Organic-rich Samples Research Articles

Nanoscale confined gas and water multiphase transport in nanoporous shale with dual surface wettability

Fluid transport in nanoporous shale is known to be affected by the nanoscale fluid transport mechanisms, surface wettability and heterogeneous pore structure. The pores of shale are believed to be dual surface wettability with gas-wet organic pores and water-wet inorganic pores. Investigation on the nanoscale multiphase transport behavior in dual surface wettability nanoporous shale has practical implication in understanding inject water distribution during injected water flow in and flow back process. In this study, we propose a nanoscale gas and water multiphase pore network transport model to study nanoscale confined gas and water transport behavior in dual wettability nanoporous shale. A 3-D shale pore network model is constructed from 3-D image that is reconstructed from 2-D shale SEM image of organic-rich sample. Water transport considers the boundary slip length determined by the contact angle. Bulk gas transport in inorganic pores considers slip effect while bulk gas transport and surface diffusion for adsorbed gas are both considered in organic pores. Injected water flow in process is modeled by water displacing gas process while injected water flow back process is modeled by gas displacing water process. Gas and water relative permeabilities during injected water flow in and flow back process at different TOC volumes and inorganic pore contact angle are analyzed in detail and are compared with relative permeabilities without nanoscale transport mechanisms. Study results reveal that nanoscale gas and water relative permeabilities are influenced by the total organic carbon (TOC) in volumes and inorganic pore water contact angle while nanoscale transport mechanisms influence on the relative permeabilities can be neglected.

Comparing optical and Raman spectroscopic investigations of phytoclasts and sporomorphs for thermal maturity assessment: the case study of Hettangian continental facies in the Holy cross Mts. (central Poland)

Abstract Thermal maturity of six organic rich samples from Jurassic continental successions cropping out in the Holy Cross Mountains in Central Poland, has been characterized by classic thermal maturity indicators, micro-Raman spectroscopy and Palynomorph Darkness Index, in order to create a multi-method workflow for complex palynofacies thermal maturity assessment. Transmitted light observations on dispersed organic matter define a Hettangian lacustrine depositional environment, characterized by periods of reducing/oxidizing conditions and variable sedimentation rates. Thermal maturity detected by classical maturity indicators and PDI indicates an early maturation stage of hydrocarbon generation and is in agreement with spectroscopic analyses performed on phytoclast groups. Moreover, Raman parameters in the sporomorph group indicate a systematic shift toward a lower degree of aromatization compared to the phytoclast group. Finally, the multivariate statistical analysis performed on Raman spectra is found to be a promising tool to define and predict the heterogeneity of dispersed organic matter in sediments.

An Experimental Study on Interactions Between Imbibed Fracturing Fluid and Organic-Rich Tight Carbonate Source Rocks

Summary Carbonate reservoirs dominate oil (70%) and gas (90%) reserves in the Middle East, and imbibition is the main mechanism for fracturing-fluid uptake during the hydraulic-fracturing stimulation process. Because of the highly heterogeneous nature of tight carbonate source rocks, it is crucial to understand the effects of imbibed fluid on the mechanical, morphological, and flow properties of carbonate rocks. Although the influence of imbibed fluids on the wettability of carbonate reservoir has been studied extensively, research regarding the effects of imbibed fluids on the texture and mineralogy of carbonate rocks is still very limited. This paper aims to provide a conceptual approach and work flow to characterize and quantify microstructure and mineralogy changes in carbonate rocks caused by imbibed fluids. A thin section of a low-permeability organic-rich carbonate-rock sample [7×7×0.3 mm (length×width×thickness)] was used in the study. The sample was submerged into 2%-KCl (pH = 7.1) fluid from one end to simulate the spontaneous-imbibition process. A scanning electron microscope (SEM) was used to capture the sample's morphological changes before and after spontaneous imbibition. Energy-dispersive-spectroscopy (EDS) maps were measured before and after fluid treatment to investigate changes in various elemental distribution. In addition, inductively coupled plasma (ICP) equipped with an optical-emission-spectrometer (OES) detector was used to quantify dissolved-ion concentration in the treatment fluid. Permeability and porosity were measured using core plugs with dimensions of 1.0×1.5 in. (diameter×length) before and after fluid treatment. During the imbibition process, approximately one-half of the sample was submerged in the treatment fluid. The SEM images for the thin-section sample showed three zones with distinct fluid-uptake characteristics. In Zone I, which was fully submerged in the testing fluid, a significant amount of mineral dissolution was observed. In Zone III, which was above the testing-fluid level, considerable mineral precipitation was detected. While in the transition zone just above the water/air interface (Zone II between the previous two zones), only a minor level of mineral dissolution was observed. Elemental-distribution changes resulting from the fluid treatment were identified by EDS analysis in all three zones. Gypsum and calcite crystals dissolved into imbibed fluids upon reaction. Gypsum was found reprecipitated on the rock surface in the zones above fluid level. The observed gypsum formation likely resulted from the dissolution of the gypsum from the rock matrix, then reprecipitation later from the imbibition experiment caused by water evaporation. Absolute-permeability and porosity measurements for core-plug samples have shown that both were increased after the imbibition process.

Multielemental composition and arsenic speciation in low rank coal from the Velenje Basin, Slovenia

Coal is one of the main fossil fuel resources and remains the most important contributor to global power generation but coal utilization has severe negative impacts, mostly due to the release of CO2 and toxic elements into the environment. This means that determining the quality of coal is important to address the environmental and health problems related to coal combustion. Coal samples from the Velenje coalmine, one of the largest actively mined coal basins in Central Europe, were divided into organic rich and inorganic rich fractions according to the percentage of carbon. Oxides of the major elements (SiO2, Al2O3, Fe2O3 MgO, CaO, Na2O, K2O, TiO2, P2O5, MnO, Cr2O3), toxicologically and environmentally relevant elements (As, Ba, Co, Cu, Hg, Mo, Ni, Pb, Th, U, V, Zn, Se) and other trace elements (Ce, Cs, Dy, Er, Eu, Gd, La, Nb, Nd, Pr, Rb, Sm, Sr, Tb, Y, Zr) were measured in sample digests using inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of the majority of analyzed elements were either equal to or below the global average for coal. Exceptions were Sr (1090 ± 510 μg g−1, 9 times higher), Ba (301 ± 184 μg g−1, 2 times higher) and Pb (9.12 ± 17.0 μg g−1, 1.4 times higher) in inorganic rich coal and Mo (7.76 ± 4.76 μg g−1, 3.5 times higher) and U (5.24 ± 3.23 μg g−1, 1.8 times higher) in organic rich samples. Eighteen elements (Ag, Au, Be, Bi, Cd, Ga, Hf, Ho, Lu, Sb, Sc, Se, Sn, Ta, Ti, Tm, Yb, W) were below the limit of detection in >70% of the samples. Speciation analysis revealed the presence of several organoarsenic compounds in the organic rich samples, with the tetramethylarsonium ion (TETRA, 0.01–1.10 μg g−1) and trimethylarsine oxide (TMAO, 0.01–0.29 μg g−1) as the most abundant. A comparison with coal samples from the Senovo, Kanižarica, and Trbovlje coal mines in Slovenia and from the Sokolov Basin, (Czech Republic) revealed that Velenje contains much higher amount of organoarsenic compounds (34.8 ± 16.9%) in comparison to the others (4.45 ± 4.19%).

Simulating the timing of petroleum generation and expulsion from deltaic source rocks: Implications for Late Cretaceous petroleum system in the offshore Jiza-Qamar Basin, Eastern Yemen

The Mukalla Formation is one of the important petroleum source rocks in the Jiza-Qamar Basin, Eastern Yemen. In this study, coal, coaly shale and shale samples from three wells (Al-Fatak-1, 16/G-1 and 16/E-1) in the offshore Jiza-Qamar Basin were studied. The organic matter richness, kerogen type, thermal maturity, and petroleum generation potential of the Mukalla organic-rich samples were investigated.The analysed Mukalla source rock samples are potentially rich in organic matter of 1.01–84.40% TOC, ranging from good to excellent source rocks. The Mukalla source rocks primarily contain Types II, II–III and III kerogen, anticipating generating mainly oil and gas. This is generally consistent with the pyrolysis–gas chromatography results and further indicates that the Mukalla source rocks can produce high wax oil and condensate/gas. The thermal maturity parameters reveal the Mukalla source rocks are in mature to late-mature and are capable of generating oil and wet gas at the present time. In addition, this study was integrated the geochemical, geo-thermal and geological data in term of basin modeling study to simulate the timing of hydrocarbon generation and expulsion from the Mukalla deltaic source rocks.The basin model results indicate that the oil generation commenced during the late Eocene to late Oligocene, with a conversion ratio of 10–50 TR%. Furthermore, based on TRs of more than 50%, the most of oil was expelled along the micro-fracturing of the Mukalla source rocks and then trapped in the Mukalla Formation itself during the latest Oligocene and continued to present-day.

Relative Permeability and Production-Performance Estimations for Bakken, Wolfcamp, Eagle Ford, and Woodford Shale Formations

Summary Relative permeability and residual water/gas saturations of organic-rich samples were determined from low-pressure nitrogen-adsorption/desorption measurements on shale samples. Adsorption/desorption isotherm measurements were performed on 100 organic-rich shale samples from the Bakken, Wolfcamp, Eagle Ford, and Woodford Formations, and were then interpreted to obtain pore-size distribution (PSD) using a modified Barrett-Joyner-Halenda (BJH) method (Barrett et al. 1951). A bimodal-fractal model was then applied to the PSD estimates to compute percolation and fractal parameters, which were incorporated in percolation theory (PT), effective-medium theory (EMT), and critical-path analysis (CPA) models to generate the relative permeability curves and residual saturations of the 100 samples. Robustness of this relatively new estimation technique for shale-reservoir samples is evaluated in this study. Petrophysical correlations of relative permeability and residual saturations with kerogen maturity, kerogen content, and kerogen removal through sample cleaning are developed for improved understanding of the saturation-dependent transport behavior of shale reservoirs. These correlations enable comparative predictions of intrawell- and interwell-production performance. Kerogen maturity was found to be the dominant factor affecting the relative permeability and residual-saturation estimates. Irreducible-water-saturation and residual-hydrocarbon-saturation estimates increased from 30 to 60% and 20 to 40%, respectively, with an increase in maturity from oil window to late-condensate window. Estimates of relative permeability of hydrocarbon and aqueous phases at 80% saturation of the corresponding phases increased from 0.7 to 0.75 and 0.05 to 0.2, respectively, with the decrease in kerogen maturity. When the samples were cleaned with a toluene/methanol mixture to remove soluble, dead hydrocarbons and bitumen from the samples, the hydrocarbon-phase residual-saturation estimates decreased by 20%, whereas the aqueous-phase irreducible-saturation estimates increased by 9%. The estimated relative permeability curves allow us to predict the production performance using a numerical simulator. Lower Bakken and Wolfcamp oil formations have the best hydrocarbon-transport characteristics because of lower thermal maturity. These formations show slower decline in hydrocarbon-production rates and more-uniform increase in water production. Hydrocarbon-phase production of the Lower Bakken oil formation is better than that of the Wolfcamp oil formation, but the water-production trend is significantly reversed. On the contrary, Wolfcamp and Woodford late-condensate formations exhibit the poorest transport characteristics and lower cumulative hydrocarbon and water production because of higher thermal maturity. Wells in these higher-maturity formations should show steeper decline in hydrocarbon-production rates and faster and sudden increase in water production after longer periods of sustained lower water production.

Impact of Different Experimental Heating Rates on Calculated Hydrocarbon Generation Kinetics

Four organic-rich samples from four basins in China have been analyzed, using open-system bulk pyrolysis with heating rates ranging from 0.7 K/min to 40 K/min. The resulting programs have been digitized and first-order Arrhenius kinetics have been optimized using groups of different heating rate ranges. Low heating rate optimization was performed for data generated at 5 K/min, 2 K/min, and either 0.7 or 1 K/min. High heating rate optimization used experimental rates of 15, 25, and 40 K/min. Optimization was also completed for wide heating rate ranges at 40 K/min, 15 K/min, and either 1 or 2 K/min. The kinetics solutions were then used to calculate bulk hydrocarbon generation at a geological heating rate of 3 K/Ma, in order to determine the impact of using different experimental approaches. The results showed that low versus high and narrow versus wide heating rates did not yield systematically different results. The highest predicted geological temperature was observed for the low heating rates (Huadian, ...

A new bomb-combustion system for tritium extraction

Quantitative extraction of tritium from a sample matrix is critical to efficient measurement of the low-energy pure beta emitter. Oxidative pyrolysis using a tube furnace (Pyrolyser) has been adopted as an industry standard approach for the liberation of tritium (Warwick et al. in Anal Chim Acta 676:93–102, 2010) however pyrolysis of organic-rich materials can be problematic. Practically, the mass of organic rich sample combusted is typically limited to <1 g to minimise the possibility of incomplete combustion. This can have an impact on both the limit of detection that can be achieved and how representative the subsample is of the bulk material, particularly in the case of heterogeneous soft waste. Raddec International Ltd (Southampton, UK), in conjunction with GAU-Radioanalytical, has developed a new high-capacity oxygen combustion bomb (the Hyperbaric Oxidiser; HBO2) to address this challenge. The system is capable of quantitatively combusting samples of 20–30 g under an excess of oxygen, facilitating rapid extraction of total tritium from a wide range sample types.

Organic petrology of peak oil maturity Triassic Yanchang Formation lacustrine mudrocks, Ordos Basin, China

An organic petrology evaluation and a determination of solid bitumen reflectance [Formula: see text] were completed for organic-rich Triassic Yanchang Formation mudrocks ([Formula: see text]) from the Ordos Basin, north-central China, as part of a larger investigation of “shale gas” resources. These data were integrated with information from Rock-Eval programmed pyrolysis to show that the samples are in the peak oil window of thermal maturity and that organic matter is dominated by solid bitumen with minor amounts of type III kerogen (vitrinite and inertinite) from vascular land plants. Describing a “kerogen type” for these rocks based strictly on parameters determined from programmed pyrolysis is misleading because the original organic matter has converted to hydrocarbons (present as solid bitumen), a large proportion of which may have been expelled into adjacent reservoir facies. However, based on the comparison with immature-early mature lacustrine mudrock (Garden Gulch Member of Green River Formation) and marine shale (Boquillas Formation), we suggest that the original organic matter in the organic-rich samples examined for our study may have been type I/II kerogen with hydrogen index values of [Formula: see text] TOC.

A Suite of Robust Radioanalytical Techniques for the Determination of Tritium and Other Volatile Radionuclides in Decommissioning Wastes and Environmental Matrices

Tritium is ubiquitous in and around nuclear plants, being formed via neutron capture by 2H, 6Li, 10B and 14N and via ternary fission. The highly mobile nature of 3H species results in widespread distribution of the radionuclide. Predictive modeling of 3H activity concentrations is challenging and direct measurement of 3H activities in materials is the preferred approach to underpin waste and environmental assessments. For well over a decade, the UK nuclear industry has engaged in a significant program of site decommissioning of its first generation reactors. This has resulted in a high demand for the rapid characterization of 3H in a diverse range of matrices, including concretes, metals, plastics, sludges, resins, soils and biota. To support such assessments, it has been necessary to develop dedicated instrumentation in parallel with robust radioanalytical methodologies; namely a multi-tube furnace and a high-capacity, closed (pressurized) oxygen combustion system. Data are presented on the development and validation of these instruments, designed specifically to enable the quantitative extraction of 3H (and other volatile radionuclides) from diverse sample types. Furthermore the furnace system has been employed as a tool to gain insights into the 3H association in decommissioning and environmental matrices exposed to 3H arising from nuclear power plant operations through tritium evolution with temperature profiling. The impact of the chemical speciation of 3H on analytical strategy is discussed. A major benefit of the multi-sample furnace is its ease of use and applicability to 3H determination in virtually any sample type. The complementary HBO2 oxygen combustion system has been developed for the quantitative oxidation of organic-rich samples (e.g. wood, plastic, oil, biota) and analytical data prove its effectiveness.

The impact of composition on pore throat size and permeability in high maturity shales: Middle and Upper Devonian Horn River Group, northeastern British Columbia, Canada

Shale reservoirs of the Middle and Upper Devonian Horn River Group provide an opportunity to study the influence of rock composition on permeability and pore throat size distribution in high maturity formations. Sedimentological, geochemical and petrophysical analyses reveal relationships between rock composition, pore throat size and matrix permeability.In our sample set, measured matrix permeability ranges between 1.69 and 42.81 nanodarcies and increases with increasing porosity. Total organic carbon (TOC) content positively correlates to permeability and exerts a stronger control on permeability than inorganic composition. A positive correlation between silica content and permeability, and abundant interparticle pores between quartz crystals, suggests that quartz may be another factor enhancing the permeability. Pore throat size distributions are strongly related to TOC content. In organic rich samples, the dominant pore throat size is less than 10 nm, whereas in organic lean samples, pore throat size distribution is dominantly greater than 20 nm. SEM images suggest that in organic rich samples, organic matter pores are the dominant pore type, whereas in quartz rich samples, the dominant type is interparticle pores between quartz grains. In clay rich and carbonate rich samples, the dominant pore type is intraparticle pores, which are fewer and smaller in size.High permeability shales are associated with specific depositional facies. Massive and pyritic mudstones, rich in TOC and quartz, have comparatively high permeability. Laminated mudstone, bioturbated mudstone and carbonate facies, which are relatively enriched in clay or carbonate, have fairly low permeability.

Measurement of Shale Matrix Permeability and Adsorption with Canister Desorption Test

Permeability estimation is a crucial part of the shale mudrock characterization because it affects the production rate, the pace of recovery and the technically recoverable hydrocarbons. Shale permeability typically falls in the low range of tens of micro-Darcy down to nano-Darcy. Accurate measurement of permeability is a challenging task because the measurement errors are more likely to occur in the low permeability range. This paper presents a common and accurate experimental method for permeability measurement of shale mudrocks known as the canister desorption test. We describe the experiment procedure and present the governing partial differential equation (PDE). The impact of linear and nonlinear adsorption isotherms and the corresponding analytical and numerical solutions are discussed. A workflow for the estimation of permeability and the gas adsorption parameter is presented. The workflow is used to measure the shale matrix permeability and the adsorption parameter for an organic-rich sample from the Marcellus formation. The measurements are in good agreement with an independent study on the Marcellus organic-rich core samples. The results show that the measurement of the adsorption parameter is as important as the permeability measurement in the organic-rich shales. In addition, given that the measurements of permeability and adsorption parameter are performed at a small scale and the gas properties are approximated by average values, it is recommended to perform multiple experiments at different pressures to constrain the uncertainty of the permeability and adsorption measurements. The limitations of the proposed workflow are discussed.

Organic petrology and geochemistry of mudrocks from the lacustrine Lucaogou Formation, Santanghu Basin, northwest China: Application to lake basin evolution

Exploration for tight oil in the frontier Santanghu Basin of northwest China has resulted in recent commercial discoveries sourced from the lacustrine Upper Permian Lucaogou Formation, already considered a “world class source rock” in the Junggar Basin to the west. Here we apply an integrated analytical program to carbonate-dominated mudrocks from the Lucaogou Formation in Santanghu Basin to document the nature of organic matter (OM) in the context of an evolving lake system. The organic-rich samples (TOC 2.8–11.4wt%; n=10) were widely spaced from an ~200m cored section, interpreted from textural and mineralogical evidence to document transition from a lower under-filled to an overlying balanced-filled lake. Organic matter is dominated by moderate to strongly fluorescent amorphous material with Type I geochemical signature (HI values 510–755; n=10) occurring in a continuum from lamellar stringers, 10–20μm thick, some ≥1mm in length (possible microbial mat; preserved only in lower under-filled section) to finely-disseminated amorphous groundmass intimately intermixed with mineral matrix. Biomarkers for methanotrophs and photosynthetic cyanobacteria indicate a complex microbial consortium. A unicellular prasinophyte green alga(?), similar to Tasmanites in marine rocks, is present as discrete flattened discs 50–100μm in diameter. Type III OM including vitrinite (some fluorescent) and inertinite also is abundant. Solid bitumen, indicating local kerogen conversion, fills voids and occurs throughout the cored section. Vitrinite reflectance values are 0.47–0.58%, consistent with strong OM fluorescence but may be “suppressed”. Other proxies, e.g., biomarker parameters, indicate the Lucaogou Formation is in the early oil window at this location. On average, slightly more amorphous OM and telalginite are present in the lower section, consistent with a shallow, stratified, saline environment with low sediment dilution. More inertinite is present in the upper section, indicating greater terrestrial influx and consistent with higher quartz and plagioclase content (dominantly authigenic chalcedony and albite). Laminated mudstones in the upper section indicate anoxia prevented bioturbation from benthic grazing, also indicating stratified water column conditions. A decrease upsection in authigenic dolomite with reciprocal increase of ankerite/siderite is consistent with decreasing salinity, as is an overall decrease in gammacerane index values. These observations suggest evolution from a shallow, stratified evaporative (saline) setting to a deeper, stratified freshwater basin with higher water input during Lucaogou deposition. The evolution from an under-filled to balance-filled lake in Santanghu Basin is similar to Lucaogou deposition in Junggar Basin, suggesting similar tectonic and climatic controls. Paleoclimate interpretations from other researchers in this area suggested an evolution from semi-arid to humid conditions during the Roadian; we interpret that the evolution from an under-filled to balanced-filled lake seen in our data is in response to climate change, and may represent increased groundwater delivery to the Santanghu Basin.

Black carbon assessment using benzene polycarboxylic acids: Limitations for organic-rich matrices

For the assessment of black carbon (BC), its oxidation to benzene polycarboxylic acids (BPCAs) is an established method. However, doubts about biological precursors remain and not all published data were obtained at low carbon concentration. We hypothesised that a considerable proportion of BC may be produced during sample treatment in the presence of a high amount of organic carbon (OC). We therefore tested whether and to which degree (i) BC-free material from stems of Zea mays L. (maize straw) and leaves of Capsicum annuum L. (bell pepper), as well as (ii) cyclic and non-cyclic carbon forms (chlorophyllin, ellagic acid and β-carotene) afford BPCAs when method protocols are overloaded with a sample above the recommended amount of 5mg OC. The results showed that small amounts (<2g/kg OC) of BPCAs with three and four carboxyl groups may be formed even at low sample weight (<5mg OC), thereby falsely representing biological BC production. When this threshold was exceeded, all BPCA forms were detected. The artificial BPCA production yield in g OC increased with increasing amount of OC (R2⩾0.81), adding up to 8.7g/kgOC (19.7gBC/kgOC) artificial production. We therefore strongly recommend that a threshold of 5mg OC sample concentration be maintained in future studies and that future BC assessments be restricted to BPCAs with five and six carboxyl groups. This constrains the application of the BPCA method for organic rich samples and for samples expected to contain a relatively low amount of BC.

Source rock characterisation of under-explored regions of Queensland

Historically, petroleum exploration in Queensland has focused on the Bowen-Surat and Cooper-Eromanga basins, with only cursory examination of other basins across the state. As part of the Queensland Industry Priorities Initiative, two projects (Round 1 and 2) were submitted to the Geological Survey of Queensland (GSQ) to examine the geochemical characteristics of potential petroleum source rocks throughout Queensland. The analysis conducted provides a better understanding of generative potential for petroleum, and predicts the timing, volume, composition, and physical state of hydrocarbons retained in and expelled from source rocks. It is an integral component to petroleum systems analysis used to identify the potential for undiscovered accumulations of petroleum from conventional and unconventional reservoirs. Of particular interest were the Georgina, Drummond, Eromanga, and Maryborough basins. Of these, the Georgina and Maryborough basins have known hydrocarbon shows identified through exploration drilling, though no commercial discoveries have yet been made. The Drummond Basin was targeted to identify a potential source for oil and gas shows encountered in drilling within the Galilee Basin. The Toolebuc Formation in the Eromanga Basin has been noted as having the potential for a shale oil play and this study is supporting further assessment to identify optimal areas for future exploration through predictive modelling. This report details the results from Round 1 of the study for samples taken from the Georgina Limestone and Scartwater, Ducabrook, Mount Hall, Toolebuc, and Maryborough formations, where limited analysis of source rock characteristics has historically been undertaken. Ninety-seven samples were chosen from nine wells and sent to Geos4 in Potsdam, Germany, for source rock analysis. All samples were screened for suitability of further analysis using Rock-Eval and TOC by LECO, with immature and organic-rich samples being preferentially selected for further testing. Screened samples were analysed using pyrolysis gas chromatography (n=27), thermovaporisation (n=23), bulk kinetics (n=5), compositional kinetics (n=4), late gas analysis (n=14), and biomarker and bulk isotope analysis (n=15). These results have been integrated with existing analyses to better understand the prospectivity of the under-explored basins of Queensland.

Nano-scale pore structure and fractal dimension of organic-rich Wufeng-Longmaxi shale from Jiaoshiba area, Sichuan Basin: Investigations using FE-SEM, gas adsorption and helium pycnometry

Much attention have been recently paid to the upper Ordovician Wufeng shale (O3w) and lower Silurian Longmaxi shale (S1l) in the Jiaoshiba area of Sichuan Basin, which is now the largest producing shale gas field in China. Field emission scanning electron microscopy (FE-SEM), low pressure gas (N2 and CO2) adsorption, helium pycnometry, X-ray diffraction and geochemical analyses were performed to investigate the pore structure and fractal dimension of the pores in O3w-S1l shale formation in the Jiaoshiba area. FE-SEM images show that organic matter (OM) pores are dominant in the organic-rich samples and these pores are often irregular, bubble-like, elliptical and faveolate in shape, while in organic-poor samples, limited and isolated interparticle (interP), intraparticle (intraP) and OM pores are observed. Reversed S-shaped isotherms obtained from nitrogen adsorption are type Ⅱ, and hysteresis loops indicate that the shape of micropore in the samples is slit-or plate-like. BET surface areas and total pore volume vary from 12.2 to 27.1 m2/g and from 1.8 × 10−2 to 2.9 × 10−2 cm3/g, with an average of 19.5 m2/g and 2.3 × 10−2 cm3/g, respectively. Adsorption volume from both N2 and CO2 adsorption increases with respect to TOC contents. Porosities obtained from helium porosimetry are comparable with these from gas (CO2 and N2) adsorption in O3w-S1l shale. However, porosity determined by quantitative FE-SEM analysis is much smaller, which is mainly related to limited resolution and the small areas of investigation.Based on the Frenkel-Halsey-Hill (FHH) model of low-pressure N2 adsorption, fractal dimensions of the pores varied from 2.737 to 2.823. Relationships between pore structure parameters and TOC content, mineral composition and fractal dimension reveal that the fractal dimension is mainly associated with micropores. Samples with higher TOC content, higher quartz content and lower clay content tend to contain more heterogeneous micropores, resulting in higher fractal dimensions and more complicated pore structure in shales. Therefore, fractal dimension is an effective parameter to reflect the complexity of pore structure and the degree of micropore development in O3w-S1l shale.

Methane dual-site adsorption in organic-rich shale-gas and coalbed systems

Herein, we report on a novel dual-site adsorption model to determine state of methane in shale formations and coalbeds. By considering two surface energy terms, the model distinguishes methane adsorption in the intrinsic micropore volume of the organic matter from that of the inorganic structure. One energy term describes methane adsorption in micropores (<2nm) whereas the other one concerns with meso-macropores. This study showed that at storage conditions, 150bar and 50.4°C, where methane is in supercritical state, the surface energy plays the major role to influence methane adsorption capacity whereas the effect of lateral forces between adsorbed molecules has a minor influence on overall adsorption capacity. We applied the model on experimental methane adsorption data in three organic-rich samples including Woodford shale, Woodford isolated kerogen and Cameo coal to quantify their methane storage state. Textural properties of shale and coal samples were analyzed by N2 porosimetry at −196°C. The samples had a weak micropore peak followed by a broader peak stretched over meso and macropore regions up to nearly 120nm. Fitting the dual-site model to experimental methane adsorption data revealed that, among all, the coal sample, with the average heat of adsorption of 24.5kJ/mol, had the highest interaction energies with both micro- and mesopore sites. Further, we find that 37%, 81% and 83% of methane was stored in the organic matter of Woodford shale, Woodford isolated kerogen and Cameo coal, respectively. Considering low maturity, and hence low micropore volume of the samples, and based on significant methane storage in the organic content we estimated that more than 50% of methane gas is being dissolved into the organic matter rather than just being adsorbed into the micropore volume.

Effect of nontronite smectite clay on the chemical evolution of several organic molecules under simulated martian surface ultraviolet radiation conditions.

Most of the phyllosilicates detected at the surface of Mars today are probably remnants of ancient environments that sustained long-term bodies of liquid water at the surface or subsurface and were possibly favorable for the emergence of life. Consequently, phyllosilicates have become the main mineral target in the search for organics on Mars. But are phyllosilicates efficient at preserving organic molecules under current environmental conditions at the surface of Mars? We monitored the qualitative and quantitative evolutions of glycine, urea, and adenine in interaction with the Fe(3+)-smectite clay nontronite, one of the most abundant phyllosilicates present at the surface of Mars, under simulated martian surface ultraviolet light (190-400 nm), mean temperature (218 ± 2 K), and pressure (6 ± 1 mbar) in a laboratory simulation setup. We tested organic-rich samples that were representative of the evaporation of a small, warm pond of liquid water containing a high concentration of organics. For each molecule, we observed how the nontronite influences its quantum efficiency of photodecomposition and the nature of its solid evolution products. The results reveal a pronounced photoprotective effect of nontronite on the evolution of glycine and adenine; their efficiencies of photodecomposition were reduced by a factor of 5 when mixed at a concentration of 2.6 × 10(-2) mol of molecules per gram of nontronite. Moreover, when the amount of nontronite in the sample of glycine was increased by a factor of 2, the gain of photoprotection was multiplied by a factor of 5. This indicates that the photoprotection provided by the nontronite is not a purely mechanical shielding effect but is also due to stabilizing interactions. No new evolution product was firmly identified, but the results obtained with urea suggest a particular reactivity in the presence of nontronite, leading to an increase of its dissociation rate.

Palynology of the Middle Miocene—Pliocene Novo Remanso Formation, Central Amazonia, Brazil

In order to reassess and amend previously published palynological data drawn from the Novo Remanso Formation, central Amazonia, we revisited the organic-rich samples from their work using systematic and quantitative biostratigraphic analyses, with possible implications in the biodiversity and paleoecological studies of Late Neogene deposits of Amazonia. In the study area, the Novo Remanso Formation is characterized by sandstones and mudstones deposited in a meandering river environment. The palynological assemblage comprises 46 morphotypes, including 21 species of pollen and 25 species of spores. Polypodiisporites usmensis, Retitrescolpites? irregularis and Mauritiidites franciscoi var. franciscoi, typical of lowland freshwater forests and shallow water lakes dominate the assemblage. Also, lower frequencies of Grimsdalea magnaclavata co-occur with Psilatricolporites crassoexinatus, Proteacidites triangulatus, Bombacacidites simpliciriloensis, Pteridaceoisporis gemmatus and Cingulatisporites laevigatus. Based on quantitative biostratigraphic techniques, the Novo Remanso Formation Is assigned a middle Miocene—Pliocene age In our study area. The presence of Elaterosporites klaszii (Albian-Cenomanian) In the Novo Remanso deposits strongly indicates Cretaceous rocks in the source-area, whereas absence of Andine palynomorphs may be indicative of a nearby source area for the deposits during the deposition of the Novo Remanso Formation.

Independent measurement of biogenic silica in sediments by FTIR spectroscopy and PLS regression

We present an independent calibration model for the determination of biogenic silica (BSi) in sediments, developed from analysis of synthetic sediment mixtures and application of Fourier transform infrared spectroscopy (FTIRS) and partial least squares regression (PLSR) modeling. In contrast to current FTIRS applications for quantifying BSi, this new calibration is independent from conventional wet-chemical techniques and their associated measurement uncertainties. This approach also removes the need for developing internal calibrations between the two methods for individual sediments records. For the independent calibration, we produced six series of different synthetic sediment mixtures using two purified diatom extracts, with one extract mixed with quartz sand, calcite, 60/40 quartz/calcite and two different natural sediments, and a second extract mixed with one of the natural sediments. A total of 306 samples—51 samples per series—yielded BSi contents ranging from 0 to 100 %. The resulting PLSR calibration model between the FTIR spectral information and the defined BSi concentration of the synthetic sediment mixtures exhibits a strong cross-validated correlation ( $$ {\text{R}}^{ 2}_{\text{cv}} $$ = 0.97) and a low root-mean square error of cross-validation (RMSECV = 4.7 %). Application of the independent calibration to natural lacustrine and marine sediments yields robust BSi reconstructions. At present, the synthetic mixtures do not include the variation in organic matter that occurs in natural samples, which may explain the somewhat lower prediction accuracy of the calibration model for organic-rich samples.