•A variety of models are applied to fit the experimental adsorption isotherms.•Performance of low-pressure gas adsorption using different adsorbents.•DFT-based CO2–Ar analysis is the best method to characterize the pore structures of shales.Low-pressure gas adsorption analysis based on density functional theory (DFT) or non-local DFT (NLDFT) has become an increasingly reliable method for the characterization of nanopore structures in porous materials. The accuracy of the characterization of nanoporous structures in organic-rich shales can be improved by using a variety of probe molecules and models: carbon dioxide, nitrogen, and argon are considered using the CO2-DFT, N2/Ar-DFT, N2/Ar-NLDFT models and so on. Three types of shale sample with different maturity levels (i.e., mudstone, oil shale, and gas shale) are studied to investigate the effect of soluble components and maturity on the nanopore structure. The results show that the CO2-DFT and N2/Ar-DFT models are more suitable than the other models based on DFT or NLDFT and that composite CO2–Ar adsorption analysis is the most accurate for assessing micro-, meso-, and macropores in the shales over the complete nanopore range (∼0.33–100 nm). The best method (i.e., DFT-based CO2–Ar analysis) is applied to characterize the pore structures of original and extracted shales, revealing that solvent extraction can increase the total pore volume (i.e., micro-, meso-, and macropore). However, the effect is not obvious in overmature gas shale. Micropores in the three shales display favorable pore size distributions of 0.4–0.7, 0.7–0.9, and 1.0–2.0 nm, respectively.Download high-res image (160KB)Download full-size image

•Anhydrous pyrolysis simulating high maturity was conducted on extracted and non-extracted shales.•Low-pressure gas adsorption was used to characterize nanopore structure.•Nanopore development in shales at high maturity can be divided into two stages.•Oil expulsion efficiency controls gas generation and pore development.•Pressure has an impact on pore (diameter< 10 nm) evolution.The influence of oil-expulsion efficiency on nanopore development in highly mature shale was investigated by using anhydrous pyrolysis (425–600 °C) on solvent-extracted and non-extracted shales at a pressure of 50 MPa. Additional pyrolysis studies were conducted using non-extracted shales at pressures of 25 and 80 MPa to further characterize the impact of pressure on pore evolution at high maturity. The pore structures of the original shale and relevant artificially matured samples after pyrolysis were characterized by using low-pressure nitrogen and carbon-dioxide adsorption techniques, and gas yields during pyrolysis were measured. The results show that oil-expulsion efficiency can strongly influence gas generation and nanopore development in highly mature shales, as bitumen remained in shales with low oil expulsion efficiency significantly promotes gaseous hydrocarbon generation and nanopore (diameter < 10 nm) development. The evolution of micropores and fine mesopores at high maturity can be divided into two main stages: Stage I, corresponding to wet gas generation (EasyRo 1.2%–2.4%), and Stage II, corresponding to dry gas generation (EasyRo 2.4%–4.5%). For shales with low oil expulsion efficiency, nanopore (diameter < 10 nm) evolution increases rapidly in Stage I, whereas slowly in Stage II, and such difference between two stages may be attributed to the changes of the organic matter (OM)’s mechanical properties. Comparatively, for shales with high oil expulsion efficiency, the evolution grows slightly in Stage I, not as rapidly as shales with low efficiency, and decays in Stage II. The different pore evolution behaviors of these two types of shales are attributed to the contribution of bitumen. However, the evolution of medium–coarse mesopores and macropores (diameter >10 nm) remains flat at high maturation. In addition, high pressure can promote the development of micropores and fine mesopores in highly mature shales.

•The N2 adsorption isotherms of organic-rich shale are a composite of Types I(b), II, and IV(a).•The hysteresis loops show similar shapes to Type H2(a).•The CO2 adsorption isotherms are similar to Type I(b), but appear to increase without limit when p/p0 = 0.03.•The composited N2 and CO2 NLDFT method is a suitable method for gas physisorption analysis in shale research.•The most suitable detection range (∼0.33–100 nm) is allowed to be explored by NLDFT.Low-pressure gas adsorption is widely used for pore size analysis of porous materials, and has been employed to characterize pore systems in shale. However, the complexity of shale pore structures means that different methods and models may lead to distinct interprets for adsorption data. Non-local-density functional theory (NLDFT) analysis based on N2 and CO2 composited adsorption isotherms is used here to investigate the pore structure of nanopores in marine organic-rich shale and compare with the results from some conventional methods in this paper. The results indicate that (1) The N2 adsorption isotherms of organic-rich shale are a composite of Types I(b), II, and IV(a), according to the IUPAC (2015) classification of physisorption isotherms. The hysteresis loops show similar shapes to Type H2(a). Delayed capillary condensation is observed in the adsorption isotherms, and the desorption step is shifted to the lower relative pressure of ∼0.45 characteristic of the cavitation mechanism, indicating ink-bottle pores with narrow necks. The CO2 adsorption isotherms are similar to Type I(b), but appear to increase without limit when p/p0 = 0.03 because of the occurrence of meso- and macropores in the shales. (2) NLDFT method based on N2 and CO2 composited adsorption isotherms is the most suitable and accurate method for using gas physisorption when considering the entire size distribution of nanopores, which allows a suitable range of critical pore sizes (∼0.33–100 nm) to be explored.The N2 adsorption isotherms of organic-rich shale are a composite of Types I(b), II, and IV(a), according to the IUPAC (2015) classification of physisorption isotherms. The hysteresis loops show similar shapes to Type H2(a). The CO2 adsorption isotherms are similar to Type I(b), but appear to increase without limit when p/p0 = 0.03 because of the occurrence of meso- and macropores in the shales. NLDFT method based on N2 and CO2 composited adsorption isotherms is the most suitable and accurate method for using gas physisorption when considering the entire size distribution of nanopores, which allows a suitable range of critical pore sizes (∼0.33–100 nm) to be explored.

Thermal maturation-related variations in the yields of lower diamondoids (adamantanes and diamantanes) in source rock were investigated by thermal simulation experiments based on a marine shale and kerogens obtained from the shale via isolation and artificial maturation, representing different maturity stages of the oil generation window. The simulations show that lower diamondoids are formed and destroyed during thermal maturation of the shale. For example, adamantanes are generated mainly in the maturity range of 0.8%–1.8% EasyRo, then they begin to degrade at 1.8% EasyRo. Diamantanes are produced mainly during the maturity range of 1.0%–2.2% EasyRo and begin to degrade at 2.2% EasyRo. The mineral matrix of shale may have a strong effect on the destruction of diamondoids, leading to a reduction in the peak yield and a reduction in the maturity level corresponding to the peak yield of diamondoids. A comparison of the diamondoid yields from four kerogens at different maturity levels indicates that the lower diamondoids are derived mainly from secondary cracking of extractable organic matter (bitumens) occurring in the source rock. For instance, at the peak stage of adamantane formation (2.1% EasyRo), 75.6% of the total adamantanes is generated from the cracking of bitumens and the remaining 24.4% is from the primary cracking of kerogens. Similarly, the yield of diamantanes generated from the secondary cracking of bitumens accounts for 87.8% of the total diamantanes at the peak stage of diamantane formation (2.5% EasyRo). Almost no diamondoids are detected in the pyrolysates of more mature kerogen (1.3%EasyRo), suggesting that 1.3% EasyRo is the upper limit of maturity for the generation of diamondoids from kerogen. Diamondoid isomerization ratios are maintained at relatively constant levels during the formation stage of diamondoids, whereas a linear correlation with maturity occurs during the destruction stage, suggesting that isomerization ratios of diamondoids are controlled by their thermal stability just in the destruction stage and are unaffected by hydrocarbon generation and expulsion of source rock at early thermal stages. This finding indicates that these diamondoid indices are a potential tool for evaluating the thermal maturity of source rocks at highly mature stages.

The delta carbonate (ΔC) method is a common used surface geochemical exploration technique for oil and gas geochemical surveys in China. However, its application effectiveness is unsatisfactory because of the unidentified origins of CO2 that form ΔC. In this study, a gold tube pyrolysis technique, coupled with gas chromatography (GC) and gas chromatography–isotope ratio mass spectrometry (GC–IRMS) detection, was employed to simultaneously measure the concentration and carbon isotopic composition of ΔC in surface soils. Experimental conditions for the ΔC analysis were determined by condition experiments, and the reproducibility and repeatability of the method were tested and found to be satisfactory. Subsequently, this new and improved ΔC method was applied to the Duoshiqiao area of the Jiyang depression in Bohai Bay Basin, China. Halo anomalies are found based on the concentrations and carbon isotopic compositions of ΔC. These are consistent with the actual distribution of known oil and gas accumulations in the region. The carbon isotopic composition of ΔC in the study area ranges from −4‰ to −8‰ [Vienna Peedee belemnite (VPDB)], possibly representing a special hydrocarbon seepage model with a relatively rapid leakage rate and partial chemical or biochemical oxidation of these migrated hydrocarbons from the subsurface. Combining concentration measurements of ΔC with its carbon isotopic values may be a promising method for accurately locating subsurface hydrocarbon seepage.

•Diamondoids concentration depends on oil type, evaporation extent, boiling point.•Evaporation has different effects on adamantane and diamantane indices.•Some diamondoid ratios can be used to deduce the evaporation extent of oils.•Slight-moderate evaporation extent can be estimated using diamantane concentrations.Diamondoids are commonly found in petroleum and sediments and have an inherent resistance to thermal and biological destruction, which means they can provide useful information in situations where conventional biomarkers cannot. Here, we present the results of an investigation of the effects of atmospheric evaporation on the concentration and distribution of low molecular weight diamondoids in four petroleum fractions (gasoline, condensate, diesel and fuel oil). These experiments indicate that both adamantanes and diamantanes evaporate with the other light hydrocarbons from oils and that variations in the concentrations of these compounds during evaporation are controlled by the type of petroleum fraction, the extent of evaporation and the boiling point of the diamondoid compounds within the oil. Evaporation has a significant effect on adamantane concentration ratios, whereas no changes in diamantane concentration ratios occur, suggesting that diamantane-based concentration and distribution indices can be used for the correlation of oils and determination of maturity even if oils have undergone evaporation. Some diamondoid concentration ratios, such as adamantane/1-methyladamantane, 1-methyladamantane/2-methyladamantane, 1-methyladamantane/1-ethyladamantane, 1-methyladamantane/4-methyldiamantane, adamantane/diamantane and 1,3-dimethyladamantane/4,9-dimentyldiamantane, progressively decrease with ongoing evaporation and are independent of petroleum fraction type, indicating that given the original unaltered index value, these indices can be used to deduce the relative extent of oil evaporation. The study also indicates that slight to moderate evaporation of oils leads to an increase in diamantane concentrations that is nearly proportional to the extent of oil evaporation, indicating that these compounds can be used as indices to estimate the extent of oil evaporation.

In this study, changes in the abundance and distribution of diamondoids in petroleum with thermal maturity were investigated by a simulation oil cracking experiment. Highly sensitive and selective gas chromatography–triple quadrupole mass spectrometry (GC–MS–MS) was employed to quantify diamondoids at ppm and sub-ppm levels. The results indicate that diamondoids were generated primarily within the maturity range 1.0–2.1% EasyRo and destroyed at high thermal maturity (>2.1% EasyRo). Hence, the occurrence of high concentrations of diamondoids probably corresponds to the maturity range from the wet gas to the early dry gas stage (i.e., 1.5–2.5% EasyRo). Good correlations were observed between a few ratios of diamondoids (i.e., EAI, DMAI-1, DMDI-1 and TMAI-1) and EasyRo. This finding indicates that these parameters may be useful maturity indices for organic matter from the late oil window to the dry gas window.Highlights► Diamondoids in the pyrolysates of oil cracking can be quantified by GC–MS–MS. ► Diamondoids experience generation, enrichment, destruction during oil cracking. ► EAI, DMAI-1, DMDI-1 and TMAI-1 are useful to assess the maturity of oils.

In this study, a simple solvent dilution followed by highly selective and sensitive gas chromatography–triple quadrupole mass spectrometry (GC–MS–MS) detection was employed to quantify diamondoids in crude oils. Runtime parameters, i.e., parent and daughter ions, collision energy (CE) and scan time, were optimized to obtain maximum selectivity and sensitivity for target analytes. Under optimum conditions, the reproducibility and accuracy of the method were tested and found to be satisfactory. Comparison of GC–MS–MS and GC–MS methods for the determination of diamondoids indicates that GC–MS–MS yields higher sensitivity (method quantitation limits of 0.08–0.37 μg/g oil) and better selectivity than GC–MS (method quantitation limits of 0.78–8.44 μg/g oil) due to the elimination of matrix ion interferences using the selected reaction monitoring (SRM) mode. In addition, quantitative data confirm that group separation has a considerable effect on the quantification of diamondoids and the effect appears to depend on multiple factors. Two crude oils (TZ261 and TD2) from the Tarim Basin, China were used to evaluate the GC–MS–MS method. The results prove that the GC–MS–MS method is a promising tool for quantitative analysis of diamondoids in crude oils, especially for oil samples with low diamondoid concentrations.Highlights► A simple solvent dilution combined with GC–MS–MS is used to determine diamondoids. ► GC–MS–MS is a promising tool for quantitative analysis of diamondoids in oils. ► Group separation has a considerable effect on the quantification of diamondoids.

In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography–isotope ratio mass spectrometry (GC–IRMS), was employed to determine compound specific carbon isotopic values (δ13C) of gasoline range hydrocarbons. The reproducibility of the method was found to be satisfactory. By comparison with the δ13C values of the twelve target compounds determined using direct injection of their n-C16 solution, no obvious isotopic fractionation was observed during the HS-SDME procedures. Some parameters that could affect the carbon isotopic fractionation, such as ionic strength of working solutions and inlet split ratio, were examined. The results also suggest that these factors had no significant effect on the carbon isotopic determination of gasoline range hydrocarbons. The application of HS-SDME to a crude oil sample proved that this method could be a promising tool for the determination of carbon isotopic values of gasoline range hydrocarbons in oils or aqueous samples.Highlights► HS-SDME is an efficient extraction method for gasoline range hydrocarbons (GRH). ► This method has good reproducibility and causes no obvious isotopic fractionation of GRH. ► The ionic strength of working solutions and the injection split ratio have no evident effect on the δ13C values of GRH. ► The application proved that HS-SDME is a promising tool for determining δ13C values of GRH in oils.

In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography-flame ionization detection (GC-FID) was tested to determine C6–C12 light hydrocarbons (LHs) in petroleum and aqueous samples. Several significant experimental parameters, such as drop solvent type, drop volume, sample solution ionic strength, agitation speed and extraction time were optimized. Under optimum extraction conditions, specifically, a 1.5 μl microdrop of n-hexadecane, 30 min extraction of a 5 ml aqueous sample placed in a 10 ml vial, and stirring at 1000 rpm at room temperature, the reproducibility and accuracy of this method were found to be satisfactory. Two examples using this method indicated that HS-SDME is a simple, efficient and promising technique for the determination of volatile C6–C12 LHs in complex matrices.

In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography-flame ionization detection (GC-FID), was employed to determine short-chain fatty acids (SCFAs) in ruthenium tetroxide (RuO4) oxidation products of asphaltenes. Several significant parameters, such as drop solvent type, drop volume, sample solution ionic strength, agitation speed, extraction time, and ratio of headspace volume to sample volume were optimized. Under optimum extraction conditions (i.e., a 3-μL drop of 1-butanol, 20 min exposure to the headspace of a 6 mL aqueous sample placed in a 10 mL vial, stirring at 1000 rpm at room temperature, and 30% (w/v) NaCl content), the reproducibility and accuracy of the method have been tested and found to be satisfactory. The analysis of a real asphaltene sample using this method proved that HS-SDME can be a promising tool for the determination of volatile SCFAs in complex matrices.

In this study, hollow fiber based liquid-phase microextraction (HF-LPME), coupled with GC, GC–MS and GC–IRMS detections, was employed to determine petroleum hydrocarbons in spilled oils. According to the results, the HF-LPME method collected more low-molecular weight components, such as C7–C11n-alkanes, naphthalene, and phenanthrene, than those collected in conventional liquid–liquid extraction (LLE). The results also showed that this method had no remarkable effect on the distributions of high-molecular weight compounds such as >C18n-alkanes, C1–C3 phenanthrene, and hopanes. Also, the carbon isotopic compositions of individual n-alkanes in the two preparation processes were identical. Accordingly, HF-LPME, as a simple, fast, and inexpensive sample preparation technique, could become a promising method for the identification of oil spill sources.

The Bohai Bay Basin is one of the most important oil-producing provinces in China. Molecular organic geochemical characteristics of Lower Paleozoic source rocks in this area have been investigated by analyzing chemical and isotopic compositions of solvent extracts and acid-released organic matter from the Lower Paleozoic carbonate rocks in the Jiyang Sub-basin of the Bohai Bay Basin. The results indicate that enclosed organic matter in carbonate rocks has not been recognizably altered by post-depositional processes. Two end-member compositions are suggested for early organic matter trapped in the Lower Paleozoic carbonate rocks: (1) a source dominated by aquatic organisms and deposited in a relatively deep marine environment and (2) a relatively high saline, evaporative marine depositional environment. In contrast, chemical and isotopic compositions of solvent extracts from these Lower Paleozoic carbonate rocks are relatively complicated, not only inheriting original characteristics of their precursors, but also overprinted by various post-depositional alterations, such as thermal maturation, biodegradation and mixing. Therefore, the integration of both organic matter characteristics can provide more useful information on the origin of organic matter present in carbonate rocks and the environments of their deposition.

•Sample particle sizes could significantly affect experimental results during gas adsorption experimental process.•Samples with smaller particle size had a greater effect for N2 low-pressure adsorption.•The 60–140 mesh particle-size range is recommended for N2 low-pressure adsorption.•The sample particle size had insignificant effect in the 60–200 mesh range for CO2 low-pressure adsorption.•Overall, the 60–140 mesh particle-size range can be used for both N2 and CO2 low-pressure adsorption measurements.The combination of low-pressure N2 and CO2 adsorption could provide an effective approach for characterizing the pore structure of shales. Although gas adsorption methods generally do not destroy the pore structure during experimental process, sample particle sizes could significantly affect experimental results that can approach or deviate from the real value. Therefore, the determination of pore structure is closely related to the sample particle size. In the current study, 4 fresh core samples of different compositions and total organic carbon (TOC) ranges collected from the Sichuan Basin were analyzed to elucidate the effect of sample particle size on the determination of pore structure parameters. Samples were ground and then sieved into seven groups based on particle size ranges, i.e., < 60, 60–80, 80–100, 100–120, 120–140, 140–200 and > 200 mesh, for measurements of low-pressure N2 and CO2 adsorption, TOC contents, and X-ray diffraction (XRD) mineralogy.TOC results show a slight enrichment whereas XRD minerals vary irregularly, with sample particle size decreases. Meanwhile, the TOC and mineral contents show insignificant statistical relation with pore structure parameters in all sample particle size ranges. Therefore, variations in organic matter content and mineral composition that result from sieving are unlikely to have a significant influence on the pore structure of shale. Rather, sample particle size may be the most important control on pore structure characteristics in the samples analyzed in this study.The relative standard deviations (RSDs) for Brunauer–Emmett–Teller (BET) N2 surface areas, Dubinin–Radushkevich (D–R) CO2 micropore surface areas and non-local density functional theory (NLDFT) N2 and CO2 nanopore surface areas measurements are < 5%, within analytical error. Therefore, in the studied grain size range (60–200 mesh), the sample particle size shows insignificant effects on surface area results. However, samples with smaller particle size have a greater effect on pore volume and pore size, especially for pore size distribution (PSD) of N2 low-pressure adsorption. The RSDs of the Barrett–Joyner–Halenda (BJH) pore volumes and BET pore sizes of all samples in the 140–200 mesh range are obviously greater than the values of other mesh ranges. Moreover, in the dV/dlogw plots of PSD analysis, high N2 peaks and new N2 peaks appeared in the 10–100 nm pore-width range, particularly for samples in the > 140 mesh range. The 60–140 mesh particle-size range is therefore recommended for N2 low-pressure adsorption. Finally, the sample particle size has insignificant effect on the pore system parameters for grains in the 60–200 mesh range for CO2 low-pressure adsorption. Overall, the results confirm that the 60–140 mesh particle-size range can be used for both N2 and CO2 low-pressure adsorption measurements.

•Sample particle sizes could significantly affect experimental results during gas adsorption experimental process.•Samples with smaller particle size had a greater effect for N2 low-pressure adsorption.•The 60–140 mesh particle-size range is recommended for N2 low-pressure adsorption.•The sample particle size had insignificant effect in the 60–200 mesh range for CO2 low-pressure adsorption.•Overall, the 60–140 mesh particle-size range can be used for both N2 and CO2 low-pressure adsorption measurements.The combination of low-pressure N2 and CO2 adsorption could provide an effective approach for characterizing the pore structure of shales. Although gas adsorption methods generally do not destroy the pore structure during experimental process, sample particle sizes could significantly affect experimental results that can approach or deviate from the real value. Therefore, the determination of pore structure is closely related to the sample particle size. In the current study, 4 fresh core samples of different compositions and total organic carbon (TOC) ranges collected from the Sichuan Basin were analyzed to elucidate the effect of sample particle size on the determination of pore structure parameters. Samples were ground and then sieved into seven groups based on particle size ranges, i.e., < 60, 60–80, 80–100, 100–120, 120–140, 140–200 and > 200 mesh, for measurements of low-pressure N2 and CO2 adsorption, TOC contents, and X-ray diffraction (XRD) mineralogy.TOC results show a slight enrichment whereas XRD minerals vary irregularly, with sample particle size decreases. Meanwhile, the TOC and mineral contents show insignificant statistical relation with pore structure parameters in all sample particle size ranges. Therefore, variations in organic matter content and mineral composition that result from sieving are unlikely to have a significant influence on the pore structure of shale. Rather, sample particle size may be the most important control on pore structure characteristics in the samples analyzed in this study.The relative standard deviations (RSDs) for Brunauer–Emmett–Teller (BET) N2 surface areas, Dubinin–Radushkevich (D–R) CO2 micropore surface areas and non-local density functional theory (NLDFT) N2 and CO2 nanopore surface areas measurements are < 5%, within analytical error. Therefore, in the studied grain size range (60–200 mesh), the sample particle size shows insignificant effects on surface area results. However, samples with smaller particle size have a greater effect on pore volume and pore size, especially for pore size distribution (PSD) of N2 low-pressure adsorption. The RSDs of the Barrett–Joyner–Halenda (BJH) pore volumes and BET pore sizes of all samples in the 140–200 mesh range are obviously greater than the values of other mesh ranges. Moreover, in the dV/dlogw plots of PSD analysis, high N2 peaks and new N2 peaks appeared in the 10–100 nm pore-width range, particularly for samples in the > 140 mesh range. The 60–140 mesh particle-size range is therefore recommended for N2 low-pressure adsorption. Finally, the sample particle size has insignificant effect on the pore system parameters for grains in the 60–200 mesh range for CO2 low-pressure adsorption. Overall, the results confirm that the 60–140 mesh particle-size range can be used for both N2 and CO2 low-pressure adsorption measurements.