Bakken Shale Samples Research Articles

Application of aromatic/aliphatic ratios analysis in revealing migration pathways in unconventional reservoirs: A case study of the Bakken Shale

The ratio of aromatic to aliphatic (AR/AL) hydrocarbons was used to investigate migration pathways in tight unconventional reservoirs. Analytical methods, including organic petrography and geochemistry, programmed pyrolysis, solid bitumen reflectance, and liptinite autofluorescence, were employed to generate regional thermal maturity maps. While AR/AL ratios are known to increase with thermal maturity, a comparison of AR/AL ratios with thermal maturity indices as well as regional thermal maturity maps shows no significant correlation to other known thermal maturity proxies (i.e., Tmax or VRo-Eq) for the Bakken Shale samples. Taking into account the concept of “size and shape” filtration from a previous study, the observed inconsistency between AR/AL ratios and thermal maturity indicators could be explained reasonably, where, based on this concept, the aliphatics are able to migrate more easily through the shale's tight media compared to the aromatics, which would remain in situ or migrate only very short distances because of their size and shape. This indicates that the inconsistency between AR/AL ratios and thermal maturity indicators is related to the disproportionate rate of migration of aliphatic and aromatic hydrocarbons in the Bakken Shale. To evaluate the potential application of using AR/AL ratio chemical analyses to reveal the possible migration pathways, initial values for total organic carbon (TOCo) and S1 (S1expelled) were calculated for the thermally immature Bakken source rock. Comparing the regional distribution maps of S1expelled and the present values of S1 could reveal the geospatial displacements of generated hydrocarbons and, therefore, the possible pathways of hydrocarbon primary migration. Comparing the AR/AL ratio regional maps with the attained migration pathway based on S1 and S1expelled regional maps reveals an acceptable correlation in terms of the general trend. In this regard, AR/AL ratios exhibit higher values in the locations of the higher concentrations of calculated S1expelled distribution maps and lower values in the location of the higher concentrations of programmed pyrolysis-derived S1 distribution maps. These outcomes confirm that AR/AL ratios could be used as a potential proxy in revealing the possible migration pathways in unconventional reservoirs.

Determination of transition metal ions in fossil fuel associated wastewaters using chelation ion chromatography

This study outlines the development and subsequent validation of a method using chelation ion chromatography (CIC) pretreatment followed by traditional ion chromatography (IC) and post column UV/vis detection to measure transition metals in fossil fuel wastewaters, such as oil & gas (O&G) brines and coal mine drainage (CMD) waters. Measurement of transition metals is often an important characterization step in the research of environmental and energy systems. IC represents one way to measure these metals with the advantages of being versatile, simple and relatively low cost compared to other analytical methods. However, high concentrations of alkali and alkaline earth metals present in fossil fuel wastewaters will decrease IC detectability of transition metals in these waters. In this study, a CIC method was developed for the analysis of transition metal ions (Fe3+, Cu2+, Ni2+, Zn2+, Co2+, Mn2+, and Fe2+) in fossil fuel associated wastewaters such as Appalachian CMD and O&G wastewaters from the Permian and Bakken shale basins in the United States. CIC system incorporated an on-line chelator column (e.g., the MetPac CC-1) with high selectivity for transition metals over alkali and alkaline earth metals for salt matrix removal prior to transition metal separation and detection. Additional method developments also included acidifying all samples to 2% v/v HCl and using gradient elution rather than isocratic. The recoverability of transition metals in simple salt solutions commonly found in CMD and brine samples (e.g. NaCl, Na2SO4, CaCl2) using CIC was evaluated and compared to that using traditional IC. Our results found that the CIC system significantly improved transition metal recoveries for samples in 10,000 mg/L CaCl2 matrix, reaching 87%-108% recovery for all analytes, as opposed to 2–323% recovery in traditional IC. The limits of detection in this study achieved 10.09–161.2 μg/L, comparable to reported values in similar IC studies. The developed method was also verified with certified water samples, resulting in 89%–111% recoveries in samples with higher analyte concentrations (i.e. >4x the LoDs). The developed method achieved 87%–112% recoveries for most analytes in CMD samples and 72%–138% recoveries for Bakken shale samples, relative to ICP-MS values. Overall, the current IC method can be a very good screening tool for fast and cheap analysis for transition metals at mg/L level, to facilitate selection of samples for more detailed ICP-MS analysis.

Maturation-induced modification of organic matter in shales: Implications for geological CO2 storage

Shales have low to ultra-low porosity and permeability, which makes Bakken shales an attractive candidate for CO2 storage or as a caprock. The organic material within source rocks consists of extremely complex macromolecules with a host of other minor components. In this study, we experimentally investigated how the artificially-induced thermal maturation of organic matter in shales will impact geological CO2 storage (GCS). We artificially induced Bakken shale samples to thermal maturity (Tmax) for an exposure period of 0–48 hr, with incremental Tmax. Firstly, we conducted source rock analyses to evaluate and analyze the modified geochemical properties due to artificially-induced thermal maturation. Secondly, we utilized X-ray diffraction to assess the mineralogical changes in the immature and mature samples. Subsequently, we conducted scanning electron microscopy (SEM) for microstructural characterization of the morphological modifications and distribution of organic matter. Our post-treatment geochemical results showed a decrease (-42%) in the total organic carbon content (TOC) and a decrease (-26%) in quartz content, and these alterations continued with an increasing Tmax. The results of the microstructural analyses also suggest changes in the pore structure of the shale samples. Further, we assessed the implications of the modified geochemical and microstructural properties for GCS. The observed modifications in the properties of Bakken Shales due to the impact of artificially-induced thermal maturation of organic matter may improve its suitability as a caprock in conventional trapping scenarios, or as a CO2 storage unit.

Quantifying Fluid‐Wettable Effective Pore Space in the Utica and Bakken Oil Shale Formations

AbstractCombined (ultra‐) small angle neutron scattering measurements [(U)SANS] and a contrast matching technique were employed to quantify the porosity and pore size distribution from 1 nm to 10 μm and to differentiate accessible (open) pores and inaccessible (closed) pores with respect to organophilic and hydrophilic fluids for two Utica and two Bakken shale samples. The results indicate that around 40–70% of the pores in the Utica oil shales (mixed carbonate mudstone) are accessible to oil, and 34–37% of the pore surfaces are water‐wet. In contrast, the Bakken oil shales (mixed siliceous mudstone and carbonate/siliceous mudstone), which have high total organic carbon contents, have a higher proportion of isolated pore space that is not preferentially wet by oil or water, with less than 36% of the pores accessible to both fluids. In addition, for both formations, pores less than 3 nm in diameter are not oil accessible (organic matter related) but water accessible (clay tactoids related).

Insight into the nanoscale pore structure of organic-rich shales in the Bakken Formation, USA

The Bakken Shale has attracted much attention because of its huge potential for shale oil. To characterize the nanoscale pore structure of the Bakken shale, ten samples were selected from the upper/lower Bakken Shale for N2 and CO2 adsorption, which can provide the pore structure parameters, such as pore size, specific surface area (SSA) and pore volume. The controlling factors of the pore structure were studied through the analysis of the mineral composition, total organic carbon (TOC) content, and thermal maturity (VRo-eq). Also, we investigated the fractal characteristics of samples based upon N2 adsorption. The results exhibited that the composition of the Bakken shale was mainly quartz and clay; the content of clay minerals varies between 24.07% and 53.92%; the content of quartz is between 38.04% and 63.59%. The average value of TOC content of the Bakken shale was 11.49 wt% and the vitrinite equivalent reflectance varied between 0.62 and 1.11%. Micropores (<2 nm) and mesopores (2–50 nm) are the dominant pores in the Bakken shale samples. Quartz, clay and TOC are the key factors controlling the pore structure of Bakken shale. The fractal dimensions D1 ranged from 2.606 to 2.896 and D2 ranged from 2.243 to 2.296. D1 shows a strong positive correlation with D2, with an R2 value of 0.8. Higher clay and TOC content lead to more complex pore systems in the Bakken shales. Smaller pore size leads to larger fractal dimensions; hence, the larger the SSA is, the larger fractal dimensions are.

Distribution of adsorbed water in shale: An experimental study on isolated kerogen and bulk shale samples

Bakken shale samples were studied for distribution of adsorbed water using low-pressure nitrogen sorption. By comparing results between dry and wet samples, the distribution of adsorbed water in shale was determined. Two of the isolated kerogen samples show a striking change of pore size distribution (PSD) in large pores (>16 nm), indicating the pronounced distribution of adsorbed water in large pores of organic matter. As for the bulk shale, water can adsorb in both small (<16 nm) and large pores (>16 nm) depending on hydrophilic sites. However, hydrophilic sites in small pores are mainly contributed by inorganic matter, while hydrophilic sites in large pores are composed of inorganic or organic matter. The overall results therefore clarify the contribution of inorganic and organic matter to water adsorption in shale and provide a better understanding of the significance of adsorbed water in shale.

A comprehensive pore structure study of the Bakken Shale with SANS, N2 adsorption and mercury intrusion

Small angle neutron scattering (SANS) analysis was performed on six Bakken Shale samples with different maturities to reveal the complexities in the pore structure. Pore size distribution (PSD), porosity and specific surface area (SSA) were calculated from SANS data via the Polydisperse Spherical Pore (PDSP) model and compared with the data from N2 adsorption and mercury intrusion. The results showed that the Bakken samples have a very small porosity value (less than 1%) and a very larger specific surface area (larger than 180995 cm−1) in the measuring pore size range (pore diameter: 1–200 nm). SANS and N2 adsorption can detect pores in the similar size range (2–200 nm). The SSA measured by SANS and mercury intrusion was found larger than the one detected by N2 adsorption. Pore structure information that is obtained from SANS, N2 adsorption, and mercury intrusion methods exhibited a fractal and multifractal behavior. Moreover, the pore size distribution that is calculated from SANS data was the most heterogeneous. Finally, the effects of rock composition on pore structures demonstrated that organic matter hosts some isolated pores while clay minerals do not host a large quantity of pores that are either connected or isolated.

Multi-scale assessment of mechanical properties of organic-rich shales: A coupled nanoindentation, deconvolution analysis, and homogenization method

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.

Nanopore structures of isolated kerogen and bulk shale in Bakken Formation

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.

Nanoscale pore structure characterization of the Bakken shale in the USA

Understanding the pore structures of unconventional reservoirs such as shale can assist in estimating their elastic transport and storage properties, thus enhancing the hydrocarbon recovery from such massive resources. Bakken Shale Formation is one of the largest shale oil reserves worldwide located in the Williston Basin, North America. In this paper, we collected a few samples from the Bakken and characterized their properties by using complementary methods including X-ray diffraction (XRD), N2 and CO2 adsorption, and Rock-Eval pyrolysis. The results showed that all range of pore sizes: micro (<2nm), meso (2–50nm) and macro-pores (>50nm) exist in the Bakken shale samples. Meso-pores and macro-pores are the main contributors to the porosity for these samples. Compared with the Middle Bakken, samples from Upper and Lower Bakken own more micro pore volumes. Fractal dimension analysis was performed on the pore size distribution data, and the results indicated more complex pore structures for samples taken from the Upper and Lower Bakken shales than the Middle Bakken. Furthermore, the deconvolution of the pore distribution function from the combination of N2 and CO2 adsorption results proved that five typical pore size families exist in the Bakken shale samples: one micro-pore, one macro-pore and three meso-pore size families. The studies on the correlations between the compositions and the pore structures showed that mostly feldspar and pyrite affect the total pore volume of samples from Middle Bakken Formation whereas clay dominates the total pore volume of samples from Upper/Lower Bakken Formation. TOC and clay content are the major contributors to the micro-pore size family in the Upper/Lower Bakken. Also, it was observed that the increase of hard minerals could increase the percentage of macro-pore family in the Middle Bakken Formation.

Fracture-Induced Anisotropy of the Stress–Strain Response of Shale at Multiple Scales

Author(s): Xu, H; Busetti, S; Arson, C | Abstract: This paper investigates deformation and stiffness anisotropy induced by damage propagation in a rock brittle deformation regime. Specifically, a finite-element-based Continuum damage mechanics model is used to capture sample size effects and the influence of intrinsic anisotropy on the stress-strain response of shale. The differential stress-induced damage (DSID) model previously proposed by the authors is calibrated against triaxial compression tests performed on North Dakota Bakken shale samples. Laboratory tests simulated with the FEM reproduce deformation and damage localization phenomena and capture the increase of boundary effects expected in larger samples. Simulations performed for various initial states of damage are used to investigate the role of the dominant fabric anisotropy of the rock: bedding planes in shale are modeled by a smeared damage zone with the DSID model and by a discrete crack plane. The continuum approach successfully captures the development of microcrack propagation and energy dissipation at the early stage of the strain hardening process observed in triaxial compression tests. Additionally, using initial anisotropic damage can effectively account for various types of mechanical anisotropy in shale.

A new method to determine Biot's coefficients of Bakken samples

In recent years, the development of oil and gas from shale has proceeded quickly in the world through the use of multistage fracturing technology in horizontal well. However, without the knowledge of rock poroelastic characteristics, the successful rate of hydraulic fracturing will be low. Among those poroelastic characteristics, effective stress is required for creating artificial fractures in the shale formation. In this study a new method is proposed to measure the Biot's coefficient, which is one of the key poroelastic parameters for calculating the effective stress.The Biot's coefficient is obtained after the variations of both the confining and the pore pressures are recorded with a simplified measuring procedures. The Bakken shale samples from Williston Basin are tested. The experiment results show that the Biot's coefficient of Bakken samples obtained from horizontal drilling and vertical drilling are significantly different from each other. This provides a solid base to scientists and engineers for Bakken in-situ stress analysis during multistage hydraulic fracturing and reservoir depletion due to production.

Porosity evolution in oil-prone source rocks

The origin of porosity and mechanisms of fluid flow in the presence of organic matter and clay minerals in source rocks are poorly understood. Burial and maturation of the source rock modify or create the pore systems in these rocks. Kerogen decomposition and consequent shrinkage may change the load bearing state of the minerals and organic matter and affect pore system since early stages of maturation. Geochemical evidence confirms that the hydrocarbon expulsion process (i.e. primary migration) is not 100% efficient. Expulsion of hydrocarbon is mainly driven by (1) pressure increase in the source rock due to solid kerogen conversion and volume increase and (2) continued compaction of the sediment. Converted organic matter is partly retained in the source rock diverse framework constituents. Quantitative measurement of and determining producibility of the retained hydrocarbon in the source rock is to date highly debated. The source rock hydrocarbon storage capacity is controlled by pore-hosting particles, pore system topology and rock–fluid interactions.The presence of organic matter and clay minerals affect log responses by generally overestimating porosity, because the low density kerogen is not accounted for, and together with low resistivity caused by presence of clay minerals can result in erroneous saturation calculations; thus, accurate reserve estimation often is challenged if the impact of low organic matter density is not explicitly addressed.In order to understand porosity evolution and the interaction of organic byproducts (i.e. bitumen and pyrobitumen) with rock minerals during thermal maturation, one must study source rock samples with different maturities. For this reason, ten Bakken Shale samples with varying maturity and mineralogy were selected in this study. Pore size distributions (PSD), specific surface areas (SSA) and geochemical characteristics of the samples were measured in native state and after successive solvent extraction.The PSD and SSA measured after each extraction shows recovery of the pore system with successive cleaning. Most significant was the recovery of kerogen-hosted pores with removal of soluble, oil-like organic material. Using successive extractions we are able to determine the evolution of organic matter porosity through maturation which is otherwise not feasible using visual techniques or other conventional laboratory procedures.

Petroleum Generation and Expulsion in the Constrained Bakken Petroleum System and Its Relevance to the Formation of Fractures: ABSTRACT

The lack of direct and unequivocal evidence for the existence of fractures in the Bakken petroleum system (Williston Basin, U.S.A./Canada) has led to controversial discussions with respect to the magnitude, orientation and frequency of fractures and especially their formation mechanism. Amongst other hypotheses, the creation of overpressuring as a result of high petroleum generation rates was called upon to induce fracturing. On the basis of a comprehensive organic geochemical study incorporating source rock and reservoir data, the present contribution provides evidence for an alternative pressure-controlled fracture formation mechanism: Mass balance calculations performed on Bakken Shale samples covering a maturity spectrum from 0.3% to 1.1% R{sub o} has revealed that the main phase of petroleum formation took place very early during catagenesis (ca. 0.4 - 0.8% R{sub o}) and that its potential has already been realized in those areas of the basin which currently produce from Bakken reservoirs. (1) It has been shown recently that the Bakken petroleum system represents a closed system, i.e. the Bakken sourced oils have not entered the Madison reservoirs. Hence, overall high expulsion efficiencies indicate that Bakken Shale over- and underlying units might represent the principal reservoirs and, most importantly, the strata where overpressuring should bemore » expected. (2) Detailed analysis of Bakken sourced oils which are produced from Bakken reservoirs imply that their bulk and molecular composition may have been altered by significant in-situ thermal alteration which took place after the main phase of expulsion. (3) The hypothesized process of reimpregnation of petroleum from the reservoir back into the source rock system may account for locally occurring high concentrations of solvent extractable organic matter in Bakken Shales.« less

Application of the 187Re- 187Os system to black shale geochronometry

The decay of 187Re to 187Os provides a tool for determining depositional ages of black shales. Re and Os concentrations and Os isotopic compositions of whole rock samples of the Bakken Shale, a Mississippian/Devonian boundary black shale, yield a whole rock isochron with an age of 354 ± 49 Ma. This age is in agreement with the accepted age of the Bakken Shale (≈360 Ma). The initial 187Os 186Os ratio of the isochron is 6.2 ± 3.3. This value is indistinguishable from the 187Os 186Os ratios observed in modern Black Sea sediments. The concentration of common Os in Bakken Shale samples is strongly correlated with total nitrogen, indicating that a large fraction of the Os in these samples is associated with a hydrogenous component, which overwhelms any Os supplied to the sediment in association with detrital material or cosmic dust. The dominance of the hydrogenous component imparts a relatively homogeneous initial 187Os 186Os ratio to the sediment at the time of deposition. The degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity. The pattern of scatter is consistent with postdepositional mobilization of Re and/ or Os on a small spatial scale. We suggest that this mobilization may be a consequence of petroleum formation and migration in the Bakken Shale.

Application of the pyrolysis-carbon isotope method for determining the maturity of kerogen in the Bakken shale

A maturity indexing procedure based on the isotopic difference between the total accumulated methane produced by exhaustive pyrolysis and the kerogen (Δ 13C) and the mole ratio of methane to kerogen carbon (CMR), has been tested by applying a standardized technique, i.e. exhaustive pyrolysis (600°C for 120 hr) of extracted-powdered samples and measurement of the amounts and isotopic composition of the methane and kerogen carbon, on a suite of 15 Bakken shale samples. A linear relation was found between the carbon mole ratio of pyrolysis-derived methane and total organic carbon and the δ 13C difference between the pyrolysis-derived methane and total organic carbon ( r = −0.79); and between the amount of CH 4 generated from exhaustive pyrolysis and H/C atomic ratios ( r = +0.91).