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Solid organic matter (OM) in sedimentary rocks produces petroleum and solid bitumen when it undergoes thermal maturation. The solid OM is a ‘geomacromolecule’, usually representing a mixture of various organisms with distinct biogenic origins, and can have high heterogeneity in composition. Programmed pyrolysis is a common method to reveal bulk geochemical characteristics of the dominant OM, while detailed organic petrography is required to reveal information about the biogenic origin of contributing macerals. Despite the advantages of programmed pyrolysis, it cannot provide information about the heterogeneity of chemical compositions present in the individual OM types. Therefore, other analytical techniques such as Raman spectroscopy are necessary. In this study, we compared geochemical characteristics and Raman spectra of two sets of naturally and artificially matured Bakken source rock samples. A continuous Raman spectral map on solid bitumen particles was created from the artificially matured hydrous pyrolysis residues, in particular, to show the systematic chemical modifications in microscale. Spectroscopic data was plotted for both sets against thermal maturity to compare maturation rate/path for these two separate groups. The outcome showed that artificial maturation through hydrous pyrolysis does not follow the same trend as naturally-matured samples although having similar solid bitumen reflectance values (%SBRo). Furthermore, Raman spectroscopy of solid bitumen from artificially matured samples indicated the heterogeneity of OM decreases as maturity increases. This may represent an alteration in chemical structure towards more uniform compounds at higher maturity. This study may emphasize the necessity of using analytical methods such as Raman spectroscopy along with conventional geochemical methods to better reveal the underlying chemical structure of OM. Finally, observation by Raman spectroscopy of chemical alteration of OM during artificial maturation may assist in the proposal of improved pyrolysis protocols to better resemble natural geologic processes.

The Bakken Formation is a major unconventional shale play in North America, which lacks an independent calibration for accurately correlate thermal maturity from programmed pyrolysis (via temperature of maximum pyrolysis yield, Tmax) with optical methods (e.g., bitumen reflectance). In the present study, several samples from the upper and lower members of the Bakken Formation in North Dakota were analyzed by detailed organic petrography, bitumen reflectance, and Rock-Eval 6 pyrolysis. Organic petrography showed that the organic matter consists of various types of bitumen, amorphous matrix bituminite, liptodetrinite, acanthomorphic acritarch, marine alginite, granular micrinite, and inertinite macerals. Fluorescence color under UV light of macerals from the liptinite group was used to confirm the thermal maturity level. Due to the scarcity/absence of primary vitrinite, RO measurements on solid bitumen particles were converted to equivalent vitrinite reflectance (VRO-Eq) using a published correlation equation from the coeval New Albany Shale. Overall, geochemical analysis from Rock-Eval pyrolysis reveals almost similar trends for the upper and lower members, which allowed proposing a single correlation for VRO-Eq to Tmax for the Bakken Shale. Comparing the observed relationship for the Bakken Shale with the previously established models for the Devonian Duvernay Shale (Canada) and the Mississippian Barnett Shale (United States) shows discrepancies. Results confirmed the necessity of developing a specific equation for the Bakken Shale members to relate vitrinite and solid bitumen reflectance data to Tmax from Rock-Eval pyrolysis. Furthermore, the outcome of this study indicated that linear trends cannot accurately represent the relationship between these two parameters, considering the kerogen kinetics and non-linear relationship between transformation ratio (TR) and Tmax. Therefore, a polynomial correlation, a better fit to the data, was proposed to more accurately represent the nature of this relationship.

The boom of unconventional shale plays has brought significant attention during the last decade to shale rocks. Elastic properties of shale are one of the most important parameters to be known for hydraulic fracturing success which is necessary for production from organic-rich unconventional reservoirs. In this study, we combined experimental methods including, XRD, Rock-Eval, and nanoindentation on eight Bakken Shale samples with various thermal maturity, taken from different wells and depths in the Williston Basin, ND, to examine their mineralogy, and geochemistry with the emphasis on variations on the elastic properties. Expectation Maximization (EM) based statistical deconvolution was applied to the nanoindentation modulus data to distinguish individual mechanical phases. Then homogenization method was used to upscale the elastic properties to macro-scale, and the data were compared with the values reported in the literature. The results showed that the elastic properties of individual mechanical phases could be distinguished and evaluated through a statistical deconvolution approach. Three main mechanical phases were recognized for each shale sample. It was found that homogenized samples have Young's modulus values in the range of 18.2 GPa–38.8 GPa, which were consistent with the range of mechanical property values of the Bakken Shales reported in the literature. Correlative relationship analysis between mineralogy and thermal maturity with elastic properties showed that Young's modulus decreases with increasing TOC content and increases with the increasing maturity.

In order to analyze the creep behavior of shale rocks, nanoindentation, a common and widely used method was employed in this study. During the experiments, an abnormal displacement behavior was observed in the holding stage which has rarely been reported. It was observed that the displacement increases with holding time followed by a decrease. Further analysis of the results showed that the reduction in the displacement could be due to elastic recovery during the holding period. The dynamic mechanical properties such as storage modulus and hardness were found to first decrease and then increase after the holding time exceeds a certain value which is inferred to elastic recovery. These findings indicate that at the beginning of the holding period, creep behavior would dominate the process while as the holding time proceed, the elastic recovery plays a more important role. Finally, we proposed a new model which includes elastic recovery to quantify the changes in displacement, storage modulus and hardness as a function of holding time.

Understanding organic matter properties in terms of maturity and production potential are crucial for the initial assessment of unconventional plays. This is important since the amount of hydrocarbon that can be generated is a function of organic matter type and content in the formation and its thermal maturity. The complexity of shale plays in terms of constituent components has demonstrated that new analytical methods should be acquired to better understand hydrocarbon generation processes. In this study, a few samples from the upper and lower members of the Bakken Formation in the USA were selected from different depths and maturity levels. The samples were analyzed by a high frequency (22 MHz) nuclear magnetic resonance (NMR relaxometry) equipment, followed by Rock-Eval pyrolysis (using the Basic/Bulk-Rock method for all samples and a multi-heating rate method, MHR, for the two least mature samples) and bitumen reflectance evaluations. Results showed NMR can detect different hydrogen populations within the samples and distinguish among phases, such as solid organic matter, hydrocarbons (mobile oil), and water by T1-T2 mapping. We were also able to relate different identified areas on NMR T1-T2 maps to geochemical parameters of the organic matter obtained from Rock-Eval pyrolysis (such as S1, S2, and HI) and with thermal maturity (vitrinite reflectance-equivalent).

In order to analyze and compare the differences in pore structures between shale gas and shale oil formations, a few samples from the Longmaxi and Bakken Formations were collected and studied using X-ray diffraction, LECO TOC measurement, gas adsorption and field-emission scanning electron microscope. The results show that samples from the Bakken Formation have a higher TOC than those from the Longmaxi Formation. The Longmaxi Formation has higher micropore volume and larger micropore surface area and exhibited a smaller average distribution of microsize pores compared to the Bakken Formation. Both formations have similar meso-macropore volume. The Longmaxi Formation has a much larger meso-macropore surface area, which is corresponding to a smaller average meso-macropore size. CO2 adsorption data processing shows that the pore size of the majority of the micropores in the samples from the Longmaxi Formation is less than 1 nm, while the pore size of the most of the micropores in the samples from the Bakken Formation is larger than 1 nm. Both formations have the same number of pore clusters in the 2–20 nm range, but the Bakken Formation has two additional pore size groups with mean pore size diameters larger than 20 nm. Multifractal analysis of pore size distribution curves that was derived from gas adsorption indicates that the samples from the Longmaxi Formation have more significant micropore heterogeneity and less meso-macropore heterogeneity. Abundant micropores as well as meso-macropores exist in the organic matter in the Longmaxi Formation, while the organic matter of the Bakken Formation hosts mainly micropores.

Pores that exist within the organic matter can affect the total pore system of bulk shale samples and, as a result, need to be studied and analyzed carefully. In this study, samples from the Bakken Formation, in conjunction with the kerogen that was isolated from them, were studied and compared through a set of analytical techniques: X-ray diffraction (XRD), Rock-Eval pyrolysis, Fourier Transform infrared spectroscopy (FTIR), and gas adsorption (CO2 and N2). The results can be summarized as follows: 1) quartz and clays are two major minerals in the Bakken samples; 2) the samples have rich organic matter content with TOC greater than 10 wt%; 3) kerogen is marine type II; 4) gas adsorption showed that isolated kerogen compared to the bulk sample has larger micropore volume and surface area, meso- and macropore volume, and Brunauer–Emmett–Teller (BET) surface area; 5) deconvolution of pore size distribution (PSD) curves demonstrated that pores in the isolated kerogen could be separated into five distinct clusters, whereas bulk shale samples exhibited one additional pore cluster with an average pore size of 4 nm hosted in the minerals. The comparison of PSD curves obtained from isolated kerogen and bulk shale samples proved that most of the micropores in the shale are hosted within the organic matter while the mesopores with a size ranging between 2 and 10 nm are mainly hosted by minerals. The overall results demonstrated that organic matter-hosted pores make a significant contribution to the total porosity of the Bakken shale samples.

In petroleum industry, creep behavior of rocks can affect the fracture conductivity, well productivity and ultimate recovery of the reservoir, in shale formations in particular. To get a better insight into this phenomenon, in this study, we applied grid nanoindentation method as a function of time to quantify creep behavior of shale rocks which is a complex material. The deconvolution results from statistical analysis of the data showed that shale samples could be distinguished by three mechanical phases where the mechanical phase with the largest hardness value exhibits the least creep deformation. Burgers models was applied to characterize the creep behavior of our shale samples. We realized as creep time increases, the creep time constant value increases, therefore, a logarithmic function can be used to quantify their correlations. This study showed that as the creep time increases, Young's modulus, hardness, and fracture toughness will decrease. Finally, we concluded, shale samples become softer and more prone to fracture growth as the creep time increases.

Retrieving standard sized core plugs to perform conventional geomechanical testing on organic rich shale samples can be very challenging. This is due to unavailability of inch-size core plugs or difficulties in the coring process. In order to overcome these issues, statistical grid nanoindentation method was applied to analyze mechanical properties of the Bakken. Then the Mori-Tanaka scheme was carried out to homogenize the elastic properties of the samples and upscale the nanoindentation data to the macroscale. To verify these procedures, the results were compared with unconfined compression test data. The results showed that the surveyed surface which was 300 μm ×300 μm is larger than the representative elementary area (REA) and can be used safely as the nanoindentation grid area. Three different mechanical phases and the corresponding percentages can be derived from the grid nanoindentation through deconvolution of the data. It was found that the mechanical phase which has the smallest mean Young's modulus represents soft materials (mainly clay and organic matter) while the mechanical phases with the largest mean Young's modulus denote hard minerals. The mechanical properties (Young's modulus and hardness) of the samples in X-1 direction (perpendicular to the bedding line) was measured smaller than X-3 direction (parallel to the bedding line) which reflected mechanical anisotropy. The discrepancy between the macromechanical modulus from the homogenization and unconfined compression test was less than 15% which was acceptable. Finally, we showed that homogenization provides more accurate upscaling results compared to the common averaging method.

Understanding pore heterogeneity can enable us to obtain a deeper insight into the flow and transport processes in any porous medium. In this study, multifractal analysis was employed to analyze gas adsorption isotherms (CO2 and N2) for pore structure characterization in both a source (Upper-Lower Bakken) and a reservoir rock (Middle Bakken). For this purpose, detected micropores from CO2 adsorption isotherms and meso-macropores from N2 adsorption isotherms were analyzed separately. The results showed that the generalized dimensions derived from CO2 and the N2 adsorption isotherms decrease as q increases, demonstrating a multifractal behavior followed by f(α) curves of all pores exhibiting a very strong asymmetry shape. Samples from the Middle Bakken demonstrated the smallest average H value and largest average α10−-α10+ for micropores while samples from the Upper Bakken depicted the highest average α10−-α10+ for the meso-macropores. This indicated that the Middle Bakken and the Upper Bakken have the largest micropore and meso-macropore heterogeneity, respectively. The impact of rock composition on pore structures showed that organic matter could increase the micropore connectivity and reduce micropore heterogeneity. Also, organic matter will reduce meso-macropore connectivity and increase meso-macropore heterogeneity. We were not able to establish a robust relationship between maturity and pore heterogeneity of the source rock samples from the Bakken.

Research on unconventional shale reservoirs has increased dramatically due to the decline of production from conventional reserves. Geochemical properties and pore microstructures are known to be important factors that affect the storage capacity and nano-mechanical properties of self-sourced organic- rich shales. In this study, eleven shale samples were collected from the Upper and Lower Members of the Bakken Formation for the analysis of mineralogy, geochemical properties, and pore structure. Bulk pyrolysis analysis was conducted using the default method and two modified methods, namely the reservoir and the shale reservoir methods. Although all three methods showed the Bakken samples to be organic-rich and to have considerable remaining hydrocarbon generating potential, it was the shale reservoir method that gave the highest hydrocarbons yield because it captured most of the lighter thermo-vaporizable hydrocarbons. Thus, the shale method is considered to be more appropriate for the geochemical analysis of the Bakken samples. This method also showed that most of the remaining potential is due to the cracking of heavy hydrocarbons, NSO compounds (Resins + Asphaltenes) and kerogen. The organic matter in the samples is mixed II/III type (oil and gas-prone), is thermally mature, and plots at the peak of the oil window. The VRo-eq values, based on solid bitumen Ro measurements and conversion, ranged from 0.85% to 0.98%. The pore structures obtained from the image analysis method showed that total surface porosity of the samples ranged from 3.89% to 11.56% and that organic porosity is not the main contributor of total porosity for the samples analyzed. The pore structures of the samples are heterogeneous due to differences in lacunarity values. Results of the impact of mineralogical composition on pore structures demonstrate that clay minerals and feldspar have a positive influence on porosity while quartz, pyrite, and that TOC has a negative impact.

Samples taken from the Upper and Lower members of the Bakken Formation in four wells that were drilled in a northeast - southwest trend along the eastern margin of the Williston Basin in central-western North Dakota were investigated in order to present an overview of source-rock quality and depositional environment conditions for the main purpose of establishing an organofacies model. Several techniques such as Rock-Eval 6 pyrolysis, X-Ray fluorescence elemental analysis, vitrinite reflectance, organic petrography and visual kerogen assessment using reflected and transmitted white light and UV light microscopy on whole-rock pellets were combined to draw the best possible conclusions. The results indicate that kerogen is mainly marine type II with increasing in maturity towards the central and SW portions of the basin. Detailed organic petrography of the samples showed that solid bitumen, amorphous matrix bituminite, granular bitumen, alginite, acritarchs, and liptodetrinite are the most abundant macerals. In order to properly determine the Bakken organofacies, the original hydrogen index (HIo) was restored using various mathematical models and empirical methods. The mathematically restored HIo from thermally mature samples (HIo-Calculated) was then compared to the HI values from thermally immature samples (Tmax

Although organic-rich oil-producing mudrocks have been studied extensively during the last decade, kerogen, as one the main constituents, is not thoroughly understood. The unknowns about kerogen elevate when it comes to its modulus of elasticity. Since kerogen is not as stiff as inorganic minerals, its presence can have a significant impact on the initiation and propagation of fractures in kerogen-rich formations that should undergo stimulation. This study proposes an approach to estimate modulus of elasticity of kerogen with different thermal maturities using Raman spectroscopy. Various shale samples from the upper and lower members of the Bakken Formation were picked from several wells within the Williston Basin in North Dakota, USA. These samples were analyzed using Rock-Eval (RE) pyrolysis and vitrinite reflectance (%Ro) for thermal maturity. In addition, Raman spectroscopic measurements were made on samples and followed by PeakForce AFM for Young's modulus estimation of the organic matter. First, the Raman responses were correlated with the thermal maturity and then, a correlation was established to show the potential relationship between elastic modulus of organic matter and its Raman response based on the maturity levels.

In order to study the mechanical properties of shale samples from Bakken Formation, nanoindentation method, an imaging technique borrowed from other engineering disciplines, was used. Different types of nanoindentation curves were analyzed and the applicability of the nanoindentation theories to study mechanical properties of shale samples at nanoscale was demonstrated. Elastic modulus and Hardness of different samples were calculated, compared and related to their mineral compositions and microstructures which are detected by 2D XRD and FESEM methods, respectively. Results showed that samples with more clay minerals (mainly composed of illite) and larger pore structures have less Young's modulus. In addition, based on the energy analysis method, the fracture toughness at nanoscale was estimated and its relationships with Young's modulus was quantified. It was observed that fracture toughness increases linearly with Young's modulus. This paper presents the results and main findings of this study.

Bacteria are ubiquitous in all depositional environments, especially in marine environments where anoxic/euxinic conditions prevail. In such environments, sulfate-reducing bacteria play a critical role to supply sulfur as a biogenic source for H2S through biomass degradation. In the biodegradation process, chemical and mechanical properties of the organic matter alter. In order to document these variations in-situ, selected samples from a deeply buried mudrock (Bakken Formation), were examined through microscopy analysis. Two separate but adjacent telalginite particles were selected; An unaltered telalginite and a bacterially degraded telalginite, which still contains relicts of the parent Tasmanites. A combination of AFM-based IR spectroscopy with high-resolution amplitude-frequency modulation was used to evaluate and compare the physicochemical variations across these two particles at the nanoscale. Results indicate that all aromaticity indexes increase for both particles but at a higher rate as a result of bacterial degradation. Furthermore, it was found that bacterial degradation imposes a major mechanical heterogeneity to the organic matter under study, which was detected through phase imaging and modulus mapping captured from submicron to micron-scale level, which exposed the remnants of the parent Tasmanites. This study reveals that bacterial degradation can accelerate the maturation process, thus the generation of hydrocarbons from the kerogen to happen at the earlier stages of thermal adavance.