Scott E. Johnson

Geology

Ph.D.
36.07

Publications

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    Christopher Gerbi · Scott E. Johnson · Alden Cook · Senthil S. Vel
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    ABSTRACT: The strength of a polyphase aggregate comprising power-law materials is a function of the constitutive laws of the phases present, the arrangement of those phases and environmental conditions such as temperature. Primarily for geological applications,we consider the degree to which the arrangement of the phases has a significant influence on bulk strength. Calculations based on current single-mineral experimental data indicate that the absolute and relative strength differences between the upper and lower theoretical bounds vary widely with mineral pair, environmental conditions and strain rate. For example, at 850 ◦C, some pairs, such as plagioclase–clinopyroxene, are highly sensitive to phase morphology, whereas others, such as quartz–plagioclase, are not. Using a finite-element implementation of asymptotic expansion homogenization, we have calculated the bulk strength of natural and synthetic microstructures across macroscale strain gradients.We find that phase morphology does not change sufficiently in most cases to be the dominant factor in bulk strength variation. Thus on its own, phase morphology in an aggregate of power-law materials does not appear to be a major control on bulk strength under typical viscous geological conditions. However, phase morphology does affect microscale stress and strain rate patterns, which in turn can induce microscale variations in constitutive laws and diffusional pathways. These factors, including reactions and changing deformation mechanisms, are strongly influenced by phase morphology and do cause strength variation in rocks. As a result, any parametrization of rock strength needs to account for evolving modal mineralogy and deformation mechanisms in addition to morphological changes alone.
    Geophysical Journal International 01/2015; 200(1):374-389. DOI:10.1093/gji/ggu388 · 2.72 Impact Factor
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    A. C. Cook · S. S. Vel · C. Gerbi · S. E. Johnson
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    ABSTRACT: The constitutive laws of polyphase aggregates dominantly depend on the operative deformation mechanisms, phase morphology and modes, and environmental conditions. Each of these factors has the potential to dramatically affect bulk mechanical properties as well as the local stress and strain rate distributions. To focus on the effects of phase morphology, we have developed a rigorous multiscale approach based on Asymptotic Expansion Homogenization. The proposed methodology has two fundamental goals: (1) accurately predict bulk behavior in aggregates by explicitly taking into account phase morphology, and (2) calculate detailed distributions of strain rates, stresses and viscosities in heterogeneous materials. The methodology is able to consider general nonlinear phase constitutive laws that relate strain rates to stresses, temperature, and other factors such as water fugacity and grain size. We demonstrate the approach by analyzing power law creep of computer-generated and natural polyphase systems and benchmarking the results against analytical solutions. As an outcome of this analysis, we find that the approximation of an aggregate as a power law material is reasonable for isotropic, homogeneous phase distributions, but breaks down significantly with higher degrees of phase organization. We also present distributions in strain rate, stress, and viscosity for different applied loading conditions. Results exhibit areas of high internal stresses and substantial localization. We describe and provide a freely-available software package supporting these calculations.
    Journal of Geophysical Research: Solid Earth 09/2014; 119(9). DOI:10.1002/2014JB011197 · 3.44 Impact Factor
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    Tectonophysics 12/2013; 587:1–3. DOI:10.1016/j.tecto.2012.11.027 · 2.87 Impact Factor
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    Ben M. Frieman · Christopher C. Gerbi · Scott E. Johnson
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    ABSTRACT: The kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements. Here, we explore the effects of microstructural and rheological heterogeneity in porphyroblastic schists. Electron backscatter diffraction and petrographic analyses reveal asymmetric microboudinage of staurolite, indicating relative rotation of staurolite porphyroblasts synchronous with bulk non-coaxial strain. Boudinage and relative rotation both require porphyroblast–matrix shear coupling. Based on 2D optical observations, the extent of the coupling appears related to the initial and boudinaged staurolite grain shape and orientation as well as the geometry of heterogeneities such as mica domains or shear bands.We designed 2D finite element numerical models to assess the role of microstructural variation and rheological heterogeneity on the degree of porphyroblast–matrix shear coupling and bulk viscous strength. Model results indicate that the bulk strength of a three-phase system comprising inclusion, weak domain, and matrix is sensitive to the relative proximity of weak and strong domains, particularly at high viscosity contrasts (i.e. ηmatrix/ηweak > 10). The threshold for bulk weakening below the matrix strength occurs over a narrow range of weak domain viscosities (ηmatrix/ηweak = 2.6–5.5), regardless of the relative abundance and spatial distribution of weak domains. Kinematic decoupling of porphyroblasts occurs at low viscosity contrasts when weak domains are proximal (ηmatrix/ηweak = 2–5), but for all other spatial distributions and modal abundances investigated, kinematic decoupling occurs at viscosity contrasts of ηmatrix/ηweak = 15–20. These data indicate that bulk weakening due to rheological heterogeneity is not necessarily coincident with kinematic decoupling.
    Tectonophysics 03/2013; 587:63–78. DOI:10.1016/j.tecto.2012.11.007 · 2.87 Impact Factor
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    Z Q Fan · Z.-H Jin · S E Johnson
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    ABSTRACT: The present work describes a multi-physics model to investigate subcritical propagation of initially oil-filled, sub-horizontal collinear microcracks driven by the excess pressure induced by the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance, propagation duration, crack coalescence and excess pressure in the crack are determined using a finite difference scheme that couples linear elastic fracture mechanics, oil-gas transformation kinetics and an equation of state for the gas. The numerical results for a shale source rock with typical properties show that when the crack spacing parameter b/a0 is greater than 3, where a0 is the half crack length and b the half distance between the crack centers, the cracks do not coalesce, and the duration of gas-driven crack propagation is governed by the transformation kinetics because the oil-gas conversion rate is much slower than the subcritical crack propagation rate. The collinear cracks coalesce for smaller crack spacing and the crack propagation duration may reduce significantly due to crack interactions. The multi-physics model presented in this work together with our previous model for crack propagation during the conversion of solid kerogen to oil indicates that self-propagating microcracks resulting from the buildup of excess fluid pressure during hydrocarbon generation may serve as effective pathways for primary migration of hydrocarbons.
    International Journal of Fracture 01/2013; 185(1-2). DOI:10.1007/s10704-013-9901-9 · 1.35 Impact Factor
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    Z. Q. Fan · Z.-H. Jin · S. E. Johnson
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    ABSTRACT: To investigate primary petroleum migration through microfracturing of source rocks, we develop a theoretical multiphysics model incorporating simultaneous generation of oil and gas from kerogen, elastic anisotropy of the source rock and propagation of microcracks filled with oil and gas. The variations of excess fluid pressure in the crack and crack propagation distance with time are determined. A detailed parametric analysis is performed to study the sensitivity of petroleum migration behaviour to changes in the input parameters including kerogen type (chemistry of kerogen to oil/gas conversion), elastic anisotropy of source rocks, geothermal gradient and burial rate. Numerical results show that a microcrack in type III kerogen-bearing source rocks can attain greater length than in rocks containing type I and II kerogen which have higher oil potentials and transform to oil/gas much faster than type III kerogen. Elastic anisotropy of source rocks has a profound influence on the crack propagation distance, but only a marginal effect on the duration of crack propagation and the excess fluid pressure. The simulation also shows that higher geothermal gradients and fast burial reduce the crack propagation duration significantly, but excess pressure and final crack length are not sensitive to the variations of geothermal gradient and burial rate.
    Geophysical Journal International 07/2012; 190(1):179-187. DOI:10.1111/j.1365-246X.2012.05516.x · 2.72 Impact Factor
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    Samuel G. Roy · Scott E. Johnson · Peter O. Koons · Zhihe Jin
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    ABSTRACT: This paper examines the development of a subvolcanic magmatic breccia located along the contact of a granitic intrusion using fractal analysis and thermal-elastic modeling. The breccia grades from clast-supported, angular clasts adjacent to unfractured host rock to isolated, rounded clasts supported by the granitic matrix adjacent to the intrusion. Field observations point to an explosive breccia mechanism, and clast size distribution analysis yields fractal dimensions (Ds > 3) that exceed the minimum value known to result from explosion (Ds > 2.5). Field observations, clast size distribution data, clast circularity data, and boundary roughness fractal dimension data suggest that the clast sizes and shapes reflect post-brecciation modification by partial melting and thermal fracture. Numerical modeling is employed to explore the possible thermal-elastic effects on the size distribution of clasts. Instantaneous immersion is assumed for metasedimentary clasts of a fractal size distribution in a superheated granitic matrix for different matrix volume percentages. Thermal analysis is restricted to conductive heat transfer corrected for latent heat. Partial melting of metasedimentary clasts is an effective secondary modification process that was probably responsible for markedly altering the clast size distribution for clast populations adjacent to the intrusion. Diabase clasts experienced late-stage fracture due to the instantaneous thermal pulse in which angular clasts with high surface area to volume ratios were preferentially fractured, although this process does not appear to have markedly influenced the clast size distribution.
    Geochemistry Geophysics Geosystems 05/2012; 13(5):5009-. DOI:10.1029/2011GC004018 · 3.05 Impact Factor
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    Z.Q Fan · Z-H. Jin · S.E. Johnson
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    ABSTRACT: In this paper, we investigate subcritical propagation of an initially oil-filled, sub-horizontal microcrack driven by the excess fluid pressure associated with the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance and propagation duration (the time required for the crack to propagate during conversion of all oil to gas), as well as the excess pressure inside the crack, are determined using a finite difference scheme that couples linear elastic fracture mechanics, oil-gas transformation kinetics and an equation of state for the gas. The effects of the source-rock temperature at the initial depth of the microcrack and fracture properties of the source rock are also considered. Our numerical results show that higher burial rates significantly reduce the crack propagation duration. However, the influence of the geothermal gradient on the propagation duration and distance is only marginal. Similar to the results for the oil-driven crack propagation during kerogen-oil conversion, the duration of gas-driven crack propagation is also governed by transformation kinetics because the subcritical crack propagation rate is much faster than the oil-gas conversion rate.
    Petroleum Geoscience 04/2012; 18(2):191-199. DOI:10.1144/1354-079311-030 · 0.80 Impact Factor
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    ABSTRACT: On a strand of the Norumbega fault system, a Paleozoic, subvertical, seismogenic fault system in northeastern New England, USA, we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite/phyllonite layers. We consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone.The Norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of ~ 10–15 km, some of which preserve evidence for mutually-overprinting pseudotachylyte and mylonite. Along one of these, the Sandhill Corner shear zone, all stages of the pseudotachylyte to ultramylonite/phyllonite transformation are preserved, from (1) primary pseudotachylyte structures to (2) initial mineral crystallization, (3) grain coarsening and reactions, and (4) viscous deformation. Our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin. Using these features, we estimate that ~ 5–50% of the rock volume in the Sandhill Corner shear zone (mean ~ 30%, locally > 50%) is deformed pseudotachylyte, suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone.The Sandhill Corner shear zone localized along the contact between two rheologically-contrasting units. Mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core, indicating that seismic rupture also localized there. A decrease in grain size promoting grain-size-sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers. The formation of multiple generations of weak, deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core.
    Tectonophysics 01/2012; s 518–521:63–83. DOI:10.1016/j.tecto.2011.11.011 · 2.87 Impact Factor
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    ABSTRACT: The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic rela-tions from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending devel-opment of the underlying Twelve Mile Bay shear zone (TMBSZ). Intact granulites preserve ca. 1145 Ma ages, slightly younger than those farther to the north in the interior PSD, but similar to that of the underly-ing granulite-to-amphibolite facies Parry Sound shear zone. Weaker zircon HREE enrichment in sheared and retrogressed samples with partially resorbed garnet porphyroblasts (containing abundant zircon inclusions) suggests that the 1128–1143 Ma ages common to sheared rocks in the southern PSD record an earlier phase of metamorphism and deformation, and not the timing of shear zone development. Deformed pegmatite dikes bounded by sheared and retrogressed wall rocks from across the southern interior PSD consistently record ca. 1100 Ma ages, indicating synchronous pegmatite emplacement across the transect and constraining the bounding amphibolite-facies shear zones to ≤1100 Ma. TMBSZ samples retain evidence for (1) the older (ca. 1145 Ma) high-grade metamorphic event, (2) pegmatite emplace-ment and shear deformation at ca. 1100 Ma, and (3) a later (ca. 1070 Ma) shearing event. Combined with published structural and petrologic data, these ages indicate that pegmatite emplacement and shearing in the TMBSZ was synchronous with that in the southern interior PSD; further confirming published models for TMBSZ development. Additionally, the data suggest that the Twelve Mile Bay assemblage (within the TMBSZ) experienced high-grade metamorphism synchronously with the interior PSD, and may therefore be correlative with the basal PSD to the north.
    Precambrian Research 01/2012; 192(195):142-165. DOI:10.1016/j.precamres.2011.10.017 · 6.02 Impact Factor
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    ABSTRACT: Structural, microstructural and petrological data have enabled determination of the mechanical and geochemical processes involved in dynamic weakening and fabric transposition along the margins of a granulite nappe [the Parry Sound domain (PSD)] during transport to mid-crustal levels of the Grenville Orogen. The data establish a genetic link between outcrop-scale structures in the southern PSD and the development of the underlying Twelve Mile Bay shear zone (TMBSZ). Following granulite facies metamorphism ($11 kbar ⁄ $850 °C) in the southern PSD, the emplacement of pegmatite dykes resulted in hydration reactions within adjacent wall rocks and the development of thin (<1 m) amphibolite facies ($6.5 kbar ⁄ $700 °C) shear zones. The shear zones exhibit bulk H 2 O and K 2 O enrichment and oxygen isotope values similar to the adjacent pegmatites, suggesting metasomatic alteration by pegmatite-derived fluids. Phase-equilibrium models indicate that the destabilization of the pre-existing pyroxene and garnet-bearing assemblages, as observed within discrete shear zones in the southern PSD and the TMBSZ, requires H 2 O-saturated conditions at these (amphibolite facies) P–T conditions. The spacing between discrete shear zones and the depth of hydration into the adjacent wall rock are of comparable length-scales ($metres), suggesting that this type of reworking process can be an effective means of hydrating kilometre-scale areas of crust relatively rapidly. Furthermore, considering the well-established effects of hydrous fluids on the creep strength of anhydrous minerals, a fracture-initiated, localized hydration-and-shearing process may be an efficient mechanism for weakening strong, dry rocks (e.g. granulites) in the middle to lower orogenic crust.
    Journal of Metamorphic Geology 12/2011; DOI:10.1111/j.1525-1314.2010.00913.x · 4.37 Impact Factor
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    ABSTRACT: The anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks. Phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust. Crenulation cleavage is one of the most common microstructural fabrics found in multiply-deformed, phyllosilicate-rich, crustal rocks. We calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development: an initial planar foliation, a moderately-developed crenulation cleavage, and a well-developed crenulation cleavage. Mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method. The difficulties involved with sample preparation and data acquisition of phyllosilicate-rich rock samples are also discussed. We compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map, namely Voigt and Reuss averages. These averages do not account for grain-scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization. Our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic, with P-wave anisotropies up to 30.9% and S-wave anisotropies up to 34.2%, whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic, with a P-wave anisotropy of 15.5% and S-wave anisotropy of 10.7%. Progressive development of the fabric also causes the orientations of P- and S-wave velocity maxima and S-wave polarization directions to change markedly. Despite the high anisotropy imparted by a planar schistosity, the variety of folds and fabrics typically found in phyllosilicate-rich rocks within larger-scale crustal volumes will tend to mute the anisotropy, possibly to the point of appearing nearly isotropic.Highlights► We employ a new method (AEH) for calculating seismic anisotropy from EBSD data. ► We use AEH to explore the effects of crenulation cleavage on seismic anisotropy. ► AEH results differ from the most commonly used method by up to 19.4%. ► We show that crenulation cleavage development strongly mutes seismic anisotropy. ► Wave-speed geometries also change markedly relative to the kinematic frame.
    Earth and Planetary Science Letters 11/2011; 311(3-4):212-224. DOI:10.1016/j.epsl.2011.08.048 · 4.72 Impact Factor
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    Zuan Chen · Z.-H. Jin · S.E. Johnson
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    ABSTRACT: We present a finite difference/perturbation method to investigate transient dike propagation from a magma chamber 1–2 km below the level of neutral buoyancy in oceanic crust and explore the role of density gradation in dike propagation arrest. The dike is modeled as a magma-filled blade crack and the host rock is assumed to be a linear elastic medium with graded mass density. An integral equation approach is used to obtain the dike propagation velocity and stress intensity factor at the propagating dike tip. The finite difference/perturbation method is applicable generally to mafic dikes with ηV less than 50 Pa-m, where η is the magma viscosity and V the dike propagation velocity. Numerical results show that dike propagation velocity initially increases with dike length, reaches a peak value when the dike tip reaches a position well above the level of neutral buoyancy and then rapidly decreases to zero after the dike propagates further into the region above the level of neutral buoyancy, which indicates that density gradation is an effective mechanism for dike arrest. The effects of host-rock fracture toughness and magma chamber depth relative to the level of neutral buoyancy are also discussed.
    Journal of Volcanology and Geothermal Research 06/2011; 203:81-86. DOI:10.1016/j.jvolgeores.2011.03.005 · 2.52 Impact Factor
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    ABSTRACT: We study the influence of microstructural variables on seismic wave speed anisotropy in crustal rocks. The bulk elastic properties and corresponding wave velocities are calculated for synthetic rock samples with varying amounts of muscovite and quartz, different muscovite and quartz grain orientations and varying spatial distributions of the muscovite grains to investigate the sensitivity of seismic wave speed anisotropy on these characteristics. The asymptotic expansion homogenization method combined with finite element modelling is used to calculate bulk stiffness tensors for representative rock volumes and the wave velocities are obtained from these tensors using the Christoffel equation. The aim of this paper is to (1) demonstrate how wave speeds computed from the rigorous asymptotic expansion homogenization method compare with those generated using stiffness tensors derived from commonly applied analytic estimates, and (2) explore how different microstructural variables influence seismic wave speeds. Our results show that the muscovite grain orientations have a significant influence on the wave speeds. Increasing the modal fraction and alignment of muscovite grains leads to greater seismic anisotropy of the rock. The P-wave speed at an incidence angle of 45° between the foliation and seismic wave path is dependent on all tested microstructural variables, with the orientation distribution of muscovite grains having the largest effect. This so-called P45 effect is an important measure of wave speed anisotropy and here we provide the first analysis of its sensitivity to microstructural variables. Although we have explicitly considered only muscovite grains in this study, the methodology and observations are expected to apply in general to other phyllosilicates.
    Geophysical Journal International 05/2011; 185(2):609-621. DOI:10.1111/j.1365-246X.2011.04978.x · 2.72 Impact Factor
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    S E Johnson · Z.-H Jin · F M J Naus-Thijssen · P O Koons
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    ABSTRACT: The 370–377 Ma Mooselookmeguntic igneous complex was emplaced at ~14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca. 400–405 Ma during the Acadian orog-eny. Gravity and drill-hole data, isograd geometry, thermal modeling, and struc-tural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east-dipping, tabular-shaped intrusion. This roof preserves a classic example of low-pressure, high-temperature metamorphism caused by the underlying intrusion. Ther-mal modeling indicates that the roof is no more than 1000 m thick over an area exceed-ing 100 km 2 . Unlike other roofs described in the literature, the Mooselookmeguntic igneous complex roof preserves a thick (~600 m) emplacement-related strain gradi-ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense, nondifferentiated folia-tion approximately parallel to the intrusive contact and perpendicular to the steeply dip-ping, regional Acadian-age foliation. Fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution-precipitation creep, which is a linear viscous deformation mechanism that can occur at lower differ-ential stress than, for example, dislocation creep. As the crenulation cleavage evolves, a stage is reached where further deformation requires a change in deformation mecha-nism, which may lead to "hardening" of the rock. Development of crenulation cleavage requires a mica-rich foliation at an initially high angle to the emplacement-related fl at-tening plane, and this confi guration may be the primary reason why the strain aureole in the Mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs.
    Geological Society of America Bulletin 03/2011; 123(5-6). DOI:10.1130/B30269.1 · 4.40 Impact Factor
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    Heather A. Short · Yvette D. Kuiper · Scott E. Johnson · Dazhi Jiang
    Journal of Structural Geology 01/2011; 33(1):59-59. DOI:10.1016/j.jsg.2010.11.004 · 2.42 Impact Factor
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    Z. Q. Fan · Z.-H. Jin · S. E. Johnson
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    ABSTRACT: We conduct a parametric study on the subcritical propagation of an oil-filled, penny-shaped microcrack induced by the pressure increase caused by transformation of kerogen to oil. The excess oil pressure on the crack surfaces, and the subcritical crack propagation distance and duration, are obtained using a coupled model of fracture mechanics and kerogen-oil transformation kinetics. The numerical results show that the excess oil pressure and crack propagation distance/duration are significantly influenced by the temperature and elastic/fracture properties of the source rock, and the initial kerogen particle size. The subcritical propagation behaviour is relatively insensitive to the volume expansion rate associated with the conversion of kerogen to oil. Because the subcritical crack propagation rate is much faster than the kerogen-oil conversion rate, the crack propagation duration is primarily determined by the transformation kinetics.
    Geophysical Journal International 09/2010; 182(3):1141-1147. DOI:10.1111/j.1365-246X.2010.04689.x · 2.72 Impact Factor
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    Scott E. Johnson
    Geology 03/2010; 38(4). DOI:10.1130/G30835Y.1 · 4.64 Impact Factor
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    ABSTRACT: The finite element method was used to investigate how the elastic interactions of quartz and muscovite minerals affect grain-scale stress and strain distributions at different stages of crenulation cleavage development. The polymineralic structure comprises individual grains that were each assigned their own 3D stiffness tensor and orientation. Gradients in mean stress and volumetric strain within quartz grains develop between the limbs and hinges of microfolds at the earliest stages of crenulation development, with higher values in the microfold limbs. These gradients decrease with development of the crenulation cleavage, as the microfold limbs become phyllosilicate-rich (P) domains and the hinges become quartz- and feldspar-rich (QF) domains. Crystallographic orientations of the quartz grains have a relatively minor effect on the mean stress and volumetric strain distributions.Our findings are broadly consistent with both pressure solution and strain-driven dissolution models for crenulation cleavage development. However, because crenulation cleavage development typically involves metamorphic reactions, we favor a model in which dissolution is driven by those reactions, and mass transfer leading to development of the mineralogically segregated fabric is driven by pore fluid pressure gradients that follow gradients in volumetric strain. Local concentrations of stress and strain across mineral interfaces may identify sites of enhanced reaction.
    Journal of Structural Geology 03/2010; 32(3):330–341. DOI:10.1016/j.jsg.2010.01.004 · 2.42 Impact Factor
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    Z.-H. Jin · S. E. Johnson · Z. Q. Fan
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    ABSTRACT: We use a fracture mechanics model to study subcritical propagation and coalescence of single and collinear oil-filled cracks during conversion of kerogen to oil. The subcritical propagation distance, propagation duration, crack coalescence and excess oil pressure in the crack are determined using the fracture mechanics model together with the kinetics of kerogen-oil transformation. The propagation duration for the single crack is governed by the transformation kinetics whereas the propagation duration for the multiple collinear cracks may vary by two orders of magnitude depending on initial crack spacing. A large amount of kerogen (>90%) remains unconverted when the collinear cracks coalesce and the new, larger cracks resulting from coalescence will continue to propagate with continued kerogen-oil conversion. The excess oil pressure on the crack surfaces drops precipitously when the collinear cracks are about to coalesce, and crack propagation duration and oil pressure on the crack surfaces are strongly dependent on temperature.
    Geophysical Research Letters 01/2010; 37(1). DOI:10.1029/2009GL041576 · 4.46 Impact Factor

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