Fracture Infill Research Articles

Poly-phase alteration history of the kaolinitized ‘Cava di Caolino’ volcanics (Lipari Island, southern Italy)

AbstractA 65 m thick altered volcanic profile was studied in the Cava di Caolino (Lipari Island) in order to (1) identify the alteration event(s), (2) model these events, and (3) propose estimates of the degree of chemical alteration (CIA, chemical index of alteration). Two mineral parageneses were identified: one comprising silico-aluminous phases, with well crystallized kaolinite (type 1), and the other comprising sulphates, with kaolinite as fracture infilling (type 2). The geochemistry of fluids analysed from a local hot spring (40°C, pH ≈ 8) allowed modelling of the observed silico-aluminous paragenesis. A later fumarolic event is suggested to be responsible for the sulphate paragenesis. The main stage of hydrothermal alteration lasted for ~50 ky, was marked by an increase in the CIA from the protolith (~52) to the alterites (~98), and resulted in the alteration of a 65 m thick series. Such data could be used to predict the alteration of volcanic rocks around underground nuclear waste repositories.

Multi-Azimuth Three-Component Surface Seismic Modeling in Cracked Monoclinic Media

The snapshots of the seismic wave propagation in monoclinic media are simulated using high-order staggered-grid finite-difference method. The monoclinic medium is a typical anisotropic model in fractured reservoir which includes two sets of non-orthorhombic vertical fractures in isotropic background and with different fracture infill. The modeling results show that the velocities of wave propagation in anisotropic models vary greatly in different directions, and the property of the fracture infill has a strong effect on velocity anisotropy. Then the axial rotation method is used in the modeling of multi-azimuth three-component surface seismic data in layered monoclinic media. The results illustrate that seismic velocities vary in directions not only with the polar angle, but also with the azimuth angle. Furthermore, the numerical modeling results may provide a theoretical basis for the inversion of anisotropic parameters and the fracture parameters by using the surface multi-azimuth seismic attributes.

Hydrogeology and pedology of saprolite formed from sedimentary rock, eastern Tennessee, USA

Groundwater flow in sedimentary rock saprolite, and in soils derived from this material, is strongly influenced by sedimentary layering and fractures inherited from the parent bedrock. The main objectives of this study were to determine whether parent bedrock lithology and infilling of fractures and other macropores with pedogenically derived clays and Fe/Mn oxides also play major roles in controlling hydraulic conductivity and groundwater flow. The study was carried out by measuring profiles of saturated and unsaturated hydraulic conductivity, K sat and K( ψ), and comparing them to soil and saprolite pedology and lithology in a 3.4 m deep pit excavated in interbedded limestone and shale saprolite. In the depth interval of 50 to 100 cm, there was an abrupt decline in K sat and K(0) by a factor of up to 250. This corresponds to the occurrence of a zone where almost all of the fractures and other macropores are occluded with pedogenic clays and Fe/Mn oxides. Below a depth of 100 cm, both degree of pore infilling and hydraulic conductivity tend to vary, with a spatial regularity that is similar to the thickness of sedimentary layering in the saprolite. However, variations in hydraulic conductivity do not always correspond to changes in lithology, suggesting that hydraulic conductivity is a result of the complex interaction of several factors, including parent bedrock lithology and the degree of infilling of the macropores. Similarities between hydrogeologic conditions at this site and at research sites in weathered sedimentary shale/siltstone and carbonate rock settings at the Oak Ridge Reservation in Tennessee and in weathered crystalline rock in North Carolina indicate that macropore infilling plays an important role in controlling hydraulic conductivity and groundwater flow for a variety of different types of parent bedrock.

Characterization of dipping fractures in a transverselyisotropic background

ABSTRACTAlthough it is believed that natural fracture sets predominantly have near‐vertical orientation, oblique stresses and some other mechanisms may tilt fractures away from the vertical. Here, we examine an effective medium produced by a single system of obliquely dipping rotationally invariant fractures embedded in a transversely isotropic with a vertical symmetry axis (VTI) background rock. This model is monoclinic with a vertical symmetry plane that coincides with the dip plane of the fractures.Multicomponent seismic data acquired over such a medium possess several distinct features that make it possible to estimate the fracture orientation. For example, the vertically propagating fast shear wave (and the fast converted PS‐wave) is typically polarized in the direction of the fracture strike. The normal‐moveout (NMO) ellipses of horizontal reflection events are co‐orientated with the dip and strike directions of the fractures, which provides an independent estimate of the fracture azimuth. However, the polarization vector of the slow shear wave at vertical incidence does not lie in the horizontal plane – an unusual phenomenon that can be used to evaluate fracture dip. Also, for oblique fractures the shear‐wave splitting coefficient at vertical incidence becomes dependent on fracture infill (saturation).A complete medium‐characterization procedure includes estimating the fracture compliances and orientation (dip and azimuth), as well as the Thomsen parameters of the VTI background. We demonstrate that both the fracture and background parameters can be obtained from multicomponent wide‐azimuth data using the vertical velocities and NMO ellipses of PP‐waves and two split SS‐waves (or the traveltimes of PS‐waves) reflected from horizontal interfaces. Numerical tests corroborate the accuracy and stability of the inversion algorithm based on the exact expressions for the vertical and NMO velocities.

Origins, closed system formation and preservation of calcites in glaciated crystalline bedrock: evidence from the Palmottu natural analogue site, Finland

A study of the geochemistry of fracture filling calcite from the Palmottu research site, a U–Th deposit located about 100 km NW of Helsinki, Finland, was part of a comprehensive natural analogue research project considering the migration of U in the crystalline bedrock environment. Fracture filling minerals act as records of the paleohydrological and chemical conditions and therefore provide insight into magmatic processes, ore deposits and long-term stability of the crystalline bedrock environment in relation to deep disposal of radioactive waste. An earlier investigation of calcite and associated U minerals identified a recent origin for these fracture infillings. Uranium series disequilibrium data indicated that relatively recent, glacial water had penetrated to a depth of 62 m. A fluid inclusion study, combined with the isotope geochemistry of several generations of fracture calcite, was undertaken to further study this aspect and to better understand the thermal and fluid history in the crystalline rock environment in general. The study revealed that at least 3 fluids were recorded by fracture calcites: 1) A crystalline calcite precipitated at 139–238 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin, 2) A massive calcite (with high salinity fluid inclusions) precipitated at 136–141 °C from a high salinity, Ca–Na–Cl fluid of magmatic or metamorphic origin and 3) A massive calcite (with low salinity fluid inclusions) precipitated at 43–286 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin that underwent equilibrium fractionation during cooling in a rock dominated system. The preservation of high temperature infillings and the cooling trend, indicating isotopic re-equilibration of water due to a very low water/rock ratio, demonstrate that the downward percolation of geologically recent waters is limited, despite the reworking of highly soluble U compounds in the upper tens of metres of the bedrock that was shown by U series disequilibrium studies. The veins in which these calcites occur have not experienced a later phase of fluid activity that dissolved or altered the calcites. Therefore, it is likely that they have not been reactivated as transmissive fractures since that initial hydrothermal episode, even during the Quaternary history in which the formation was subjected to ice sheet loading and unloading.

Mineral Matter and Trace Elements in Miocene Coals of the Tuncbilek-Domanic Basin, Kutahya, Turkey

The Tuncbilek-Domanic basin, which is one of the most productive coal basins of western Anatolia-Turkey, contains a thick and lateral extensive coal bed at the base of the Miocene Tuncbilek Formation. The coal bed sampled in this study has a thickness of 3–10 m. The coals are produced both by underground and mainly open-cast mining methods. Eight channel samples, which represent the full thickness of the coal bed, were collected from different coal mines and evaluated in this study. The samples show that the coal bed, on an air-dried basis, has high ash yields (29.8–55.5%), a large range of total sulfur contents (0.61–4.48%), and calorific values (2,433–4,834 kcal/kg). The mean values of random reflectance, in oil, of ulminite, which are between 0.42 and 0.51%Rr (av. 0.46%), and nonagglomerating character imply that the coals are in a subbituminous coal rank. The mineral matter of the whole-coal samples was identified by X-ray powder diffraction. This study shows that the samples are mainly made up of clay minerals (smectite, illite, and kaolinite/chlorite) and quartz and minor/trace amounts of siderite, dolomite, and pyrite. Moreover, some samples contain appreciable contents of feldspar, calcite, gypsum and opal-CT, and jarosite. Petrographic studies show that some massive pyrites, calcites, and dolomites were detected in cleat and fracture infillings. Elemental results were determined using an energy dispersive, polarized X-ray spectrometer (EDP-XRF) on a whole-coal dry basis. Ten trace elements (As, Co, Cr, Mn, Ni, Pb, Sb, Se, Th, and U), considered as potentially hazardous air pollutants, are present in low to moderate concentrations. The Cr and Ni concentrations in all of the samples and As, Co, Mn, Se, Th, and U concentrations in some samples display relative enrichments in comparison to other coals in the world.

Mineral Matter and Trace Elements in Miocene Coals of the Tuncbilek-Domanic Basin, Kutahya, Turkey

The Tuncbilek-Domanic basin, which is one of the most productive coal basins of western Anatolia-Turkey, contains a thick and lateral extensive coal bed at the base of the Miocene Tuncbilek Formation. The coal bed sampled in this study has a thickness of 3–10 m. The coals are produced both by underground and mainly open-cast mining methods. Eight channel samples, which represent the full thickness of the coal bed, were collected from different coal mines and evaluated in this study. The samples show that the coal bed, on an air-dried basis, has high ash yields (29.8–55.5%), a large range of total sulfur contents (0.61–4.48%), and calorific values (2,433–4,834 kcal/kg). The mean values of random reflectance, in oil, of ulminite, which are between 0.42 and 0.51%Rr (av. 0.46%), and nonagglomerating character imply that the coals are in a subbituminous coal rank. The mineral matter of the whole-coal samples was identified by X-ray powder diffraction. This study shows that the samples are mainly made up of clay minerals (smectite, illite, and kaolinite/chlorite) and quartz and minor/trace amounts of siderite, dolomite, and pyrite. Moreover, some samples contain appreciable contents of feldspar, calcite, gypsum and opal-CT, and jarosite. Petrographic studies show that some massive pyrites, calcites, and dolomites were detected in cleat and fracture infillings. Elemental results were determined using an energy dispersive, polarized X-ray spectrometer (EDP-XRF) on a whole-coal dry basis. Ten trace elements (As, Co, Cr, Mn, Ni, Pb, Sb, Se, Th, and U), considered as potentially hazardous air pollutants, are present in low to moderate concentrations. The Cr and Ni concentrations in all of the samples and As, Co, Mn, Se, Th, and U concentrations in some samples display relative enrichments in comparison to other coals in the world.

Stable isotope relationships of mineralisation in theNorth Eastern German Basin

Stable isotope systematics ( δ 34S, δ 18O, δ 13C) of carbonates, sulphates and sulphides of hydrothermal veinsand fracture infills in the North Eastern German Basin (NEGB) give evidence of a complex history of fluid input from at least two sources: fluids equilibrated with the Zechstein evaporites and fluids ascending from deeper sources such as the Rotliegend and Upper Carboniferous volcanics. The different fluid events can be related to mineralising events of vein deposits in the Harz Mountains and the polymetallic Kupferschiefer mineralisations.

Geomechanical Properties of Shear Zones in the Eastern Aar Massif, Switzerland and their Implication on Tunnelling

¶Rock zones containing a high fracture density and/or soft, low cohesion materials can be highly problematic when encountered during tunnel excavation. For example in the eastern Aar massif of central Switzerland, experiences during the construction of the Gotthard highway tunnel showed that heavily fractured areas within shear zones were responsible for overbreaks in the form of chimneys several metres in height. To understand and estimate the impact of the shear zones on rock mass behaviour, knowledge concerning the rock mass strength and deformation characteristics is fundamental. A series of laboratory triaxial tests, performed on samples from granite- and gneiss-hosted shear zones revealed that with increasing degree of tectonic overprint, sample strength decreases and rock behaviour shows a transition from brittle to ductile deformation. These trends may be explained by increasing fracture densities, increasing foliation intensity, increasing thickness of fine-grained, low cohesion fracture infill, and increasing mica content associated with the increasing degree of tectonic overprint. As fracture density increases and the influence of discrete, persistent discontinuities on rock mass strength decreases, behaviour of the test samples becomes more and more representative of rock mass behaviour, i.e. that of a densely fractured continuum. For the purpose of numerical modeling calculations, the shear zones may be subdivided with respect to an increasing fracture density, foliation intensity and mica content into a strongly foliated zone, a fractured zone and a cohesionless zone, which in turn exhibit brittle, brittle-ductile and ductile rock mass constitutive behaviour, respectively.

Fluid transfers at the basement/cover interface: Part I. Subsurface recycling of trace carbonate from granitoid basement rocks (France)

Basement rocks usually contain trace amounts of disseminated carbonate minerals and sometimes also fractures/veins filled with carbonates which may have a variety of origins. Extensive drilling has been performed of a granitoid body (Vienne, western part of the French Massif Central) overlain by a carbonate sedimentary sequence, a situation which is representative of most of the Variscan basement in western Europe. These unique samples provide the opportunity to investigate the behaviour of basement trace carbonate across the basin/basement interface and the mass transfers throughout a series of fluid migration ranging from high (ca. 450 °C) to low (down to 50 °C) temperature conditions. Study of quartz and carbonate found in fracture infillings reveals three major stages of fluid circulation, namely: (1) high to medium temperature late Hercynian fluids which circulated in fractures developed during basement uplift; (2) medium temperature Mesozoic basinal brines migration laterally along the basement/cover contact; (3) late infiltration of diagenetic/marine waters in the upper part of the granitoid body. A majority of analyzed vein carbonates from the three stages displayed a rather restricted range of C isotopic compositions ( δ 13C between −14‰ and −9‰/PDB) whatever the stage in which they were deposited and despite large variations of O isotopic ratios ( δ 18O in the range +3‰ to +30‰/SMOW). Carbon in pervasively altered rocks displays the same features and is distributed among the two broad groups defined using O isotopic ratios in veins and fluid inclusions data, namely the Hercynian and the Mesozoic carbonates. Isotopic data argue that carbon was introduced as carbonates in the early stages of the retrograde Hercynian metamorphism and was systematically reworked by subsequent fluids without any significant carbon introduction from external sources. The only exception corresponds to the stage III calcite veins which are located in the upper sections of the cores, at depths lower than 350 m beneath the basement/cover boundary and are the only witnesses of element transfer from the sediments towards the basement. The present data show that trapping of carbon by Ca-rich plutonic rocks during their cooling history confers on these rocks an efficient self-sealing capacity during later fracturing and fluid flow because carbonates possess high solubility and rapid dissolution/precipitation kinetics. Fluid circulation occurred not only in the macroscopic system of fractures but also within some volumes of the granitic “matrix” in order to scavenge and redistribute the early granitic trace carbonate. In the event of poly-cyclic carbonate redistribution, the use of carbon isotopic composition to unravel the fluid origins must be exercised with caution, as early C could be easily redistributed during later fluid migration episodes.

Depositional Environments of Oligo/Miocene Coal-Bearing Strata and Coal Quality from the Oltu-Balkaya Basin, Northeastern Turkey

This study addresses the depositional environments of the Oligo/Miocene coal-bearing Susuz Formation and Komurlu Formation from the Oltu-Balkaya Basin, northeastern Turkey, and coal quality of the Ince and Susuz seams from the Susuz Formation, which was formed in a mainly meandering fluvial environment. The Komurlu Formation that conformably overlies the Susuz Formation was developed in a mainly lacustrine environment. Six channel samples from the Ince seam and 13 channel samples from the Susuz seam exploited in the underground mines around Balkaya were collected and evaluated in this study. The samples show that the Ince and Susuz seams, on an air-dried basis, respectively, average high ash yields (38.9 and 31.2%), high total sulfur contents (3.49 and 2.89%), and relatively high calorific values (3956 and 4775 kcal/kg). The mean random ulminite reflectance (0.54 and 0.56% Rr) and nonagglomerating character imply that these coals are in a subbituminous-A coal rank. The mineral matter of the studied samples is made up of smectite, quartz, pyrite, calcite, feldspar, and traces of illite and gypsum. Petrographic studies show that some of pyrite and all calcites were detected in cleat and fracture infillings.

Palladseite and its oxidation: evidence from Au-Pd vein-type mineralization (jacutinga), Cauê iron-ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

AbstractPalladseite (palladium selenide) and palladian gold occur as euhedral inclusions in specularite and as infill of microfractures and interstices in magnetite aggregates (host rock fragments) within specularite-rich auriferous veins (jacutinga). Palladseite has minor amounts of Pt (0.6–3.5 wt.%), Cu (2.9–3.5 wt.%), Hg (0.9–1.9 wt.%), and Ag (0.3–0.5 wt.%). The palladian gold contains up to 6 wt.% Pd, and minor Cu and Ag. Isomertieite (Pd11Sb2As2) and sudovikovite (PtSe2) are also recorded. The veins cross-cut the main tectonic foliation of the wallrock (itabirite). The feature of fracture infill and the Se-Sb-As-Hg-Cu-Ag geochemical/mineralogical signature of the Au-Pd mineralization point to a post-ductile deformation and possibly low-temperature (epithermal) origin of the jacutinga mineralization style.Palladseite is replaced locally by a Pd-rich oxidation phase. Electron microprobe analysis of alteration halos around palladseite yields 76–80 wt.% Pd. Oxygen calculated by stoichiometry accords with a monoxide compound of the type PdO. However, the measured oxygen content is lower than expected for the PdO stoichiometry, suggesting loss of volatile species, and a more ‘hydrated’ compound, such as an oxyhydroxide.

Modelling of Biochemical Processes in Rocks: Oxygen Depletion by Pyrite Oxidation – Model Development and Exploratory Simulations

ABSTRACTThe purpose of this work is to gain understanding about one of the potential oxygen consuming processes occurring in deep subsurface crystalline rocks. A model for the depletion of oxygen by microbially mediated oxidation of pyrite along a deep subsurface conducting fracture was developed. In the model, a previously undisturbed fracture is suddenly invaded by oxygenated water. This could happen after a glacial period when the melt water penetrates down in the deep rock. We assume that no degradable organic matter is present. The oxygen in the water will be scavenged by the pyrite minerals in the rock matrix and in secondary minerals in any fracture infill. In this study the intrusion depth of oxygenated water is simulated assuming microbially mediated oxidation of pyrite mineral in the rock matrix as the main oxygen consuming reaction. Bacterial growth was modelled using Monod's dual limitation equation with oxygen and dissolved pyrite as the two limiting substrates. The bacteria grow as a biofilm on the fracture surface and are also present in the flowing water. The oxygen is transported to the microbes in the biofilm by the flowing water in the fracture. The ferrous iron and the sulphur from the pyrite move by molecular diffusion in the rock matrix. The rock matrix, near the fracture, is gradually depleted of pyrite and a dissolution front propagates away from the fracture.

Orthogonal cross joints: do they imply a regional stress rotation?

Orthogonal cross joints extend across intervals between systematic joints in brittle sedimentary strata and abut the systematic joints at about 90° angles. These joints typically form a ‘ladder-like’ pattern if viewed on a bedding surface. A common interpretation is that orthogonal cross joints define the orientation of the regional stress field during their formation: least compressive stress perpendicular to the joints. It follows that they indicate a rotation of regional principal stresses by 90° after the formation of the systematic joints. Using a three-dimensional boundary element code (Poly3D), we considered a simple geologic case of vertical systematic fractures developing in horizontal strata under a triaxial remote load with: the maximum principal tensile stress being horizontal and perpendicular to the strike of the fractures, the intermediate principal stress being horizontal and parallel to the strike of the fractures, and the least principal tensile stress (i.e. maximum compressive stress) being vertical. The results show that the local maximum principal stress is first perpendicular, and then parallel to, the strike of the systematic fractures as the ratio of fracture spacing to height changes from greater than to less than a critical value when the horizontal remote principal stress ratio, the ratio of the intermediate remote principal stress to the maximum remote principal stress under the sign convention of positive for tensile stresses, is greater than a threshold value (∼0.2). Thus, the fracturing process changes from infilling of systematic fractures to the formation of orthogonal cross fractures. This provides an alternative mechanism for the formation of orthogonal cross joints that does not require a systematic rotation of the regional stress field by 90°. The critical spacing to height ratio for the local principal stress switch is independent of the least remote principal stress (i.e. overburden). It increases nonlinearly with increasing ratio of the horizontal remote principal stresses, and decreases nonlinearly with increasing Poisson's ratio of the material.

Illumination of vein quartz textures in a porphyry copper ore deposit using scanned cathodoluminescence: Grasberg Igneous Complex, Irian Jaya, Indonesia

Vein quartz from the Grasberg Igneous Complex in Irian Jaya, Indonesia, displays a wide range of features when observed using scanned cathodoluminescence imaging on the electron microscope. Examination of seventeen samples revealed concentric growth zoning, quartz-filled microfractures, dark luminescence near sulfides, changes in crystal size and orientation from vein edges to vein centers, turbid growth zoning, and the truncation of concentric growth zoning. Individual features observed along two large photomosaic traverses from vein margins to vein centers coupled with trace element analyses from the electron microprobe allow determination of a detailed history of fracture infilling. The two traverses indicate a simple, two-stage history of crystal growth into an open fracture followed by mechanical closing of the fracture with resulting microfractures. Electron microprobe trace-element analyses show that the Fe content of quartz reaches values of up to 4900 ppm in dark-luminescent regions near sulfide crystals and decreases smoothly to ~200 ppm along traverses away from the sulfides. The solution to the diffusion equation coupled with existing experimental data about Fe diffusion in SiO2 indicate the duration of the diffusion was perhaps on the order of thousands of years.

Closely spaced fractures in layered rocks: initiation mechanism and propagation kinematics

Spacings of opening-mode fractures (joints and veins) in layered sedimentary rocks often scale with the layer thickness. Field observations reveal that the ratio of fracture spacing to the thickness of the fractured layer, S/ T f, ranges from less than 0.1 to greater than 10. There is a critical spacing to layer thickness ratio that defines the condition of fracture saturation, and explains the observed spacing ratios between 0.8 and 1.2. Values of S/ T f>1.2 are explained as the results of the fracturing process having not reached the saturation level. To explain ratios of S/ T f<0.8, we study the possibility for further fracture infilling, by considering flaw distributions between adjacent fractures loaded by extension of the layer. Results show that infilling fractures grow more easily from flaws located near the interface than from those in the middle of the fractured layer. The propagation of a flaw located in the middle of the fractured layer is unstable, but for the flaw to propagate toward the interfaces, its height has to be greater than a critical size. This critical size decreases with increasing S/ T f. The propagation behavior of a flaw with one of its tips at the interface depends on S/ T f. The propagation is unstable when S/ T f is greater than a critical value. When S/ T f is less than this critical value, the propagation is first unstable, then stable, and then unstable again. An infilling fracture can cut through the fractured layer only if S/ T f is greater than another critical value, otherwise the infilling fracture can only partially cut the fractured layer. For models with the same elastic constants for the fractured layer and the neighboring layers, this critical value is 0.546, and the minimum spacing to layer thickness ratio of fractures formed by the in filling process under extension is 0.273.

Equivalent medium representation of fractured rock

A similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks. Fractures can be modeled using an empirical linear slip condition; however, natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip (cracks) (model 1) or, conversely, as planar distributions of imperfect interfacial contacts (model 2). An alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material (model 3). We present analytic expressions for the fracture compliances for these three models and, using these analytic results, compute the effective compliances and stiffnesses of the fractured material. As a result, it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture, given appropriate a priori information on the fracture surfaces. The results for model 1 are equivalent to a volume distribution of cracks as studied by Hudson [1980, 1981] for small crack density; the results for model 2 are basically the same as those given by White [1983] for a packing of spheres; and finally, the results for model 3 are in agreement with those given by Backus [1962] for combinations of two constituent layers. These results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock. Finally, it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill.

Fracture spacing in layered rocks: a new explanation based on the stress transition

Opening-mode fractures (joints and veins) in layered sedimentary rocks often are periodically distributed with spacings linearly related to the thickness of the fractured layer. To better understand this linear relation, we have investigated the stress distribution between two adjacent opening-mode fractures as a function of the fracture spacing to layer thickness ratio using a three-layer elastic model with a fractured central layer. The results show that when the fracture spacing to layer thickness ratio changes from greater than to less than a critical value (approximately 1.0) the normal stress acting perpendicular to the fractures changes from tensile to compressive. This stress state transition precludes further infilling of fractures unless there are existing flaws and/or the fractures are driven by an internal fluid pressure or other mechanisms. Hence, for fractures driven by tectonic extension, the critical fracture spacing to layer thickness ratio defines a lower limit, which also defines the condition of fracture saturation. The critical value of the fracture spacing to layer thickness ratio is independent of the average strain of the fractured layer, and it increases with increasing ratio of Young's modulus of the fractured layer to that of the neighboring layers. The critical value increases with increasing Poisson's ratio of the fractured layer, and with increasing overburden stress (depth), but it decreases with increasing Poisson's ratio of the neighboring layers. For representative variation of the elastic constants of the fractured layer and the neighboring layers, and overburden stress, the critical fracture spacing to layer thickness ratio varies between 0.8 and 1.2. This range encompasses the often cited spacing to layer thickness ratios in the literature for well-developed fractures sets.

Spacing of edge fractures in layered materials

Opening-mode fractures developed from a free surface in a layered material often terminate at the interface that divides the fractured layer and the underlying layer. They also display regular spacing that is of the same order of magnitude as the thickness of the fractured layer. We have investigated the stress distribution between two adjacent edge fractures as a function of the ratio of fracture spacing to thickness of the fractured layer using a two-layer elastic model with a fractured top layer. The results show that when the ratio of fracture spacing to the layer thickness changes from greater than to less than a critical value the normal stress acting perpendicular to the fractures near the free surface changes from tensile to compressive. This stress state transition precludes further infilling of fractures unless they are driven by mechanisms other than a pure extension, or there are flaws that significantly perturb the local stress field between the fractures. Hence, the critical fracture spacing to layer thickness ratio defines a lower limit for fractures driven by extension, which also defines the condition of fracture saturation. The critical value of the fracture spacing to layer thickness ratio is independent of the average strain of the fractured layer, and it increases with increasing ratio of Young's modulus of the fractured layer to that of the underlying layer. The critical value increases with increasing Poisson's ratio of the fractured layer, but it decreases with increasing Poisson's ratio of the underlying layer. For the case with the same elastic constants for the fractured layer and the underlying layer, the critical spacing to layer thickness ratio is about 3.1. Delamination between the fractured layer and the underlying layer makes the critical spacing to layer thickness ratio much greater. Infilling fractures grow more easily from flaws located near the bottom of the fractured layer than from those located near the free surface when the spacing to layer thickness ratio is less than the critical value. The propagation of an edge flaw between adjacent edge fractures is unstable, but for the flaw to propagate to the interface, its height has to be greater than a critical size, that decreases with increasing fracture spacing to layer thickness ratio. The propagation behavior of an internal flaw with its lower tip at the interface depends on the edge fracture spacing to layer thickness ratio. The propagation is unstable, when the fracture spacing to layer thickness ratio is greater than a critical value; stable, when the fracture spacing to layer thickness ratio is less than another critical value; and first unstable, then stable, and/or unstable again, when the fracture spacing to layer thickness ratio is between these two critical values.

Mineral matter and trace elements in coals of the Gunnedah Basin, New South Wales, Australia

The concentrations of major and trace inorganic elements in a succession of Permian coals from the Gunnedah Basin, New South Wales, have been determined by X-ray fluorescence techniques applied to both whole-coal and high-temperature ash samples. The results have been evaluated in the light of quantitative data on the minerals in the same coals, determined from X-ray diffraction study of whole-coal samples using a Rietveld-based interpretation program ( Siroquant™), to determine relationships of the trace elements in the coals to the mineral species present. Comparison of the chemical composition of the coal ash interpreted from the quantitative mineralogical study to the actual ash composition determined by XRF analysis shows a high degree of consistency, confirming the validity of the XRD interpretations for the Gunnedah Basin materials. Quartz, illite and other minerals of detrital origin dominate the coals in the upper part of the sequence, whereas authigenic kaolinite is abundant in coals from the lower part of the Permian succession. These minerals are all reduced in abundance, however, and pyrite is a dominant constituent, in coals formed under marine influence at several stratigraphic levels. Calcite and dolomite occur as cleat and fracture infillings, mostly in seams near the top and bottom of the sequence. The potassium-bearing minerals in the detrital fraction are associated with significant concentrations of rubidium, and the authigenic kaolinite with relatively high proportions of titanium. Zirconium is also abundant, with associated P and Hf, in the Gunnedah Basin coal seams. Relationships exhibited by Ti, Zr, Nd and Y are consistent with derivation of the original sediment admixed with the seams from an acid volcanic source. Pyrite in the coals is associated with high concentrations of arsenic and minor proportions of thallium; no other element commonly associated with sulphides in coals, however, appears to occur in significant proportions with the pyrite in the sample suite. Small concentrations of Cl present in the coal are inversely related to the pyrite content, and appear to represent ion-exchange components associated with the organic matter. Strontium and barium are strongly associated with the cleat-filling carbonate minerals. Ge and Ga appear to be related to each other and to the coal's organic matter. Cr and V are also related to each other, as are Ce, La, Nd and Pr, but none of these show any relationship to the organic matter or a particular mineral component.