Amorphous Organic Matter Research Articles

Organic geochemical characteristics and depositional setting of Paleogene oil shale, mudstone and sandstone from onshore Penyu Basin, Chenor, Pahang, Malaysia

The eastern Chenor Pahang area of the Penyu Basin contains an untapped potential, in part due to poor knowledge of its organic facies characteristics The sedimentary succession of eastern Chenor Pahang area is dominated by fine to coarse grained sandstone occurring in association with oil shale and mudstone in a predominantly fining upward sequence. Detailed geochemical and petrographic characterization was carried out on oil shale and mudstone samples to evaluate the origin and type of organic matter (OM), as well as the paleoenvironmental conditions and organic richness of the Paleogene sedimentary succession. The facies are only enriched in total organic carbon (TOC), potential hydrocarbon yield (S2) and extractible organic matter (EOM) the types of organic matter are important index reflecting the potential of the source rock(s). Based on elemental ratios, the analyzed samples are shown to contain Types I and II kerogens. This finding is supported by the high hydrogen index (HI) of >300 mg HC/g TOC and the presence of abundant liptinitic materials. Biomarker signatures [n-alkanes >n-C25, high C29/C30 17α (H) hopane ratios (0.8–3.3) and high C29 steranes (49–78%)]. Low total sulfur and V/(V + Ni) ratio] suggest a predominantly terrestrial source input with some contribution from aquatic microorganisms as indicated by the common occurrence of amorphous organic matter. Combining this results with n-alkane patterns, kerogen types and organic petrography, predominance of C29 sterane probably indicates a contribution of aquatic organisms such as green and brown algae (consistent with Botrycoccus braunii race A and Pediastrum contribution) and some influence of higher plants OM. Integration of biomarker studies with bulk geochemical and petrographic analysis suggests that the analyzed samples were deposited under suboxic conditions in a fresh water lacustrine environment. This finding is corroborated by the ratios of pristane/phytane, V/Ni and Sr/Br. High bioproductivity and a stratified fresh water column with suboxic bottom water condition enhanced OM preservation in the analyzed mudstone and shale facies. The high OM content in these samples maybe related to the fine-grained matrix of the mud rocks and the paleoclimate in the area.

Petroleum geochemical properties of the oil shales from the Early Cretaceous Garau Formation, Qalikuh locality, Zagros Mountains, Iran

A suite of petroleum geochemical techniques including Rock-Eval VI pyrolysis, carbon analyzer, kerogen petrography and palynofacies, vitrinite reflectance (VRr), elemental analysis of kerogen (CHONSFe), gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), pyrolysis ((PY)-GC) and carbon isotope (δ13C) analyses were performed on 123 outcrop samples from 10 locations to characterize the properties of organic matter and the petroleum potential of the oil shales from the Early Cretaceous Garau Formation at Qalikuh locality in the High Zagros (Iran). To characterize the mineralogy and chemistry of the oil shales X-ray diffraction (XRD) and X-ray fluorescence (XRF) were performed.Together with our previous work (Rasouli et al., 2015), this is the first study in which organic geochemical and petrographical investigations on the Qalikuh oil shales were performed. Total organic carbon (TOC) content of the oil shales varies from 3.83 to 26.4 wt%, with a mean value of 16.5 wt%, suggesting excellent TOC content. The mean value of hydrogen index (HI = 557 mg HC/g TOC) and petroleum potential (S2 = 89 mg HC/g rock) for the oil shales is high, indicating the presence of highly oil-prone organic matter. Elemental analysis of kerogen shows the oil shales are predominantly enriched in Type II-S kerogen. The results of kerogen microscopy observations reveal that homogenous bitumen (as widespread cement by impregnation) and amorphous organic matter are the main organic components. The oil shales of the Garau Formation are mainly consisting of calcite minerals and therefore they are classified as organic-rich/highly bituminous limestone. Additional data from molecular geochemistry and biomarker parameters of GC and GC–MS (e.g. CPI, Pr/Ph, Pr/nC17, Ph/nC18, DBT/Phen and C29/C27 20R sterane) as well as carbon isotope data of extracted bitumen confirm the presence of marine Type II kerogen. These data also show the oil shales were deposited within a marine carbonate environment under anoxic/euxinic conditions.Based on equivalent VRr, elemental analysis, microscopy evidence and biomarker maturity parameters the investigated oil shales from the Garau Formation are early mature and located in the oil generation window. The revised oil-yield calculation from Fischer Assay results confirms the occurrence of moderate to good commercial oil shales with oil yield of 83 l/ton rock on average. Moreover, preliminary analyses of PY-GC data indicate that the Qalikuh oil shales have also very good shale gas potential at over-mature state (i.e. VRr >1.3%).

Paleogene-early Neogene paleoenvironmental reconstruction based on palynological analysis of ODP Hole 959A, West Africa

Abstract Five paleoenvironmental intervals (interval 1-interval 5) are established in the late Paleogene-early Neogene of the Ocean Drilling Program (ODP) Site 959 (Hole 959A), Cote d'Ivoire-Ghana Transform Margin in the eastern Equatorial Atlantic. The intervals are based on dinoflagellate cyst and palynofacies analyses of 30 samples covering a 273.3-m interval. We observed an abundance of the Filisphaera group (Filisphaera filifera and Bitectatodinium tepikiense) which is interpreted as an Arctic migration to equatorial regions during a narrow interval in the Early Oligocene (early Rupelian). In addition, we document one hiatus within interval 3 in the Early Miocene (Aquitanian). The dominance of typically restricted marine species of the Polysphaeridium group (Homotryblium plectilum and Polysphaeridium zoharyi) at several horizons in the open-oceanic sediments of ODP Hole 959A appear to be very distinctive events. Upon integrating dinoflagellate cyst data with prior lithologic and microfossil data, we interpret these dominance events as being due to offshore transportation by turbidity currents (intervals 1 and 2), hiatus event (interval 3), or hyperstratification conditions (intervals 3 and 4). In addition, the dominance of Cribroperidinium spp. (C. giuseppei and C. tenuitabulatum) in the upper part of the section within paleoenvironmental interval 5 is suggestive of cold-water masses during a strong upwelling period in the latest Early Miocene (Burdigalian). The general abundance of amorphous organic matter alongside the dinoflagellate cyst assemblage supports an outer neritic-oceanic depositional environment for the studied site. The consistent occurrence of degraded phytoclasts in the sediments also suggests fluvial outflows to this offshore site.

Organic petrography and pore structure characterization of low-mature and gas-mature marine organic-rich mudstones: Insights into porosity controls in gas shale systems

Analyses of both type II organic-rich low-mature mudstones from the Kimmeridge Clay Formation (Yorkshire, England) and gas-mature mudstones from the Vaca Muerta formation (Argentina) were performed using petrographic and SEM observations to characterize the organic and inorganic components of the rocks and their relationship with porosity. The porosity analyses using nitrogen adsorption measurements and mercury intrusion porosimetry were then evaluated as a function of both composition and thermal maturity. Despite the absence of variation in the pore size distribution and the total pore volume with maturity, which is controlled by total organic carbon content (TOC), this study demonstrates that the pore network of these marine mudstones varies considerably with thermal maturity. Contrary to low-mature samples, whose porosity depends essentially on mineral interparticle pores, the porosity of thermally-mature rocks is mainly influenced by organic-matter-hosted pores located within secondary solid bitumen. Organic matter (OM) porosity, which is rare in the low-mature stage, appears to increase during thermal maturation in response to the thermal cracking of kerogen and oil, to become predominant in gas-mature rocks. It can therefore be considered that thermal maturation is a major process for the development of the OM porosity. However, the OM of high-TOC (>5.5 wt%) gas-mature samples contains smaller pores than gas-mature low-TOC mudstones probably due to a difference in the original OM composition between these high- and low-TOC samples. By analogy with the Kimmeridge Clay formation, which is often considered as time equivalent in terms of OM composition (Uliana et al., 1999), the high-TOC Vaca Muerta samples appear to have originally contained greater proportions of oil-prone amorphous OM and thus a better OM intrinsic quality for oil generation. This difference of OM composition and petroleum potential between Vaca Muerta marine mudstones appears to have dramatically influenced the OM-hosted pore genesis during maturation. Consistent with other studies, these results show that thermal maturity is not the only process that can explain OM-hosted pore genesis in gas shale systems. The OM composition can greatly influence the evolution of this porosity during maturation. Organic composition and OM thermal maturity may therefore have a complementary effect, explaining the diversity of trends observed on porosity in natural formations.

Solid bitumen in shales: Petrographic characteristics and implications for reservoir characterization

The presence of solid bitumen strongly affects hydrocarbon storage and expulsion from a source rock as it might either cause blockage of pore throats leading to lower effective gas permeability, or contribute to hydrocarbon storage and provide migration pathways when a continuous network of hydrocarbon-wet organic matter (OM) pores is formed. Furthermore, organic matter transformation reactions are suggested to influence mineral diagenesis as well. In an attempt to characterize different solid bitumen types and transformation stages over a broad maturity interval (0.5–2.7%Ro) and for varying primary kerogen compositions, we reviewed optical and scanning electron microscopy (SEM) data of 35 solid bitumen-rich shale samples with a Cambrian to Triassic age. We were able to identify in-situ pre-oil solid bitumen, as well as remobilized post-oil solid bitumen at various maturity stages from the early oil window onwards. Solid bitumen is the main host for SEM-visible organic matter porosity; onset of porosity development in solid bitumen differs considerably between predominantly oil-prone (e.g., alginites, amorphous OM from algal and bacterial precursors) and gas-prone (vitrinite-rich) kerogen compositions. Furthermore, solid bitumen (pyrobitumen) in rocks with a terrestrially dominated OM composition seems to be considerably less mobile within the source rock compared to pre- and post-oil solid bitumen in oil-prone rocks, and less reactive in terms of porosity generation. In most samples, several solid bitumen populations with varying fluorescence properties and bitumen reflectance were observed, complicating the use of these petrographic maturity indicators. The apparently different solid bitumen populations often form continuous networks at the SEM-scale. Microstructural features such as irregularly distributed sponge-like porosity or detrital and authigenic mineral inclusions in the sub-micrometer scale were found to have a great influence on texture and reflectance under reflected light microscopy. The formation of authigenic minerals (quartz, various carbonate phases with different Ca/Mg/Fe proportions, magnetite in Cambrian samples) was observed frequently in post-oil solid bitumen of oil-prone rocks, indicating a close genetic relationship between transformation products formed during hydrocarbon generation (e.g., acetate, carbon dioxide and methane) and the dissolution and precipitation of minerals during diagenesis. In some cases, stylolite-like features in the sub-micrometer scale were found, showing that processes well-known from reservoir characterization at core-scale also play a role at the micrometer-scale. Furthermore, the observed strong interaction between organic matter transformation and mineral authigenesis indicates a substantial aqueous component even in pores filled apparently exclusively with solid bitumen.

Palynofacies, micropalaeontology, and source rock evaluation of non-marine Jurassic–Cretaceous boundary deposits from northern Germany - Implications for palaeoenvironment and hydrocarbon potential

Abstract The Jurassic–Cretaceous boundary interval of northern Germany is characterised by non-marine deposits attributed to the Purbeck and Wealden facies. These organic-rich sediments were deposited under restricted, brackish-lacustrine conditions in a palaeogeographically isolated basin (Lower Saxony Basin). Rock-Eval and δ13Corg measurements were performed to characterise the composition and distribution of the organic matter, to determine the kerogen type chemically, and to define the quantity and maturity of the organic matter. A strong correlation between the optical characteristics, documented by the palynofacies analysis, and the chemical kerogen analyses have led to a better understanding of the palaeoenvironment and the processes which caused the organic matter enrichment. The samples studied from the basin center show a high petroleum generation potential, their estimated maturity is close to the onset of oil generation. Total organic carbon (TOC) concentrations of up to 18% have been measured, hydrogen index values reach up to 1185 mg HC/g TOC. The kerogen assemblages in two basinal successions are dominated by granular amorphous organic material (AOM), dinoflagellate cysts, and Botryococcus algae. In a more marginal succession continent-derived debris (phytoclasts, pollen and spores) is dominant. The AOM is interpreted to have originated either from phytoplankton (probably dinoflagellate cysts and Botryococcus), or its bacterial/archaeal degradation products formed under anoxic conditions. TOC-rich samples which are dominated by these amorphous kerogens show the highest petroleum potential, the AOM therefore plays a key role for the petroleum generation. The long-term climatic and hydrological evolution of the Lower Saxony Basin, which is based on palynomorphs, ostracods, and benthic foraminifera, is here interpreted by a five-stage model. Fluctuations in the salinity of the water bodies are directly reflected by the ratio between different freshwater and brackish-marine organisms as well dinoflagellate cyst diversity; five different cyst morphogroups are used for palaeoenvironmental interpretations. The deposition of TOC-rich beds in the German Wealden can be best explained by both high primary productivity and the establishment of prolonged phases of bottom water anoxia in a brackish-lacustrine depositional environment. Ecological conditions supporting enhanced productivity and preservation of the algal/bacterial-derived organic matter were most favourable during the mid–late Berriasian (Wealden 1–4).

Geochemical and petrographic analyses of new petroleum source rocks from the onshore Upper Jurassic and Lower Cretaceous of Lebanon

The recent interest in the Lebanese petroleum potential led to new studies mainly concerning marine petroleum source rocks, without taking in consideration the dysodiles and other associated facies from the Upper Jurassic and the Lower Cretaceous. These organic-rich outcrops of metric thickness (max. 3 m) are of limited lateral extension and often found next to volcanic deposits. Sixty-three samples were collected from 25 different localities, and were subdivided into three facies that are presented in this paper: (i) dysodiles formed by the superposition of millimetric layers that are easy to separate; (ii) dysodiles-like with distinct, but more compacted layers; (iii) non-dysodiles where the layering is not visible. The aim of this work is to assess the origin of these organic-rich rocks, and to evaluate their source rock potential. We used herein Rock-Eval pyrolysis, elemental analysis, palynofacies observation, organic petrography and molecular geochemical analyses. Rock-Eval pyrolysis reveals a very good to excellent source-rock potential for all above cited rock types, with average total organic carbon (TOC) of 14 ± 7% for the dysodiles, 19 ± 12% for the dysodiles-like and 13 ± 9% for the non-dysodiles. Hydrogen index (HI) and oxygen index (OI) values indicate a type I to II kerogen for the dysodiles (average HI = 649 ± 162 mg HC/g TOC), type I to II kerogen for the dysodiles-like (average HI = 340 ± 182 mg HC/g TOC) and type III kerogen for the non-dysodiles (average HI = 145 ± 42 mg HC/g TOC). The dominance of fluorescent amorphous organic matter (AOM) and alginite macerals confirm an algal/bacterial source of organic matter for the dysodiles. The presence of opaque AOM, phytoclasts, sporinites and cutinites macerals, suggests a variable terrestrial organic matter input for the dysodiles-like and non-dysodiles. Maturity parameters such as Tmax values (on average 428 ± 7 °C for dysodiles, 429 ± 6 °C for dysodiles-like and 424 ± 10 °C for non-dysodiles), vitrinite reflectance (<0.5%) and Carbon Preference Index (>1) indicate that the organic matter is still thermally immature. The present study sheds light on the long forgotten dysodile shales and similar material, and their importance as potential source rocks or oil shales. A comparison with local and regional source rocks is provided in order to emphasize the specific character of the dysodile deposits.

Organic matter content and type variation in the sequence stratigraphic context of the Upper Devonian New Albany Shale, Illinois Basin

Organic matter quantity and type are important parameters in conventional source rock evaluation and unconventional shale oil/gas reservoir characterization. Understanding the stratigraphic distribution of organic matter content and type in black shale successions is critical for identifying potentially productive intervals, because organic matter can adsorb large amounts of oil and gas. Detailed examination of total organic carbon content, organic petrographic composition, and high-resolution (8 cm spacing) geochemical proxies in a sequence stratigraphic framework were conducted on an early mature (0.55% Ro) New Albany Shale core of the Illinois Basin to study the influence of relative sea-level fluctuations on paleoproductivity, clastic supply, bottom-water redox conditions, and their combined control on total organic carbon distribution pattern and organic maceral variation.Marine organic matter including amorphous organic matter and alginite (mainly derived from Tasmanites cysts) is the dominant organic matter in the New Albany Shale. Terrestrial organic matter accounts for <10% of total organic matter. Within a sequence stratigraphic context, total organic carbon content increases in transgressive systems tracts, reaches a maximum before the maximum flooding surface, and shows relatively low values in highstand systems tracts. Comparatively, low total organic carbon contents at maximum flooding surfaces reflect a combination of low sedimentation rates, elevated bottom-water oxygenation, and high biogenic silica dilution.Stratigraphically, amorphous organic matter content increases in transgressive systems tracts, reaches a maximum near the maximum flooding surface, and decreases in highstand systems tracts. Enrichment of broken Tasmanites cysts and their detrital infills are an indicator of high-energy environments and could possibly indicate lowstand systems tracts. The stratigraphic distribution of organic matter content and type may result in cyclic stratigraphic variations of hydrocarbon generation potential and oil saturation, and influence the development of secondary organic pores when the New Albany Shale is within the oil and gas windows.

Middle Jurassic palaeoenvironment and palaeobiogeography of the Tabas Block, Central Iran: palynological and palaeobotanical investigations

We performed a palynological study on Jurassic successions of the Hojedk Formation, Central Iran. The sampled material was recovered from well no. 233, South Kuchak-Ali area belonging to the Tabas Block. The lithology comprises 475.80 m of green–grey shales and sandstones. Palynological assemblages were recovered including spores, pollen grains, dinoflagellate cysts, woody debris and amorphous organic matter (AOM). Furthermore, 22 taxa of plant macrofossils were identified. We analysed 48 samples which resulted in identification of 13 spore species (belonging to 11 genera) and three pollen species (from three genera) with fair preservation quality. Trilete spores dominate while pollen grains only represent 11.5% of the total particulate organic matter. Representatives of the fern spore Klukisporites are particularly abundant and the K. variegatus acme zone, which is of biostratigraphical importance, is identified in the host strata. The abundance of K. variegatus and K. scaberis suggests a Middle Jurassic (Bajocian–Bathonian?) age for the host strata. This age designation corroborates previous stratigraphic interpretations, mainly based on plant macrofossils for this formation. The palynomorphs indicate deposition in an oxygenated environment close to the shoreline in a deltaic system. The presence of subordinate marine palynomorphs (proximate dinoflagellate cysts) is probably indicative of intermittent sea level transgression. The affinity of the palynological assemblages from the Hojedk Formation indicates a diverse parental flora of, in descending quantitative order, ferns, bryophytes and gymnosperms, the latter mainly represented by conifers and ginkgophytes. Comparisons with modern plant ecology indicate accumulation of palynomorphs under a moist warm climate during the Middle Jurassic (Bajocian–Bathonian?) time. The comparison of parental plants with previous palaeofloristic studies indicates that the Tabas Block was located in the Mid-Asian part of the Indo-European floristic province of the Northern Hemisphere.

Morphology and gut contents of anguillid and marine eel larvae in the Sargasso Sea

The head morphology and body shapes of anguilliform larvae (leptocephali) vary widely by having fewer, much longer teeth in the small larvae and more numerous, but relatively smaller teeth in larger developmental stages. The feeding ecology of leptocephali has been difficult to understand because they do not appear to feed on typical zooplankton like other fish larvae but are mostly found with amorphous organic material in the gut. Observations of the basic morphology of mainly the head and gut contents of anguillid larvae and several families of larger marine eel larvae were made using photographs taken during three recent sampling surveys for leptocephali in the Sargasso Sea spawning area of the European, Anguilla anguilla, and American, Anguilla rostrata, eels. The gut contents of leptocephali consisted of amorphous material that sometimes flowed out of the intestine, and like in the Indo-Pacific, appendicularian houses and fecal pellets and other visible objects were sometimes present. High-magnification microscope images showed the presence of many spherical objects, amorphous and other materials that were likely related to bacteria, protists, fungi, or other organisms. The presence of filter structures confirmed that large oval objects were appendicularian houses, and possible hydrozoan objects were seen in Eurypharynx pelecanoides and Avocettina infans gut contents. The gut contents of A. anguilla, E. pelecanoides, and Kaupichthys hyoproroides leptocephali appeared to contain round ≤40 μm heterotrophic thraustochytrid protists (class Labyrinthulomycetes) that likely colonized marine snow materials consumed by the larvae. These observations support the hypothesis that leptocephali, whose teeth structure and relative teeth sizes change with growth, primarily target overlapping size ranges of marine snow particles as a food source in the Sargasso Sea, and the particles contain a wide range of components that aggregate from the food-web.

Palynological analysis of Upper Ordovician to Lower Silurian sediments from the Diyarbakir Basin, southeastern Turkey

ABSTRACT This paper reports on a palynological analysis of 41 core and 21 cutting samples from a well drilled through an Upper Ordovician–Lower Silurian sequence, belonging to the Bedinan and Dadas formations, of the Diyarbakir Basin of southeastern Turkey. The samples yield abundant and well-preserved marine palynomorphs (acritarchs, chitinozoans and scolecodonts) although non-marine palynomorphs (spores/cryptospores) are extremely rare or absent. The Upper Ordovician sediments of the Bedinan Formation have low organic content but contain abundant palynomorphs whereas the Lower Silurian sediments of the Dadas Formation have high organic content, dominated by amorphous organic matter, with relatively rare palynomorphs. Three chitinozoan assemblages are identified and attributed a late Katian (merga Biozone), Hirnantian (moussegoudaensis Biozone) and Llandovery (?alargada Biozone) age. Two acritarch assemblages are identified and attributed a Katian–Hirnantian and Llandovery (Aeronian–Telychian) age. The chitinozoan and acritarch age determinations are compatible and suggest that the Bedinan Formation is of Katian–Hirnantian age and is separated by an unconformity from the Dadas Formation that is of Llandovery (Aeronian–Telychian) age. These findings confirm the presence of an unconformity at the Ordovician–Silurian transition in the southeastern Turkey. Palynofacies analysis suggests that the Bedinan Formation accumulated on an offshore shelf that was initially well oxygenated but became increasingly anoxic, whereas the Dadas Formation accumulated in an offshore basin that was anoxic. Palynomorph assemblages recorded in the Bedinan and Dadas formations indicate northern Gondwana affinity.

Palynofacies and Kerogen Analyses of Upper Cretaceous (Campanian-Danian) Shales of Ugwueme Section of Anambra Basin, Southeastern Nigeria

Three samples of shale recovered from late Campanian to Danian sediments around Ugwueme area, Anambra Basin were studied for their particulate organic matter distribution and hydrocarbon potentials. Two major palynofacies were identified and designated palynofacies A and palynofacies B. Palynofacies A occupies the basal unit (Nkwe mudstone unit of Nkporo Shale) and the topmost unit; Umuaku shale unit of Nsukka Formation).Palynofacies A is dominated by abundant amorphous organic matter (AOM) and some phytoclastmaterials majorly of macerol marine exinite and some terrestrial influenced materials. Palynofacies B represents the middle unit bed (AcharaUgwueme shale unit of Mamu Formation) constitutes abundant phytoclasts with opaque debris of well-preserved terrestrial influenced plant fragments. The kerogen investigation result shows Nkwe mudstone unit of Nkporo Shale Formation and Umuaku shale unit of Nsukka Formation to be type II/III kerogens known to be oil-gas prone material. The study reveals that AcharaUgwueme shale unit of Mamu Formation is of type III kerogen known to be gas-prone. Estimation of the maturation stage of the samples using the exinecolours of pale-yellow to yellow, the sporomorphs shows (1+ to 2+)thermal alteration index(TAI) and reflectance values of 0.3% for both Nkwe mudstone and AcharaUgwume shale unit, thus are thermally immature to generate hydrocarbons while Umuaku shale unit with exinesporomorphscolour of yellow –light brown, (2to2+), thermal alteration index (TAI) and reflectance value of 0.5% to be thermally immature to early mature (early oil phase of hydrocarbon generation).

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.

Biostratigraphy of the Hareelv Formation (Upper Jurassic) in the Blokelv-1 core, Jameson Land, central East Greenland

The Hareelv Formation in the Blokelv-1 core is biostratigraphically subdivided by means of ammonite and dinoflagellate cyst stratigraphy. The succession ranges from the Oxfordian C. densiplicatum Chronozone to the Volgian P. elegans Chronozone. The mudstones of the Blokelv-1 core are characterised by large amounts of amorphous organic matter. This hampers the preparation and identification of dinoflagellate cysts, which are also commonly degraded and corroded. Ammonites, on the other hand, are common and well-preserved in the core, contrasting with that observed in the equivalent facies and stratigraphic interval at outcrop. Integration of the ammonite and dinoflagellate cyst biostratigraphical data yields a robust chronostratigraphic subdivision of the middle Oxfordian – lowermost Volgian cored section.

Palaeoenvironmental Reconstruction of the Puerto Madryn Formation (Middle to Late Miocene), Northeast of Patagonia: Palynology, Nannofossils and Stratigraphy

A 20 m thick interval of the Miocene Puerto Madryn Formation, composed of nearshore to shelf mudstones and sandstones was the subject of a detailed, multiproxy study, including sedimentological and stratigraphical observations as well as marine-derived amorphous organic matter (AOM), phytoclast, terrestrial and marine palynomorph and calcareous nannofossil assessments. Three palynofacies were defined based on qualitative and quantitative analysis of the palynological organic matter. Based on their sedimentology and microfossils, the section was divided into four intervals: Interval 1 accumulated in relatively stable, warm water conditions, without salinity fluctuations, in an inner shelf; Intervals 2 and 3 show a marked upward fining trend and a progressive change towards a phytoclast-dominated (continental) palynofacies; Interval 4 represents a new reinstauration of normal marine conditions with a regressive trend. The accumulation of the finest-grained deposits on the shelf corresponds to the most continental signature of the succession which seems to contradict the classic grain-size stratigraphic trends for shallow marine sedimentary successions. We interpret this as the result of the progressive increase of fluvial, mud-laden freshwater input into the depositional system, interpreted as produced by switching of deltaic channels processes. The dinoflagellate cyst assemblages reflect overall warm-water conditions. Nevertheless, the presence of cool-water indicators species in some levels suggests the influence of Antarctic cold–waters in the area. Based on diagnostic dinoflagellate cyst and nannofossil taxa, the age of the studied interval was estimated between the Serravalian and the Tortonian.

Factors controlling shale microstructure and porosity: A case study on upper Visean Rudov beds from the Ukrainian Dneiper–Donets Basin

The present contribution aims for a characterization of microstructure and pore-space distribution of upper Visean Rudov beds, considered the main source rock for conventional oil deposits in the Ukrainian Dneiper–Donets Basin and a prospect for unconventional hydrocarbon production in recent years. Broad ion beam–scanning electron microscopy (SEM) mapping revealed a remarkably heterogeneous microstructure controlled by diagenetic precipitates (Fe/Mg carbonates, albite). Formation of these precipitates is likely triggered by organic matter decomposition and represents an important influencing factor for overall porosity and permeability. Furthermore, shale diagenesis also influences mechanical properties, as suggested by nanoindentation tests. The SEM-visible organic matter porosity is restricted to solid bitumen; although pores less than 2–3 nm in vitrinites of overmature samples are indicated by focused ion beam–SEM results, they cannot be resolved clearly by this method. Pore generation in solid bitumen that likely formed in situ in primary amorphous organic matter already starts at the early oil window in samples from the basinal oil-prone organofacies, whereas most porous solid bitumen at peak oil maturity was interpreted as relicts of primary oil migration, representing an earlier oil phase that predominantly accumulated in quartz-rich layers and became nanoporous during secondary cracking. In the terrestrially dominated transitional to marginal organofacies, pore generation in pyrobitumen resulting from gas generation occurs significantly later and is less intense. Formation of authigenic clay and carbonate minerals within pyrobitumen is likely related to organic acids formed during bitumen decomposition and implies the presence of an aqueous phase even in pores that are apparently filled exclusively with solid bitumen.

Geochemical and micropaleontological evidence of paleoenvironmental change in the Late Cretaceous Western Interior Seaway from the Niobrara to the Pierre Shale Formation

Deposition of the Upper Cretaceous Niobrara Formation and overlying Sharon Springs Member of the Pierre Shale occurred during transgressive phases within the Western Interior Cretaceous (WIC) Seaway, however their respective geochemical and biological differences are significant. The Niobrara Formation is an alternating calcareous chalk/marl system, biologically defined by fecal pellets and robust foraminifers including Heterohelix globulosa, Globigerinelloides ultramicrus, Hedbergella, Giimbelina and two Archaeoglobigerina species. However, the Sharon Springs is primarily argillaceous in composition with lesser amounts of biogenic calcite and silica. Larger foraminifers within the Sharon Springs are primarily arenaceous whereas foraminifers of the same species found in the Niobrara are dwarfed in the Sharon Springs. Dwarfism of foraminifers species and the presence of dispersed diatoms, which flourish in cold waters, indicate a shift in paleocurrents. Deposition of the Niobrara occurred during times when warm Gulfian currents allowed for carbonate production through the Early Campanian. Paleogeographic maps of the Middle Campanian indicate a southerly restriction of the WIC during the time of the southerly Claggett Transgression. The influx of cold, southerly waters resulted in an environment conducive to diatoms and environmentally stressful to foraminifers. Lithological and geochemical evidence from the Sharon Springs indicates that the cold-water Claggett Transgression resulted in a stratified water column in the middle of the WIC as well as increased organic matter production. Petrographic analysis of preserved organic matter show clumped floccules or flattened amalgamations of floccules in laminated facies. While dispersed amorphous organic matter is also present, flocculation had a major influence on organic matter preservation. Two possible depositional mechanisms, each dependent on water density, probably produced these water conditions. If density contrasts existed between the cold water influx and the warm waters present during the deposition of the Niobrara, the cold waters would have progressed along the bottom of the basin leading to upwelling along the basin margin and basin stratification. If no significant density contrast existed, caballing would occur at the mixing front, leading to downwelling, rapid transport of organic matter to the sea floor, better organic matter preservation and basin stratification.

Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: insights from organic petrography and geochemical techniques

This paper investigates chemical functional groups of the two extracted bitumen fractions in shales of the Jurassic to early Cretaceous Vaca Muerta Formation of the Neuquén Basin in Argentina, South America. The results indicate that Bitumen I is strongly aliphatic and appears to be genetically related to fluorescent amorphous organic matter. In contrast, Bitumen II consists of highly condensed, aromatic hydrocarbons, and has some correspondence to nonfluorescent amorphous organic material. Comparison of Rock-Eval VI pyrolysis data (S1 and S2) with the bitumen yields suggests that Bitumen I relates to S1 but also to S2. In addition, Bitumen I has a positive correlation with light liquid hydrocarbons (C5–C29), but also partially with heavier hydrocarbons (above C30). This suggests that Bitumen I corresponds to the majority of lighter hydrocarbons up to C29 and some portion to heavier hydrocarbons. These results have implications for the assessment of the mobility of generated hydrocarbons and their availability for production.

Integration of the outcrop and subsurface geochemical data: implications for the hydrocarbon source rock evaluation in the Lower Indus Basin, Pakistan

The present study details the hydrocarbon source rock geochemistry and organic petrography of the outcrop and subsurface samples of the Middle Jurassic Chiltan Formation and the Lower Cretaceous Sembar Formation from the Sann #1 well Central and Southern Indus Basin, Pakistan. The total organic carbon (TOC), Rock–Eval pyrolysis, vitrinite reflectance (Ro) % and Maceral analysis techniques were used and various geochemical plots were constructed to know the quality of source rock, type of kerogen, level of maturity and migration history of the hydrocarbons. The outcrop and Sann #1 well data on the Sembar Formation reveals poor, fair, good and very good quality of the TOC, type II–III kerogen, immature–mature organic matter and an indigenous hydrocarbon generation potential. The outcrop and Sann #1 well data on the Chiltan Formation show a poor–good quality of TOC, type II–III kerogen, immature–mature source rock quality and having an indigenous hydrocarbon generation potential. The vitrinite reflectance [Ro (%)] values and Maceral types [fluorescent amorphous organic matter, exinite, alginite and inertnite] demonstrate that maturity in both Sembar and the Chiltan formation at surface and subsurface fall in the oil and gas generation zone to cracking of oil to gas condensate zone. Recurrence of organic rich and poor intervals within the Sembar and Chiltan formation are controlled by the Late Jurassic thermal uplift preceding the Indo-Madagascar separation from the Afro-Arabian Plate and Early Cretaceous local transgressive–regressive cycles. From the current study, it is concluded that both Sembar and Chiltan formation can act as a potential hydrocarbon source rock in the study area.

Formation and occurrence of organic matter-hosted porosity in shales

Porosity within organic matter (OM) is considered to be the main site for gas storage (via fluid phase saturation of the pores and sorption on the pore walls) and thus its presence or absence is very critical for in-place gas assessment for organic-rich rocks. Numerous workers have suggested that OM-hosted porosity increases with thermal maturity mainly related to the process of bituminized organic matter cracking to gas. Comprehensive reviews of published literature enable us to conclude that organic porosity dominantly develops within bituminized organic matter (i.e., that portion that is petrographically identified, mainly based on its morphology, as solid bitumen (Mastalerz et al., 2018)) and primary (i.e, structured or amorphous) organic matter (kerogen) is mostly deficient in porosity. We show that in the same shale sample, structured kerogen shows no porosity whereas solid bitumen contains abundant porosity. It has been previously reported that sample preparation for SEM by ion milling may alter the organic matter. In this study, some adverse effects of ion milling have been observed by comparing SEM-visible pores of the same sample prepared by mechanical grind polish (MGP) and ion milling (IM) methods. Since solid bitumen is more labile, and probably more prone to chemical/physical alterations, we recommend that SEM observations of ion milled samples to be conducted with more attentiveness.We show that 1) MGP is a suitable sample preparation method for SEM observations, 2) the overwhelming majority of SEM-visible organic matter-hosted pores develop within solid bitumen and not in structured organic matter (kerogen), and 3) we reaffirm that shale mineral content is a key factor in preservation or destruction of solid bitumen-hosted porosity.Finally, we propose a conceptual model for the formation, preservation and destruction of OM-hosted porosity for shales. Accordingly, we suggest that, all other factors being equal, a quartz-rich brittle shale will preserve more SEM-visible solid bitumen-hosted (meso and micro) porosity compared to that of a clay-rich ductile shale. Therefore, gas storage capacity of a quartz-rich shale will be relatively high compared to that of a clay-rich shale.