Black Carbonaceous Shale Research Articles

Mobility and environmental impact of cadmium (Cd) during weathering of carbonaceous black shales in western Hunan, China

Carbonaceous black shale generally contains high concentration of Cd, with weathering leading to Cd release to environment. In this study, the mobility of Cd during weathering was quantified using geochemical assessment on black shale from western Hunan, China. Results suggested that Cd was heterogeneously distributed in shale profiles with concentrations ranging from 0.16 to 109.9 (mg/kg). Cd distribution was heterogeneous resulting from the parent shale inheritance and the mobility of Cd during weathering. Black shales weathered to a moderate degree with Cd mobility characterized by both enrichment in and release from weathered shales. Cd enrichment in weathered shales resulted from the re-enrichment of Cd in secondary minerals formed during the initial stage of carbonate (and phosphorite) dissolution, and the secondary stage of sulfide oxidation. The release of Cd was caused by decomposition of the secondary Cd-bearing minerals. Cadmium was extensively released during pedogenesis, and Cd release mass flux was estimated to range from 1.26 to 9.50 (g/m2) with a mean of 6.60 g/m2. Thus, black shale weathering may lead to the releasing of large amount of Cd resulting in Cd contamination to local environments.

Geochemistry of Cryogenian Datangpo manganese deposits in the southeastern Yangtze Platform of South China: implications for the origin of metallogenesis and depositional environment

ABSTRACT Sedimentary manganese (Mn) deposits in the southeastern Yangtze Platform of China have been newly discovered with considerable potential for resource exploration. Such deposits formed due to dynamic changes in the global environment, including the break-up of the supercontinent Rodinia, Snowball Earth glaciation, and the Neoproterozoic Oxygenation Event. The Mn-bearing sequences are hosted in the basal Datangpo Formation and are interbedded with black carbonaceous shale in a series of graben sub-basins in the Nanhua Rift Basin. We investigated the origin of these Mn deposits by analysing the major and trace elements, total organic carbon, and stable carbon and oxygen isotopes. Positive Eu anomalies were observed, indicating that the Mn was derived from a hydrothermal system. Redox-related elements, including Ce, Mo, U, V, S, and P, indicated that the depositional environment of the Mn layers was oxidative and that Mn2+ was initially oxidized to Mn oxides. However, the host rock (i.e. black carbonaceous shale) was deposited under dysoxic – anoxic and even sulfidic water conditions. The alternating distribution of Mn ore and host rock suggested alternating redox conditions in the sub-basins. A negative correlation between the Mn content and δ 13Ccarb values (−10‰–−7‰) and a deficiency of sulphide minerals indicate that the diagenetic degradation of organic matter (δ 13Corg from − 34‰ to − 31.9‰) took place through Mn oxide reduction rather than microbial sulphate reduction and can contribute to a considerable source of bicarbonates. The approximately homogeneous δ 13Ccarb depletion and their apparent discrepancy with δ 13Corg indicate that the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a considerable contribution from seawater-derived carbon sources.

Spontaneous combustion of carbonaceous shale at an iron ore mine, South Africa

Spontaneous combustion during coal mining operations is a major problem that affects the health and safety of workers and causes environmental problems. The phenomenon is associated with the presence of coal, coal shale, and pyrite. In 2020, a premature detonation incident occurred at an iron ore mine where the waste material contains black carbonaceous shale units known to be associated with pyrite. The spontaneous combustion propensity and properties of samples of the black carbonaceous shales from the mine were examined and compared with samples from the Witbank Coalfield. The spontaneous combustion liability indexes of these samples were correlated with X-ray fluorescence (XRF) and proximate and ultimate analyses using linear regression. The Wits-Ehac Index classification results show that the samples were between medium and high risk. The linear regression analysis showed very poor correlations between the Wits-Ehac Index results and the XRF and proximate and ultimate results. The most valuable relationship found is between the presence of relatively high sulphur (greater than 3%) and ground reactivity with nitrate-bearing explosive emulsion. Keywords: coal, spontaneous combustion, carbonaceous shale, linear regression, premature detonation, iron ore mine.

A study on benthic molluscs and stable isotopes from Kutch, western India reveals early Eocene hyperthermals and pronounced transgression during ETM2 and H2 events

The early Eocene greenhouse Earth experienced several transient global warming events, indicated by sharp negative excursions in the stable isotope ratios of carbon and oxygen. A huge amount of CO2, enriched with 12C, was released in the ocean–atmosphere system leading to warming. The Paleocene–Eocene boundary is demarcated by the most significant and well-known hyperthermal event, Paleocene–Eocene thermal maximum (PETM). The PETM is documented to be accompanied by a transgression. The later hyperthermals are relatively less studied. Information on the hyperthermals from the palaeo-tropical basins are relatively few. Here, we present a high-resolution litho-, bio- and isotope–stratigraphic analysis of the early Eocene succession from the Kutch Basin, western India. Stable isotopes of carbon and oxygen were analysed from sediments (δ13Corg) and mollusc shells (δ13Ccarb and δ18Ocarb). The succession, prevailingly with lignite, along with carbonaceous black shale and plenty of fossil plant remains, is primarily a product of terrestrial environment. A pronounced marine transgression, characterised by marine mollusc bearing glauconitic shale in the middle of the succession, indicates a coastal transitional setting between the ocean and land. The δ13C curve of organic carbon reveals five negative excursions, which are identified as the PETM, Eocene thermal maximum 2 (ETM2)/H1, H2, I1 and I2 in ascending order. The hyperthermal pair of ETM2–H2 corresponds with the marine interval. δ13Ccarb and δ18Ocarb from the middle part of the succession reveal concomitant negative excursions. The association between these hyperthermals and transgression appears to be regionally and globally valid, which strongly suggests a causal link between them.

New Evidence of Submarine Exhalative Sedimentation in the Uranium-Polymetallic Phosphorite Deposit in Baizhuyu, Hunan, China

There are as many as 25 kinds of minerals (including non-ferrous metals, ferrous metals, rare and dispersed elements, precious metals, non-metallic and energy minerals) enriched in uranium-polymetallic fertile beds in black rock series, which is therefore widely attracting scholars all over the world. However, there is still great controversy in terms of the metallogenic mechanism in such beds. The black rock series have been systematically sampled from the Baizhuyu deposit in northwestern Hunan Province, China based on field geological and radioactivity surveys. Major and trace elements as well as rare earth elements (REE) of uranium-polymetallic phosphorite and its wall rocks were analyzed. Furthermore, carbon and oxygen isotopes, Sm-Nd isotopes, and mineralogy of the Baizhuyu deposit were studied. The results show that dolomite is a normal marine sediment, while and uranium-polymetallic elements were pre-enriched in phosphorites and black carbonaceous argillaceous shales and slates that formed from marine sedimentation and submarine exhalative sedimentation. Hydrothermal reworking to uranium-polymetallic phosphorites is significant as a result of submarine exhalative sedimentation. The research results of this paper can support a better understanding of metallogenesis and the future exploration of uranium-polymetallic phosphorite in the Lower Cambrian Niutitang Formation in the study area.

Trilobites, Biostratigraphy, and Geochemistry of the Middle Cambrian Kuonamka Formation (Northeastern Siberian Platform, Kyulenke River)

Abstract —We studied the middle Cambrian unit of the Kuonamka Formation section on the Kyulenke River (Siberian Platform) and performed its biostratigraphic subdivision based on trilobites. The middle Cambrian section has intervals corresponding to the regional zones of the Amginian Stage. Six levels with mass accumulation of fauna remains have been identified: Two levels are located within the Ovatoryctocara Zone; the third level is at the boundary between the Ovatoryctocara and Kounamkites zones; the fourth layer is confined to the roof of the Triplagnostus gibbus Zone; and the fifth and sixth levels are located within the Tomagnostus fissus–Paradoxides sacheri Zone. The composition of rocks and bitumens of their organic matter (OM) has been studied, including the geochemical specifics of the mineral components of rocks (iron, sulfur, and CO2) and of saturated hydrocarbons of bitumens as well as noncarbonate carbon isotopes in the OM. It has been established that the OM sedimentation took place under normal aeration of the sea basin waters, without hydrogen sulfide contamination of the bottom waters. The intensity of chemical and biochemical transformations of mineral and organic components during diagenesis was controlled by the contents of organic carbon and sulfate ion, the activity of the anaerobic prokaryote community, and the rate of sediment mineralization. We have also established relationships between the content of organic carbon in potentially oil source rocks and the contents of iron oxide, total sulfur, and sulfide and sulfate sulfur as well as the ratios of saturated hydrocarbons. The alternation of highly carbonaceous black shales and carbonaceous rocks is apparently due to a change in the composition of biologic communities of microorganisms (sources of hydrocarbon biomarkers) and in the intensity of OM transformation during diagenesis. We assume that the OM transformation included sulfate reduction and dealkylation of high-molecular steroids in the unconsolidated OM-enriched marine sediments with the participation of bacteria. The intensity of these processes depended on the mass of the primary OM, the amount of sulfate ion, and, hence, the pH and Eh of the medium.

Assessment of the tectonically induced Quaternary landforms and active deformation in the area between Main Boundary Thrust and Himalayan Frontal Thrust, south of the Siang Antiform, Arunachal Himalaya, India

The present study area represents one of the active segments of the frontal part of the Arunachal Himalaya. The major structural boundaries traversing the area are the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) and other local faults are tectonically active, as evident from the landforms that evolved in the fault zones. Strath terraces and uplift fluvial terraces are observed in the hanging‐wall block of the HFT and MBT, and along the HFT straight to curvilinear mountain fronts are observed. Normal faults both transverse and parallel to the strike of HFT are observed in the hanging‐wall block of the HFT. West of Pasighat the sub‐Himalaya terrain resembles a duplex, where there is gradual rise in elevation from the Brahmaputra Plain to the low‐lying hills of the sub‐Himalayan and sudden rise in elevation across the MBT. Drainage aligning to the bedrock structures and stream migrations are observed in the fault zones. Along the MBT, the sub‐Himalayan rocks are thrusted over either by quartzite or black carbonaceous shales, in some sections the MBT is behaving as a back‐thrust that dips towards the south. The MBT has also been displaced by almost N–S‐trending transverse faults where highly sheared bedrocks are also observed. Tilting of Quaternary fluvial deposits are observed in the hanging‐wall blocks of intraformational thrusts at two locations. Signatures of paleoseismic activities in the form of various soft‐sediment deformational structures are observed in Quaternary deposits in the HFT zone and also in the hanging‐wall block of the MBT. To determine the deformation and strain rate accumulation, we have re‐analysed the available GNSS results of the area, which indicate a deformation rate of up to 16 mm/year in the vicinity of the Pasighat region. The GPS‐derived deformation rates are well corroborated with the existing results of peak ground acceleration (PGA) anomalies. The component–wise motion indicates higher fault perpendicular motion in this part, which reveals an accumulation of more strain along the major Himalayan faults.

Factors controlling organic-matter accumulation in the Upper Ordovician-Lower Silurian organic-rich shale on the northeast margin of the Upper Yangtze platform: Evidence from petrographic and geochemical proxies

Organic-rich shales are widespread in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations on the northeastern margin of the Upper Yangtze platform, and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in China. However, organic matter accumulation in these shales needs further research, especially in the areas which experienced an extensive upwelling and volcanism. Here, this work integrates detailed petrographic and geochemical proxies, namely redox-sensitive trace elements (RSTEs) and ratios of Si/Al & Ti/Al, from two Ordovician-Silurian sections on the northeast margin of Upper Yangtze platform, in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the Ordovician-Silurian transition. Data of Si/Al and Ti/Al ratios indicate that the deposition of black-gray siliceous mudstones in the Lower Wufeng Formation commenced with a transgression triggered by the Boda warming event which led to a reduced detrital input. Based on RSTEs, the black carbonaceous shales of the Upper Wufeng Formation are organic-rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation. Abundant nutrients supplied by the post-glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column, which lead to high concentrations of organic matter in Lower Longmaxi Formation. Although the water column conditions were not conducive to the accumulation of organic matter, it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle Longmaxi Formation. The RSTEs (especially Cd/Mo, Co × Mn) reveal that the organic matter accumulation of the Wufeng-Longmaxi Formation during Ordovician-Silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input.

Geological conditions and gas-bearing evaluation of the Niutitang Formation shale in the Xixiang–Zhenba area, China

Black carbonaceous shale of the Niutitang Formation in the Xixiang–Zhenba area was found to be well developed and abundant in high-quality shale gas. However, few studies have been conducted in this area, and its reservoir-forming conditions are not clear. This study aims to analyse source rocks, reservoir characteristics and storage conditions of the Niutitang Formation shale in the Xixiang–Zhenba area using field investigations, drilling and logging, supplemented with laboratory experiments. Results of this study shows that the Niutitang Formation shale is deeply buried at 1500–4500 m and is mainly 10-110 m thick. The shale is widely distributed, thicker in the west, thinner in the east and is a highly mature organic rich source rock with kerogen type I and II. The shale stratum features a high content of brittle minerals, many natural cracks, and a low content of clay minerals which are mainly illite, followed by a mixed layer of illite and smectite. The shale reservoir is characterized by low porosity and permeability. In addition, the lithology is dense and acts as a good seal in this area while it has a high adsorption capacity and high gas-bearing potential. Overall, within the study area, the western, central, and southern parts have good structural preservation conditions. The Niutitang Formation in the study area shows excellent organic matter characteristics for shale gas generation as well as good geological conditions for shale gas accumulation.

Fluid Inclusion Study of the Penjom, Tersang, and Selinsing Orogenic Gold Deposits, Peninsular Malaysia

Ore-forming fluids in the auriferous district of the Central gold belt in Peninsular Malaysia were studied for their temperature, salinity, and relationship to the surrounding geology. Microthermometric analysis carried out showed homogenisation temperatures range from 210 to 348 °C (Tersang), between 194 and 348 °C (Selinsing), and from 221 to 346 °C (Penjom). Salinities range from 2.41 to 8.95 wt % NaCl equiv (Tersang), between 1.23 and 9.98 wt % NaCl equiv (Selinsing), and from 4.34 to 9.34 wt % NaCl equiv (Penjom). Laser Raman studies indicated that at the Tersang gold deposit, most inclusions are either pure or nearly pure CO2-rich (87–100 mol %), except for one inclusion, which contains CH4 gas (13 mol %). In addition, at Selinsing, most inclusions are CO2-rich (100 mol %). However, an inclusion was found containing CO2 (90 mol %), with minor N2 and CH4. Additionally, at the Penjom gold deposit, most fluid inclusions are CO2-rich (91–100 mol %), whereas one fluid inclusion is N2-rich (100 mol %) and another one has minor N2 and CH4. At a basin scale, homogenisation temperatures against salinity suggests an isothermal mixing of fluids. Most fluids are CO2-rich and are interpreted to be of metamorphic origin. The evidence further indicates involvement of magmatic fluids that is supported by the association of sandstone and carbonaceous black shales with magmatic rocks, such as rhyolite, rhyolite-dacite, and trachyte-andesite at the Tersang and Penjom orogenic gold deposits.

Geological field characteristic of the Kroh Formation in the Upper Perak shales, western Peninsula Malaysia

Unconventional hydrocarbon resource has become an interesting field of study; specially for its source rocks which can also be turned into excellent reservoirs to generate and accumulation hydrocarbons. Exploration and production from fine-grained rocks, such as shale, contemporaneous with advancement in technology facilitated the use of black shales with reduced operation cost. Twenty five percentage of sedimentary rocks of the Peninsular Malaysia are made up of Paleozoic black shales. This work is focused on understanding the geological and geochemical characteristics of the carbonaceous black shales from the Paleozoic Kroh Formation in the Western Belt of Peninsular Malaysia. Description of lithology and structures of outcrops and total organic carbon (TOC) analyses have been carried out for six representative black shale samples from the Kroh Formation. Detailed lithology, structures such as bedding plane, joint, fault, fold, lamination and iron stain were identified. The TOC values of the shales range from 2.02% to 7.01% and this range of TOC indicates that the studied shales have a very good potential to serve as source of hydrocarbons.

Characteristics of wufeng-longmaxi formation shale gas sweet layer: a case study of weiyuan-rongchang block in the south of sichuan basin, China

Taking the typical shale well of the Wufeng-Longmaxi Formation in the Weiyuan shale gas block of the South Sichuan Basin as an example, based on core observation, core slice, ar-ion milling SEM, physical property test, X-ray diffraction, geochemical analysis and on-site gas content test, the author systematically evaluated the Wufeng-Longmaxi geological sweet layer from the aspects of petrology, mineral composition, organic geochemistry, hydrocarbon-generating organism, physical property, pore type and gas content.The research shows that the lithology of the geological sweet layer in Weiyuan area is mainly composed of black carbonaceous shale, and laminae are developed. Compared with the Jiaoshiba area, the carbonate mineral content is higher and the quartz mineral content is relatively lower. The sweet layer has high TOC, and is at a stage of high maturity. It is rich in hydrocarbon-generating organism such as graptolites, algae, sponge bones and bacteria. The inorganic pores and organic pores of the sweet layer are well developed. The inorganic pores are mainly clay mineral pores, feldspar alteration pores and intercrystalline pores of pyrite. In terms of gas content, it has the characteristics of high gas content and high Initial dissipation rate, which provides a possibility for high yield and stable production of fracturing in this layer.The geological sweet layer has high organic carbon content, is rich in hydrocarbon-generating organisms, has a large potential for gas generation, and generated enough gas after a period of high mature evolution; at the same time, a large number of inorganic pores and organic pores generated in diagenetic evolution, these pores provide space for shale gas generation, as the same time, it increases the permeability of the sweet layer. The study area has good preservation conditions, it has favorable top and bottom Stratum conditions, and does not have large destructive fractures and destructive fluid activities, The above factors make the shale gas enriched and preserved, which provides the possibility of high and stable yield after fracturing.

Mobilisation of arsenic, selenium and uranium from Carboniferous black shales in west Ireland

The fixation and accumulation of critical elements in the near surface environment is an important factor in understanding elemental cycling through the crust, both for exploration of new resources and environmental management strategies. Carbonaceous black shales are commonly rich in trace elements relative to global crustal averages, many of which have potential environmental impacts depending on their speciation and mobility at surface. This trace element mobility can be investigated by studying the secondary mineralisation (regolith) associated with black shales at surface. In this study, Carboniferous shales on the west coast of Ireland are found to have higher than average shale concentrations of As, Cd, Cu, Co, Mo, Ni, Se, Te and U, similar to the laterally equivalent Bowland Shales, UK. Groundwater penetration and oxidative weathering of these pyritic black shales produces oxide deposits, dominated by goethite and jarosite, which are significantly enriched in As (44–468 ppm), Se (12–184 ppm), U (6–158 ppm) and other trace elements, compared to concentrations in the parent shales. Major elemental abundances vary in composition from 3.5 to 29.4% sulphate, 0.6–9.1% phosphate and 36.6–47.2% iron-oxide. Phosphate substitution within jarosite is observed in these samples, formed under ambient pressure and temperature conditions.The major and trace elements forming these secondary deposits are predominantly sourced from the underlying black shales through mobilisation by groundwater. This discovery is critical for the environmental assessment of black shale lithologies during shale gas exploration and production, where the demonstrated mobility of in situ elemental enrichments may indicate a significant source of produced- and groundwater contamination during and after hydraulic fracturing processes. The proportions of the major oxide phases exhibit a clear control on the trace elemental enrichments within the secondary deposits, where increasing Se and As concentrations correlate with increasing phosphate content of the jarosite mineralisation. This has implications for the remediation of acid mine drainage seeps, where phosphate-rich jarosite phases could be utilised for more efficient trace element removal.

Influence of volcanic activities on redox chemistry changes linked to the enhancement of the ancient Sinian source rocks in the Yangtze craton

Volcanic eruptions and related submarine hydrothermal activities were very extensive during the Precambrian periods in the Yangtze craton in southern China. How these geological events affected redox chemistry changes linked to the enhancement of mudstone/shale hydrocarbon source rocks in ancient strata is an important concern for oil and gas exploration in the Precambrian strata. Samples were collected from two Sinian (Ediacaran) outcrops in the northern Sichuan Basin, the Zhaiziyan and Yanhe outcrops, which revealed Doushantuo Formation (Z1ds) shales approximately 30.5 m in thickness, and shales in the third member of the Deying Formation (Z2dn3) approximately 30.0 m in thickness. TOC contents of the black carbonaceous shales in the Doushantuo Formation of the Zhaiziyan outcrop are mostly greater than 5.0% (up to 12.9%). Positive correlations were observed between TOC contents and trace and rare earth element concentrations in the shales: samples with high concentrations of trace elements such as V, Ni, Cu, Zr, Rb, and Pb and total rare earth element (∑REE) concentrations had relatively high TOC contents. The shales are rich in volcanic ash interlayers and thin siliceous layers and have positive Eu anomalies, indicating that volcanic activities and related hydrothermal activities significantly impacted the development of black shale source rocks. In the rifting background of the Sinian cratonal marginal, extensive volcanic activities brought rich nutrients to the seawater, which may have promoted the breeding of cyanobacteria and multicellular algae. At the same time, the volcanic activities released large amounts of H2S and SO2, which could cause the seawater to become a euxinic environment in which the bottom seawater was sulfurized and anoxic. Massive amounts of organic matters formed in the surface seawater entered the anoxic bottom seawater and were well preserved in the shale sediments. Therefore, under the influence of volcanic activities, ancient Sinian organic-rich high-quality source rocks were widely developed and had a large hydrocarbon supply potential for the Sinian dolomite reservoirs.

Origin of bedding-parallel fibrous calcite veins in lacustrine black shale: A case study from Dongying Depression, Bohai Bay Basin

The bedding-parallel fibrous calcite veins in black shales are common in sedimentary basins and retain significant information about shale diagenesis and organic matter evolution. However, the origin of the bedding-parallel fibrous calcite veins in black shales is still of great controversy. Carbonaceous black shales of Es4s–Es3x interval are the main source rock in Dongying Depression, Bohai Bay Basin, and fibrous calcite veins parallel to bedding are widely spread in these shales. Petrographic examination, fluid-inclusion microthermometry, and isotopic analyses were conducted to study the timing, diagenetic fluid conditions and formation mechanism of fibrous calcite veins. The bedding-parallel fibrous calcite veins include beef veins and cone-in-cone structures. The beef veins occur as short lenses and are filled with sub-vertical fibrous calcite. They contain a “median line” defined with brown granular calcite, scattered host rock fragment and pyrite framboids. Several adjacent beef veins may combine and grow together as a cone-in-cone structure and black shale laminas involved in it occur as sinusoidal solid inclusions in slices. Primary two-phase inclusions in fibrous calcite have homogenization temperatures (Th) between 86.4 °C and 117.4 °C. The δ13C composition of the micritic calcite in host shale ranges from +3.7‰ to +6.3‰VPDB and that of granular calcite ranges from −0.2‰ to +1.4‰VPDB, but the fibrous calcite has a moderate δ13C values ranging from +1.8‰ to +5.0‰ VPDB. The δ13C compositions of these calcite suggest an evolving carbon source at different burial stages. The fibrous calcite likely precipitated from modified pore water at elevated temperature, which is supported by its low δ18O values (−13.7‰ to −11.4‰ VPDB). The veins formed over two stage and the fibrous calcite growth was continuous and at least partly driven by the force of crystallization. The bicarbonate responsible for the fibrous calcite was derived from a mixed source including inorganic carbon from previous carbonate dissolution, and organic carbon from both fermentation and thermal decarboxylation.

Kerogen typing using palynofacies analysis in Permian Barren Measures Formation in Raniganj sub–basin, East India

The objective of the study is the simple assessment of kerogen type in Barren Measures Formation shale using palynofacies. The Barren Measures Formation sandwiched between the coal–rich formations consist of organic–rich black carbonaceous shales. These ̴ 1000 m thick, black shales are in the productive gas window having kerogen types III with excellent gas generation potential as proven in recent pyrolysis studies. In this study, the palynological analysis of the four core samples of the Barren Measures Formation shale was done. The analysis has revealed three types of kerogen assemblages, viz. palynomorphs, phytoclasts and amorphous organic matter. The palynomorph assemblage consists of spores and pollen; phytoclasts consist of secondary xylem wood macrophyte plant debris and amorphous organic matter is a higher plant decomposed product. Based on the application of published kerogen classification and correlation of Tyson (1995), the pure kerogen type in these shales is categorized as mixed type; type II (oil prone) and type III (gas prone). However, the gas–prone kerogen assemblage gets relatively dominant in the samples from greater depth in the studied borehole. Our results are analogous to the previously published outcomes of kerogen typing evaluated using Rock–Eval pyrolysis experiments.

Oil Shale Prospects of Imo Formation Niger Delta Basin, Southeastern Nigeria: Palynofacies, Organic Thermal Maturation and Source Rock Perspective

The search for oil and gas in the Nigerian inland basins and efficacy of the unconventional oil shale in hydrocarbon generation have necessitated the palynofacies characterization, thermal maturation and source rock evaluation of the Imo Formation, updip Niger Delta basin to determine its palynofacies constituents and oil shale potential. The standard acid maceration technique, palynofacies description as well as palynomorph colour index chart systematically compared to thermal alteration index and vitrinite reflectance index values were utilized for the study. The palynofacies components of the Imo Formation is dominant of yellow and brown amorphous organic matter, marine taxa, opaque particles, with few dark brown structured phytoclasts and terrestrial microflora. The source rock content is interpreted as Kerogen Type II - oil prone with abundant marine palynomorphs, amorphous organic matter and few terrestrial microfossils. The spore/pollen colour of the strata is light brown signifying mature and oil generation zone. This correlates to thermal alteration index (TAI) 2+ to 3 and vitrinite reflectance index (%Ro) values of 0.5% to 1.0%. The abundance of the amorphous organic matter of marine origin suggests good source rock which indicates good oil shale prospects. The oil shale prospects of the Imo Formation in the study area are hindered by the low abundance of the palynofacies associations. Field evidence indicate that the grey to slightly light grey shales in hand specimen, as against black carbonaceous shales rich in organic matter suggests that the environmental conditions affected the quality of the organic kerogen. The oil yields may be compensated by the great volume of marine shale deposited in the formation and the abundance of amorphous organic matter of marine origin which is known to be an excellent indicator of oil. The palynofacies model enhances the understanding of palynofacies events and oil shale quality which can be tied to the development of unconventional oil shale, oil fields and subsequent exploration drive in the area.

Hydrocarbon source rock potential evaluation of the Late Paleocene Patala Formation, Salt Range, Pakistan: Organic geochemical and palynofacies approach

Organic geochemical and palynofacies analyses were carried out on shale intervals of the Late Paleocene Patala Formation at Nammal Gorge Section, western Salt Range, Pakistan. The total organic carbon content and Rock-Eval pyrolysis results indicated that the formation is dominated by type II and type III kerogens. Rock-Eval \({T}_{\mathrm{max}}\) vs. hydrogen index (HI) and thermal alteration index indicated that the analysed shale intervals present in the formation are thermally mature. \(S_{1}\) and \(S_{2}\) yields showed poor source rock potential for the formation. Three palynofacies assemblages including palynofacies-1, palynofacies-2 and palynofacies-3 were identified, which are prone to dry gas, wet gas and oil generation, respectively. The palynofacies assessment revealed the presence of oil/gas and gas prone type II and type III kerogens in the formation and their deposition on proximal shelf with suboxic to anoxic conditions. The kerogen macerals are dominated by vitrinite and amorphinite with minor inertinite and liptinite. The kerogen macerals are of both marine and terrestrial origin, deposited on a shallow shelf. Overall, the dark black carbonaceous shales present within the formation act as a source rock for hydrocarbons with poor-to-moderate source rock quality, while the grey shales act as a poor source rock for hydrocarbon generation.

Significance of stable carbon isotope trends in carbonate concretions formed in association with anaerobic oxidation of methane (AOM), Middle and Upper Devonian shale succession, western New York State, USA

Carbonate concretions are concentrated along discrete stratigraphic horizons within Middle and Upper Devonian carbonaceous black and organic-deficient gray shale of western New York State. Textural characteristics preserved throughout studied concretions, including a clay grain microfabric typical of flocculated clay and spherical algal cysts, are consistent with a model entailing the formation of low-density, compaction-resistant calcium carbonate masses at shallow burial depth, within the zone of bacterial sulfate reduction. Burial of nascent concretions to the sulfate methane transition zone (SMTZ) was accompanied by the nearly complete infilling of porosity, the diagenetic consequence of the anaerobic oxidation of methane (AOM). Each concretionary horizon, then, defines the position of the SMTZ stabilized during an episode of reduced sedimentation rate. Most analyzed concretions (75%) of the Middle Devonian Marcellus Formation into the Upper Devonian Gowanda Formation display center-to-edge profiles of increasing bulk δ13C; far fewer concretions are characterized by radial profiles of decreasing δ13C. The common radial profiles of increasing δ13C can be attributed to AOM-related kinetic carbon isotope fractionation under diagenetic conditions of limited methane, which favors the production of dissolved inorganic carbon of increasing δ13C. Less common concretions displaying center-to-edge profiles of diminishing δ13C may record the anaerobic oxidation of more strongly 13C-depleted methane produced by carbon dioxide reduction, methanogenesis related to the degradation of organic matter within and near the SMTZ, or AOM back flux, an enzyme-mediated equilibrium isotope effect. These processes are favored by sulfate-limited diagenetic conditions more typical of the lower SMTZ and immediately underlying upper methanogenic zone (MEZ). Contradictory isotope profiles of texturally similar concretions can be accounted for by diagenesis associated with an extended SMTZ (ESMTZ) comprising the methane-limited SMTZ and the underlying sulfate-deficient upper MEZ. Vertical shifts of the diagenetic horizons of the ESMTZ induced by variable methane flux would have juxtaposed concretions displaying seemingly incompatible isotopic histories.

Comparison between Datangpo-type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on rare earth elements

Datangpo-type sedimentary manganese deposits, which are located in northeastern Guizhou province and its adjacent areas, are Mn carbonate-type deposits hosted in black carbonaceous shale that represent a series of medium to large deposits containing a huge tonnage of reserves. PAAS-normalized rare earth element distribution patterns of manganese ores record “hat-shaped” REY (REE+Y) plots characterized by pronounced middle rare earth element enrichment, evident positive Ce anomalies, weak to strong positive Eu anomalies and negligible negative Y anomalies. These REY geochemical characteristics are different from those of country rocks and record the processes and features of sedimentation and diagenesis. Manganese was precipitated as Mn-oxyhydroxide particles in oxidized water columns with the sorption of a certain amount of rare earth elements, subsequently transforming from Mn-oxyhydroxides to rhodochrosite and redistributing REY in reducing alkaline pore-water during early diagenesis. A number of similarities can be observed through a comparison of Datangpo-type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on their rare earth elements. The precipitation of Datangpo-type manganese ores is similar to that of hydrogenetic crusts and nodules based on their positive Ce anomalies and relatively higher total REY concentrations. However, several differences also exist. Compared to hydrogenetic crusts and nodules, Datangpo-type manganese ores record smaller positive Ce anomalies, lower total REY concentrations, unobvious fractionation between Y and Ho, and weak to strong positive Eu anomalies. These were caused by quicker sedimentary rates in the oxic water columns of the shallower basin, after which pore water became strongly reducing and alkaline due to the degradation of organic matter in the early diagenetic stage. In addition, compared to typical deposits in the world, Datangpo-type manganese ores are similar to hydrogenetic deposits and different than hydrothermal deposits. All of these characteristics of manganese ores indicate that Datangpo-type manganese ores, the principal metallogenic factors of which include oxidation conditions during deposition and reducing conditions during early diagenetic stages, represent hydrogenetic deposits.