This study was performed to evaluate pore systems of reservoir lithofacies within the Devonian Three Forks Formation in the Williston Basin through micro-scale pore characterization. These ...lithofacies are from the Upper Three Forks section, which is a prominent drilling target within the Bakken-Three Forks Petroleum System. Samples from the Formation were examined by (1) physical core description, (2) petrographic thin section microscopy, (3) x-ray diffractometry (XRD) minerals analysis, (4) scanning electron microscopy (SEM), and (5) porosity measurements from helium porosimetry, nuclear magnetic resonance (NMR), gas adsorption and mercury intrusion porosimetry (MIP). These were done to provide better understanding of the local variations in pore structures and how such structures impact reservoir quality within the Three Forks Formation. Seven reservoir lithofacies were identified and described, including laminated lithofacies, massive dolostone, mottled dolostone, massive mudstone, mottled mudstone, mudstone conglomerates, and brecciated mudstone. Samples show a diverse variation in mineralogical composition, pore types, porosity, and pore-size distribution. Six types of pores were identified: interparticle, intercrystalline, intracrystalline, vuggy, microfractures, and mudstone microporosity. Dolostone-rich lithofacies have abundant dolomite and less siliciclastic minerals such as quartz, feldspar, and clays. They also have relatively low porosity and generally larger pore size. A general positive trend exists between porosity with clay minerals and feldspar, in contrast to a negative trend with dolomite, and no clear relationship with quartz content. Results from the gas adsorption analysis, NMR and MIP pore-size distribution confirm an abundance of macropores (>50 nm in diameters) in dolostone dominated lithofacies while other lithofacies generally have abundant mesopores (2–50 nm).
Biogenic gas shales, predominantly microbial in origin, form an important class of organic-rich shale reservoirs with a significant economic potential. Yet large gaps remain in the understanding of ...their gas generation, storage, and transport mechanisms, as previous studies have been largely focused on mature thermogenic shale reservoirs. In this study, the pore structure of 18 Antrim Shale samples was characterized using gas adsorption (CO
2
and N
2
). The results show that most of the Antrim Shale samples are rich in organic matter content (0.58 wt.% to 14.15 wt.%), with highest values found in the Lachine and Norwood members. Samples from the Paxton Member, characterized by lower organic content, have smaller micropore surface area and micropore volume but larger meso-macro pore surface area and volume. The deconvolution results of the pore size distribution from the N
2
adsorption indicate that all of the tested Antrim Shale samples have similar pore groups. Organic matter in the Antrim Shale hosts micro pores instead of meso-macro pores, while clay minerals host both micro and meso-macro pores. Mineral-related pores play a primary role in the total porosity. The biogenic Antrim Shale, therefore, has different pore structures from other well-studied thermogenic gas shales worldwide.
The elements of Bakken Petroleum System consist of two source rocks with high underlying burial depths for significant hydrocarbon generation. However, this deep hydrocarbon generation process is ...dependent on its kinetic properties, thermal maturity, and geochemical properties. The statistical compensation effect is a complicating factor in the kinetic analyses of the Bakken Formation. In this study, we experimentally determined the kinetics of the Bakken formation source beds, observed the presence of the residual compensation effect, and numerically established a correlation between the kinetic parameters, thermal maturity indices (T max), and the vitrinite reflectance (VRo) and bitumen reflectance (BRo). First, we conducted source rock analysis to determine kinetic properties and the organic geochemical assays of reactive kerogen in the Bakken source beds. Finally, we incorporated previous established studies to generate numerical correlation for T max in terms of VRo and BRo reflectance. Our kinetic results show evidence of the residual compensation effect in the Bakken Formation when samples are repeatedly analyzed. The simultaneous linear expression of the residual compensation effect and the regression analysis of the solutions to the Kissinger equation for heating rate, yielded a kinetic parameter solution that correlates with T max. Furthermore, recalculated T max values established a correlation between the kinetic parameters, T max, VRo, and BRo. The application of state-of-the-art numerical correlations to measure subsurface kinetics, source rock richness, and burial-depth temperatures will enhance the accuracy of reservoir exploration and hydrocarbon production within the Bakken Formation.
The Lower Bakken Shale is a key member of the Bakken Petroleum System, which is a prolific unconventional accumulation in North America. Unconventional accumulations have unpredictable lateral ...variations in hydrocarbon production due to a variety of factors including porosity, permeability, and other rock properties. Therefore, understanding the geochemical (source rock potential), petrological, and petrophysical properties of these units is essential in evaluating the hydrocarbon potential for the Lower Bakken Shale. This study utilized cores from four wells within three fields in Divide County, North Dakota, with samples collected for Rock- Eval pyrolysis, organic petrology, petrographic thin section studies, XRD, SEM, porosity, pore size, and pore fluid distribution. Helium porosimetry and NMR T2 porosity techniques were used to estimate porosity and also to check the quality of the results and avoid discrepancies. Results showed that maturity in the study area varies from immature to early mature using Tmax and solid bitumen reflectance. Organic petrology showed the dominance of solid bitumen and marine alginites, which confirms the Type II kerogen identified from pyrolysis and marine depositional environment in the study area. Major organic matter types identified from SEM studies involve stringy OM, OM-mineral admixture, particulate OM and pure OM which host the majority of the organic matter pores. Furthermore, pore types identified from SEM include mineral matrix pores, organic matter pores, and microfracture pores. Porosity values based on both helium and NMR varies but the difference was nominal and attributed to the presence of abundant clay minerals. Pore sizes are distributed within micropores, mesopores, and macropores with thermal maturity, TOC, and clay mineral proportion having a major influence on pore distribution. Clay-bound water was identified to be the dominant fluid within the shale samples using the T2 cutoff values and supporting evidence from the abundance of clay matrix porosity.
•The organic matter in the source rock is Type II with maturity ranging from immature to mature.•The stringy organic matter, OM-mineral admixture, particulate OM and pure OM were identified and characterized.•Pore types identified includes mineral matrix pores, organic matter pores and microfracture pores.•Porosity values from NMR methods were higher than values from Helium porosimetry method.•Micropores are controlled by organic matter abundance while the mesopores and macropores are influenced by mineral matrix.•Pore fluid characterization from NMR T2 cutoff shows clay-bound water is the dominant fluid in the Lower Bakken Shale pores.
Using crushed shale samples to obtain pore information from the gas adsorption experiments is a widely used method. Previous studies have evaluated the impact of the particle size on the pore size ...distribution, but potential pore shape damage during the crushing process has not been thoroughly investigated. In this paper, we crushed and sieved the same shale samples into five different particle sizes and studied their pore structures using nitrogen adsorption. The results demonstrate that, as the particle size decreases, the shape of the hysteresis loop changes from H2 type (ink-bottle pore shape) to H3 type (slit pore shape). The hysteresis index decreases as the particle size decreases, indicating potential damage of the ink-bottle pores during crushing. As the particle size becomes smaller, the pore complexity first decreases and then remains steady. We, therefore, conclude that crushing shale samples into smaller particle sizes could damage the ink-bottle-shaped pores, which needs to be considered in sample preparation protocols for the gas adsorption analysis.
Evaluating pore structures of shale samples can be done with high accuracy with adsorption experiments. However, in order to obtain reliable results, proper guidelines of experimental parameters ...should be followed, including the number of test points and allocated time for equilibrium at each test interval. These parameters are somehow ignored, and researchers simply use default parameters that are in the manuals or suggested by the developer of these apparatus without being aware of how they can impact the results. Hence, to understand the effects of these two mentioned parameters on the N2 adsorption results, in this study, the number of test points and the equilibrium interval time were examined in two separate experimental approaches. Experiments were conducted on Antrim Shale where two separate groups of samples were subjected to N2 adsorption: in the first one, we changed the number of test points and in the second one, the equilibrium interval time. The results showed that if the experimental time is shorter due to either the smaller number of test points or limit in the equilibrium interval time, then the pore structures such as the surface area, pore volume, and pore complexity (derived from fractal analysis) might not be accurately obtained. Ultimately, the number of test points and/or the equilibrium interval time should be larger than a critical value, and then the pore information that is acquired from the adsorption data stays unchanged. Collectively, we emphasize the importance of choosing appropriate experimental parameters rather than following default values in the instrument settings in gas adsorption protocols for more meaningful pore structure analysis.
The Devonian Antrim Shale is an unconventional biogenic gas accumulation with a technical recoverable resource of 19.9 Tcf. However, major knowledge gaps remain regarding understanding of the source ...rock potential, organic facies assemblages and paleo-depositional conditions of the Antrim Shale members. This work utilized Rock-Eval pyrolysis, reflected light microscopy and solid bitumen reflectance to characterize the source rock quality, organo-facies assemblages, and thermal maturity of the various Antrim Shale members at three different localities in the Michigan Basin. Results showed that the Lachine and Norwood members are richer in organic matter (up to 24 wt%) than the Upper and Paxton members (<8 wt%). Organic matter is mainly dominated by marine Type II kerogen in the black shales of the Lachine and Norwood members, and by Type II and Type II/III in the Paxton Member. Telalginite, which is represented mainly by Tasmanites and Leiosphaeridia cysts, is the dominant organic matter in the black shale members where they account for about two-thirds of the organic matter composition. Solid bitumen, which accounts for less than one-third of the organic matter composition, is second after alginite. Both alginite and solid bitumen populations decline in abundance progressively in the Upper and Paxton members at the expense of inertinite and vitrinite. The dominant organofacies groups in the studied Antrim Shale members can be assigned to the BP type B and type D/E. Organic matter maturity determined from Rock-Eval Tmax and bitumen reflectance varies from immature to marginally mature across the Michigan Basin. The results confirmed that sediment burial depth and lateral position in the basin controlled organic facies assemblages within the Antrim Shale members.
Fracture toughness is an important parameter in the hydraulic fracturing design, which is the major tool in the development of unconventional resources. Laboratory techniques for fracture toughness ...measurements usually require intact core samples and time-consuming sample preparation. The objective of this study is to compare fracture toughness values obtained by two less conventional methods: nanoindentation test and scratch test, which could facilitate the evaluation of this important parameter on smaller samples and at different scales. A set of 5 Antrim shale samples characterized by different mineral compositions is used to test this approach. For the scratch test, the fracture toughness and hardness are linearly correlated and show the same changing trend along the scratch length on all tested samples. For the nanoindentation test, the fracture toughness also increases with the increasing hardness. Most importantly, the results show that the fracture toughness values derived from these two methods are very similar, despite the difference in the scale of the measurements. This study is the first to compare fracture toughness between scratch and nanoindentation tests. Our results suggest that these two methods can be used to quickly evaluate fracture toughness from the shale core intervals containing both intact and nonintact parts.
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During the Late Devonian, the Michigan Basin in the Eastern Interior Seaway was characterized by phases of well-developed bottom water anoxia, triggering extensive accumulation of organic matter-rich ...black shales. Palynological analysis for paleoenvironmental reconstruction and biostratigraphy of the Upper Devonian rocks has been previously performed in other basins of eastern North America, but it is lacking in the Michigan Basin. Here, the Upper Devonian Antrim Shale Formation is investigated for biostratigraphic, paleoenvironmental, and sea level reconstructions, organic matter characterization, and source rock potential. 35 core samples from three drill holes across the Michigan Basin spanning the Norwood, Paxton, Lachine, and Upper members. Palynological analysis showed a moderately diverse assemblage of 31 genera represented by 53 species of relatively well-preserved prasinophyte phycomata and acritarchs, with sparse records of plant spores and freshwater algae. Based on marker prasinophytes and acritarchs, an age of late Frasnian was assigned to the Norwood and Paxton members, while an early Famennian age was proposed to the Lachine and Upper members of the Antrim Shale. Two palynofacies assemblages (PFA) were identified from the variation in the particulate organic matter (POM) composition, both of which indicate deposition in a distal inner neritic shelf environment, but the PFA-2 took place in slightly shallower conditions than the PFA-1. Geochemical screening indicated high organic matter content (up to 25 wt%, avg. 8.5 wt%) and excellent hydrocarbon generating potential of kerogen Type II for the Norwood, Lachine, and Upper members. Only the Paxton Member was dominated by poor to fair organic richness (<0.6 wt%) and low hydrocarbon potential with a mixed Type II/III kerogen. Organic petrography indicated that the dominant organic matter assemblage consists mainly of unicellular marine Tasmanites telalginite and Leiosphaeridia, followed by solid bitumen of the initial oil type. Thermal maturity, determined from VRo-eq and Tmax, indicated that all samples are in the oil window, except for some intervals in the Paxton Member.
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•An assemblage of prasinophytes and acritarchs defined a late Frasnian-early Famennian age.•Distal inner neritic shelf was the main depositional environment of the Antrim Shale.•Organic matter accumulation occurred under dysoxic-anoxic, well-stratified conditions.•The Antrim Shale has excellent OM content and hydrocarbon potential of kerogen Type II.•Organic matter is in the oil window, except for the TOC-lean Paxton Member that is immature.
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