Low Total Organic Carbon Values Research Articles

Organic geochemistry of the middle Paleozoic Ora Formation in Iraq: Implications for source rock assessment and hydrocarbon potentiality

The Ora Formation (late Devonian–early Carboniferous) is thought to be a potential source rocks for the Paleozoic petroleum system of Iraq. The source potential from the Ora Formation is evaluated for the first time ever in this study from western and northern Iraq which integrates data from organic geochemistry including Total Organic Carbon (TOC) analysis, HAWK pyrolysis, gas chromatography (GC), and gas chromatography mass spectrometry (GC-MS) and mineralogical X-ray diffraction and scanning electron microscopy. The shale and muddy carbonate succession within the Ora Formation from surface section in northernmost Iraq and subsurface section from two wells (Akkas-1 and Akkas −3) from western Iraq have been employed to assess the source rock potentiality, thermal maturity, kerogen type, organic content, and depositional environment. In addition to organic geochemical analyses, mineralogical XRD and SEM-EDS were used to support the paleoenvironmental interpretation of the Ora Formation. The results from TOC and HAWK analyses reveal that the Ora Formation ranges from poor to good as a source rock. However, the HAWK data suggests that the surface samples from northernmost Iraq are highly mature, highly weathered, or both. Kerogen analysis revealed that the Ora Formation contains immature type III and mixed II-III kerogens. Low TOC values were attributed to factors such as significant clastic input, weathering effects, and the prevailing oxic environment during deposition. The presence of detrital influx of quartz and feldspars, along with the occurrence of illite and kaolinite clay minerals, suggest a detrital input with weathering influence under hot arid and warm humid conditions. Biomarker analysis of the light hydrocarbons using GC and GC-MS revealed that these light hydrocarbons were generated from marine planktonic algae sources, possibly with some contributions from terrestrial and/or microbially reworked organic matter. These high mature light hydrocarbons in subsurface section were originated from anoxic marine shale source rocks. They were most likely from the Cambro-Ordovician Khabour Formation and were contaminated from another source.

Shallow burial shale gas accumulation pattern of the Wufeng–Longmaxi Formations in the northern Guizhou area, western Yangtze platform

Although continuous breakthroughs of economic shale gas exploitation have been made globally within the middle–deep buried depth (∼1500–3500 m), the shallow–buried (less than 1500 m) shales are also widely distributed, showing excellent exploration prospects. However, the shale gas accumulation (SGA) pattern of the shallow burial shale remains unknown, limiting future shale gas exploration. To investigate the SGA pattern and the exploration potential of shale gas in shallow–buried shales, experiments such as X–Ray Diffractometer, N2 and CO2 adsorption, Focused Ion Beam Scanning Electron Microscopy (FIB-SEM), total organic carbon (TOC) content, gas chromatography–mass spectrometry, and field gas content test were carried out based on three wells penetrated the Wufeng–Longmaxi Formation with the buried depths of less than 700 m in the Northern Guizhou area, western Yangtze Platform. The geochemical and mineral records indicate that the organic matter is enriched as a result of global climate and regional tectonic activity, indicating the sufficient gas generation potential. The results of low pressure N2 and CO2 adsorption, as well as images of the FIB–SEM show that the mesopores are controlled by the organic matter, and the micropores, which are associated with minerals (primarily clays), can provide much more specific surface area and pore space in shallow–buried shales. Furthermore, carbonates with low porosity and permeability from the Songkan, Jiancaogou and Baota Formations, and the shale interval with low TOC values, weak brittleness, and low porosity, can serve as good roofs and floors for the SGA. Despite the fact that multi–stage tectonic movements resulted in the formation of faults and fractures, the good preservation, adequate gas storage space, and sufficient hydrocarbon generation potential contribute to the SGA in the shallow–buried shale.

Occurrence of Mid Jurassic, organic-rich, lacustrine deposits on the southern Utsira High

A detailed biostratigraphic and organic geochemical investigation was undertaken of an organic-rich, lacustrine interval at ∼2622–2639 m in well 16/4–10, in the Fosen Prospect on the Utsira High in the Norwegian North Sea. A Bajocian to Aalenian age is suggested, similar to that for the immediately overlying Sleipner Fm, whereas the underlying strata were effectively barren and could not be dated with confidence. The dark grey claystone of the lacustrine unit is dominated by Botryococcus contributions, as indicated by palynofacies analysis. The kerogen exhibits a typical algaenan signature with a dominant single activation energy value of ∼54.5 kcal/mol for its bulk kinetics. Total organic carbon (TOC) increases from ∼2% at the base to ∼11% at the top, although the hydrogen index (HI) varies throughout (∼400–∼800‰ HC/Corg). The unit is on the verge of oil generation (Tmax ∼446 °C; estimated vitrinite reflectance 0.5–0.6%; spore colour index 5). Overall, it is similar to lacustrine intervals reported in the Møyfrid Discovery (North Sea wells 2/2-1 and 2/2–4) and Kap Stewart (Jameson Land, East Greenland). Freshwater/brackish lacustrine environment indications include a high tetracyclic polyprenoid (TPP) ratio and particularly abundant 4-methyl-24-ethylcholestanes. Specific botryococcal markers are absent, but the abundant 24-ethylcholestanes probably derive from this organism. Algal contributions increase upwards (at the expense of woody higher plants), as do 3-methylhopanes, whereas the pristane/phytane ratio (Pr/Ph) decreases (from ∼4 to 2), which may reflect increasing methanogenic/methylotophic activity. The immediately overlying Sleipner Fm has much lower TOC and HI values and a greater woody higher plant contribution, in what appears to be a proximal marine setting. Beneath the lacustrine facies is ∼10 m of dominantly sandstone, underlain by brown claystone, exhibiting the characteristics of a highly oxic marine environment, with very low TOC and HI values, but high oxygen index (OI) and Pr/Ph. The Fosen lake seems to correspond to one of numerous mini-basins in the area attributed to Zechstein salt withdrawal during the Jurassic. Like the Møyfrid lake, it appears to lie near the periphery of the sub-aerial, central North Sea volcanic dome. It is possible that organic-rich lacustrine facies developed in other, deeper mini-basins and have expelled some oil. Their subtle contributions to accumulations bearing other Jurassic charges may have been overlooked in the absence of obvious biomarker characteristics.

FÓSSEIS MOLECULARES DA MATÉRIA ORGÂNICA SILURIANA DA FORMAÇÃO TIANGUÁ (BACIA DO PARNAÍBA), NORDESTE DO BRASIL: IMPLICAÇÕES PALEOAMBIENTAIS E MATURAÇÃO TÉRMICA

MOLECULAR FOSSILS FROM SILURIAN ORGANIC MATTER OF THE TIANGUÁ FORMATION (PARNAÍBA BASIN), NORTHEASTERN BRAZIL: PALEOENVIRONMENTAL IMPLICATIONS AND THERMAL MATURATION. The Parnaíba Basin covers an area of 665,888 km2 between the northern and northeastern regions of Brazil. It is completely made up of siliciclastic sedimentary and volcanic rocks with ages varying between the Silurian and the Cretaceous. Among the formations of the Basin, the Tianguá Formation is considered a potential source rock. However, there are few geochemical and/or geological studies and no research work on the molecular parameters of the organic matter (OM) present in its outcropping rocks (OR). Thus, the objective of the present work is to characterize the OM, concerning the depositional paleoenvironment and the degree of thermal evolution, through molecular parameters and OM content. The OM was extracted using the Soxhlet system, chromatographed by classical methods, and analyzed using GC-MS and GC-MS/MS. The samples exhibited low total organic carbon values (TOC <1) and the molecular parameters of saturated and aromatic hydrocarbons suggested a marine depositional environment, with algae, bacteria, and terrestrial plants input. The C30 biomarkers (24-n-propylcholestane), identified using synthetic standards and usually associated with the Chrysophyte algae, corroborated the marine origin of the depositional environment. In addition, some molecular parameters suggested little thermal evolution.

Simple harpacticoid composition observed at deep hydrothermal vent sites on sea knoll calderas in the North-west Pacific

AbstractDirivultid copepods (Siphonostomatoida), one of the most successful meiobenthic organisms found at deep-sea hydrothermal vents, have been the focus of most previous ecological studies among meiofauna in these habitats. The ecology of Harpacticoida, a major benthic copepod group in typical deep-sea floor, however, is not well understood in terms of variations in community structure and controlling factors at venting sites. The spatial heterogeneities in benthic harpacticoid composition and their association with environmental parameters were investigated at hydrothermal vent chimney structures in the calderas of three neighbouring sea knolls (Bayonnaise Knoll, Myojin Knoll and Myojin-sho Caldera) in the western North Pacific. While a previous study had reported the distribution of dirivultids was strongly associated with spatial differences in stable carbon isotopic signatures (δ13C) of organic matter in the detritus on active chimneys in the field, multivariate analyses detected no significant corelation between the parameter and harpacticoid composition in this study. Instead, high associations of the harpacticoid composition with differences in water depth and total organic carbon (TOC) concentration were detected. Ectinosomatidae dominated at vent sites with lower TOC values in the shallowest Bayonnaise Knoll, while they were less prevalent at deeper vent fields in the other knolls, where Miraciidae was the most abundant family. This study indicated the availability of vent chemoautotrophic carbon is not a primary factor controlling the composition of harpacticoids even in the habitats on the hydrothermal vents, but instead by the food amount, regardless of its resources (including marine snow from the sea surface), in the study area.

Palynology, palynofacies and organic geochemistry analysis of the late Eocene shale from Meghalaya, Northeast India

Here the depositional environment and hydrocarbon source rock potential of the Kopili Formation is investigated using palynological analysis and Rock-Eval pyrolysis on samples from a borehole section (Borehole BUM14) collected at Umphyrluh area in the Jaintia Hills, Meghalaya. In these Kopili shales, amorphous organic matter is often associated with structural terrestrial organic matter, biodegraded organic matter, charcoal, black carbon debris, dinoflagellate cysts, and spores. The palynotaxa are mainly composed of dinoflagellate cysts comprising eight genera and twelve referable species. Based on the palynological data, the sediments of the study area were deposited in a shallow marine setting under oxygen deficient conditions in an environment that received a continuous terrestrial influx throughout the succession. Rock-Eval pyrolysis and Total Organic Carbon (TOC) analysis determine the quantity, type, and thermal maturity of the associated organic matter. TOC values range from 0.03 to 0.45 wt.% (averaging 0.28 wt.%) and the Genetic Potential (GP) and Hydrogen Index (HI) values vary from 0.04 to 0.24 mg HC/g rock and 22–100 mg HC/g TOC, respectively. These values imply that all the shale samples have very low TOC values (< 0.5%), S1, S2, and Hydrogen Index (HI) values. Although most of the samples are in a mature stage as the average Tmax value is 428.16°C and the Production Index (average 0.16) indicates a potential for oil generation, low Genetic Potential (S1 + S2) and TOC concentrations suggest there is limited potential for oil generation. The HI vs. OI plot and HI vs. Tmax plot show that most of the shale samples fall in the predominantly gas prone domain (mostly Type III and Type IV), because the organic matter is generally derived from a terrestrial source. Thus, the source rock potential for the Kopili shales of the Umphyrluh area is considered to be poor for gaseous hydrocarbons.

Source rock characteristics and hydrocarbon generation potential in Brunei-Muara district, Brunei Darussalam: a comparative case study from selected Miocene-Quaternary formations

A comparative analysis on source rock properties has been carried out on the Miocene-Pliocene formations as well as the Quaternary terrace deposits using Rock–Eval pyrolysis results and organic petrography as well as some biomarkers results. Samples were obtained from outcrops of the Quaternary terrace deposits, Pliocene-aged Liang Formation together with the Miocene Miri and Setap Shale formations in Brunei-Muara district, with sample lithologies ranging from coal, coaly shale, shale and lignitic sand. High total organic carbon (TOC) and S2 values ranging from 41.8 to 62.4% and 7.40 mg HC/g rock to 122 mg HC/g rock, respectively, are identified in coals of the terrace deposit, Liang and Miri formations, making these as the best potential source rock due to the “good to excellent” generating potential. Meanwhile, a “fair to poor” potential is exhibited for the coaly shale, shale and lignitic sand samples as a result of their low TOC, HI and S2 values. The organic matter is composed of kerogen type III (gas prone) and type II-III (mixed oil and gas prone). Organic matter in all studied formations originate from a terrestrial-source, as proven by the abundance of huminite. Organic petrographical and biomarkers studies suggest that the coals and lignitic sand samples were deposited in a mangrove-type mire in a lower delta setting, under oxic and limnic to limnotelmatic conditions, except sample DD2-1, which is deposited in a less water-saturated environment. The samples display the presence of bi-modal and normal distribution of n-alkanes. For all of the samples, the dominating plant types in the palaeomire are of soft, herbaceous plants and this is supported by the low vegetation index and moderate Paq values. All the studied samples are thermally immature to early mature, as exhibited by the Tmax values that range from 300 to 437 °C and vitrinite reflectance readings of 0.22% to 0.46%.

Effects of Igneous Intrusion on the Organic Content of Irati Formation, Paraná Basin, in Sapopema (PR)

The thermal effects of an igneous intrusion on organic-rich sedimentary rocks can be considering an important source of maturation of organic matter. The Permian Irati Formation of Paraná Basin (Brazil) is a carbonatic and organic-rich shale sequence intruded by Jurassic-Cretaceous basic rocks. This study reports possible effects of igneous intrusion on the organic matter content of Irati Formation, in Sapopema region (Paraná State). Total organic carbon (TOC), total sulfur (S) and insoluble residue (IR) data were combined with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The relatively low TOC values recorded in well where Irati Formation is in contact with 60 m of diabase sill (SP-58-PR) are residuals, associated with the depletion of organic carbon, caused by the thermal effect from the overlying intrusive rock. It was responsible to promote the cracking of the organic matter and reduced those values in relation to the original ones, observed in SP-32-PR (without thermal influence). When comparing the TOC peaks of the Assisting Member in both wells, it was observed that there was a decrease between 80.7 and 84% in the SP-58-PR. SEM images reveal that organic matter in Taquaral Member is sub-rounded and regular shape, while the organic matter in Assistência Member presents a characteristic pattern of thermally evolved organic matter.

The Middle Jurassic to lowermost Cretaceous in the SW Barents Sea: Interplay between tectonics, coarse‐grained sediment supply and organic matter preservation

AbstractSyn‐rift successions contain both prolific organic‐rich and coarse‐grained rocks, but these rocks are usually studied independently. Thus, the interplay among organic matter deposition, tectonics and sediment supply is not commonly assessed. In this study, we use well logs, Total Organic Carbon (TOC) content, biostratigraphy and seismic data from the SW Barents Sea to unravel the variability of organic‐rich successions and its interaction with potential reservoir rocks in areas affected by active normal faulting versus tectonically stable areas. Four Transgressive‐Regressive (T‐R) sequences were defined for the Middle to Upper Jurassic Fuglen Formation and for the organic‐rich Upper Jurassic to lowermost Cretaceous Hekkingen Formation. Three TOC trends, controlled by palaeobathymetric variations, and organic matter dilution, were identified within the two main organic‐rich sequences (sequences 2 and 3): (a) wells with the highest TOC values (&gt;10 wt %) at the base of the succession; (b) wells with the highest TOC values at the top of the succession and (c) wells with high TOC values at the base and top of the succession. Our interpretation indicates that fault activity controlled the TOC trends in two different ways: Firstly, by creating a sharp topographic contrast‐triggering hyperpycnal flows, and shallowing the footwalls, where higher oxygen content and less developed stratified waters lowered organic matter preservation. Secondly, by significantly increasing the input of clastic material, which inherently led to dilution of organic matter and resulted in lower TOC values (&lt;6 wt %). We interpret that sand deposition was controlled by the size and geomorphology of the sediment source areas. The western part of the study area (i.e. the Loppa High) was characterized by uplifted footwall islands and localized sands along their flanks, whereas the southern part (i.e. the Finnmark Platform) constituted a larger sediment source area for the Volgian age. This work has implications for a better understanding of the distribution of reservoir and source rocks in active rift basins.

Sea-level changes control organic matter accumulation in the Longmaxi shales of southeastern Chongqing, China

Fine-grained sediments in the Ordovcian–Silurian (O-S) Longmaxi Formation on the Yangtze Platform represent a significant shale gas source and reservoir in South China. The formation of the organic-rich black rock has been attributed to high primary productivity, suboxic/anoxic environments, and/or low detrital dilution. However, these claims remain to be debated. Here, we investigated total organic carbon (TOC) contents, iron speciation, major and trace elements of the O–S Longmaxi shale in the inner-shelf Qianqian #1 drillcore (southeastern Chongqing, South China). Our results show that the Longmaxi Formation is composed of black shale with high TOC values (4.7% ± 0.6%) in the lower member (LM: 0–6 m) and silty shale with low TOC values (1.3% ± 0.5%) in the upper member (UM: 6–77 m). The data from the LM suggests high primary productivity, ferruginous conditions punctuated with episodes of intermittent euxinia, and low terrigenous fluxes. In contrast, data for the UM indicate reduced primary productivity, ferruginous conditions, and higher terrigenous fluxes leading to dilution of the organic matter. A good coupling of local redox conditions with primary productivity and P/Al ratios, rather than water mass restriction, suggests that water-column anoxia was mainly driven by high primary productivity, and had a minor effect on primary productivity through marine P cycling. Changes in primary productivity might result from variable upwelling intensities associated with the fluctuations of sea levels. Variations in local redox conditions and terrigenous fluxes are mainly controlled by sea-level fluctuations. This study thus suggests that sea-level fluctuations and related environmental effects play key roles in organic matter accumulation through primary productivity, redox state, and detrital dilution. These findings highlight that multiple factors are responsible for organic matter accumulation at this location during the O-S transition in South China.

The effects of organic carbon content and thermal maturity on acoustic parameters in a black shale: Woodford Shale, Permian Basin, West Texas

We have derived relationships between organic carbon and acoustic properties in the Woodford Shale, an Upper Devonian organic-rich shale in the Permian Basin, Texas. Extensive core data enable us to relate measured total organic carbon (TOC) on core samples to gamma log response; in turn, gamma log-calculated TOC is compared with P-wave [Formula: see text] and S-wave [Formula: see text] velocities, density, and derivative properties including [Formula: see text], Young’s modulus, Poisson’s ratio, [Formula: see text], and [Formula: see text]. Because we sampled two wells at very different thermal maturity (early oil window and wet gas window), we can examine the effect of thermal maturity on these parameters. In both wells, [Formula: see text] and [Formula: see text] decrease with increasing TOC, exhibiting a rapid decrease from 0% to approximately 1.5% TOC, and a slower decrease above 1.5% TOC, similar to results from ultrasonic experiments on core samples, that we attribute to a shift from a mineral grain load-bearing to an organic matter load-bearing fabric. The [Formula: see text] decreases by 28% and [Formula: see text] by 20% over the range of 0.5%–12% TOC. [Formula: see text] and [Formula: see text] correlate more strongly to neutron porosity than to TOC in the less mature well, related to the development of a bitumen phase that is relatively soft and less dense and has greater impact on rock physical properties than kerogen; this effect disappears in the high-maturity well in which bitumen was further cracked to petroleum and physical properties of bitumen changed with increased aromatization. The effect of increased thermal maturity is to increase [Formula: see text] by approximately 12%, [Formula: see text] by 25%, and P-wave impedance by 12%. [Formula: see text] ratios decrease with increasing TOC, contrary to the behavior expected for the incorporation of a soft material into a rock. We suggest that the rock develops a horizontal planar fabric at lower TOC values that absorbs S-wave energy, counteracting the effect of the organic matter.

Sedimentological and Geochemical Analysis of the Middle Jurassic Shinawari Formation, Upper Indus Basin, Pakistan: Implications for Palaeoenvironmental and Hydrocarbon Assessment

Microfacies, palynofacies, and hydrocarbon source rock potential of Toarcian–Bathonian sediments of the Shinawari Formation are investigated in the Chichali Nala Section, Baroch Nala, Gulla Khel Nala, Surghar Range, and Askari Cement Factory Section, Nizampur Kala Chitta Range, Pakistan. The Shinawari Formation is dominated by a mix of limestone, sandstone, shale, marls, siltstone, and mudstone units with the association of laterite, hard grounds, coal layers, and coal disseminations. Microfacies analysis suggests that deposition of the Shinawari Formation occurred in peritidal lagoon, beach shoal to distal shelf setting. This interpretation is supported by sub-types of standard Tyson (in: Sedimentary organic matter, Springer, Berlin, 1995), palynofacies, which we have defined as palynofacies SFPF-A, attributed to a marginal dysoxic to an anoxic basinal setting, and palynofacies SFPF-B, deposited in heterolithic proximal platform settings. Some of the intervals have low ( $$\le 0.5\%$$ ) total organic carbon (TOC) values except for intervals with fair values (0.5–2.09%) in the Shinawari Formation in different stratigraphic sections. Despite some lower TOC values, and the presence of kerogen types III and II, spore colouration index of 5.0–8.0, thermal alteration index of 2–3+, and vitrinite reflectance (VR) of 1.16–1.35 indicate possible hydrocarbon source rock potential. The various geochemical plots obtained from TOC (%), VR and Rock-Eval pyrolysis data indicate that the Shinawari Formation specifies kerogen type III having Tmax ( $${}^{\circ }$$ C) range of 430–450 $${}^{\circ }$$ C and thermal maturity lies in the oil window.

Organic matter accumulation in the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, from sequence stratigraphic analysis and geochemical proxies

We present a model for organic carbon accumulation in the Upper Devonian Duvernay Formation of the Western Canada Sedimentary Basin, relating total organic carbon (TOC) concentrations to a high resolution geochemical and organic petrologic database and comparing these to a core-based sequence stratigraphic interpretation. Many previous source rock studies have suggested that organic matter is enriched in transgressive systems tracts or at maximum flooding surfaces, an observation that is commonly attributed to an assumed relationship between water depth and oxygen levels. Our data set enables us to associate total organic matter content and organic assemblages with particular systems tracts and to examine how the organic assemblages change across the basin, tested against proxies for redox conditions and bioproductivity to relate relative sea level to specific mechanisms for organic accumulation.The Duvernay Formation comprises three third-order depositional sequences, superimposed on a second-order late transgressive systems tract and early highstand systems tract. Sequences are composed almost entirely of transgressive systems tracts and highstand systems tracts, with a lowstand systems tract only at the base of the upper cycle that marks the second-order sea level turnaround. Depositional facies generally vary from bioturbated carbonate-rich siltstones to siliceous mudstones from the platform margins to basin center. Organic petrologic analysis records predominantly amorphous organic matter and solid bitumen, with much less abundant vitrinite and inertinite. Organic matter type indicated by Rock-Eval analysis shows that in most cases, high total organic carbon (TOC) content is associated with better quality organic matter (high hydrogen index) and vice versa, indicating more reducing conditions or higher bioproductivity were responsibility for organic enrichment. However, near carbonate reefs, hydrogen index is uncorrelated with TOC, indicating that here, dilution by carbonate minerals was the primary control on organic enrichment. Across most of the basin, highest TOCs are recorded just above the second-order maximum flooding surface. Secondary peaks in TOC are recorded above the maximum flooding surface in the upper third-order cycle and in the transgressive systems tract of lower third-order cycle. Low TOC values are recorded in the lowstand systems tract and in the middle and upper third-order transgressive systems tracts almost up to the maximum flooding surfaces.Throughout much of the basin, proxies for relative sea level, restriction of water masses (Mo/TOC), redox conditions (Mo/Al and S/Fe) and bioproductivity (biogenic silica) coincide. These relationships demonstrate that organic carbon accumulation resulted from influxes of nutrient-rich upwelled water during high sea level; thus, TOC values are highest in the upper part of transgressive systems tracts and lower highstand systems tracts. Anoxia typically developed as a result of bioproductivity and enhanced organic matter accumulation but was not itself a trigger for organic sedimentation. Near carbonate reefs, however, varying sea level regulated carbonate sedimentation rate and organic matter dilution, and bioproductivity had less impact on organic matter concentrations; thus, TOC was high when carbonate deposition was low during transgressions, which forced carbonate reefs to backstep, limiting carbonate sedimentation in deeper water area.

Benthic foraminiferal paleoecology and depositional patterns during the Albian at DSDP Site 327 (Falkland Plateau)

The present paleoenvironmental study uses a spectrum of analytical methods, such as benthic foraminiferal assemblages, total organic carbon (TOC), calcium carbonate (CaCO3) contents and magnetic susceptibility (MS), to monitor variations in primary productivity, bottom-water oxygenation and depositional patterns within the Albian interval recovered at DSDP Site 327, Hole A, Falkland Plateau. Thirty-three benthic foraminiferal species were identified in the studied section and, based on the abundances of morphogroups (epifaunal and shallow infaunal), two distinct associations were identified. Stratigraphic intervals dominated by the epifaunal morphogroup can be interpreted as indicative of bottom-waters with low-oxygen content. However, these decreases in oxygenation were not vigorous enough to establish a dominance of deep-infaunal morphotypes, as supported by the low TOC values. Intervals dominated by the shallow infaunal morphogroup were interpreted as subjected to moderate to high nutrient flux to the ocean floor. These intervals are associated with high MS values and low CaCO3 content, suggesting that dissolution processes, rather than increased primary productivity, controlled CaCO3 accumulation in the studied section. Furthermore, faunal analysis points to deposition in an outer neritic to upper bathyal paleoenvironment.

The effect of biomass variations of Spartina alterniflora on the organic carbon content and composition of a salt marsh in northern Jiangsu Province, China

The relationship of the organic carbon content and surface sediment composition to the litter mass of Spartina alterniflora was analyzed to investigate the responses of organic carbon content and composition to biomass changes in a salt marsh after the introduction of S. alterniflora. The vertical distribution of sediment organic carbon and its relationship with S. alterniflora underground biomass was also examined. The results show that the total organic carbon (TOC), recalcitrant carbon (RC) and labile carbon (LC) contents were significantly greater in the surface sediments of the S. alterniflora flat, which showed different intra-annual variation trends compared to a bare flat. In particular, the majority of S. alterniflora-derived organic carbon was LC in the S. alterniflora flat sediments, indicating that the introduction of S. alterniflora resulted in a significant increase in surface sediment LC content. In addition, the results of S. alterniflora litter decomposition revealed a rapid decrease of 40% in the organic carbon stocks within the first 2 months, and the TOC and LC contents showed intra-annual variation trends similar to the surface litter mass but with a two-month “phase difference” in time. Further analysis indicated, S. alterniflora litter controlled the content and proportion of the S. alterniflora-derived TOC input, and it ultimately affected the TOC content variations in the sediments. However, due to the LC mainly originated from S. alterniflora, the surface litter exerted more significant impact on the variation of LC content. In the bare flat core, the relatively low TOC, RC and LC content values were significantly influenced by the grain control effect, and the origins of these components were mainly derived from non-S. alterniflora sources. However, in the S. alterniflora flat cores, the relationships between organic carbon content, grain size and underground biomass, together with the analysis of the organic carbon origins, indicated that the S. alterniflora underground biomass significantly affected the origin of organic carbon. In addition, Spartina alterniflora introduction significantly increased the carbon sequestration capacity of the tidal salt marsh in North Jiangsu. Furthermore, the TOC, LC and RC accumulation rate analyses also indicated that S. alterniflora introduction improved the salt marsh soil and was beneficial to long-term carbon sequestration of the organic carbon pool.

Organic geochemical characteristics of black shales across the Ordovician–Silurian boundary in the Holy Cross Mountains, central Poland

Black shales in the Holy Cross Mountains area of Poland provide a record of environmental change across the Ordovician–Silurian boundary. The changing depositional conditions have generated a variation in organic matter contents above and below the boundary. Investigating the organic constitution of these black shales has the potential to reveal how their organic matter contents were generated and how suitable these rocks and their lateral equivalents may be for exploitation as shale gas reservoirs. One outcrop at the Holy Cross Mountains with a continuous section across the Ordovician–Silurian boundary occurs near the village of Bardo Stawy in the Kielce region, and contains sandy-silty mudstones, as well as grey and black shales. Organic geochemical analyses of samples at Bardo Stawy reveals low Total Organic Carbon (TOC) contents for Ordovician samples and higher TOC values for Silurian samples. Organic biomarkers indicate that the Ordovician rocks were deposited in a shallow-marine shelf setting, while the Silurian rocks were deposited in a deeper marine environment. The progressive increase in TOC from the uppermost Ordovician to the lowermost Silurian rocks reflects increasingly oxygen-poor depositional conditions during the post-glacial transgression. Following the deposition and preservation of organic matter in the Ordovician and Silurian rocks, these rocks were buried and subjected to thermal maturation. Rock Eval and biomarker thermal maturity parameters all indicate that the organic matter is mature and lies within the oil window. The Ordovician and Silurian shales have direct relevance to recent attempts to discover and exploit shale gas reservoirs in Poland. Our data and interpretations suggest that the relatively low TOC values (<2%) and low maturities for gas generation render these rocks unsuitable for commercial shale gas production. The progressive improvement in conditions for preserving organic matter across the Ordovician–Silurian boundary does, however, leave the possibility that more suitable deposits occur in Early Silurian rocks in the other parts of the basin.

Hydrocarbon potential of Ordovician–Silurian successions in Akkas field, western desert of Iraq

The shale beds of the Khabour and Akkas Formations (Ordovician–Silurian) in Akkas field of western Iraq have been studied to determine their hydrocarbon-generation potential. The total organic carbon (TOC) values of the Khabour Formation were generally low and associated with low S2 and hydrogen index (HI) values indicating that this formation is not a hydrocarbon source, although this could reflect advanced thermal maturity. The gray-green shales of the upper part of the Akkas Formation also have low TOC and S2 values. On the other hand, the TOC, S2, and HI values of the black shales of the lower part of the Akkas Formation were high. The values indicate that the gray-green shales of the upper part of the Akkas Formation are not petroleum sources, whereas the black shales of the lower part can be regarded as potential hydrocarbon source rocks. Organic petrology studies reveal that marine amorphous organic matter is predominant, and no significant differences were observed between Khabour and Akkas samples in terms of organic-matter type. Molecular geochemical data also indicate that the kerogen of the two formations is of similar origin. The normal alkane distribution is unimodal, with a maximum at , indicating marine algal organic matter. Rock-Eval and biomarker data indicate that the organic matter of the black shales of the lower part of the Akkas Formation is early mature, whereas the Khabour Formation is highly mature in the Akkas field.

Late Pleistocene–Holocene vegetation and Indian summer monsoon record from the Lahaul, Northwest Himalaya, India

The high resolution Holocene paleomonsoon records from Northwest (NW) Himalaya are limited. The carbon isotope (δ13C), Total organic carbon (TOC) and pollen analysis were therefore carried out from a peat-lake sediment sequence developed in alpine meadows of the Chandra valley, Lahaul, NW Himalaya, in order to reconstruct centennial to millennial scale vegetational changes and Indian summer monsoon (ISM) variability during the Holocene. The chronology of peat-lake sediments is constrained with 9 AMS 14C dates. The recovered non-arboreal pollen (NAP) suggested that during Holocene alpine desert-steppe, meadows and shrubs growing along the stream had developed in the Lahaul valley whereas arboreal pollens (AP) e.g. Pinus, Quercus, Cedrus and Ulmus presently growing in the southern hill slopes of Pir Panjal range indicated moisture carrying monsoonal air flow from the South. The increased δ13C and low TOC values between ∼12,880 and 11,640 calibrated years before present (cal yr BP) suggested weakening of ISM and low organic carbon production corresponding to the Younger Dryas (YD) cold event. The gradual depletion in carbon isotope ratio from ∼11,640 to 8810 cal yr BP indicated enhanced precipitation in the Chandra valley in response of increased ISM strength in early Holocene. The short spell of cold and dry climate with gradual decrease in ISM intensity between ca 10,398 and 9778 cal yr BP is closely linked with Bond event-7. The other prominent cold-dry events recorded in present study are (i) ∼8810 to 8117 cal yr BP roughly corresponding to global 8.2 ka cold event, (ii) ∼4808 to 4327 cal yr BP closely preceding the global 4.2 ka cold-arid period, and (iii) ∼1303 to 1609 cal AD corresponding to Little Ice Age (LIA) event. The expansion of thermophillous broad leaved taxa viz. Betula utilis, Alnus nepalensis, Quercus semicarpifolia and Juglans regia and effective growth of meadow vegetation such as grasses, Caryophyllaceae and Artemisia along with marshy elements i.e. Polygonum and Liliaceae between ∼6732 and 3337 cal yr BP marked warm and wet Holocene climate optimum (HCO) period. The warm and moist climate from ∼1158–647 cal yr BP corresponded with global Medieval Warm Period (MWP).

SHALE‐GAS POTENTIAL OF THE MID‐CARBONIFEROUS BOWLAND‐HODDER UNIT IN THE CLEVELAND BASIN (YORKSHIRE), CENTRAL BRITAIN

The shale‐gas potential of mid‐Carboniferous mudrocks in the Bowland‐Hodder unit in the Cleveland Basin (Yorkshire, northern England) was investigated through the analysis of a cored section from the uppermost part of the unit in the Malton‐4 well using a multidisciplinary approach. Black shales are interbedded with bioturbated and bedded sandstones, representing basinal‐offshore to prodelta – delta‐front lithofacies. The total organic carbon (TOC) content of the shales ranges from 0.37 to 2.45 wt %. Rock‐Eval pyrolysis data indicate that the organic matter is mainly composed of Type III kerogen with an admixture of Type II kerogen. Tmax (436–454°C), 20S/(20S+20R) C29 sterane ratios, and vitrinite reflectance values indicate that organic matter is in the mid‐ to late‐ mature (oil) stage with respect to hydrocarbon generation. Sedimentological and geochemical redox proxies suggest that the black shales were deposited in periodically oxic‐dysoxic and anoxic bottom waters with episodic oxic conditions, explaining the relatively low TOC values. The Rock‐Eval parameters indicate that the analysed mudrocks have a limited shale‐gas potential. However, burial and thermal history modelling, and VRr data from other wells in the region, indicate that where they are more deeply‐buried, the Bowland‐Hodder shales will be within the gas window with VRr &gt; 1.1 % at depths in excess of 2000 m. Therefore although no direct evidence for a high shale‐gas potential in the Cleveland Basin has been found, this cannot be precluded at greater depths especially if deeper horizons are more organic rich.

Paleoenvironmental and paleoceanographic changes across the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) as indicated by foraminiferal assemblages from the eastern margin of the Cretaceous Western Interior Sea

Two sites near the eastern margin of the Cretaceous Western Interior Sea (WIS) were investigated. The section at Cuba, Kansas (CK) is ~630km east of the GSSP for the Cenomanian/Turonian boundary at Rock Canyon, Colorado (RC), and the section near Sioux City, Iowa (SCI) is ~315km northeast of the Cuba site. Surprisingly, planktic foraminifera dominate all the studied samples despite the relative proximity of the sites to the paleo-shoreline and presumed neritic water depths. Such dominance suggests inhospitable benthic environments, and benthic foraminiferal assemblages indicate that seafloor oxygen in the WIS decreased eastward. Therefore, the hypothesis of an influx of freshwater into the WIS from the western margin forming a fresh water cap that diminished eastward is not supported. Benthic foraminifera are scarce in most studied samples and species diversity is low. In the CK section, diversity and abundance increase abruptly in the uppermost Cenomanian Benthonic Zone at the initiation of the δ13Corg positive excursion marking the onset of Oceanic Anoxic Event (OAE 2). In the SCI section, however, the Benthonic Zone is recognized only by presence of Nodosaria bighornensis that has a stratigraphic range restricted to this interval; other benthic taxa are very rare or absent. The favorable conditions of the Benthonic Zone were short-lived. While relatively diverse planktic foraminiferal assemblages, including keeled species, characterize the initial δ13C excursion at the CK site, dwarfed specimens of Heterohelix and Hedbergella dominate the assemblages at the more proximal Sioux City site. Furthermore, most of the biostratigraphic events recognized in the basin center RC section can be traced into the CK section, but not into the SCI section, including the Rotalipora spp. and Globigerinelloides bentonensis extinctions, the Heterohelix shift event, and the brief benthic recovery event. Correlation between eastern WIS sites shows that the SCI section is thicker, suggesting a major sediment source to the east. This may explain the inhospitable benthic conditions and development of a ‘dead zone’ (seasonal hypoxia) analogous to the modern shelf of the northern Gulf of Mexico proximal to active discharge of the Mississippi River. Development of estuarine circulation in the WIS provided the eastern part of the basin with input of calcareous plankton from the south including planktic foraminifera. Foraminiferal assemblages, total organic carbon (TOC), and δ13C data of the studied sections suggest a two-fold history of Greenhorn transgression and OAE 2 development. The initial phase is a global signal characterized by a positive δ13C excursion, low TOC values, and high foraminiferal species diversity. The second phase is characterized by overprinting by local conditions in the WIS, including high fluvial input, relatively high TOC values, and low foraminiferal species diversity. These findings suggest that the eastern margin records unique depositional and biotic environments further revealing the dynamic and complex nature of the WIS and its sedimentary cycles.