Dispersed Organic Matter Research Articles

Petrographic-geochemical characterization of the dispersed organic matter in menilite shales from the Silesian Unit in the Carpathian Mountains of SE Poland

Petrographic-geochemical characterization of the dispersed organic matter in menilite shales from the Silesian Unit in the Carpathian Mountains of SE Poland

Characteristics of source rocks of the Datangpo Fm, Nanhua System, at the southeastern margin of Sichuan Basin and their significance to oil and gas exploration

In recent years, much attention has been paid to the development environment, biogenetic compositions and hydrocarbon generation characteristics of ancient source rocks in the deep strata of the Sichuan Basin because oil and gas exploration extends continuously to the deep and ultra-deep strata and a giant gas field with the explored reserves of more than 1 × 1012 m3 was discovered in the Middle and Upper Proterozoic–Lower Paleozoic strata in the stable inherited paleo-uplift of the central Sichuan Basin. Based on the previous geological research results, outcrop section of the Datangpo Fm, Nanhua System, at the southeastern margin of the Sichuan Basin was observed and the samples taken from the source rocks were tested and analyzed in terms of their organic geochemistry and organic petrology. It is shown that high-quality black shale source rocks of the Datangpo Fm are developed in the tensional background at the southeastern margin of the Sichuan Basin between two glacial ages, i.e., Gucheng and Nantuo ages in the Nanhua Period. Their thickness is 16–180 m and mineral compositions are mainly clay minerals and clastic quartz. Besides, shale in the Datangpo Fm is of high-quality sapropel type source rock with high abundance at an over-mature stage, and it is characterized by low pristane/phytane ratios (0.32–0.83), low gammacerane abundance, high-abundance tricyclic terpane and higher-content C27 and C29 gonane, indicating that biogenetic compositions are mainly algae and microbes in a strong reducing environment with low salinity. It is concluded that the Datangpo Fm source rocks may be developed in the rift of Nanhua System in central Sichuan Basin. Paleo-uplifts and paleo-slopes before the Caledonian are the favorable locations for the accumulation of dispersed liquid hydrocarbons and paleo-reservoirs derived from the Datangpo Fm source rocks. In addition, scale accumulation zones of dispersed organic matter cracking gas and paleo-reservoirs originated from the Datangpo Fm source rocks may be discovered in the stable area inside the Sichuan Basin.

Source rock evaluation of Kharita and Bahariya formations in some wells, North Western Desert, Egypt: Visual palynofacies and organic geochemical approaches

Palynofacies analyses were applied on ninety-one samples from the subsurface Albian – Cenomanian succession represented by Kharita and Bahariya formations, encountered in El-Noor, and South Sallum wells, located in the North Western Desert, Egypt, to visually characterize the content of dispersed organic matter, as well as, organic geochemical characterization to reveal the depositional paleoenvironments and source rock potentiality. The result recognized of five palynofacies associations in the studied interval. The deposition of Kharita Formation took place mainly in a steady and a relatively stable deltaic to marginal environment continued as well in the lower part of Bahariya Formation with minor changes. The marine influence became more common in the upper part of Bahariya Formation showing the exceptional high hydrocarbon potential recorded in the studied interval. This indicates marine transgression by the end of the early Cenomanian (Upper Bahariya) age. Samples from the Kharita Formation contain abundant brown phytoclasts which suggest gas-prone kerogen type III and IV. While Bahariya Formation includes translucent, brown cuticles and woody tracheid phytoclasts pointing to more promising gas-prone kerogen type III. The organic geochemical analysis shows poor to fair gas-prone source rock potential within the study section., Thermally, the color of the spore grains in Kharita and Bahariya formations show that dark yellow to orange, indicates immature besides their general little poor hydrocarbon generation potentiality.

Mechanisms of generation of hydrogen sulphide, carbon dioxide and hydrocarbon gases from selected petroleum fields of the Zechstein Main Dolomite carbonates of the western part of Polish Southern Permian Basin: Isotopic and geological approach

In this paper we present the results of analyses of molecular and isotopic (34S/32S in H2S, 13C/12C in CO2, CH4, C2H6 and C3H8, and 2H/1H in CH4) compositions of natural gas and stable sulphur isotope composition of S-organic compounds of oils from the Zechstein Main Dolomite carbonate reservoir of PZ2 cycle and sulphates of Basal Anhydrite of PZ2 and Upper Anhydrite of PZ1 cycles of the western part of Polish Upper Permian Basin. These results related to geological and palaeogeological conditions reveal that: (i) H2S could be generated mainly during thermochemical sulphate reduction (TSR) at the turn of Jurassic and Cretaceous and, in negligible volumes, during microbial sulphate reduction (MSR) and also during thermal decomposition of S-containing compounds of dispersed organic matter and oil; (ii) CO2 was generated during microbial and thermal transformations of organic matter, TSR and MSR processes, and during carbonate dissolution by fluid migration; (iii) the hydrocarbon components were generated by both microbial and thermogenic processes. The microbial methane was generated by fermentation and also during carbon dioxide reduction. Thermogenic hydrocarbons were generated during a single generation process of early low-temperature stage of “oil window” from type II kerogen.

Hydrocarbon composition of bitumen from deeply buried terrestrial organic matter (zone of apocatagenesis)

We present new results catagenetic alteration of dispersed organic matter (DOM) and individual hydrocarbon composition of bitumen extracts from the lower mesocatagenesis and apocatagenesis zone, based on the study of core samples from superdeep well SV-27 (Vilui syneclise), which penetrated Permian and Upper Carboniferous coal-bearing strata. The vitrinite reflectance data show a progressive increase in the thermal maturity of DOM at a depth of ~4 km. The major biomarker indicators of the thermal history of DOM in the central part of the Vilyui syneclise appear to play only a minor role below this depth, thus reflecting the early stages of mesocatagenesis. The pristane/phytane ratios begin to invert at the same depth. Bitumen extracts from core samples collected from this depth contain new hydrocarbons in trace amounts, which increase with depth. The identified compounds include homologous series of alkenes, 2,4- and 2,7-dimethylalkanes, and alkylcyclohexanes with a prevalence of compounds with an odd carbon number predominance. Unusual differentiation of aliphatic and cyclic hydrocarbons with even and odd carbon-numbered chains occurs at the maximum depth (6458 m). Among aromatic hydrocarbons, four new diastereomers are identified: 17-desmethyl–23-methylmonoaromatic steroids C27. Considerable variations in the composition of bitumen extracts from the apocatagenetically altered core samples appear to reflect the termination of hydrocarbon generation from kerogen and further thermolysis of residual bitumens, including their asphaltene components. This explains the low values of maturity biomarker indicators corresponding to the apocatagenesis grade, since in this case they reflect the composition of hydrocarbons generated during the early stages and later occluded and adsorbed by asphaltenes. “Deep-seated” microoil from Permian, Carboniferous, and Cambrian deposits did not participate in the formation of oil rims and major gas condensate pools in the Vilyui petroleum area.

Identification of alginite and bituminite in rocks other than coal. 2006, 2009, and 2011 round robin exercises of the ICCP Identification of Dispersed Organic Matter Working Group

The paper presents results of round robin exercises on photomicrograph-based identification of dispersed organic matter in source rocks that represent a range of marine and lacustrine deposits from worldwide localities and cover a range of thermal maturities. The round robin exercises were conducted by the Identification of Dispersed Organic Matter Working Group (IDOM WG) of the International Committee for Coal and Organic Petrology (ICCP). The round robin exercises aimed to (1) assess the applicability of the established ICCP definitions of bituminite, (2) identify deficiencies and improve the existing nomenclatures, and (3) provide a basis for the revision of the bituminite and alginite definitions in the ICCP Handbook (Taylor et al., 1998). Three round robin exercises performed in 2006, 2009 and 2011 involved 18 participants from research laboratories at universities and within government and industry. Participants were asked to identify macerals on the basis of existing definitions in 129 photomicrographs taken in incident white light and fluorescent mode and also in fluorescence light mode at prolonged (15min) blue light irradiation. The results indicate that the definition of telalginite permits its positive and satisfactory agreement among the participants. In contrast, the descriptive characteristics of lamalginite referring to film-like appearance (filamentous) and lack of inner or outer structure are insufficient and inadequate to enable an unequivocal discrimination between it and telalginite. Furthermore, based on the amorphous nature of bituminite and lack of adherence to its established description and character (Taylor et al., 1998), the highest discrepancies were observed in its identification. Differentiation of bituminite from a fluorescent groundmass and in some cases from lamalginite proved to be particularly challenging. The findings of these round robin exercises are useful for improving the identification of sedimentary organic matter in source rocks and may be an important foundation for the modification of the ICCP definitions of alginite and bituminite.

Hydrocarbon composition and structural parameters of resins and asphaltenes of naphthenic oils of northern West Siberia

The composition of saturated and aromatic hydrocarbons and the structural-group characteristics of resins and asphaltenes in four oils of northern West Siberia have been studied. As follows from the distribution of hydrocarbons, the Cenomanian oils (pools in the beds PK and K2c) of the Russkoe, Barsukovskoe, and Pangodinskoe fields are naphthenic and the Pangodinskoe oil sample from the Lower Cretaceous pool (bed BN9) is methanoic. The hydrocarbon composition pattern of the saturated fraction and the characteristics of the heterocyclic components of these oils point to their formation from dispersed organic matter of mixed genesis in the oil window. Hence, the oil source rocks are localized at depths much greater than the present-day level of the oil pools. Secondary (cryptohypergene) alterations (oxidation and biodegradation) in the shallow-depth oils determined their current predominantly naphthenic composition and considerably hamper reconstruction of the types of initial dispersed organic matter. We assume that the mixed genetic type of the studied oils is not only due to their heterogeneous source but also due to the reorganization of primary oil pools at the Cenozoic stage of tectogenesis. The Cenomanian Russkoe and Pangodinskoe naphthenic oils are saturated with adamantoids, which can be selectively accumulated during biodegradation.

Molecular geochemistry of organic matter of Triassic rocks in the northeastern part of the Barents Sea: the influence of tectonic and magmatic processes

Samples of Triassic sediments were collected from cores of parametric boreholes drilled on the Admiralteiskii Rise, Franz Josef Land Archipelago, and Fersman structure in the northeastern part of the Barents Sea sedimentary basin. The composition and distribution of hydrocarbon biomarkers (n-alkanes, isoprenoids, cyclic alkanes, and polyaromatic hydrocarbons) were studied using the GC-MS method and provided information about the genesis, facies conditions of sedimentation, and catagenetic maturity stage of dispersed organic matter (DOM). The Late Mesozoic-Cenozoic uplifting of the region and active magmatic processes significantly influenced the thermal transformation of sediments. The revealed trend is best observed in the sediment section of the Franz Josef Land Archipelago, where the thermal effects led to a radical increase in DOM transformation, up to the AC1 stage.

Geochemistry of organic matter of the Bazhenov Formation in the north of the Khantei anteclise

Results of comprehensive geochemical studies of dispersed organic matter of the Bazhenov Formation are presented. The maximum total contents of organic carbon are found in the areas of the Surgut arch and South Nadym megamonoclise. The contents of organic matter are high (up to 24%) at the top of the formation section and gradually decrease to its bottom. By pyrolysis parameters and the composition of hydrocarbon biomarkers, the organic matter is referred to as type II kerogen (sapropelic organic matter) and is localized in the oil window.

Geochemistry, composition, and structure of protoasphaltenes in organic matter of recent lacustrine sediments

Asphaltenes and asphaltogene acids of organic matter (OM) of lacustrine sediments in the Novosibirsk and Tomsk Regions of West Siberia have been investigated by applying physical and physicochemical methods (elemental and X-ray diffraction analyses, IR, NMR, and EPR spectroscopy, Rock Eval pyrolysis, and electron microscopy). Compared with asphaltenes of dispersed OM at the early catagenesis stages, asphaltenes of OM at the diagenesis stage have a high number of hydrogen atoms and heteroatoms, a low degree of aromaticity, a low concentration of paramagnetic centers, a large number of carbon atoms in the long paraffin chains and oxygen-containing groups. Tetravalent vanadium is absent from asphaltenes of recent lacustrine sediments, and only complex compounds of copper have been identified. The sediments contain not asphaltenes but their precursors, protoasphaltenes. These are less condensed compounds with a looser nongraphite structure and a rather high petroleum potential. At the diagenesis stage, protoasphaltenes are immature protokerogen blocks.

Thermal methods implementation in analysis of fine-grained rocks containing organic matter

The thermal methods, including TG and DSC techniques, were implemented to determine the composition of the fine-grained rocks with dispersed organic matter. The analyses were carried out in inert and oxidising atmosphere. The analysed samples represented Palaeozoic siltstones, shales, claystones and mudstones from Baltic Basin, Lublin Basin and Upper Silesian Coal Basin (Poland). There was detected the presence of some minerals as: clay minerals, muscovite, quartz, calcite, dolomite and pyrite. A comparison of the reactions occurring in an oxidising and inert atmosphere can give a solution to the problem of some effects overlapping, such as distinguishing pyrite decomposition from organic matter combustion. The analysis of DSC curves of heated rocks under oxidative atmosphere allows to determine the temperature regions of the organic matter combustion and to conclude on its maturity level. There was also found a strong correlation between the mass loss resulted from organic matter combustion, and the mass loss as a result of pyrolysis.

Maturity of dispersed organic matter in bituminous formations of the Ionian Zone (Epirus region, NW Greece)

The main objective of this paper is to study by means of Organic Petrology techniques, the maturity of the dispersed organic matter from certain sedimentary formations of the Ionian Zone, such as the Bituminous Shale, the Upper Siliceous Vigla Formation and the Bituminous Sandstone. The samples were collected from outcropping sites located in the region of Epirus. Initially they were treated with acids (HCl-HF) to remove most of the carbonate and silicate minerals. Then a ZnCl2 solution was used to concentrate the organic-rich fraction. Total Organic Carbon (TOC) content was determined applying dichromate oxidation. Polished blocks were prepared from the concentrated organic matter mounted in epoxy resin and examined under the coal-petrography microscope. Emphasis was given to maceral identification and vitrinite reflectance (R) measurements, which provide information regarding the quality and the maturity of the organic matter respectively, with implications for the petroleum generation potential regardless the level of alteration. The TOC and Rr values (4.74% and 0.68%, respectively) confirm to the oil potential of the Lower Jurassic Posidonia Shale. Nevertheless, it is suggested that detailed and higher resolution sampling focusing on the Lower Posidonia Shale, as well as organic petrography analyses coupled with Rock-Eval pyrolysis should be carried out in order to accurately determine its quality as petroleum source rocks.

A conceptual framework for mixing structures in individual aerosol particles

AbstractThis study investigated the particle size‐ and age‐dependent mixing structures of individual particles in clean and polluted air. Aerosols were classified into eight components: sea salt, mineral dust, fly ash, metal, soot, sulfates, nitrates, and organic matter (OM). Based on our aerosol classification, a particle that consists of two or more aerosol components can be defined as an internally mixed particle. Otherwise, it is considered to be an externally mixed particle. Within the internally mixed particle class, we identified four heterogeneous mixing structures: core‐shell, dumbbell, OM coating, and dispersed OM, as well as one homogeneous‐like mixing structure. Homogeneous‐like mixing mainly occurred in fine particles (<1 µm), while the frequency of heterogeneously mixed particles increased with particle size. Our study demonstrated that particle mixing structures depend on particle size and location and evolve with time. OM‐coating and core‐shell structures are important indicators for particle aging in air as long as they are distant from specific emission sources. Long‐range transported particles tended to have core‐shell and OM‐coating structures. We found that secondary aerosol components (e.g., sulfates, nitrates, and organics) determined particle mixing structures, because their phases change following particle hydration and dehydration under different relative humidities. Once externally mixed particles are transformed into internally mixed particles, they cannot revert to their former state, except when semivolatile aerosol components are involved. Categorizing mixing structures of individual particles is essential for studying their optical and hygroscopic properties and for tracing the development of their physical or chemical properties over time.

Thermal maturity of Silurian deposits in the Baltic Syneclise (on-shore Polish Baltic Basin): contribution to unconventional resources assessment

Shale gas is envisaged to contribute in the next future to the European energy mix in the prospective of lowering CO2 emissions. Poland is by far one of the most prospective countries in Europe. In the Golden Belt, potential productive levels are Early Paleozoic in age and the reliable assessment of their thermal maturity is crucial for evaluating hydrocarbon generation/expulsion scenarios.When exploring Early Paleozoic targets that are devoid of vitrinite macerals, uncertainties in thermal maturity evaluation can occur according to commonly adopted parameters (e.g., vitrinite reflectance). These uncertainties can negatively influence targets assessment.We adopted a multi-method approach to assess thermal maturity of the Silurian sections encountered in three wells deep between 2.9 and 3.3 km, recently drilled in the Polish Baltic Basin.The methodological strategy consists of: (i) measurement of organoclasts (mainly graptolites) reflectance; (ii) FT-IR spectroscopy on bulk dispersed organic matter; (iii) X-ray diffraction on

Solid bitumen, bituminite and thermal maturity of the Upper Jurassic-Lower Cretaceous Chia Gara Formation, Kirkuk Oil Field, Zagros Fold Belt, Kurdistan, Iraq

The Upper Jurassic-Lower Cretaceous Chia Gara Formation, a proven marine source rock characterised by predominance of amorphous organic matter (AOM) is known from few outcrops in NE Iraq and a number of exploration wells in the Zagros Fold Belt. This study presents comprehensive organic petrographic and mineralogical investigations of the dispersed organic matter and rock-forming minerals of the Tithonian-Berriasian Chia Gara Formation intersected by the Kirkuk well K-109 drilled in the giant Kirkuk oil field, NE Iraq. The organic petrographic analyses, including incident light microscopy and confocal laser-scanning microscopy (CLSM) were applied to carbonate source rocks with low to moderate organic carbon contents. Our data demonstrate the predominance of bituminite and solid bitumen among the examined dispersed organic matter followed by oil exudates and oil globule inclusions as well as the presence of minor contributions of alginite (i.e., Tasmanites and Leiospheridia), liptodetrinite and vitrinite. Based on optical characteristics and CLSM spectral fluorescence measurements, examinations of bituminite point towards the presence of bituminite type III. The bituminite type III encloses dark orange fluorescent amorphous components suggesting a genetic relationship either with liptodetrinite and/or with solid bitumen. The qualitative assessment of solid bitumen reveals several dissimilar forms identified in the analysed samples. The photometric and spectral fluorescence measurements point to distinct reflectance values ranging between 0.19% and 0.43% SBRr and lambda max values extending from 515nm to 590nm, respectively. The corresponding results of the solid bitumen exhibit (1) a lack of a relationship with depth and, (2) suggest its poor property as a reliable maturity indicator for the examined samples. The presence of oil exudates, oil globule inclusions and their association with solid bitumen suggests a genetic relation with the examined residues of liquid hydrocarbon phase. The performed thermal maturities assessments of dispersed organic matter point towards early-mature to peak-mature oil window with vitrinite reflectance values ranging between 0.52% VRr at 2801m to 0.83% VRr at 3089m depth. Mineralogical analyses revealed the predominance of calcite over dolomite in the examined limestone to marl lithofacies (e.g., clear rimmed dolomite) with localised intensive dolomitisation. In the phyllosilicate-rich matrix, predominance of calcite and siderite is observed, followed by zinc blende and pyrite. The composition of organic matter in the examined Chia Gara Formation with predominance of bituminite and solid bitumen makes the major marine source rock of Iraq's Cretaceous petroleum systems distinctive in terms of hydrocarbon generation and migration at an oil window stage as it provides suitable explanations for its generally low hydrogen index (HI) with only minor terrestrial input. In addition, the results presented herein can be helpful for future evaluation of thermal maturities and characterisation of optical appearances of dispersed organic matter in carbonates by incident light microscopy and especially by CLSM during hydrocarbon prospecting.

Trace elements and REE geochemistry of Middle Devonian carbonate mounds (Maïder Basin, Eastern Anti-Atlas, Morocco): Implications for early diagenetic processes

Trace and rare earth elements (REEs) have proven their utility as tools for assessing the genesis and early diagenesis of widespread geological bodies such as carbonate mounds, whose genetic processes are not yet fully understood. Carbonates from the Middle Devonian conical mud mounds of the Maïder Basin (eastern Anti-Atlas, Morocco) have been analysed for their REE and trace element distribution. Collectively, the carbonates from the Maïder Basin mud mounds appear to display coherent REE patterns. Three different geochemical patterns, possibly related with three different diagenetic events, include: i) dyke fills with a normal marine REE pattern probably precipitated in equilibrium with seawater, ii) mound micrite with a particular enrichment of overall REE contents and variable Ce anomaly probably related to variation of pH, increase of alkalinity or dissolution/remineralization of organic matter during early diagenesis, and iii) haematite-rich vein fills precipitated from venting fluids of probable hydrothermal origin.Our results reinforce the hypothesis that these mounds were probably affected by an early diagenesis induced by microbial activity and triggered by abundance of dispersed organic matter, whilst venting may have affected the mounds during a later diagenetic phase.

Alteration of organic matter by ion milling

Ion milling is increasingly used as a mean of sample surface preparation for scanning electron microscopy (SEM) to provide high-resolution imagery needed for studying the nanoscale structure of mudstones. Application of this technique has introduced new insight into nano-structural properties within organic matter (i.e., organic porosity) as an important reservoir control factor in unconventional shale/tight rocks. The continuous bombardment of the sample surface by ions (e.g., Gallium, Argon) results in the formation of kinetic heat energy, hence heating the milled surface of the rock. While this technique has proven to provide a spectacular surface for studying the fine structures, the effect of initial heat transfer to the organic matter (OM) is not well understood. Optical properties of OM (reflectance and fluorescence spectrometry) respond to the degree of aromatization of organic molecules, caused by thermal alteration. The effects of ion milling on dispersed OM were studied on four different macerals (i.e., solid bitumen, pyrobitumen, zooclasts, and alginate) within mudrocks of different thermal maturity levels. These mudrocks were milled with a cryogenic broad ion beam (C-BIB) and a focused ion beam (FIB). The random reflectance (Ro), bi-reflectance and the fluorescence red/green quotient (Q) of the macerals were measured and compared before and after the milling processes.The results show that initial heating caused by ion bombardment results in significant thermal alteration of the OM surfaces. The C-BIB milling resulted in 10 to 53% increase in the Ro values of the studied macerals, depending on their initial chemical refractoriness. The higher energy FIB milling increased Ro values of the same macerals by one order of magnitude. The solid bitumen appears to be more susceptible to thermal alteration than more refractory macerals (e.g., zooclasts and pyrobitumen). Ion milling did not shift the fluorescence spectra in immature alginate, suggesting that thermal alteration caused by ion milling is limited to the surface and not the entire body of the maceral. This study also documents significant morphological and possible chemical changes in pyrite due to the ion beam incident thermal alteration in most samples.Since most ion beam SEM studies focus on organic nanoporosity within the OM, thermal alteration caused by ion milling may have serious implications for image-based physical analysis of pores. The possibility of increased nanoporosity due to rapid devolatilization of lighter organic compounds in macerals could potentially have a significant impact on organic porosity, especially at lower thermal maturities (i.e., oil and wet gas windows).

Fossilization of organic matter, formation of oil and gas, and emigration of hydrocarbons in carbonate rocks

Based on the comparison of conditions of organic matter (OM) accumulation in modern carbonate sediments in Paleozoic shallow-water carbonate sediments, it is shown that drastic disproportion in the degree of preservation of the primary dispersed OM (DOM) in the clayey and “pure” varieties of carbonate rocks is not caused by its loss due to the diagenetic oxidation in the shallow-water setting, its disintegration due to the vital activity of microorganisms, or due to its consumption for the reductive oxide forms of Fe. It has experimentally been proven that a great significance in oil and gas formation in carbonate rocks belongs to OM, which occurs in the carbonate component of the chloroform bitumen (CBCCR) and is not determined by the conventional analytical methods (e.g., incineration of the HCl-insoluble rock remnant). Higher concentration of hydrocarbons (HC) in CBCCR relative to CBA is confirmed by the oil-generating properties of “pure” carbonate rocks. The release and emigration of HC from carbonate rocks are promoted by their secondary transformations.

Comparative study of vertical suspension fluxes from the water column, rates of sedimentation, and absolute masses of the bottom sediments in the White Sea basin of the Arctic Ocean

A new approach using dispersed organic matter of the water column in sedimentation traps in comparison with the surface layer of the bottom sediments is applied for the study of marine sedimentation. This approach provides the opportunity for an in situ (by fluxes of sedimentary matter in the water column) study of modern sedimentation in the surface layers of the bottom sediments and tracing the changes in the environment and climate at a new technological level. This also allows us to choose the reverse task: to reconstruct the fluxes of the matter and chemical elements in ancient seas by the rates (or absolute masses) of sedimentation.

Graptolite-derived organic matter in the Wufeng–Longmaxi Formations (Upper Ordovician–Lower Silurian) of southeastern Chongqing, China: Implications for gas shale evaluation

The organic-rich shales of the Wufeng–Longmaxi Formations (Ordovician-Silurian) are one of the most important hydrocarbon source rocks and shale gas exploration targets in China. Graptolites are abundant in these shales, but their contribution to the dispersed organic matter (OM) has been overlooked. This lack of attention could seriously affect the evaluation of OM type, OM maturity, hydrocarbon generation potential, reservoir characteristics and gas content of these shales. Optical microscope and focused ion beam-scanning electron microscope (FIB-SEM) techniques were employed to describe the optical characteristics and micro-nanopores of the graptolites, to determine the contribution of the graptolites and to evaluate the thermal maturity of the Wufeng–Longmaxi Formations.Non-granular graptolites, which show a stronger anisotropy and higher reflectance under polarized light than the granular graptolites, were primarily observed in these sediments. Graptolites account for 20–93% of the dispersed OM in the Wufeng–Longmaxi Formations, and the level of their contribution has been attributed to the radiation of graptolites at the late Ordovician (Kaptian) and the recovery of graptolites at the early Silurian (Rhuddanian). The bireflectance values of the non-granular graptolites fall between 2.22 and 4.32%, indicating a behavior similar to biaxial anthracitic vitrinite. The equivalent vitrinite reflectance (EqVRo) values of the sediments in the Wufeng–Longmaxi Formations, calculated from the mean maximum reflectance of non-granular graptolite, fall in the range of 3.08–4.29%, and these values have a precision that is greater than in previous studies on the Wufeng–Longmaxi Formations. Abundant organic pores were observed in the non-granular graptolites, and they are aligned parallel to the fusellar layers of the graptolites. This indicates that the fusellar layers of non-granular graptolites controlled the development and distribution of organic pores. Such abundant organic pores played a significant role in the accumulation of shale gas in the Wufeng–Longmaxi Formations of the Upper Yangtze Platform.