrease in thickness towards marginal areas especially for the Nanhua and Sinian Systems. The Early Cambrian source rocks occurred pervasively due to a global marine transgression and those formed under deep-water anoxic conditions at slope areas were of the best quality with TOC up to 15%. These source rocks could be focuses for future exploration in the deep or ultra-deep depths in the studied blocks as well as their peripheral areas.

A climate change event, detected in Viséan oil shales from Devon Island, Arctic Canada

Important depositional characteristics indicative of climate change has been discovered in 50-m sedimentary succession consisting of oil shales with a marlstone and carbonate matrix of the Viséan shales from the Lower Carboniferous Emma Fiord Formation on Devon Island, Canada's Arctic Archipelago. Older Carboniferous (Tournaisian) shales, are however absent from Devon Island, but elsewhere such as Big Marsh, Nova Scotia, Tournaisian shales show no comparable evidence for climate change.The techniques used include reflected light microscopy, RockEval analyses, instrumental neutron activation analysis (INAA) for elemental concentration.Indeed, Viséan shales from the Lower part the Emma Fiord Formation from Devon Island have a marlstone matrix with abundant algal matter. The matrix also has low carbonate (32%) and total organic carbon (TOC) content between 33.3%and 55.6%. Moreover, the TOC show a positive correlation with authigenic uranium, and Mo, which suggests a high productivity due to the high rain, deeper water lacustrine depositional environment. Other Viséan oil shales from the Emma Fiord Formation on Devon Island have somewhat different characteristics from those with marlstone matrix. Indeed, they have a high (75%) carbonate content with calcified algal matter and low TOC (1.14–17.4%). A very low correlation between TOC and authigenic U, and Mo. Most importantly very high oil generation (98–413 L/t) potential of marlstones as compared to low yield of carbonate mudstones (4.1–17.0 L/t) shows a dramatic climatic shift to low precipitation and much dryer conditions. Such a drastic climatic condition has not been detected for the Tournaisian oil shales.

Postglacial transgressive shales of Upper Devonian–Lower Carboniferous boundary of the Parnaíba Basin

Upper Devonian-Lower Carboniferous deposits in north Brazil show a postglacial transgressive event characterized by the formation of thick layers of gray to black shales. In the Parnaíba Basin, the shales of the Longá Formation overlap the Upper Famennian non-marine glacial deposits of the Cabeças Formation marked by a ravinement surface. The Longá shales exhibit wavy-lenticular beddings in their base, grading to gray-black shales locally interbedded with thin layers of sandstones with hummocky cross-stratification and megaripple bedding. These deposits were interpreted as having formed in shallow marine upper offshore to lower shoreface with episodic storm events. The organic matter analysis of the outcropping Longá shales using Total Organic Carbon (TOC), Rock-Eval pyrolysis, and biomarkers provide several geochemical parameters for the interpretation of the paleoenvironment, the origin of the organic matter and its thermal maturation. The Longá shales presented low TOC contents (less than 1%), very low numbers of free hydrocarbons (S1), and potential generators (S2). The maximum pyrolysis temperature (Tmax), as well as the thermal maturation biomarker parameters, indicate that the Longá shales are immature. The values of pristane/n-C17, phytane/n-C18, Hydrogen Index (HI) and Oxygen Index (OI) suggest kerogen types III and IV, which are formed from conical land leaves, needles, and plant waxes; this interpretation is also supported by total sulfur (TS) < 0.2 wt%, terrigenous/aquatic ratio (TAR), C29-sterol, hopane/sterane and Methylphenanthrene (MPs). This organic matter was deposited in an epeiric sea with shallow water, oxidizing conditions, normal salinity, and low temperatures prevalence.

Factors influencing microstructure and porosity in shales of the Wufeng-Longmaxi formations in northwestern Guizhou, China

Thickness marine shale of Paleozoic Wufeng-Longmaxi Formations is deposited in the northwestern Guizhou province, which borders on the shale gas production area in southeast Sichuan Basin. However, few studies have been performed to characterize shale gas reservoir of the Wufeng-Longmaxi Formations in this area. In this study, 80 shale samples collected from the LY1 well were designed to conduct a series of analyses, aiming to obtain microstructure features and reveal their controlling factors. Shales within the Upper Longmaxi Formation characterized by low total organic carbon (TOC) content, high clay mineral content and low porosity are considered to be unfavorable interval for shale gas exploration. Whereas, a high-quantity shale (TOC>2%, clay<40%) interval with thickness of about 27 m exists within Lower Longmaxi Formation and Wufeng Formation. Pores in the studied shales are dominated by organic matter (OM) pores, followed intraparticle (intraP) within carbonates and intercrystalline pores within pyrite framboids. OM pores could be divided into three types, as OM pores within solid bitumen, in-situ amorphous kerogen and graptolite. OM pores are extremely developed in solid bitumen and in-situ amorphous kerogen, and the former have larger diameter than the latter. Rare pores are developed in graptolite. The total porosity and specific surface area of the shales within the Lower Longmaxi Formation and Wufeng Formation are twice more than those within the Upper Longmaxi Formation. Samples with higher TOC values commonly have higher porosities and specific surface areas, suggesting TOC content determines shale physical property. Clay mineral content exhibits a negative influence on microstructure and porosity but determines water saturation. Water saturation has significant negative relationships with micro-to mesopore volumes, porosity and TOC content, illustrating water molecular stores mainly in hydrophilic pores associated with clay minerals rather than OM-hosted nanopores. The gas generation, reservoir quantity and gas content of the Longmaxi-Wufeng shales in northwestern Guizhou are also compared with those of the famous Fuling and Weiyuan districts in eastern Sichuan Basin, and we considered that the shale gas prospect in northwestern Guizhou is excellent.

Lateglacial and Holocene palaeoenvironments on Bolshevik Island (Severnaya Zemlya), Russian High Arctic

The late Quaternary climatic and environmental history of the Severnaya Zemlya Archipelago in the Russian High Arctic has been the focus of many palaeoenvironmental studies since the 1970s. However, the existing data are highly fragmentary and sometimes contradictory. New information concerning the archipelago's environmental history was derived from a 2.46‐m‐long sediment core from Lake Tvyordoe, located in the northwestern part of Bolshevik Island. The core was investigated by a multi‐proxy approach that comprises radiocarbon dating as well as lithological, granulometrical, palynological and geochemical analyses. The results provide new insights into the vegetation history as well as changes in lake level, lake‐ice cover, and water and sediment input since the Last Glacial Maximum (LGM). Fine‐grained, laminated sediments at the core base, which were dated to c. 24.2 cal. ka BP, suggest lacustrine sedimentation under a permanent ice cover with low meltwater supply. This, along with a lack of glacial and glacial‐lacustrine deposits, supports earlier assumptions that Bolshevik Island was not covered by the Barents‐Kara Ice Sheet during the LGM. Furthermore, even the Mushketov Ice Cap, the closest ice mass to Lake Tvyordoe, did not expand from the denudation plateau to the lake. During the terminal Pleistocene, starting c. 14.5 cal. ka BP, warmer and wetter climate conditions resulted in higher sediment loads. The most favourable environmental conditions prevailed on Bolshevik Island throughout the Early and Middle Holocene (c. 11.7–6.5 cal. ka BP), when low shrub tundra associations with dwarf birch, willow and alder dominated the vegetation. After c. 10.0 cal. ka BP, a shift to climate drying occurred, which, after 6.5 cal. ka BP, was accompanied by a climate cooling. The Late Holocene climate, according to pollen data (suggesting sparse lichen–moss–grass cover) and relatively low total organic carbon (TOC) contents, was rather similar to that during the Lateglacial.

Environmental predictors of deep-sea polymetallic nodule occurrence in the global ocean

Abstract Polymetallic nodules found on the abyssal plains of the oceans represent one of the slowest known geological processes, and are a source of critical and rare metals for frontier technologies. A quantitative assessment of their occurrence worldwide has been hampered by a research focus on the northeastern Pacific Ocean and the lack of a global open-access data set of nodules. We have compiled a global data set of &amp;gt;10,000 seabed nodule and control samples, and combine it with digital grids of key environmental parameters to generate a predictive machine-learning model of nodule occurrence. In order of decreasing parameter ranking, we find that nodules are associated with very low sedimentation rates (&amp;lt; 0.5 cm/k.y.), moderately high oxygen values (150 and 210 mmol/m3), lithologies of clay followed by calcareous ooze, low summer surface productivity (&amp;lt;300 mgC/m2/day), low benthic biomass concentration (&amp;lt;1 log mgC/m2), water depths &amp;gt;4500 m, and low total organic carbon content (0.3–0.5 wt%). Competing hypotheses for nodule sustention and thus continued growth on the seafloor are the removal of sediment by bottom-water currents and biological activity. Using a high-resolution eddy-resolving ocean circulation model, we find that the bottom-current speeds over nodule fields are too low (&amp;lt;5 cm/s) to remove sediment, implicating the activity of epibenthic megafauna as the most likely mechanism. Our global nodule probability map combined with the assessment of a range of environmental drivers provides an improved basis for decision and policy making in the controversial area of deep-sea exploration.