Lithostatic Pressure Gradient Research Articles

Ultrafast eclogite formation via melting-induced overpressure

The conventional wisdom holds that metamorphic reactions take place at pressures near-lithostatic so that the thermodynamic pressure, reflected by the mineral assemblage, is directly correlated with depth. On the other hand, recent field-based observations and geodynamic simulations suggest that heterogeneous stress and significant pressure deviations above lithostatic (overpressure) can occur in Earth's crust. Here we show that eclogite, normally interpreted to form at great depths in subduction zones and Earth's mantle, may form at much shallower depths via local overpressure generated in crustal shear zones. The eclogites studied crop out as lenses hosted by felsic paragneiss in a sheared thrust slice and represent a local pressure and temperature anomaly in the Taconic orogenic belt, southern New England. Sharply-defined chemical zones in garnet, which record ∼5 kbar pressure rise and fall accompanied by a temperature increase of 150–200 °C, demonstrate extremely short timescales of diffusion. This requires anomalously fast compression (∼500 yrs) and decompression. We use coupled phase equilibria and garnet diffusion forward modeling to fit the observed garnet profiles and test the likely P–T–t paths using a Monte Carlo-type approach, accounting for off-center sectioning of garnet. The simulation shows that a ∼5 kbar pressure increase after the temperature peak is necessary to reproduce the garnet zoning. Remarkably, this post-peak-T compression (from 9 kbar to 14 kbar) lasted only ∼500 yrs. If the compression was due to burial along a lithostatic pressure gradient, the descent speed would exceed 30 m yr−1, defying any observed or modeled subduction rates. Local overpressure in response to partial melting in a confined volume (Vrijmoed et al., 2009) caused by transient shear heating can explain the ultra-fast compression without necessitating burial to great depth.

The impact of energy systems demands on pressure limited CO2 storage in the Bunter Sandstone of the UK Southern North Sea

National techno-economic pathways to reduce carbon emissions are required for the United Kingdom to meet its decarbonisation obligations as mandated by the Paris Agreement. Analysis using energy systems models indicate that carbon capture and storage is a key technology for the UK to achieve its mitigation targets at lowest cost. There is potential to significantly improve upon the representation of the CO2 storage systems used in these models, but sensitivities of a given reservoir system to future development pathways must be evaluated. To investigate this we generate a range of numerical simulations of CO2 injection into the Bunter Sandstone of the UK Southern North Sea, considered to be one of the most important regional aquifers for CO2 storage. The scenarios investigate the sensitivity of CO2 storage to characteristics of regional development including number of injection sites and target rates of CO2 injection. This enables an evaluation of the impact of a range of deployment possibilities reflecting the range of scenarios that may be explored in an energy system analysis. The results show that limitations in achieving target injection rates are encountered at rates greater than 2 MtCO2/year-site due to local pressure buildup. The areal location of injection sites has minimal impact on the results because the Bunter Sandstone model has good regional connectivity. Rather, the depth of the site is the most important factor controlling limits on CO2 injection due to the relationship between the limiting pressure and the lithostatic pressure gradient. The potential for model simplification is explored by comparison of reservoir simulation with analytical models of average reservoir pressure and near-site pressure. The numerical simulations match average pressure buildup estimated with the “closed-box” analytical model of Zhou et al. (2008) over a 50year injection period. The pressure buildup at individual sites is estimated using the Mathias et al. (2011) formulation and compared to the simulation response. Discrepancies in the match are mostly due to the interaction of signals from multiple injection sites and heterogeneous permeability in the numerical simulations. These issues should be the focus of further development of simplified models for CO2 storage in an energy systems analysis framework.

The origin of Canada Hill — A result of strike-slip deformation and hydraulically powered uplift at the Pleistocene/Holocene border?

Canada Hill, located in the centre of Miri City in Sarawak, is roofed by terrace deposits of Pleistocene/Holocene age, which implies a very young uplift of this complex anticline. The present-day Canada Hill structure is explained by strike-slip deformation, in conjunction with a semi-liquid pillow of Setap Formation clay. Horizontal pressure acting in NW-SE direction, and the likely presence of a strike-slip system with late inversion triggered a 'diapiric' remobilization of the clay reservoir. The flow of liquefied clay was refocused upwards, namely in direction of the lowest pressure. Miri Formation remnants, the roof of forming clay pillow, and overlying Quarternary terrace deposits, were subsequently uplifted and emerged from the Pleistocene peneplain. However, the adjacent country rock, including the main part of the Miri Field, was hardly affected. Assuming a hydraulic uplift in the order of 400 feet and a lithostatic pressure gradient of 0.9 psi/foot, lateral pressure above the 360 psi equilibrium pressure would have prompted the clay core to move. It is noted that all current oil seepages in the hill area are related to fault systems that were reactivated at the Pleistocene/Holocene border, and an extinct mud volcano of the Canada Hill was also sighted at Tanjong Lobang, this being an obvious indication that overpressured fluids had escaped from the centre of Canada Hill. In addition, other potential areas of hydraulic uplift located in Brunei and Sabah, particularly the well-documented Jerudong and Belait Anticlines are also discussed for comparison with the observations derived from Canada Hill to support the new structural model proposed in this study.

Magma deformation may induce non-explosive volcanism via degassing through bubble networks

The open-system degassing of magma controls the bifurcation of explosive and non-explosive volcanism. Permeable gas flow through connected bubbles is a potentially important mechanism of open-system degassing from viscous silicic magma. However, recent experimental study revealed that the permeability of isotropically vesiculated magma is low; this suggests that effective degassing for causing effusive eruptions requires some other mechanisms that increase the magma permeability. In this study, we experimentally demonstrate that magma deformation results in an increase in the permeability of vesiculated magma. Torsional deformation experiments of vesiculated rhyolitic melts were performed at shear strain rate of < 0.029 s − 1 up to a total strain of 34.6; then, the permeability of the run products was measured to quantify the degassing rate. The experimental results show that shear deformation dramatically increases the magma permeability parallel to the shear direction via the enhancement of bubble coalescence and the networking of tube-like bubbles. When shear strain is above 8, the permeability sharply increases at a vesicularity of 30 vol.%. The gas velocity along the direction of magma flow is estimated to be 10 − 5 to 10 − 1 m s − 1 on the basis of experimentally obtained permeabilities (10 − 15 to 10 − 10 m 2 at vesicularities of 30 to 80 vol.%) and Darcy's law, assuming a lithostatic pressure gradient of 0.03 MPa m − 1 . The gas velocity is inferred to be large enough for gas escape to be significant during magma ascent of effusive eruption. A simple model of the magma flow along volcanic conduits indicates that magma deformation results in degassing at greater depths (a few thousand meters for rhyolite with temperatures of 700–900 °C and an initial water content of 5 wt.%) than in the case where the magma is isotropically vesiculated. The ratio of the radius of the volcanic conduit to its length may control the degree of magma deformation and consequently the eruption explosivity.

新潟地域の大規模地すべりと異常高圧熱水系

High salinity Na-Cl-type geothermal waters have often been found in the anticlinal hilly terrains and landslide-prone areas of Niigata Prefecture by boring to depths of more than 1,000 meters. These fluid pressure gradients are much higher than the hydrostatic pressure gradient and approach the lithostatic pressure gradient with increasing depth. The geothermal waters in the Matsunoyama area, Tokamachi City, have the highest orifice temperature in Niigata Prefecture. Eight geothermal water wells in this area were drilled along the anticlinal axis of a nearby hilltop or higher breast of the Matsunoyama dome and ranged in depth from 170 to 1,170 meters and in temperature from 35 to 95°C. For example, the Takanoyu-1 geothermal water well is only 170 meters in depth but has an orifice temperature of 90°C. These waters show geyser action associated with methane gas, and typically have very high salinity with considerable amounts of chloride. Hydrogen and oxygen isotope values and chloride concentration of approximately 9,000mg/L suggest that the origin of the waters is altered fossil seawater trapped in organic-matter-bearing sedimentary rocks. The temperature and depth of the primary reservoir are estimated to be 139°C and 3,000 to 4,000 meters, respectively, using a Li-Mg geothermometer and the mean geothermal gradient of 30-40°C/km in the Niigata sedimentary basin. Na-Cl-type groundwaters emerged from several landslides located in the Higashi-kubiki and Naka-kubiki areas including the Matsunoyama area. Well loggings were carried out to profile the electric conductivity and hydrochemistry of groundwater in the Utsunomata landslide in the neighborhood of the Matsunoyama area. Profiles in the active landslide mass showed sharp increases of the electric conductivity and the NaCl components at around the depth of the sliding surface. In contrast, no significant variations of the profiles were recognized in the inactive landslide mass. Na-Cl-type groundwaters are formed by mixing deep Na-Cl-type geothermal waters with meteoric groundwaters. These phenomena suggest that the geothermal water injection into shallow aquifers in landslide mass generates a partially high pore water pressure around the sliding surface and causes landslides.

Tertiary subduction, collision and exhumation recorded in the Adula nappe, central Alps

Abstract The Adula nappe in the Central Alps represents a lithospheric mélange assembled in a south-dipping subduction zone during the Tertiary orogenic cycle. It consists of several heterogeneous lobes which are stacked in a forward-dipping duplex geometry. Eclogites, garnet peridotites and garnet-white-mica schists record southward-increasing peak pressure conditions which culminate at 12–17 kbar/500–600 °C in the north and 30 kbar/800–850 °C in the south. Some studies infer even higher peak pressures for the garnet peridotite body of Alpe Arami. The present-day metamorphic field gradient for peak pressures exceeds the lithostatic pressure gradient. So far, only eclogites and garnet peridotites from the Cima Lunga complex in the south and the adjacent Southern Steep Belt have yielded Tertiary metamorphic ages for the peak-pressure stage. Some recent studies propose that the Adula nappe got assembled after the formation of high-pressure assemblages in eclogites and garnet peridotites and reject regional high-pressure conditions in Tertiary times. This scenario, however, is in conflict with the observed continuity of metamorphic field gradients and post-peak-pressure structures. Amphibolite facies conditions post-date formation of the Central Alpine nappe stack. In this paper, the associated field gradient is explained through southward-increasing temperatures during near-isothermal decompression. The main mylonitic foliation in the Adula nappe post-dates peak-pressure conditions. It is associated with top-to-the-north shearing and southward-increasing amounts of decompression from eclogite facies to amphibolite facies conditions. Also, the present-day supra-lithostatic field gradient for peak pressures probably results from this deformation phase and is here related to substantial vertical flattening during northward shearing. All subsequent structures affect established nappe boundaries. Pervasive Oligocene deformation events in the Adula nappe are coeval with intense shearing along the so-called Insubric mylonites and occur during ongoing isothermal decompression to around 5 kbar. They are associated with orogen-oblique to orogen-parallel stretching of unspecified amount which may considerably contribute to the exhumation of the Lepontine dome already before the onset of the well-known Miocene extension.

Mineral precipitation associated with vertical fault zones: the interaction of solute advection, diffusion and chemical kinetics

AbstractThis article is concerned with chemical reactions that occur between two interacting parallel fluid flows using mixing in vertical faults as an example. Mineral precipitation associated with fluid flow in permeable fault zones results in mineralization and chemical reaction (alteration) patterns, which in turn are strongly dependent on interactions between solute advection (controlled by fluid flow rates), solute diffusion/dispersion and chemical kinetics. These interactions can be understood by simultaneously considering two dimensionless numbers, the Damköhler number and the Z‐number. The Damköhler number expresses the interaction between solute advection (flow rate) and chemical kinetics, while the Z‐number expresses the interaction between solute diffusion/dispersion and chemical kinetics. Based on the Damköhler and Z‐numbers, two chemical equilibrium length‐scales are defined, dominated by either solute advection or by solute diffusion/dispersion. For a permeable vertical fault zone and for a given solute diffusion/dispersion coefficient, there exist three possible types of chemical reaction patterns, depending on both the flow rate and the chemical reaction rate. These three types are: (i) those dominated by solute diffusion and dispersion resulting in precipitation at the lower tip of a vertical fault and as a thin sliver within the fault, (ii) those dominated by solute advection resulting in precipitation at or above the upper tip of the fault, and (iii) those in which advection and diffusion/dispersion play similar roles resulting in wide mineralization within the fault. Theoretical analysis indicates that there exists both an optimal flow rate and an optimal chemical reaction rate, such that chemical equilibrium following focusing and mixing of two fluids may be attained within the fault zone (i.e. type 3). However, for rapid and parallel flows, such as those resulting from a lithostatic pressure gradient, it is difficult for a chemical reaction to reach equilibrium within the fault zone, if the two fluids are not well mixed before entering the fault zone. Numerical examples are given to illustrate the three possible types of chemical reaction patterns.

Chemical reaction patterns due to fluids mixing and focusing around faults in fluid-saturated porous rocks

Chemical reaction patterns are strongly dependent on flow rates, solute diffusion/dispersion and chemical kinetics. Since ore body formation is closely associated with chemical reaction patterns, it is important to investigate an interaction between solute advection, solute diffusion/dispersion and chemical kinetics due to two fluids mixing and focusing around faults in fluid-saturated porous rocks. Therefore, the concept of the equilibrium length due to either solute advection or solute diffusion/dispersion is presented. For a permeable fault with a given chemical reaction, there exists an optimal flow rate so that chemical equilibrium may be attained between two fluids mixing and focusing within the fault. However, for rapid and parallel flows, such as those caused by the lithostatic pressure gradient, it may be impossible for a chemical reaction to reach an equilibrium state within a permeable vertical fault, if two fluids are not well mixed before entering the fault.

Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

Abstract The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya-Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya-Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhumation of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasizing its applicability to the Himalaya-Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented.

Numerical simulation of the steam-water flow instability as a mechanism of long-period ground vibrations at geothermal areas

SUMMARY We present a model for the excitation of continuous ground vibration at geothermal areas and active volcanoes, which is based on the instability of water/steam two-phase flow. Through a series of numerical experiments, we evaluate the amplitude and period of ground vibrations excited by density wave oscillation (called ‘DWO’), one of the flow-instability phenomena, which is known in the nuclear reactor engineering to occur under a wide variety of boundary conditions. We consider a vertical cylindrical pipe buried underground and 1-D flows through it driven by the lithostatic pressure gradient. Water flows into the pipe from below and is evaporated by the heat supplied from outside. When the heating rate is low, after a perturbation such as a change in heating rate is given to the flow system, a decaying oscillation of pressure, flow velocity and void fraction occurs being followed by a stationary flow. On the other hand, when the heating rate exceeds a threshold, the perturbation grows to a continuous oscillation with a fixed amplitude, forming a limit cycle DWO. We investigate how various parameters such as pipe length, pressure and heating rate, affect the stability of the two-phase flow. We then estimate the amplitudes of seismic waves excited by DWO, assuming that the pipe is embedded in a elastic half-space. We find that a longer pipe can excite a DWO of longer period and of larger maximum amplitude. For a pipe of length of 100 m with its bottom at the depth of 150 m, the DWO period is nearly 10 s. In this case the maximum pipe diameter for the excitation of limit cycle DWO for reasonable rates of stationary conductive heat supply from country rocks to the two-phase mixture inside the pipe (less than about 35 MW m−3) is 0.02 m. The maximum amplitudes of ground displacement due to DWO are of the order of 7 × 10−8 m and 4 × 10−9 m at epicentral distances of 200 and 1000 m, respectively. Typical geothermal areas and active volcanoes can supply heat and discharged water mass, which are required to excite DWO larger than typical background noises. Therefore, DWO due to a shallow water/steam flow could excite ground motions with periods longer than 10 s which is large enough to be observed with suitably designed broadband seismic experiments at geothermal fields.

EXPLORATION OPPORTUNITIES, EAST COAST BASIN, NEW ZEALAND

The Westech-Orion Joint Venture holds onshore Petroleum Exploration Permit 38329 and offshore PEPs 38325, 38326 and 38333 in the East Coast Basin, New Zealand. The Joint Venture holds 24,117 km2 covering Hawkes Bay and the Wairarapa shelf.The Westech-Orion Joint Venture has drilled six exploratory wells and five appraisal wells in the onshore East Coast Basin over a two year period. All wells encountered significant gas shows, with two wells discovering hydrocarbons in potentially commercial volumes. Each well was drilled on the crest of a seismically mapped structure, characterised by asymmetric folding over a northwest dipping thrust fault.Prior to this drilling program, the reservoir potential of the Wairoa area was inferred to be dominated by turbidite sandstones of the Tunanui and Makaretu formations (Mid-Late Miocene). The new wells show that the Mid Miocene and parts of the Early and Late Miocene pinch out across the 'Wairoa High'.One of the primary onshore reservoirs is the Kauhauroa Limestone (Early Miocene), a bryozoan-dominated, tightly packed and cemented limestone with dominantly fracture porosity. The other primary reservoir is the Tunanui Sandstone (Mid Miocene), which in well intersections to date comprises medium-thickly bedded sandstone, with net sand typically 40%. The sands have high lithic content, and are moderately sorted and subangular-subrounded.Abnormally high formation pressures were encountered in all wells, ranging up to 3,400 psi at 1,000 m. Crestal pressure gradients commonly exceed 70% of the lithostatic pressure gradient, despite the relative proximity to outcrop. The overpressure may reflect relatively young uplift of fossil pressures, with insufficient time for pressure equilibration within a generally overpressured system.The prospectivity of the area has been highgraded by recent maturation and reservoir studies in Hawkes Bay and by gas discoveries in Westech-Orion wells onshore northern Hawkes Bay. Maturation studies identified nine kitchen areas with oil migration commencing in the Late Miocene. Seismic stratigraphy and correlation with onshore wells identified offshore submarine fan deposits of Eocene, Early Miocene, Mid Miocene and Pliocene age.A 594 km2 exploration 3D seismic survey was acquired in Hawke Bay in April 1999, and 685 km of 2D seismic were acquired in March 2000. Preliminary interpretation of the 3D survey has yielded five prospects, each covering 20–90 km2. One prospect is a lowstand fan identified by stacked mounding and bidirectional downlap, correlated with the onshore Mid Miocene Tunanui Sandstone. High amplitude seismic events of Mid-Late Miocene ages are inferred to be pulses of submarine fan development, in places associated with direct hydrocarbon indicators (DHIs). High amplitude seismic events in the Pliocene include a package of high amplitude seismic reflectors interpreted as structurally trapped DHI truncated by a major unconformity.

Sediment-hosted disseminated gold mineralisation at Zarshuran, NW Iran

Mineralisation at the Zarshuran, NW Iran, occurs on the flank of an inlier of Precambrian rocks hosted in black silty calcareous and carbonaceous shale with interbedded dolomite and limestone varying in thickness from 5 to 60 m and extending along strike for approximately 5–6 km. Two major, steeply dipping sets of faults with distinct trends occur in the Zarshuran: (1) northwest (310–325) and (2) southwest (255–265). The main arsenic mineralisation occurs at the intersection of these faults. The mineral assemblage includes micron to angstrom-size gold, orpiment, realgar, stibnite, getchellite, cinnabar, thallium minerals, barite, Au-As-bearing pyrite, base metal sulphides and sulphosalts. Hydrothermal alteration features are developed in black shale and limestone around the mineralisation Types of alteration include: (1) decalcification, (2) silicification, (3) argillisation, (4) dolomitisation, (5) oxidation and acid leaching and (6) supergene alteration. The early stage of mineralisation involved removal of carbonates from the host rocks, followed by quartz precipitation. The main stage includes massive silicification associated with argillic alteration. In the late stage veining became more dominant and the main arsenic ore was deposited along fault cross cuts and gouge. These characteristics are typical of Carlin-type sediment-hosted disseminated gold deposits. The early stage of mineralisation contains only two-phase aqueous fluid inclusions. The main stage has two groups of three-phase CO2-bearing inclusions with minor CH4 ± N2, associated with high temperature, two-phase aqueous inclusions. During the late stage, fluids exhibit a wide range in composition, salinity and temperature, and CH4 becomes the dominant carbonic fluid with minor CO2 associated with a variety of two-phase aqueous fluid inclusions. The characteristics of fluids at the Zarshuran imply the presence of at least two separate fluids during mineralisation. The intersections of coexisting carbonic and aqueous inclusion isochores, together with stratigraphic and mineral stability evidence, indicate that mineralisation occurred at 945 ± 445 bar and 243 ± 59 °C, implying a depth for mineralisation of at least 3.8 ± 1.8 km (assuming a lithostatic pressure gradient). Fluid density fluctuations and the inferred depth of formation suggest that the mineralisation occurred at the transition between overpressured and normally pressured regimes. Geochronologic studies utilising K/Ar and Ar/Ar techniques on hydrothermal argillic alteration (whole rock and separated clay size fractions) and on volcanic rocks, indicates that mineralisation at Zarshuran formed at 14.2 ± 0.4 Ma, and was contemporaneous with nearby Miocene volcanic activity, 13.7 ± 2.9 Ma. It is proposed that mineralisation was the result of the infiltration of hydrothermal fluids containing a magmatic gas component, and that it was localised in the Zarshuran Unit because of the redox boundary that it provided and/or because it lay between an overpressured region at depth and a zone of circulating, hydrostatically pressured fluids above.

Analysis of pore-fluid pressure gradient and effective vertical-stress gradient distribution in layered hydrodynamic systems

A theoretical analysis is carried out to investigate the pore-fluid pressure gradient and effective vertical-stress gradient distribution in fluid saturated porous rock masses in layered hydrodynamic systems. Three important concepts, namely the critical porosity of a porous medium, the intrinsic Fore-fluid pressure and the intrinsic effective vertical stress of the solid matrix, are presented and discussed. Using some basic scientific principles, we derive analytical solutions and explore the conditions under which either the intrinsic pore-fluid pressure gradient or the intrinsic effective vertical-stress gradient can be maintained at the value of the lithostatic pressure gradient. Even though the intrinsic pore-fluid pressure gradient can be maintained at the value of the lithostatic pressure gradient in a single layer, it is impossible to maintain it at this value in all layers in a layered hydrodynamic system, unless all layers have the same permeability and porosity simultaneously. However, the intrinsic effective vertical-stress gradient of the solid matrix can be maintained at a value close to the lithostatic pressure gradient in all layers in any layered hydrodynamic system within the scope of this study.

Conduit flow and fragmentation

Abstract This chapter provides a critical overview of syn-eruptive processes between magma chamber and vent in magmatic, sustained, Plinian-type eruptions. Phreatomagmatic effects are largely neglected. The main sources of information derive from textural studies of ejecta, theoretical models and physical experiments of eruption dynamics. Textural studies commonly find bimodal vesicle populations, indicative of several discrete nucleation events. The degree of disorder increases with explosivity. Dynamical parameters, such as nucleation density and rate and bubble growth rate, can be inferred from studies of the moments of bubble size distributions. Vesicularity in pumices is observed to vary significantly both within and between deposits, which suggests that the vesicularity at fragmentation is affected by the flow dynamics. Vesicularity variations correlate most closely with changes in magmatic composition and viscosity, but not with discharge rate. Conduit flow models can be broadly grouped into first- and second-generation models; the former generally impose a lithostatic pressure gradient and a constant Newtonian viscosity, whereas the latter include equations for the rheological changes that take place during vesiculation and solve for the pressure. Second-generation models derive highly non-lithostatic pressure gradients with the result that most of the vesiculation occurs at a high rate over a short distance just prior to fragmentation. The mechanisms of brittle and ductile fragmentation have been investigated in separate studies in non-vesiculating magmas, but which mechanism operates in explosive eruptions is not known. Dynamical laboratory experiments provide observations of the physical processes operating in conduit flows. Gas-expansion experiments have shown that it is possible to generate violent explosions by unloading in cool magmatic materials. Expanding dusty flows are found to be stable only if the bulk density increases with height. Exsolution experiments have demonstrated that acceleration precedes fragmentation and that gas evolution is enhanced by advection and bubble deformation. Deformed vesicles similar to those found in ‘woody’ pumice have been generated in an analogue system that has similar rheology to that found in vesiculating magmas. Large-scale exsolution experiments suggest that explosive volcanic eruptions are inherently heterogeneous: the fluctuations in discharge rate and discrete pulses and shocks commonly observed are a consequence of the large physical scale of volcanic systems. The effect of the magma chamber and conduit geometry has also been investigated. Eruption of material from a spherical flask up a narrow cylindrical tube generates quasi-steady flow conditions after an initial transient during which the discharge rate grows, as frequently observed in volcanic eruptions. The fragmentation surface does not propagate down into the magma chamber.

Fractal analysis of fractures applied to Soultz-sous-Forets hot dry rock geothermal program

Abstract A fractal analysis has been performed on a fracture set observed on granite cores in order to determine preferential paths for fluids injected during Hot Dry Rock (HDR) geothermic experiments. This analysis was based on the sorting of fractures on criteria such as their direction according to the present stress field, their dip, their hydrothermal filling thickness (vein width) which is linked to the intensity of alteration and thus to the physico-chemical properties of the surrounding rock. The results show a global increase of the fractal dimension (D) with depth which may be related to the lithostatic pressure gradient. Analysis of the total fracture set has showed zones in which fractures are clustered (low D values) while others are characterized by regularly spaced fractures (higher D values). Fractures parallel to the maximum horizontal stress, narrow veins, and fractures with a dip greater than 40° represent 35% to 79% of the total fracture set. Their fractal dimensions are similar to those of the total fracture set. Large veins linked with high porosity zones show a cluster pattern all along the drilling. In the context of HDR geothermics, one should seek highly fractured zones especially if the fractures they contain are clustered and if they have developed wide alteration zones linked to a high porosity. These conditions may optimize the HDR heat exchanger possibilities.

Porosity-depth trends in reservoir sandstones: theoretical models related to Jurassic sandstones offshore Norway

Theoretical models of compaction and porosity modification in sandstones during burial are discussed. Theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2.5–3 km. Porosity-depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress. Time and temperature also influence the porosity-depth gradients at shallow to moderate depths, but are probably less important than effective stress and framework grain composition. At greater burial depths, below 2.5–3 km, chemical compaction, including pressure solution, can explain most of the porosity reduction in well sorted clean arenites. Theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2.5–3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km. This theory and comparisons with empirical data from Jurassic sandstones from offshore Norway indicate that high porosity is favoured by (1) overpressures approaching the lithostatic pressure gradient, (2) a short residence time below 3 km burial, (3) coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and (4) a high content of feldspar and early diagenetic secondary pores. Early hydrocarbon emplacement may retard quartz cementation and preserve porosity, but chemical compaction probably continues after oil emplacement, except at very high hydrocarbon saturations.

Fluid flow along a decollement layer: A Model applied to the 16°N section of the Barbados Accretionary Wedge

A seismic section across the whole Barbados accretionary complex at 16°N recently published by Westbrook et al. (1988) shows that the reflection marking the decollement fades between 40 and 60 km arcward of the deformation front but is present again further inland. This interruption appears to be related to a thrust cutting through the wedge. One‐dimensional steady state models of water flow along the decollement suggest a mechanism responsible for decollement disruption: a local increase of the slope either of the wedge, or of the decollement, causes the subducted layer to compact, and locally increases the friction along the decollement. If the thrust is used as a conduit for fluids to escape from the decollement, the friction along the decollement toeward of the thrust is further increased. The model also shows that the permeability of the decollement should be of the order of 10−14 m2, but may be higher near the deformation front for short periods if fluid expulsion mainly occurs in transient bursts associated with hydraulic fracturing. As a consequence of the non‐linearity of the compaction equation, pore pressure at depth in the decollement depends on the water flux, on the lithostatic pressure gradient along the decollement and on the parameters from which the physical properties of the sediment are calculated. However, it is almost independent on the pore pressure below the toe.

The influence of geometry on the ascent of magma in open fissures

During steady eruption, the flow conditions (emitted mass flux, exit velocity and exit pressure) depend on the geometry of the conduit in which the eruption occurs. This dependence is examined for the onedimensional, isothermal ascent of a homogeneous basaltic magma with an aqueous volatile phase and newtonian rheology. By fixing the geometry of the feeding fissure, the mass flux flowing in steady conditions can be determined at any depth, as well the magma pressure and vertical velocity. Flow behaviour is analysed for three fissure shapes: constant width, slowly upward narrowing and lenticular. In all the cases examined the magma arrives at the earth's surface with a pressure greater than atmospheric. The results are compared with those obtained when a lithostatic pressure gradient is assumed for the magma column. Some speculations are made, moreover, about the change in eruption style, if conduit geometry varies during a non-steady phase.

Inverse hydrologic analysis of the distribution and origin of Gulf Coast‐type geopressured zones

Inverse hydrologic analysis of compaction‐driven groundwater flow provides insights to the distribution and origin of geopressured zones in subsiding sedimentary basins, such as those found in the U.S. Gulf Coast. Occurrences of Gulf Coast‐type geopressures are most frequently attributed to “disequilibrium compaction” caused by slow rates of fluid escape from compacting sediments, “aquathermal pressuring” from thermal expansion of pore fluids, or the subsurface dehydration of smectite. This paper presents an inverse solution to the Lagrangian equation of compaction flow that includes effects of aquathermal pressuring and dehydration reactions. The solution gives the permeability profile required to maintain a lithostatic pressure gradient in a subsiding basin. Comparison of the closed‐form solution with measured permeabilities shows that geopressured zones are likely to form in shaly basins subsiding more than about 1 mm/yr but unlikely to develop in shale‐poor basins or basins subsiding less than 0.1 mm/yr. This result correctly predicts geopressures in the Gulf Coast but suggests that many important sedimentary basins were not significantly overpressured during compaction. Solutions that consider only thermal expansion of pore fluids give required permeabilities about 1.3–1.8 orders of magnitude (factor of 20–60) less than those considering only sediment compaction, indicating that aquathermal pressuring is much less important than disequilibrium compaction in causing geopressures. Solutions accounting for the effects of dehydration reactions show that release of structural water during smectite dehydration can be a significant and perhaps necessary factor in geopressuring. Hydrologic effects of a possible increase in the volume of structural water during dehydration are less significant. The most important contribution of smectite dehydration to development of geopressured zones, however, may be the accompanying reduction of host rock permeability.

Predicted Corrosion and Scale-Formation Properties of Geopressured Geothermal Waters From the Northern Gulf of Mexico Basin

Introduction Geopressured geothermal waters arc present at depths greater than about 2500 m in Cenozoic and Mesozoic sedimentary rocks beneath an area of about 350 000 km2 in coastal Texas and Louisiana. The hydraulic pressure gradients in the geopressured zones are higher than hydrostatic [10.5 kPa/m (0.465 psi/ft)], and with increasing depth approach the psi/ft)], and with increasing depth approach the lithostatic pressure gradient of 22.6 kPa/m (1.0 psi/ft). The temperature gradients in the psi/ft). The temperature gradients in the geopressured zones also are higher than the gradients in the overlying normally pressured zone. The temperature gradient in geopressured zones in south Texas is 67 deg. C/km (3.7 deg. F/100 ft); the gradients decrease northward along coastal Texas and Louisiana (Fig. 1). There currently is an intensive research and development effort by public agencies and private companies to determine the feasibility of using these geopressured waters as sources of energy. Three forms of energy may be extracted from these waters:(1)thermal energy,(2)mechanical (hydraulic) energy, because these waters will be under high pressure at ground level, and(3)energy obtained pressure at ground level, and(3)energy obtained from dissolved gases, mainly methane, which may exceed the thermal energy if the waters are saturated with natural gas under subsurface conditions. This paper describes the chemical and physical parameters of selected geopressured geothermal parameters of selected geopressured geothermal waters that are important in determining their corrosion and scale-formation properties. The equations used here for predicting corrosion and scale formation apply to geopressured waters associated with oil and gas as well as geopressured waters from geothermal reservoirs. We have carried out detailed chemical analyses of 120 formation-water samples from geopressured and normally pressured production zones that range in depth from about 1000 to 5600 m. The 25 oil and gas fields sampled are in the McAllen-Pharr, Corpus Christi, and Houston-Galveston areas of Texas and in the Lafayette, LA, area. Oil and gas production is from sandstone reservoir rocks, which are mainly in the Frio Clay of Oligocene (?) age in Texas and of Miocene age in Louisiana. Chemical Composition Chemical analyses of eight formation-water samples selected as typical of the geopressured zone in their respective areas appear in Table 1, which also gives the locations of the sampled wells, subsurface temperatures, pressures, and other pertinent data. More complete chemical analyses, as well as the field and laboratory procedures used for collection, preservation, and analysis, are given in Kharaka et preservation, and analysis, are given in Kharaka et al.