Kerogen Molecules Research Articles

An insight into quartz mineral interactions with kerogen in Green River oil shale

Green River oil shale is an organic-inorganic macromolecular composite system. Identifying the interactions between nanoscale organic kerogen and the inorganic mineral matrix in oil shale is an important step towards the design of technologies for economic and efficient recovery of shale oil. Quartz (SiO2), one of the tectosilicate minerals present in the Green River oil shale, is modeled to determine the interactions with the 3D kerogen model. The molecular dynamics simulation technique is used as the tool for modeling the interactions. Two models with different orientations of quartz mineral and kerogen are built to evaluate the role of mineral orientation on the molecular interactions. The models are simulated at room temperature and pressure (300 K and 1.013 bar). The kerogen-quartz interactions are evaluated with respect to eight kerogen fragments. We observe that different fragments within the 3D Kerogen molecules interact with the quartz depending upon the quartz orientation. In both cases, the unconstrained ammonium (NH4) ions from the adjacent layer of kerogen migrate to the quartz cavities. All the non-interacting kerogen fragments move away from quartz. The nonbonded interactions between kerogen and quartz are generally electrostatic. The presence of quartz mineral also affects the inter-fragment and inter-layer interactions within the kerogen molecule.

Molecular dynamics of methane flow behavior through realistic organic nanopores under geologic shale condition: Pore size and kerogen types

Up to date, for the purpose of simplicity, graphene-based structures, like nano-porous carbons or carbon nanotubes, have been widely utilized to investigate methane flow behavior inside shale organic nanopores. However, realistic shale organic matrix is composed of kerogen molecules, possessing complex amorphous structures and apparently different surface attributes compared with graphene-based nanopore surface, which will inevitably have a great impact on surface-methane interactions and methane flow behavior. Current research works in terms of the graphene-based material fails to capture the influence of kerogen surface, and therefore cannot accurately characterize nano-confined methane flow behavior through realistic shale organic matter. Also, shale organic nanopores with different kerogen types contain different surface compositions, while its impact on methane flow capacity has not been reported yet. To bridge this knowledge gap, this paper simulates the methane flow behavior through authentic kerogen-based circular nanopores with the use of molecular dynamics (MD) for the first time. And a novel construction method was developed to generate kerogen-based organic nanopores with desirable pore size and different kerogen types for MD simulation. Main results show that a) decrease in pore size will contribute to the enhancement of adsorption capacity for nanopores and type-III kerogen > type-II kerogen > type-I kerogen in terms of methane adsorption capacity; b) ratio of average methane density confined in nanopores to bulk-gas density ranges from 1.2 to 2.6, which will decrease with the increase of the pressure and increase with decreasing pore size; c) Under shale geological condition, the conventional theoretical model for nanoconfined gas flow will underestimate that of 0.41 time for type-I kerogen-based nanopores, 0.59 time for type-II kerogen-based nanopores, and 0.88 time for type-III kerogen-based nanopores.

A Simple Molecular Kerogen Pore-Network Model for Transport Simulation in Condensed Phase Digital Source-Rock Physics

Transport of small hydrocarbon molecules inside organic nanopores is of great interest in the oil and gas production from source rocks such as shale. Because of the small pore sizes, it is difficult to measure transport directly in the laboratory but require alternative approaches such as molecular simulation. Existing molecular models use nanoscale capillaries and do not describe the exact geometric structure of the organic nanopores in real source rocks, especially for mesopores (2–50 nm in diameter, defined by IUPAC), which widely exist in organic solids and are crucial for fluid transport in the source rocks. Here we present a new method to simplify the existing molecular kerogen models. Pore structures can be built to reflect the exact porous structure of hydrocarbon-bearing kerogen pores for fluid transport without losing accuracy. The kerogen pore-network skeleton can be populated based on 3D digital segments obtained from high-resolution TEM tomographs, combining with other material properties, including the elemental composition and density of the organic solid. The basic idea of the simplified molecular model is to ignore the detailed chemical information of the kerogen molecules and to use a uniform “representative” atom to replace all various atom types, while maintaining the geometric structure intact. To test the transport simulation accuracy using molecular dynamics for this simplified model, sample pore-network structures are populated with both the existing exact molecular model and the simplified model, and the permeability of reservoir fluid flowing through the media is simulated and compared. Specifically, simple reservoir fluids are charged into the space of the 3D porous networks with grand canonical Monte Carlo simulation at given subsurface condition; then, the fluids are driven by an external force field to flow through the network using non-equilibrium molecular dynamics simulation. The simplified kerogen model can be easily applied to larger organic porous material samples to reduce spatial uncertainty, provided enough computational resources.

Evaluation of Spatial Alignment of Kerogen in Shale Using High-Resolution Transmission Electron Microscopy, Raman Spectroscopy, and Fourier Transform Infrared

As the three-dimensional (3D) molecular structure of kerogen plays important roles in further understanding of shale gas storage and transport, accurate characterization methods for 3D kerogen structures are attracting increasing attention. Spatial alignment is important information for 3D kerogen modeling, but was usually ignored in previous studies. In this work, seven kerogen samples with different maturities were isolated from organic-rich shale using a chemical method and high-resolution transmission electron microscopy (HRTEM) was employed to quantitatively characterize the spatial alignment of these seven kerogen samples. Raman spectroscopy was used to investigate the overall structural disorder of the kerogen molecules and Fourier transform infrared (FT-IR) was conducted to study the chemical structure of these kerogen samples. The results show that immature, mature, and overmature kerogen samples all show an obvious alignment on the scale of 20 nm × 20 nm. In the immature kerogen sample Yl-1 with...

Pyrolysis characteristics analysis of Chang-7 oil shale using thermal analysis and pyrolysis-gas chromatograph-mass spectrometry

There are vast resources of oil shale in the Chang-7 section of the Upper Triasic Yanchang Formation, Ordos Basin, China. They would provide secure access to transportation fuels, if utilized in an effective, economic and environmental manner. A Chang-7 oil shale sample containing pyrite has been pyrolysed in a thermogravimetric analyzer at constant heating rates of 5, 10, 20℃/min up to 1150℃ with nitrogen as purge gas. Meanwhile, the pyrolysis-gas chromatograph-mass spectrometry experiments were performed on parallel samples of Chang-7 oil shale sample at 20℃/min in the four temperature sections, ranging from 100 to 700℃ to understand the decomposition mechanism. The initial decomposition temperature and the final decomposition temperature of hydrocarbonaceous material were about 367℃ and 521℃ (average values), respectively, which raised with the increase of heating rate. The decomposition of pyrite began to take place over 500℃, and the pyrolysates involving sulfur compounds are harmful to the environment and corrode the equipment. Thus, we recommended that the final temperature of Chang-7 oil shale pyrolysis was best not to exceed 550℃ considering the heating rate to avoid environmental pollution and damage of the instruments. Chang-7 oil shale exhibited a single stage decomposition in the range of about 367–521℃, representing the rearrangement of kerogen molecules in the temperature range of 100–350℃. The activation energies of kerogen decomposition were calculated using Coats–Redfern method at different heating rate. We found that both activation energy and pre-exponential factor increased with an increasing heating rate. Finally, the DTA curves of Chang-7 oil shale verified the reliability of the calculated activation energies.

Molecular Dynamics Simulation of Adsorption and Replacement of Methane in Kerogen Micropores

Characteristics of CH4 adsorption and CH4 replacement with CO2 in kerogen micropores were investigated by molecular dynamics (MD) simulations to obtain accurate estimates of CH4 volume and reduction of environmental load by applying multi-stage CO2 fracking for shale gas development. Firstly, CH4 density in the kerogen micropores was found to be about 1.8 times higher than in the mesopores outside the kerogen, indicating that an adsorption model accounting for the micropore filling is essential to correctly evaluate the volume of CH4 adsorption. Secondly, CO2 molecules with linear shape easily passed through the throat of the kerogen micropore, whereas CH4 molecules with regular tetrahedron shape did not. Thirdly, CH4 was easily replaced by CO2 in the kerogen micropores due to the higher affinity for CO2 than CH4 of oxygen atoms, which are much more common than other heteroatoms in the kerogen molecule. Finally, H2O molecules in the kerogen micropores and mesopores were aggregated by hydrogen bonding around the heteroatoms and prevented the replacement of CH4 by blocking the pathways.

Estimation of various chemical bond dissociation enthalpies of large-sized kerogen molecules using DFT methods

ABSTRACTKerogen is irregularly connected by a variety of chemical bonds including C–C bond, C=C bond, C–O bond, C–N bond, C–H bond and C–S bond. It is difficult to identify selective bond breaking events using the existing experimental conditions. In this study, we predict dissociation tendencies of chemical bonds in various kerogen molecules by using 554 small molecular model compounds based on the density functional theoretical (DFT) calculations. Results from our DFT study indicate that the calculated various subtypes of bond dissociation enthalpies (BDEs) changing tendency is that C–S bond < C–N bond < C–O bond < C–C bond < C–H bond < C=C bond. However, for each bond type, the BDE value has a large range and overlaps each other, and the BDE mostly relies on the local environment (the functional groups and position) of chemical bonds and radical stability. For the C–C bond, a linear relationship (R = 0.85) between C–C bond distances and BDE has been achieved. Thus, the bond distance can serve as a good indicator of bond strength for C–C bond linkages. All results support that the BDE is a good choice to evaluate the dissociation tendencies of kerogen and this work provides an effective path to reveal the nature of chemical bonding for large-sized kerogen molecules.

Novel molecular simulation process design of adsorption in realistic shale kerogen spherical pores

Adsorbed methane in shale organic nanopores is an important factor of the shale gas resource. The major component of shale organic matter, kerogen solids, consists of macro organic molecules and functions as the most important adsorbent. In this work, a novel molecular simulation workflow is proposed to generate organic pores on residue-type kerogen molecules and to simulate the gas adsorption in the pores. The molecular dynamics-based cutter atom pore generation method can construct pores with arbitrary shapes and sizes—approaching microscopy observations—2–50nm shale matrix pores with reasonable physical significance. Grand canonical Monte Carlo simulations for CH4 and CO2 are performed on kerogen pores with various pore radii using two categories of molecular potentials. The ensemble averaging density distributions in the pores are calculated and analyzed, which concludes that the free gas state becomes distinguishable from the adsorbed state unless the pore radius exceeds 1nm. Adsorbed layers at the kerogen pore surfaces are heterogeneous because of non-uniformly distributed adsorptive sites on the surfaces. Adsorption isotherms are simulated and thereafter fitted with the Langmuir equation comparing various molecular models and fluid types. The adsorption affinity indicated by the Langmuir pressure for the total adsorbed molecule number in all cases decreases as a function of increasing pore size.

Sorption of methane, ethane, propane, butane, carbon dioxide, and nitrogen on kerogen

Sorption isotherms of nitrogen, methane (in the pressure range of 0.1–40 MPa), ethane (0.1–3.7MPa), propane (0.01–1 MPa), butane (0.01–0.2 MPa), and carbon dioxide (0.1–6 MPa) are measured on two adsorbents with kerogen contents of 16 and 75% at temperatures of 303, 323, 343 K. Adsorption volumes are calculated for all adsorption systems using two independent methods. The BET technique is used to determine the surface area values of the two adsorbents on the basis of sorption data for ethane, propane, butane, and carbon dioxide. The initial and isosteric adheat of sorption values are calculated on the basis of sorption isotherms of ethane, propane, butane, carbon dioxide measured at three temperatures. It is found from comparing the dependences of isosteric heat of sorption on the two adsorbents that molecules of the above gases diffuse into its bulk (adsorbent 2) in addition to sorbing on the outside surface formed by kerogen molecules, while sorption of the same gases on the rock (adsorbent 1) is similar to sorption on a smooth hard adsorbent surface.

シェールガス貯留層内に存在するナノポアにおけるガスの振る舞い

In recent years, the exploration of unconventional shale gas has been active in North America and production amount is currently still increasing. Shale gas is a hydrocarbon generated by source rocks and it is remained in place without migration. Usually these source rocks are shale. Furthermore kerogen is present in the pores of the source rocks, and shale gas exists in the nm order kerogen nanopores. Since the nanopore diameter is small, it is necessary to consider different physical properties compares with the conventional gas. That is, knowing the adsorption and absorption properties of shale gas to kerogen and the flow properties of the gas in the nanopore inside kerogen are essential matters for the future exploration and development activities for shale gas. In order to understand details of these phenomena, we simulate the adsorption and absorption phenomena of methane molecules to the kerogen molecules by using molecular dynamics method. Also the flow of the methane molecule through the nanopores is investigated by molecular dynamics, and indicating the presence of a slip flow. In order to scale up molecular simulation results, lattice Boltzmann method is adopted. Through these, we tried to build a numerical modelling system that can reproduce the physical and flow properties of shale gas in the nanopores.

Organic-Inorganic Interactions and Sandstone Diagenesis

The maturation of organic material in hydrocarbon source rocks and inorganic diagenetic reactions in reservoir sandstones are a natural consequence when a prism of sedimentary rocks is buried. We can predict the distribution of porosity and permeability enhancement in potential hydrocarbon reservoirs by integrating the reaction processes characterizing the progressive diagenesis of a reservoir/source rock system. A variety of observations suggests that the organic solvents needed to increase aluminosilicate and carbonate solubilities in sandstones can be generated either by thermal or oxidative cracking of carbonylic or phenolic groups from kerogen in adjacent source rocks. For example, nuclear magnetic resonance (NMR) spectra of kerogen show that peripheral carbonylic and phenolic groups are released from the kerogen molecule before liquid hydrocarbons are generated. Experimental data indicate these water-soluble organic species can significantly affect the stability of both carbonates and aluminosilicates. Water-soluble organic acid anions (carboxylic) have been observed in oil-field waters in concentrations up to 10,000 ppm, and they commonly dominate the alkalinity in the fluid phase from 80° to 120°C. We can model the integration of organic and inorganic diagenetic reactions by constructing a series of potential reaction pathways with increasing temperature for a system that includes aluminosilicates, carbonates, organic chelate species (carboxylic and phenolic), and CO2. The important chemical divides in these diagenetic flow diagrams are dependent on temperature, the nature of the pH buffer (carbonate species or organic acid anion species), and the relationship between organic acid anions and PCO2 (P = partial pressure). Forward predictive capabilities result when this general diagenetic model is placed in a time-temperature framework. The detailed organic and inorganic geochemistry and the general thermal scenario used in the time-temperature ana ysis must be basin specific. Casting the diagenetic history of a sandstone into this type of process-oriented model helps us move from a descriptive mode to a predictive mode of analysis. Two types of information result: (1) general optimum conditions for porosity and permeability enhancement in sandstones are delineated and (2) specifically, the degree and potential for porosity and permeability enhancement are determined. Predictive models have been developed for several tectonic settings, including rift or pull-apart basins and intermontane or Laramide basins. From these reconstructions, we can forward-predict the porosity-enhancing potential of a diagenetic system based on an understanding of the reaction process in a time-temperature framework.

Organic-inorganic reactions during progressive burial: key to porosity and permeability enhancement and preservation

The development of porosity and permeability enhancement and preservation or both in many sandstones is a function of aluminosilicate or carbonate mineral stability. The dissolution of aluminosilicate minerals and subsequent porosity and permeability enhancement is a problem of aluminium mobility. Our experimental data demonstrate that it is possible to increase the mobility of aluminium significantly and to transport it as an organic complex in organic acid solutions. These organic acids, primarily carboxylic and phenolic acids, also have the ability to dissolve carbonate grains and cements. W. W. Carothers &amp; Y. K. Kharaka ( G. cosmochim. Acta 44, 323—332 (1980)) have shown that concentrations of carboxylic acid anions range up to 5000/10 6 (by mass) over a temperature range of 80-100 °C in some oil-field formation waters. Our experiments show that acetic acid solutions at the same concentration and over the same temperature range can increase the solubility of aluminium by one order of magnitude, whereas oxalic acid solutions increase the solubility by three orders of magnitude. Phenols such as hydroquinone and catechol are almost as effective as oxalate. The textural relations observed in the experiments are identical to those in sandstones containing porosity enhancement as a result of aluminosilicate dissolution. A natural consequence of the burial of sedimentary prisms is the maturation of organic material. Nuclear magnetic resonance (n.m.r.) studies have shown that carbonyl and phenol groups are removed from the kerogen molecule before the generation of hydrocarbons. Thermal degradation or the action of mineral oxidants, or both (the reduction of Fe 3+ released from clay diagenesis and the reduction of polysulphides are two examples) may be the mechanisms by which the peripheral groups containing the phenols and carboxylic acids are released from the kerogen molecule. The experiments suggest that the enhancement of porosity in a sandstone as the result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis. Because of the time and temperature dependence of these reactions, the actual reaction sequences will be sensitive to the thermal and stratigraphic history of the source reservoir sediments.

Predictive Models for Sandstone Diagenesis: ABSTRACT

The maturation of organic material in hydrocarbon source rocks and inorganic diagenetic reactions in reservoir sandstones are a natural consequence of the burial of a prism of sedimentary rocks. The distribution of porosity/permeability enhancement in potential hydrocarbon reservoirs can be predicted by integrating the reaction processes characterizing the progressive diagenesis of a reservoir/source rock system. A variety of observations suggests that the organic solvents necessary to increase aluminosilicate and carbonate solubilities in sandstones can be generated either by thermal or oxidative cracking of carbonyl or phenolic groups from kerogen in adjacent source rocks. For example, nuclear magnetic resonance (NMR) spectra of kerogen show that peripheral carbonyl and phenol groups are released from the kerogen molecule prior to the generation of liquid hydrocarbons. Experimental data indicate that these water-soluble organic solvents can significantly affect the stability of both carbonate and aluminosilicate minerals. Water-soluble organic acids (carboxylic) have been observed in oil field waters in concentrations up to 10,000 ppm, and they commonly dominate the alkalinity in the fluid phase over the 80°-120°C temperature range. The integration of the organic and inorganic diagenetic reactions can be modeled conceptually by constructing a series of potential reaction pathways with increasing temperature, for a system that includes aluminosilicate minerals, carbonate minerals, organic solvents (carboxylic and phenolic), and carbon dioxide. The important chemical divides in these diagenetic flow diagrams are dependent on temperature, the nature of the buffer in the carbonate system (internal or external), and the relationship between organic acids and PCO2. Forward predictive capabilities result when this general diagenetic model is placed in a time-temperature framework. The detailed organic and inorganic geochemistry and the general thermal scenario used in the time-temperature analysis ust be basin-specific. Casting the diagenetic history of a sandstone into this type of process-oriented model enables one to make the transition from a conventional descriptive mode to a predictive mode of analysis. Predictive models have been developed for several tectonic settings, including rift or pull-apart basins, and intermontane or Laramide basins. From these reconstructions, two types of information result: (1) general optimum conditions for porosity/permeability enhancement in sandstones are delineated, and (2) specifically, the degree and potential for porosity/permeability enhancement are determined. Forward prediction of the porosity-enhancing potential of a diagenetic system is possible based on an understanding of the reaction processes in a time-temperature framework. End_of_Article - Last_Page 2046------------

A Surface Vitrinite Reflectance Anomaly Related to Bell Creek Oil Field, Montana U.S.A.: ABSTRACT

Vitrinite reflectance measurements from surface samples of mudrock and coal show anomalously high values over the Bell Creek oil field. The average vitrinite reflectance (Rm) increases to a peak of 1.2% over the field against background values of about 0.3%. The Rm anomaly coincides with a geochemical anomaly indicated by ^dgr13C in carbonate-cemented sandstones. These samples were taken from the Upper Cretaceous Lance and Paleocene Fort Union formations, which form an essentially conformable sequence. The depositional environment is apparently similar in both formations, and we expect little variation in the source and composition of the organic matter. Rm should be rather constant across the field if conditions of diagenesis w re uniform. The limited topographic relief (< 1,000 ft or 305 m) over the shallow-dipping homoclinal structure of the field and the poor correlation coefficient of Rm regressed against sample locality elevation (r = 0.2) indicate that the Rm anomaly is not due to burial, deformation, and subsequent erosion. Temperature studies over local oil fields with similar geologic conditions suggest the expected thermal anomaly would be less than 10°C (50°F), which is too small to account for the significantly higher rank over the field. Coal clinkers are rare in the vicinity of the Bell Creek and widespread heating by burning of coal seams is unlikely. We suggest that activity by petroleum-metabolizing bacteria is a possible explanation of the Rm anoma y. Microseepages from oil fields support large colonies of these organisms, which could also metabolize aliphatic side-chains on the kerogen molecule. The loss of these side-chains increases the aromaticity of the vitrinite and consequently increases its reflectance. End_of_Article - Last_Page 418------------

A study of a New Zealand oil shale by differential thermal analysis

The heats of combustion of 95 samples of oil shale from the Nevis Valley have been determined from DTA peak areas obtained by combustion in 1.5 atm oxygen. The heats of combustion of the oil shale ranged from 0.2 to 8.5 MJ kg −1, with an average of 2.6 MJ kg −1, while that of the kerogen was calculated to be 34.2 MJ kg −1. Up to four peaks were obtained in the DTA combustion curve, suggesting that different parts of the kerogen molecule were being oxidised at different temperatures. In nitrogen, DTA endotherms were observed due to decarbonation of siderite (ca. 455°C), and dehydroxylation of kaolinite (ca. 570°C) and chlorite (ca. 760°C), and an exotherm due to formation of mullite (ca. 980°C).

Stable carbon isotope ratios and chemical properties of kerogen and extractable organic matter in pre-Phanerozoic and Phanerozoic sediments — Their interrelations and possible paleobiological significance

Kerogen and the polar (or “humic”) fraction of benzene/methanol extractable organic matter in various pre-Phanerozoic and Phanerozoic sedimentary rocks were analyzed for stable C isotope content. Variations in the δ 13C values as functions of the geologic age and chemical properties of the organic substances were investigated. The δ 13C values of both kerogen and extractable polar organic matter in mudstones showed a tendency to decrease with increasing geologic age, possibly indicating secular decrease in atmospheric and hydrospheric CO 2. However, the secular trend of the polar extracts has a much higher statistical significance and much less scatter than the secular trend of the kerogen. This suggests that the heterogeneity and complexity of kerogen tend to mask systematic variations, whereas the polar extract, being a specific small fraction of the organic matter, varies less erratically. Bulk isotope analyses of kerogen have limited usefulness for investigation of systematic variations; the various components of the kerogen molecule should be separated by fractionation procedures and analyzed individually. The δ 13C values of all kerogen samples, regardless of rock type, age, or depositional environment, showed a highly significant negative correlation with the ratio of aliphatic to condensed aromatic components of their respective polar extracts, and δ 13C of kerogen in carbonate rocks and cherts showed a significant positive correlation with the aromaticity of the kerogen; other data indirectly suggest that carbonyl-type groups are enriched in 13C. These results demonstrate that the aliphatic components of organic matter are isotopically lighter than the associated aromatic components, suggesting that the aliphatic chains are derived from lipids of algae. Thus, fractionation of kerogen prior to isotope analysis should at least separate the C atoms of aliphatic, aromatic and carbonyl-type groups. δ 13C of kerogen in carbonate rocks and most cherts showed significant negative correlation with concentration of OH groups in polar extracts, suggesting loss of phenolic OH groups with increase in condensed-aromatic character. The observed relationships between polar extracts and kerogen confirm previously reported evidence that kerogen and its associated extractable polar substances are chemically similar and are largely derived from the same source materials. Possibly paleobiological and paleo-environmental information coded in the molecular structure of kerogen may be obtained more easily, rapidly, and economically from polar extracts. The existence of secular variation in polar extracts and a relationship between polar extracts and kerogen supports the conclusion that the polar extracts are indigenous to the rock in which they occur, and thus are valid chemical fossils. Comparisons between δ 13C values of kerogen and polar extracts from mudstones are tentatively interpreted as indicating that terrigenous organic matter, and hence land life, originated between 1.05·10 9 and 1.3·10 9 years ago as a manifestation of the early evolution of eucaryotic algae.