Clay-rich Sandstones Research Articles

Consolidating rock-physics classics: A practical take on granular effective medium models

Granular effective medium (GEM) models rely on the physics of a random packing of spheres. Although the relative simplicity of these models contrasts with the complex texture of most grain-based sedimentary rocks, their analytical form makes them easier to apply than numerical models designed to simulate more complex rock structures. Also, unlike empirical models, they do not rely on data acquired under specific physical conditions and can therefore be used to extrapolate beyond available observations. In addition to these practical considerations, the appeal of GEM models lies in their parameterization, which is suited for a quantitative description of the rock texture. As a result, they have significantly helped promote the use of rock physics in the context of seismic exploration for hydrocarbon resources by providing geoscientists with tools to infer rock composition and microstructure from sonic velocities. Over the years, several classic GEM models have emerged to address modeling needs for different rock types such as unconsolidated, cemented, and clay-rich sandstones. We describe how these rock-physics models, pivotal links between geology and seismic data, can be combined into extended models through the introduction of a few additional parameters (matrix stiffness index, cement cohesion coefficient, contact-cement fraction, and laminated clays fraction), each associated with a compositional or textural property of the rock. A variety of real data sets are used to illustrate how these parameters expand the realm of seismic rock-physics diagnostics by increasing the versatility of the extended models and facilitating the simulation of plausible geologic variations away from the wells.

Identification and characterization of low resistivity low contrast zones in a clastic outcrop from Sarawak, Malaysia

Zones with low resistivity low contrast (LRLC) are usually overlooked due to the absence of high-end well logging techniques in the older oil/gas fields. The Nyalau Formation outcrop (stratigraphically equivalent to offshore cycles I and II) from Sarawak, offers a good opportunity to identify and characterize LRLC zones through integrated approaches. Seven facies of the Nyalau Formation had been identified, which are: 1) low angle cross-bedded sandstone (CBSS), 2) laminated sandstone (LSS), 3) heterolithic sandstone (HSS), 4) bioturbated sandstone (BSS), 5) clay-rich sandstone (CSS), 6) mudstone (MS), as well as 7) trough CBSS (TCBSS). Apparent resistivity was determined for brine (60 kpm) saturated core plugs at a temperature of 75 °F by employing the sample core induced polarization technique. The measurement of resistivity was calibrated based on offshore salinity, as well as surface and subsurface temperature of the Sarawak. The geological and petrophysical characteristics were analyzed through field observations and lab analyses. The findings of the study revealed that the cutoff value of resistivity found at LRLC zones in Nyalau Formation was 12.6 Ω-m. Accordingly; LSS, BSS, and CSS facies appeared to derive from LRLC zones. The main causes that generate such LRLC zones are laminations, thin bedding, bioturbation, and clay minerals. The dominant effect portrayed by mixed layers of illite/smectite and smectite in LRLC zones could be attributed to their high cation exchange capacity (CEC). Two varying resistivity trends were discovered based on the variances detected in clay types for different facies. A negative correlation was found between resistivity and porosity within high resistivity zones, while a positive relationship within the LRLC zones. Disparity in the resistivity-porosity correlation is deduced to be associated with clay mineralogy. This study suggests that LRLC zones are expected to be present in shallow marine clastic reservoirs of offshore cycles (I and II), where the offshore reflects Nyalau Formation.

A comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of sandstones from the Oligocene Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, China

The sandstone from the Oligocene Xiaganchaigou Formation (E3) in the Lengdong area, Qaidam Basin, China, is extensively distributed and acts as an effective regional reservoir for hydrocarbon accumulation. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD) image analysis and cathodoluminescence (CL), a comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of the Oligocene sandstone reservoirs of the Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, are studied. The sandstones in the Lengdong area are classified as argillaceous sandstone, silty sandstone, fine sandstone, medium sandstone, pebbly sandstone, conglomeratic sandstone and conglomerate based on their grain sizes, degrees of sorting, and matrix contents. Five sedimentary facies were identified in the Xiaganchaigou Formation (E3) of the Lengdong area, Qaidam Basin: distributary channel, interdistributary bay, mouth bar, central bar and distal sand sheet facies. Their major diagenetic processes include compaction, carbonate cementation and feldspar and calcite cement dissolution. The porosity and permeability values of the fine sandstone, conglomerate, and conglomeratic sandstone are the highest, followed by those of the medium sandstone and silty sandstone, and the argillaceous sandstone and pebbly sandstone have the lowest porosity and permeability values. The diagenetic sequence in the study area ranges from the eodiagenesis B stage to the mesodiagenetic A stage. Compaction has significantly reduced the primary porosity. Carbonate cements, mainly calcite cements, occlude pores by precipitating in intergranular and intragranular spaces. Dissolution largely contributes to increasing the secondary porosity. The well-sorted and relatively coarser-grained sandstones that formed in higher-energy sedimentary facies (distributary channel, mouth bar and central bar) have a better reservoir quality than the poorly sorted, clay-rich sandstones that formed in low-energy environments (interdistributary bay and distal sand sheet). Based on the comprehensive analysis of petrographic data, diagenesis and sedimentary facies, the reservoir in the Lengdong area is classified into three types: type A, type B and type C. Type A, located in the center of the study area, is the most favorable reservoir, while type C sandstone is the last to consider when looking for a good reservoir. This study can guide hydrocarbon exploration in the Lengdong area, as well as in northwestern China and similar areas throughout the world.

Performance of Smart Water in Clay-Rich Sandstones: Experimental and Theoretical Analysis

Armin Bazyari, Bahram S. Soulgani, Mohammad Jamialahmadi, Abolfazl Dehghan Monfared and Abbas Zeinijahromi

Experimental investigation of thermomechanical behaviour of clay-rich sandstone at extreme temperatures followed by cooling treatments

The aim of this study is to investigate the influence of extreme temperatures (from 25° to 1000°C) followed by two cooling methods (both rapid and slow) on the mechanical behaviour of clay-rich Hawkesbury sandstone under uniaxial conditions. A separate set of samples was tested under continuous heating conditions without cooling to compare the results with those for cooled samples. The stress-strain behaviours were analysed, with simultaneous recording of the acoustic signals and the failure mode. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were also performed to understand the changes observed in the mechanical tests. According to the results, the mechanical, mineralogical and microstructural characteristics of clay-rich sandstones are largely dependent on the thermal field, and, regardless of the cooling method, the compressive strength, Young’s modulus, crack initiation stress and crack damage stress appear to increase when the preheating temperature increase from 25 °C to 600 °C and decrease with increasing preheating temperature for temperatures greater than 600 °C. The SEM results confirmed the transformation of the initial hexagonal kaolinite mineral structure into a fibre-like (needle-type) mineral structure, which is believed to be the reason for the strengthening phenomenon observed at preheated temperatures between 25 °C and 600 °C. Progressive dehydroxylisation of kaolinite in the sandstone cement at temperatures beyond 600 °C was found to be the main reason for the weakening and softening of the sandstone which was observed with increasing preheated temperature beyond 600 °C. Apart from these mineral degradations, induced inter-granular and intra-granular cracks at preheated temperatures beyond 600 °C also play a dominant role in the weakening of clay-rich sandstone.

The permeability structure of fault zones in sedimentary basins: a case study at the Castle Cove Fault, Otway Basin

An understanding of the permeability structure and transmissibility of fault zones can have profound implications for reservoir appraisal and development within petroleum systems. Previous investigations on the permeability structure of fault zones often focus on low-porosity host rocks rather than porous sedimentary rocks which more commonly form reservoirs. We present detailed mineralogical and geomechanical data from porous Cretaceous sandstones (Eumeralla Formation) collected at the Castle Cove Fault in the Otway Basin, south-east Australia. Ten orientated sample blocks were collected in the hanging wall at distances within 0.5–225 m from the fault plane. A progressive increase in porosity (~17–24%), permeability (0.04–2.92 mD), and pore throat size and connectivity was observed as the fault plane was approached. High-resolution thin section analyses revealed an increase in grain-scale fractures and deformation of authigenic clays in sandstones adjacent to the Castle Cove Fault plane. The improvement of the permeability structure of the sandstones is attributed to the formation of grain-scale fractures and the change in clay morphology as a result of faulting. This study demonstrates the importance of detailed mineralogical and geomechanical analyses when attempting to understand the reservoir properties of high porosity, low permeability, and clay-rich sandstones.

CEC input to evaluate the butyl diammonium dichloride as a swelling reducer in clay rich material

Previous research suggest that repeated swelling of the clay minerals involve differential strain during wetting and drying cycles which may induce decay under a fatigue of the clay rich material. Scaling due to this process has been observed on some stone binding clay but also on earthen material used in architecture. Since recent years, research on the conservation of clay-rich sandstones lead to the development of bifunctional cationic surfactants as swelling reducers for sandstones with high hygric and hydric dilatation. In this paper, the butyl diammonium dichloride surfactant was experimented on earthen material to reduce important hygric dilatation which leads to fast and important decay of earthen material. It has been directly added into the clay mixture of molded adobe brick that was to be physically and mechanically tested. The aim is to further investigate the interaction of the surfactant with clay minerals. Series of experiments showed great impact on various physical-mechanical properties of the molded earthen mixtures. Some could have been expected according to the function of the surfactant, which is to block the micropores within the clay mineral structures, responsible for the swelling. Thus, the decrease of the specific surface area as well as the hygric dilatation is understandable. However, according to the concentration of surfactant used within the adobe mixture, other properties are also affected such as the ultrasonic velocity, the water uptake coefficient, the hydric shrinkage and the compressive strength. Therefore this paper underlines important influences in changes of properties of molded adobe according to the concentration of surfactant used, and the cation exchange capacity of the material. The paper will show a new approach consisting in adapting the concentration of surfactant as a function of cation exchange capacity (CEC) of the material.

How long do natural waters “remember” release incidents of Marcellus Shale waters: a first order approximation using reactive transport modeling

Natural gas production from the Marcellus Shale formation has significantly changed energy landscape in recent years. Accidental release, including spills, leakage, and seepage of the Marcellus Shale flow back and produced waters can impose risks on natural water resources. With many competing processes during the reactive transport of chemical species, it is not clear what processes are dominant and govern the impacts of accidental release of Marcellus Shale waters (MSW) into natural waters. Here we carry out numerical experiments to explore this largely unexploited aspect using cations from MSW as tracers with a focus on abiotic interactions between cations released from MSW and natural water systems. Reactive transport models were set up using characteristics of natural water systems (aquifers and rivers) in Bradford County, Pennsylvania. Results show that in clay-rich sandstone aquifers, ion exchange plays a key role in determining the maximum concentration and the time scale of released cations in receiving natural waters. In contrast, mineral dissolution and precipitation play a relatively minor role. The relative time scales of recovery τrr, a dimensionless number defined as the ratio of the time needed to return to background concentrations over the residence time of natural waters, vary between 5 and 10 for Na, Ca, and Mg, and between 10 and 20 for Sr and Ba. In rivers and sand and gravel aquifers with negligible clay, τrr values are close to 1 because cations are flushed out at approximately one residence time. These values can be used as first order estimates of time scales of released MSW in natural water systems. This work emphasizes the importance of clay content and suggests that it is more likely to detect contamination in clay-rich geological formations. This work highlights the use of reactive transport modeling in understanding natural attenuation, guiding monitoring, and predicting impacts of contamination for risk assessment.

Direct visualization of pore-scale fines migration and formation damage during low-salinity waterflooding

Formation damage due to fines migration results, potentially, in significant decreases in reservoir permeability and, hence, the recoverability of crude oil from reservoirs. On the other hand, low salinity brine injection is a promising technique for increasing oil recovery from clay-rich sandstones in an economic manner. Clay detachment at low salinity conditions, however, drastically alters fluid flow. In this work, clay-functionalized etched-silicon micromodels are used to visualize directly the mobilization of clay at low salinity conditions in (i) the absence of oil, and (ii) the presence of oil. Study results include clay mobilization and pore plugging in the absence and presence of oil visualized by saturating the clay-functionalized micromodel with high salinity brine followed by injections of reduced salinity brines. Clay detachment and migration was observed in oil-free systems for 4000 ppm NaCl low salinity injection brine. The extent of fines detachment was quantified to determine the types of clay structures affected. Furthermore, fines migration, flocculation, and re-deposition were visualized directly. The types of structures formed (i.e., pore-plugging, pore-lining, etc.) by the re-deposited clay particles are characterized to determine their impact on formation damage. Clay detachment in the presence of oil was also visualized. Initial conditions analogous to clastic reservoirs were established by allowing the crude oil, brine, and solids to interact (i.e., age). Clay detachment occurred during 4000 ppm NaCl low salinity oil recovery. Real-time, pore-level visualization revealed significant mechanisms during oil recovery processes and their influence on multiphase flow. Specifically, pore plugging particles in water-filled pores obstructed preferential flow paths and diverted injection fluid to unswept regions thereby increasing oil production.

Microspherules-Bearing White Sandstone: Implication of Cosmic Impact Event near Jurassic-Cretaceous Boundary in West Central Sinai, Egypt

The presence of glass microspherules enclosing relict grains, shattered quartz and silicon carbide in white sandstone beds near the Jurassic-Cretaceous boundary in west central Sinai indicates a cosmic impact event. Characterization of the impact microspherules and proposing a reasonable scenario for their origin are the aims of this work. Field observations, optical, binocular, scanning electron and high-resolution transmitted electron microscopy investigations and chemical analyses were carried out. The study revealed that glass microspherules have high Al2O3 and FeO contents and low CaO and MgO contents. The high content of Al2O3 indicates that the source of microtektite-like microspherules is attributed to the melting of a clay-rich sandstone and carbonaceous matter, while the high content of FeO indicates admixing with projectile matter. The reaction between silica and carbon was carried out under conditions of high temperature (T > 1000&#176C) and carbon (C/Si > 1) which resulted in the production of silicon carbide with microdiamond intergrowth. Consequently, this intergrowth is in accordance with the impact origin via rapid condensation and growth within a vapor phase. In spite of the fact that no source crater has been recognized to date in the study area, the authors propose at least a single cosmic impact event scenario for the recorded glass microspherules in west central Sinai. The impact excavated the Paleozoic siliciclastic sedimentary rocks and then the glass microspherules showered the area of study. The deposition of microtektite-like glass particles within the white sandstone beds of the Malha Formation took place in the fluvial plain terrestrial environment. This setting precluded severe post-depositional reworking, yielding preservation of the glass particles in a primary layer. Eventually, lateral migration of the braided channels led to the reworking of the microspherules layer and the spatial dispersal of the shattered quartz.

Effect of bedding planes, their orientation and clay depositions on effective re-injection of produced brine into clay rich deep sandstone formations: Implications for deep earth energy extraction

The rapidly increasing world energy demand requires the development of deep earth energy extraction methods such as unconventional oil and gas production, including coal bed methane recovery (CBM) and utilisation of hot pore fluid in deep geothermal reservoirs. However, all these deep earth energy production methods are associated with huge wastewater (brine) production. Disposal of this brine has become a major challenge in the energy industry, as direct disposal is highly undesirable due to its organic and inorganic pollutant content and its high salinity. Re-injection of brine into deep aquifers is therefore a feasible option to overcome this issue and the flowability through the sedimentary formation or its permeability plays a major role in this process. However, most of previous works mainly focus on the homogenous formations and therefore the brine permeability of deep heterogeneous sandstone aquifers is still not well understood. This study intends to obtain a comprehensive knowledge of this process by conducting a series of core flooding experiments for brine injection into two types of clay rich sandstone, WWS and WGS, obtained from Marburg formation, Queensland, Australia along different bedding orientations and under different reservoir conditions. A series of core flooding tests were conducted on 38mm diameter and 150mm long cylindrical core samples and the pressure developments along the samples were recorded over time for a range of confining pressures (10–25MPa), temperatures (30–60°C), and injection over-pressures (4–20MPa). According to the results, brine permeability in clay rich sandstone increases with increasing injection over-pressure and reduces with increasing depth and aquifer temperature. Furthermore, the flow characteristics of sandstone aquifers are largely dependent on their heterogeneous nature, and the presence of authigenic quartz, clay depositions, low permeable beddings and their orientation have significant influences on reservoir permeability. A stepped pattern was observed in the pore pressure development along the tested sandstone cores which depicts a possible pore structure variation during the fluid injection. A rapid pressure development occurs throughout the sample at the initial stage of injection may be due to the flux tends to pass through the preferable flow paths in the rock mass and then over the time this pressure development diminishes, possibly due to the gradual building of flow barriers by grain particles extracted from the initial rapid flux. These flow barriers obstruct the expected flow towards the downstream and cease the flow for some time until the pressure builds up steadily up to a certain level as it can break the flow barrier by re-arranging the pore structure. The existence of beddings further delays the demolishment of this barrier and beddings perpendicular to the flow direction creates a greater permeability reduction in the aquifer than parallel beddings, because the influence of these low permeable zones on permeability depends on the extent to which they limit the flowability across the rock mass. The experimental results reveal that the brine permeability in sandstone can be reduced significantly up to 7 and 40 times respectively for parallel and perpendicular beddings in this study compared to no-bedding types.

Effect of clay minerals on the effective pressure law in clay-rich sandstones

The relative sensitivity of permeability to pore fluid pressure Pp and confining pressure Pc of rocks can be expressed as the effective pressure coefficient nk. To investigate the effect of clay minerals on nk, permeability data were measured in clay-rich sandstones under conditions of lowering Pp at different constant-confining pressures, and the effective pressure coefficients were subsequently calculated using a method based on our observation that the effective pressure law was linear. The values of nk in I/S-sandstones and chlorite-sandstone agreed with the clay shell model, while the values of nk in kaolinite-sandstones were consistent with the clay particle model. However, the nk values of our kaolinite-sandstones were less than 1.0, which is different from previous observations showing nk greater than 1.0. The nk value of our I/S-sandstone saturated with formation water was greater than 1.0, consistent with the previous results, but the nk value of our I/S-sandstone saturated with dried nitrogen was lower than 1.0. Thus, the coefficient nk was affected by the type of clay mineral, the preparation of the samples, and the type of the pore fluid.

Determination of effective grain geometry for electrical modeling of sedimentary rocks

Accurate electrical modeling of sedimentary rocks is crucial for the interpretation of electrical survey data and must take into account the geometric information of the rock grains. We have developed an incremental model for the multiphase electrical conductivity of sedimentary rocks on the theoretical basis of differential effective medium models. The developed incremental model was first validated on clean and clay-rich sandstones and then applied to simulate the conductivity of five digitally created rock samples with different spectra of grain aspect ratios. The conductivity is calculated as a function of water conductivity at full water saturation, and an effective grain aspect ratio for a two-phase medium is determined to reproduce the electrical behavior of the digital rocks. It was found that the effective grain aspect ratio can either be determined analytically or estimated from the aspect ratio spectrum. A link between the obtained effective grain aspect ratio and the cementation coefficient widely used in petrophysics has been established. The cementation coefficient increases with the decrease of the grain aspect ratio (increasing the [Formula: see text]-axis depolarization factor) and grain conductivity and the increase of sample porosity. It showed that the cementation coefficient can be interpreted as a measure of the connectivity of pores as well as the conductivity of the grains. Our results gave us new insights into the physical meanings of the grain aspect ratio and cementation coefficient, which could be used to facilitate the electrical modeling or interpretation of reservoir rocks.

Regional variability of carbon dioxide storage potential of the Queenston Formation in New York

Initial assessments of the potential for geologic carbon sequestration rely on existing subsurface data, most of it collected for oil and gas exploration. We document the challenges of assessing the [Formula: see text] storage potential based on archived data, for the case of the Upper Ordovician Queenston Formation in New York. In central New York, the entirely subsurface Queenston Formation consists primarily of sandstones. In contrast, in western New York where the Queenston Formation crops out, it is composed of shale, siltstone, and sandstone. A foremost interpretation challenge is to obtain porosity data from the borehole logs. Intercomparisons of various measures of porosity and of the availability of those data led to the decision to use neutron porosity data for the hematite- and clay-rich sandstone. To map porosity regionally, a second challenge is to establish the physical correlation between four regionally extensive stacked petrophysical zones in central New York, each recording base-level fall trends in fluvial sandstones, and four petrophysical zones in western New York, each characterized by base-level rise deposits of marginal marine and shallow marine deposits. Two alternative correlations can be justified with differing implications for pore volumes in a transition region. This analysis estimates that the Queenston Formation of central New York can sequester up to approximately [Formula: see text] metric tons of [Formula: see text] at depths greater than 3000 ft (914 m) in sandstones with porosity exceeding 10%. The Queenston Formation is not suitable for [Formula: see text] storage in western New York.

Wettability Alteration in a Tight Oil Reservoir

In fractured reservoirs, the efficiency of water flood is governed by spontaneous imbibition of water into oil-containing matrix blocks. When the matrix is oil-wet or mixed-wet, little oil can be recovered by imbibition. The objective of this work is to identify chemicals that can be added to the injection water that can induce imbibition into an originally mixed-wet, tight, fractured sandstone reservoir. Several surfactants were evaluated for their aqueous stability at the reservoir temperature and salinity. Contact angles were measured on a clay-rich sandstone. Spontaneous imbibition tests were conducted on the reservoir rocks. It is shown that the use of dilute (0.1 wt %) anionic surfactant solution with a large number of ethoxy groups can alter the wettability from oil-wet toward more water-wet conditions on the mineral plates. Incremental oil recovery as high as 68% original oil in place is obtained through spontaneous imbibition experiments performed on tight (∼10 μD) oil-wet/mixed-wet sandstone res...

Pressure solution – The importance of the electrochemical surface potentials

“Pressure solution”, frequently found in clay-rich sandstone, is characterized by enhanced quartz dissolution at inter-grain contacts. The origin of pressure solution and many other related dissolution processes remains elusive. Using an Electrochemical Surface Forces Apparatus we visualized and measured the dissolution of silica glass surfaces close to an electrode surface. The dissolution rates correlate quantitatively with the electrode potential via the Butler–Volmer equation for corrosion. Our experimental results demonstrate that at low temperature, apparent pressure solution and many other mineral dissolution phenomena can be driven by electrochemical processes rather than a pressure-driven process. This finding highlights the role of electrochemical surface potentials in dissolution phenomena at dissimilar material interfaces, and provides new perspectives on pressure solution in particular and a new theoretical basis for predictive control of dissolution phenomena in general.

The effective pressure law for permeability of clay-rich sandstones

To study the relative sensitivity of permeability to pore pressure Pp and confining pressure Pc for clay-rich rocks, permeability measurements were performed on samples of four clay-rich sandstones. A new method (hereafter denoted the “slide method”) was developed and used for analyzing the permeability data obtained. The effective pressure coefficients for permeability nk were calculated. The values of nk were found to be greater than 1.0 and insensitive to changes in pressure. These results confirmed observations previously made on clay-rich rocks. Also, the coefficients nk obtained had different characteristics for different samples because of differences in the types of clay they contained. The effective pressure law (σeff=Pc−nkPp) determined using the slide method gave better results about k(σeff) than classic Terzaghi’s law (σeff=Pc−Pp).

Pressure effects on the joint elastic-electrical properties of reservoir sandstones

We conducted a laboratory study of the joint elastic-electrical properties of sixty-three brine-saturated sandstone samples to assess the likely impact of differential pressure (confining minus pore fluid pressures) in the range 8–60 MPa on the joint interpretation of marine seismic and controlled-source electromagnetic survey data. The samples showed a wide range of petrophysical properties representative of most sandstone reservoirs. We found that a regression equation comprising both a constant and an exponential part gave a good fit to the pressure dependence of all five measured geophysical parameters (ultrasonic P- and S-wave velocity, attenuation and electrical resistivity). Electrical resistivity was more pressure-sensitive in clay-rich sandstones with higher concentrations of low aspect ratio pores and micropores than in clean sandstones. Attenuation was more pressure-sensitive in clean sandstones with large open pores (macropores) than in clay-rich sandstones. Pore shape did not show any influence on the pressure sensitivity of elastic velocity. As differential pressure increases, the effect of the low aspect ratio pores and micropores on electrical resistivity becomes stronger than the effect of the macropores on attenuation. Further analysis of correlations among the five parameters as a function of pressure revealed potentially diagnostic relationships for geopressure prediction in reservoir sandstones.

Joint elastic-electrical properties of reservoir sandstones and their relationships with petrophysical parameters

We measured in the laboratory ultrasonic compressional and shear-wave velocity and attenuation (0.7–1.0 MHz) and low-frequency (2 Hz) electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity, permeability and clay content on the joint elastic-electrical properties of reservoir sandstones. P- and S-wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi-logarithmic scale for both clean and clay-rich sandstones; P- and S-wave attenuations showed a bell-shaped correlation (partial for S-waves) with apparent electrical formation factor. The joint elastic-electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents. The laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co-located seismic and controlled source electromagnetic surveys.

New findings on the tectono-metamorphic history of the western Rhenish Massif (Germany) by K–Ar dating of metasedimentary illite

Metapelites, clay-rich sandstones and volcanics from Cambrian, Ordovician and Lower Devonian strata of the western Rhenish Massif underwent a complex regional Variscan tectono-thermal evolution, as shown by mineralogical and K–Ar isotopic analyses of the illite to mica components from three NW-SE transects. The metamorphic degree extended from an anchimetamorphic to an epimetamorphic intensity during two major episodes of illite crystallization at 328 ± 6 and 282 ± 12 Ma. A further late orogenic or post-orogenic extensional activity could also be detected, but not precisely, around 270 Ma, probably recorded by the precipitation of illite in new or reactivated extensional faults with upward moving heat flows. Three studied cross-sections through the massif display a distribution of the illite crystallinity index that scatters within the diagenesis-to-anchizone range with increased crystallization grades along the thrust zones. The determined K–Ar ages probably frame the Variscan convergence in the western Rhenish Massif, which allows calculation of a “shortening velocity” of 140 km in 40 Ma, or of about 0.35 cm/a, that can be suggested on the basis of a total shortening of about 50% for the entire massif.