Pore Fluid Salinity Research Articles

Abstract: Relationship between overpressured compartments and spatial variations in pore fluid salinity in sediments of South Marsh Island OCS 310, offshore Louisiana

Abstract: Relationship between overpressured compartments and spatial variations in pore fluid salinity in sediments of South Marsh Island OCS 310, offshore Louisiana

Multiple Linear Regression of Complex Permittivity of a Till at Frequency Range from 200 MHz to 400 MHz

The complex permittivities of 40 compacted natural clayey till specimens at various water content, density and pore fluid salinity as independent variables are analyzed. A multiple linear regression model is constructed and the statistical analyses for the relative permittivity (e') and dielectric loss (e '')-regression equations in the frequency range from 200 to 400MHz are performed. It is concluded that the multiple linear regression model of the complex permittivity is highly significant. The coefficient of determination (R 2) and standard error (SE) of the linear regression equations vary with frequency. At 200MHz for e ' -regression and 300MHz for e '-regression, all three independent soil variables are significant at a probability of 0.95. The complex permittivity regression model with two variables at 250MHz for e 'and 350 for e '-regression equations. Upon further study on the effect of soil mineralogy to the complex permittivity, the results obtained in this study may serves as the principle of the characterization of subsurface and detection of soil and groundwater contamination.

An integrated model for transitional pressure solution in sandstones

A key aspect of the deformation of sedimentary rocks during diagenesis is pressure solution. However, it is often not clear why some rocks of a given mineralogy, depth, and pressure exhibit a great deal of pressure solution while very similar rocks under similar conditions do not. To address such difficulties, which we believe arise from the natural, non-linear physico-chemical processes, a quantitative model for pressure solution applied to sandstones has been developed. This model subsumes various factors: (1) pressure, temperature, and burial rate; (2) grain size, shape, and packing of the mineral grains; (3) the variability of water film thickness and coefficient of diffusion between any two grains in contact as a function of stress across the contact, pore fluid pressure, salinity, pH, and temperature. In this study we distinguish several types of surface sites over quartz grains in a sandstone with corresponding differences in chemical potential, thus allowing the diffusive transfer of matter from one site to another. The contact between two grains is divided into two parts. The actual true contact between two grains is a site of `water film diffusion', and the inclusions inside the contact, the free-face inclusion contacts, are sites of `free-face pressure solution'. On the pore surface site, precipitation occurs. It is shown that a kinetics transition from a reaction-limited deformation to a diffusion-limited deformation occurs in the upper crust. A geometric transition occurs too due to the textural evolution of the rock with deformation. The main goal of this paper is to show the dynamics of the shifting importance of actual and free-face inclusion contacts using a unified model including both features. The result is a predictive model for porosity variations with depth due to pressure solution in sandstones.

The concentration of deep sea gas hydrates from downhole electrical resistivity logs and laboratory data

The concentration of gas hydrate at an Ocean Drilling Program Site on the continental slope off the coast of Vancouver Island has been estimated using a combination of downhole electrical resistivity logs, resistivity and porosity of recovered core, and core pore fluid salinity. From a depth of 100 m to the base of gas hydrate stability (bottom-simulating reflector, or BSR) at about 225 m, the log resistivities have an average of about 2.0 ohmm, compared to 1.0 ohmm at a nearby deep sea reference site where no gas hydrate is present. The downhole high resistivities result from a combination of (1) high-resistivity hydrate filling part of the pore space, and (2) low-salinity in situ pore water. The low-salinity fluids are probably produced at greater depths by hydrate dissociation as the base of the stability field has moved upward with time. Both the hydrate concentration and the in situ salinity can be calculated if the hydrate concentration vs resistivity relation is known. Assuming that the effect of hydrate may be approximated by porosity reduction, the relation is given by Archie's Law with exponent about two. The hydrate affects the core pore fluid salinity through the dilution of the pure water produced by hydrate dissociation upon core recovery. The core fluid salinity results from a combination of this dilution and of in situ freshening. The in situ pore fluid salinity in the hydrate zone above the BSR is calculated to be about 80% of seawater whereas the measured salinity in the recovered cores is 60% of seawater. The computed hydrate concentration in the 100 m interval above the BSR is 25–30% of pore space (12-15% of sediment volume), in general agreement with the 20% estimated from the velocity data.

Streaming potential in porous media: 2. Theory and application to geothermal systems

Self‐potential electric and magnetic anomalies are increasingly being observed associated with hydrothermal fields, volcanic activity, and subsurface water flow. Until now a formal theoretical basis for predicting streaming potential of porous materials has not been available. We develop here a model giving both the macroscopic constitutive equations and the material properties entering these equations. The material properties, like the streaming potential coupling coefficient, depend on pore fluid salinity, temperature, water and gas saturations, mean grain diameter, and porosity. Some aspects of the model are directly tested with success against laboratory data. The streaming potential increases with temperature, grain size, and gas saturation, and decreases with salinity. At the scale of geological structures the model provides an explanation for the presence of kilometer‐scale dipolar self‐potential anomalies in geothermal systems and volcanoes. Positive self‐potential anomalies are associated with fluid discharge areas, whereas negative self‐potential anomalies are associated with fluid recharge areas. Self‐potential anomaly maps determined at the surface of active hydrothermal fields appear to be a powerful way of mapping the fluid recharge and discharge areas. In the case of free convection the vorticities of the convection pattern generate a magnetic field. The greater these vorticities, the greater the associated magnetic field. It follows that hydrothermal systems act as natural geobatteries because of the flow of pore fluids in the subsurface of these systems.

Streaming potential in porous media: 1. Theory of the zeta potential

Electrokinetic phenomena are responsible for several electrical properties of fluid‐saturated porous materials. Geophysical applications of these phenomena could include the use of streaming potentials for mapping subsurface fluid flow, the study of hydrothermal activity of geothermal areas, and in the context of earthquake prediction and volcanic activity forecasting, for example. The key parameter of electrokinetic phenomena is the ξ potential, which represents roughly the electrical potential at the mineral/water interface. We consider silica‐dominated porous materials filled with a binary symmetric 1:1 electrolyte such as NaCl. When in contact with this electrolyte, the silica/water interface gets an excess of charge through chemical reactions. Starting with these chemical reactions, we derive analytical equations for the ξ potential and the specific surface conductance. These equations can be used to predict the variations of these parameters with the pore fluid salinity, temperature, and pH (within a /pH range of 6–8). The input parameters to these equations fall into two categories: (1) mineral/fluid interaction geochemistry (including mineral surface site density and surface equilibrium constants of mineral/fluid reactions), and (2) pore fluid /pH, salinity, and temperature. The ξ potential is shown to increase with increasing temperature and pH and to decrease with increasing salinity. The proposed model is in agreement with available experimental data. The application of this model to electric potentials generated in porous media by fluid flow is explored in the companion paper.

Influence of pore-fluid salinity on pressure solution creep in gypsum

The uniaxial compaction creep behaviour of wet, granular gypsum is investigated under chemically closed, drained conditions known to favour pressure solution. The experiments were performed using applied stresses of 1 to 4 MPa and grain sizes in the range 50 to 280 μm, at room temperature. The tests were carried out using saline pore fluids with a NaCl concentration of 0.01 to 6 M, saturated with respect to the unstressed gypsum starting material. Since gypsum is almost always associated with other evaporite minerals such as halite, pore fluid compositions in gypsum will generally be highly saline. Data on the effect of pore fluid salinity on pressure solution in gypsum are thus directly relevant to the behaviour of gypsum in geological situations. The results obtained, in terms of mechanical behaviour and microstructures developed, are in good agreement with the results obtained in previous experiments performed using NaCl-free pore fluids. However, the absolute creep rates obtained here are up to 50 times faster than in the NaCl-free tests. Comparison with theoretical models for pressure solution creep and crystal growth data reveals that diffusion is unlikely to be the rate-controlling mechanism of pressure solution in the present experiments; since the growth rate of gypsum increases with nutrient salinity, the observed effect of NaCl on pressure solution is consistent with the possibility of rate control by the precipitation reaction. However, the absolute creep rates observed are one to two orders of magnitude slower than creep rates predicted by a precipitation rate-limited model, so that the detailed mechanism remains unclear.

Diagenesis in North Sea HPHT clastic reservoirs — consequences for porosity and overpressure prediction

Upper Jurassic clastic reservoirs of the Fulmar formation in the Central North Sea possess anomalously high porosities for their present day depth of burial. Reservoirs with the highest overpressures have the highest porosities, possess less macroquartz cement, and have significant secondary porosity. Quartz cementation in HPHT (High Pressure High Temperature) reservoirs has been inhibited by a combination of factors: high overpressure, limited fluid movement, presence of early grain coating cements, high pore fluid salinity, and possibly petroleum migration. Secondary porosity has contributed to reservoir quality, with an average of 4vol% extra porosity created. Quantitative prediction of porosity would require an improved depositional model for the Fulmar, accurate thermal and pressure modelling, and detailed knowledge of field filling and leakage histories. Theoretical calculations indicate that diagenetic reactions occurring in the Fulmar formation (smectite illitisation and quartz cementation), did not generate significant overpressure, because seal permeabilities were too high and the rate of volume increase associated with the reactions too small. Therefore diagenetic reactions can effectively be ignored when modelling overpressure generation in the Central North Sea, although cementation will affect rock permeability and rates of fluid dissipation.

Abstract: Spatial Variations in Formation Water Salinities, South Pelto and South Timbalier Areas, Eastern Louisiana Continental Shelf 

ABSTRACT Spatial variations in pore fluid pressure, temperature, and salinity were determined from borehole logs along a 100-km long north-south transect from the Louisiana coastline to the shelf edge in the South Pelto and South Timbalier outer continental shelf areas to help identify driving forces and pathways of regional fluid flow and solute transport, information potentially useful also in identifying pathways of hydrocarbon migration. The Pleistocene to Upper Miocene sediments were deposited in fluvial, deltaic, and open marine environments and hence contained waters of fresh to brackish to normal marine salinities, <1 to 35 g/L, at the time of their deposition. Most of these sediments, however, now contain hypersaline fluids having salinities of up to 140 to 170 g/L. Exceptions are shallow Upper Pleistocene fluvial and deltaic sediments that contain fluids less saline than sea water, even though this section is now overlain by seawater. These shallow brackish fluids probably represent the remnant of a regional freshwater lens formed during the last low-stand of sealevel. A regional plume of water having salinities in excess of 140 g/L dips gulfward to the shelf edge at depths of 3 to 5 km below the seafloor. The locus of this plume is within the sandier Pliocene section. Potential sources of dissolved salt within this plume include updip shallow salt structures and the underlying remnants of an allochthonous salt sheet at depths of approximately 6 km. Regional faults in the area may have acted as vertical conduits for upward transport of brines. Down-dip fluid flow was presumably in part gravity-driven. The distal end of this regional salinity plume near the shelf edge is now highly overpressured, and fluid flow should be in the opposite direction, i.e., updip. Hence, this part of the regional salinity structure was probably established prior to over pressuring. The results of this study establish the dynamic nature of formation water flow which has occurred in the outer continental shelf. Further work is now needed to quantify the magnitude of the driving forces and rates of these processes.

Effects of Wettability and Fluid Chemistry on Proton NMR T1 in Sands

The dependence of the proton nuclear magnetic resonance parameter, T1, on surface wettability and pore fluid chemistry was investigated using laboratory prepared silica sand samples. A pulsed NMR spectrometer, with a proton Larmor frequency of 90 MHz, was used to obtain T1 relaxation data on a set of sand samples which ranged in grain size from 115 μm to 275 μm. The T1 data were found to be dependent on the surface wettability of the sand samples. A difference in T1 of approximately 1 second was obtained between 100% water‐wet and 100% oil‐wet sand, when the pH of the pore fluid was neutral. The T1 relaxation time was found to depend on grain size in both the water‐wet and oil‐wet samples. Estimated surface relaxivities were approximately 11 μm/s and 3.4 μm/s for the water‐wet and the oil‐wet samples respectively. When experiments were conducted to assess the effect of pore fluid salinity and pH on T1 in sands, the results indicated that T1 was unaffected by salinity but was dependent on the pH of the por...

Electrical conductivity in shaly sands with geophysical applications

We develop a new electrical conductivity equation based on Bussian's model and accounting for the different behavior of ions in the pore space. The tortuosity of the transport of anions is independent of the salinity and corresponds to the bulk tortuosity of the pore space which is given by the product of the electrical formation factor F and the porosity ϕ. For the cations, the situation is different. At high salinities, the dominant paths for the electromigration of the cations are located in the interconnected pore space, and the tortuosity for the transport of cations is therefore the bulk tortuosity. As the salinity decreases, the dominant paths for transport of the cations shift from the pore space to the mineral water interface and consequently are subject to different tortuosities. This shift occurs at salinities corresponding to ξ/t(+)f ∼ 1, where ξ is the ratio between the surface conductivity of the grains and the electrolyte conductivity, and t(+)f is the Hittorf transport number for cations in the electrolyte. The electrical conductivity of granular porous media is determined as a function of pore fluid salinity, temperature, water and gas saturations, shale content, and porosity. The model provides a very good explanation for the variation of electrical conductivity with these parameters. Surface conduction at the mineral water interface is described with the Stern theory of the electrical double layer and is shown to be independent of the salinity in shaly sands above 10−3 mol L−1. The model is applied to in situ salinity determination in the Gulf Coast, and it provides realistic salinity profiles in agreement with sampled pore water. The results clearly demonstrate the applicability of the equations to well log interpretation of shaly sands.

An integrated thermal and isotopic study of the diagenesis of the Brent group, Alwyn South, U.K. North Sea

The petrography, fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore-fluid history of the Middle Jurassic Brent Group reservoir sandstones in the Alwyn South area of the U.K. North Sea. The study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels. The cement assemblage of kaolinite, quartz and illite has resulted in severe deterioration of otherwise good reservoir quality. Early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation. Temperature estimates for kaolinite precipitation of 80°C and δ 18O of ≈ + 15‰ (±3‰) suggest co-existing fluids of δ 18O ≈ −3‰. Quartz cementation overlapped both kaolinite and illite formation. Fluid inclusion data indicate that quartz cementation took place at temperatures of 109±7°C. Pore fluid salinities were ≈4 wt% NaCl with a H 2O O isotopic composition of ≈ -1 %o ± 0.5‰ SMOW. The fluids which precipitated coexisting illite were compositionally homogeneous with equilibrium δ 18O water compositions of +0.5‰ SMOW. Illite SD values range from −33 to −50‰ SMOW. These fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform, marine signature. This is consistent with the predominantly marine to paralic depositional context of the Brent Group in Alywn South. Illite precipitation was followed by hydrocarbon emplacement between the Middle Eocene and Lower Oligocene.

Effects of ion exclusion and isotopic fractionation on pore water geochemistry during gas hydrate formation and decomposition

The effects of ion exclusion and isotopic fractionation associated with gas hydrate formation and decomposition in continental margin sediments are examined using simple mass balance calculations. In a closed system pore fluid salinity can be increased to brine levels and detectable changes in interstitial waterδ18O can be caused by formation of significant amounts of interstitial gas hydrate. Time- and mass-dependent models indicate that given appropriate geometries, the diffusion of dissolved salts is sufficiently rapid and their supply is large enough to establish dissolved ion gradients that can be measured in sediments obtained from piston cores or boreholes.

The strength and deformation behaviour of model adfreeze and grouted piles in saline frozen soils

The results of a laboratory study to determine the influence of soil salinity, pile surface treatment, pile backfill material, and temperature on the adfreeze bond strength and deformation of model piles are presented. The influence of salinity and temperature on the adfreeze strength is shown to be related through the unfrozen water, and similar strength results were obtained at similar unfrozen water contents. Salinity in the backfill material of 10 and 30 ppt caused reductions in the adfreeze bond strength of 80–99%. Similar salinities within the native soil reduced its shear strength causing the location of the failure to change from the pile–backfill interface to the backfill – native soil interface when nonsaline backfill was used. Sandblasting of the pile surface to remove any surface coating resulted in adfreeze bond strengths two to three times greater than those measured on nonsandblasted piles. The use of cementitious grout to replace the sand slurry backfill resulted in greater pile load carrying capacity as the failure surface was transferred to the grout (backfill) – native soil interface instead of the pile–backfill interface. Use of backfill made from saline native soil cutting should be discouraged because of the detrimental effects of pore-fluid salinity on the adfreeze bond strength. Key words : frozen soil, saline, model pile, adfreeze, strength, deformation, grout, temperature.

Modeling the Interactions of Fluids and Rocks to Predict Reservoir Quality: Applications to the Gulf Coast Frio Formation

This study evaluates the extent to which computer models of regional paleohydrology and fluid-rock interaction can be used to predict hydrocarbon migration and the evolution of sandstone reservoir quality in the Gulf Coast Frio Formation. An integrated model was developed showing variations in fluid flow and timing of compactional overpressure along three regional cross sections in eastern Texas, southern Texas, and central Louisiana. The model suggests that the onset of extensive overpressures occurred at different times: during the early Oligocene in southern Texas, the earliest Miocene in Louisiana, and the Plio-Pleistocene in eastern Texas. These variations in overpressure timing could have resulted in significant variations in porosity evolution, since overpressured sediments experienced decreased fluid movement and consequently less fluid-rock interaction. In order to test this hypothesis and develop better methods for constraining these types of models with observational data, the mineralogy and geochemistry of authigenic cements from recent exploration wells in the Frio Formation of southern Louisiana and Texas are being analyzed. Mineralogic observations are directionally consistent with early meteoric incursion followed by minor fluid flow during burial. Modeling of fluid-rock interactions indicates that this assemblage could be precipitated by increasing pore-fluid salinity and temperature in response to burial diagenesis.more » Porosity predictions based on these calculations will be tested by future drilling of exploration wells.« less

Formation Permeability Damage Induced by Completion Brines

Summary In a laboratory study to determine permeability changes induced by floodinglarge Berea and Casper sandstone cores with NaCl and KCl brine, theconcentration of each brine was incrementally increased from 0 to 22 wt% andthen decreased from 22 to 0 wt%. In both sandstones, the permeability to KClbrines increased noticeably with increases in KCl concentration up to 1 0 wt%. Permeability remained at the higher levels throughout the remainder of eachflood until a critically low salinity level was reached, where dispersion ofthe interstitial fines took place. X-ray diffraction (XRD) analysis of theproduced formation fines and scanning electron micrographs of the sandstonepore surfaces indicated that the improved permeability was primarily a resultof alteration of illite in the clay minerals in the sandstones. TABLE 1-FORMATION MINERAL COMPOSITION (wt%)* Mineral Berea Casper Quartz 84 51 Clay minerals 10 17 Kaolinite 76 63 Illite20 34 Chlorite 4 3 Mixed layer Trace Trace 100 100 Dolomite 1 28 Calcite 1 1 Siderite 1 Trace K-Feldspars 3 3 100 100 Introduction To control formation pressure during workover operations, kill-weight fluidsare prepared by suspending insolubles in the fluid or by dissolving solublesalts in water to make a clear brine. Using suspended particles may lead toformation plugging, requiring additional acidizing of the well. Often the bestalternative to control formation pressures during workovers is a kill-weightfluid composed of a clear NaCl or KCl brine. Brine densities up to 9.8 lbm/gal[1174 kg/m] can be produced by NaCl or KCl brines. The composition ofcompletion and injection fluids rarely matches that of the formation water. Because of the difficulty and cost of duplicating the exact formation-watercomposition, a composition contrast often exists between the two fluids. Several authors have reported the effects of various completion brines onsandstone permeability. Their studies were limited to either singular brineconcentrations, a series of low brine concentrations (less than 6 wt%), orhigh-density calcium and zinc bromide brines. Coreflooding experimentsinvestigating the permeability change as NaCl and KCl brine concentrations varybetween 0 wt% and saturation have not been published. Early studies concernedprimarily the relationships between swelling and salinity. As the pore-fluidsalinity decreases, osmotic forces allow water layers to be added to theinterlayer region. The salinity level also affects the bound or crystallinewater on the clay surfaces. Baptist and Sweeney were the first to conclude thatthe absence of 2:1 montmorillonites does not guarantee that a sandstone isinsensitive to water. They reported permeability reductions up to 60% in"non-swelling" cores flooded with fresh water after saturation with 2%NaCl brine. Mungan demonstrated that dispersion of any clay is possible whenmonovalent cations are on the exchange sites. This classifies most sandstonescontaining any type of clay as water-sensitive. Khilar showed that forclay-bearing sandstones, a critical salt saturation (CSS) exists below whichclays start to disperse. Sharma et al. supported the CSS theory bydemonstrating that particle-release and particle-deposition regimes-functionsof the electrostatic potential between the clay and pore-wall surfaces and thefluid velocity exist. The electrostatic potential is controlled partly by theionic strength of the pore fluids. As the pore-fluid salinity declines, ionicstrength decreases to where the release regime dominates the depositionalregime and dispersion occurs. This point closely corresponds to the CSSdetermined by Khilar. Clay Mineral Influences on Permeability Formation permeability changes are often the result of the amount. location, and type of clay minerals in the formation. The amount of clay minerals in aformation can be a misleading indicator of potential permeability changes. Therelative abundance of specific clay types in the matrix and pore spaces must beknown in addition to the total amount of clays present. Moore demonstrated thata sand with less than 4 wt% clay minerals could have an appreciable amount ofits pore spaces (greater than 20%) filled with clay. Almon, showed that somepores may be entirely lined with authigenic clays so that the pore fluids donot contact the large quartz, feldspar, or carbonate grains. Numerous acid jobsand waterfloods have had disastrous results because the engineer or geologistfailed to recognize the importance of the location of certain clay minerals inthe formation. Grim showed the replacement order of one monovalent cation foranother to be Li+ less than Na+ less than K+ less than H+. For example, in porefluids with equal equivalent fractions of K+ and Na+, more K+ will occupy theexchange sites than Na+. Not all the Na+ will be removed. When the claylattices align in booklets, the exchange sites created in the opposing layersare about 2.8 A [0.28 nm] in diameter and 2.4 A [0.24 nm] deep. JPT P. 486^

Natural Gas Hydrates in the Alaskan Arctic

Summary The occurrence of in-situ natural gas hydrates in the arctic North Slope of Alaska is governed by several thermodynamic and geologic parameters, such as mean annual surface temperature, geothermal gradients above and below the base of the permafrost, pore-fluid salinity, permafrost base depth and temperature, subsurface pressure, and composition. Accurate knowledge of these parameters is necessary to determine the depths and thicknesses of zones of potential hydrate occurrences. The role of the parameters Is discussed in this paper. To determine the hydrate-stability zone, a nomogram has been developed and has been used for several Alaskan wells. For further delineation of gas hydrates, the neutron-transit-time crossplots have served as a valuable tool. To quantify gas hydrate deposits in terms of thickness, porosity, and saturation; use of neutron- and sonic-porosity-correction factors and Pickett crossplots is recommended.

Evaluation of the stability of gas hydrates in Northern Alaska

The factors which control the distribution of in situ gas hydrate deposits in colder regions such as Northern Alaska include; mean annual surface temperatures (MAST), geothermal gradients above and below the base of permafrost, subsurface pressures, gas composition, pore-fluid salinity and the soil condition. Currently existing data on the above parameters for the forty-six wells located in Northern Alaska were critically examined and used in calculations of depths and thicknesses of gas hydrate stability zones. To illustrate the effect of gas hydrate stability zones, calculations were done for a variable gas composition using the thermodynamic model of Holder and John (1982). The hydrostatic pressure gradient of 9.84 kPa/m (0.435 lbf/in 2ft), the salinity of 10 parts per thousand (ppt) and the coarse-grained soil conditions were assumed. An error analysis was performed for the above parameters and the effect of these parameters on hydrate stability zone calculations were determined. After projecting the hydrate stability zones for the forty-six wells, well logs were used to identify and to obtain values for the depth and thickness of hydrate zones. Of the forty-six wells, only ten wells showed definite evidence of the presence of gas hydrates.

Influence of pressure, salinity, temperature and grain size on silica diagenesis in quartzose sandstones

Based on calculated quartz solubilities for near-neutral pH conditions in the 1–6-km depth range, there is a suggested predictable affect of pressure, temperature and salinity on pressure solution in quartzose sandstones. The following results were obtained: 1. (1) Quartz solubility related to increasing grain-contact pressure with depth is ∼ 7 times greater at 4 km than at 1 km ( m s = 1.5 · 10 −3 or 90 ppm vs. 2.0 · 10 −4 or 12 ppm, respectively). 2. (2) The difference between solubilities at grain contact and hydrostatic pressures increases with depth, excluding overpressuring (at 1 km Δm s = 1.0 · 10 −6 or 0.06 ppm; at 4 km Δm s = 4.7 · 10 −5 or 2.8 ppm; and at 6 km Δm s = 2.2 · 10 −4 or 13.0 ppm). 3. (3) Increasing pore-fluid salinity decreases solubility of quartz regardless of depth (e.g., at 3 km and m NaCl = 3, m s = 8.9 · 10 −4 or 54 ppm; at 3 km and m NaCl = 6, m s = 7.5 · 10 −4 or 45 ppm). 4. (4) Temperature has the greatest influence on quartz solubility of any parameter studied. Both high heat flow and high geothermal gradient enhance quartz solubility (e.g., at 1 km and 50°C, m s = 2.0 · 10 −4 or 12 ppm; at 4 km and 140°C, m s = 1.5 · 10 −3 or 90 ppm). These results indicate that pressure solution, as well as local and non-local migration and reprecipitation of silica, and thus reservoir quality destruction in quartzose sandstones, should be enhanced under higher temperature conditions, where grain-contact pressure is near lithostatic and pore-fluid salinity is low. Under conditions of low temperature, high pore-fluid salinity and grain-contact pressure close to hydrostatic, pressure solution is discouraged and higher reservoir quality more likely preserved. Grain-size differences in quartzose sandstones also influence pressure solution. For very fine-, fine- and medium-grained sand laminae, the average pressure solution index is about 3.3, 2.5 and 2 times that of coarse sand.

Thermodynamic and structural aspects of the dehydration of smectites in sedimentary rocks

AbstractThe diagenetic conversion of smectite to illite in shales has been proposed as a mechanism for generating overpressure (fluid pressure above hydrostatic) in sedimentary basins. However, the mechanism and rate-controlling factors of the reaction and the magnitude of the resulting volume change are not known. In this paper the thermodynamics of the reversible hydration/dehydration of smectite are analysed in the pressure/temperature regime of sedimentary rocks. If the interlamellar water is more dense than bulk water, and the pressures on the solid and fluid phases are equal, the dehydration temperature should increase with increasing confining pressureviathe Clapeyron-Clausius equation. However, either fluid pressures below the pressure on the solid matrix or high pore-fluid salinities can reduce the dehydration temperature. Experiments on Ca2+-montmorillonite compacted in the presence of distilled water in a steel autoclave fitted with a sensitive pressure transducer failed to detect any overall volume change due to dehydration up to 185°C and 1400 bar. This finding, which is consistent with recent high-pressure DTA and XRD studies on Na+-montmorillonite, suggests that reversible loss of the last two water layers does not occur in sedimentary basins except as part of the more complex diagenetic conversion of smectite to illite. Some recent evidence from high-resolution TEM and microprobe analyses regarding the mechanism of the reaction is also discussed.