Electrical Conductivity Of Rocks Research Articles

Influences of Geological and Petrophysical Attributes on Electrical Resistivity–Based Reserve Evaluation: Enhancing Carbonate Reservoir Classification, Permian-Triassic Reservoirs of Southern Iran

Summary The strong heterogeneity of carbonate reservoirs makes it challenging to assess the spatial distribution of fluid behavior, geological attributes, petrophysical properties, and estimate oil reserves. The diversity in facies and diagenetic processes contributes significantly to the heterogeneity in these reservoirs. The ability to accurately characterize and manage hydrocarbon reservoirs heterogeneity hinges on a comprehensive understanding of geological and petrophysical attributes such as water saturation, porosity, permeability, and electrical conductivity. In the quest for optimizing hydrocarbon reservoir identification and management, understanding the intricate relationship between the geological, petrophysical characteristics, and electrical conductivity of reservoir rocks is paramount. Electrical conductivity serves as an indicator of the structural attributes of pore networks, reflecting sedimentary and diagenetic influences on reservoir quality. Despite the extensive use of electrical resistivity in hydrocarbon reservoir assessments, accurate interpretation of its variations remains a significant challenge, which is due to complex geological factors. Thus, acquiring a thorough insight into the impact of geological and petrophysical attributes on electrical resistivity is crucial for a dependable assessment of hydrocarbon reservoirs. In this article, we explore the complex interplay between various geological and petrophysical factors and their impact on the electrical conductivity of rocks, which serves as a vital parameter in the assessment of hydrocarbon reservoirs. The focus is on how sediment texture, pore types, depositional environments, diagenetic processes, and characteristics of pore throat radius influence the electrical properties of rocks. The objective of this comprehensive approach is to decipher the geological and petrophysical modifications using electrical data, thereby improving the analysis of electrical discrepancies. Moreover, the accuracy of categorizing rocks based on their electrical characteristics is evaluated to effectively manage reservoir heterogeneity. Different data were gathered from an exploratory well situated in the western Persian Gulf. This data set comprised evaluations of 1,370 thin sections, 1,110 porosity and permeability data, 32 scanning electron microscopy (SEM) analyses, wireline logs, 29 mercury injection capillary pressure (MICP) data, 58 formation resistivity factors (FRFs), 20 formation resistivity indexes (FRIs), and 157 Dean-Stark measurements. Rocks were classified into different groups with similar electrical behavior utilizing the electrical quality index (EQI) approach. The efficiency of this method in managing reservoir heterogeneity, as one of the techniques for determining electrical rock type, was assessed, particularly by comparing the precision of predicted water saturation with Dean-Stark saturation data. The findings of this study demonstrated that diagenetic processes, especially dolomitization and dissolution, have the most significant impact on variations in the electrical conductivity of rocks. These processes govern pore size, distribution, pore types, and the radii of pore throats. According to the findings of this research, categorizing rocks based on their electrical characteristics improves the precision of water saturation predictions. In additionally, the use of constant Archie coefficients in water saturation calculations results in an overestimation of water saturation, leading to an underestimation of hydrocarbon reserves in the reservoir. The results of this research allow for more knowledgeable decision-making concerning reservoir efficiency, hydrocarbon reserves, production techniques, and increased oil recovery.

Comparative study and discussion of diagenetic facies and conductivity characteristics based on experiments.

The study of conduction mechanisms is the key to establishing physical derivations, resistivity simulations and saturation models. The purpose of this research is to clarify conduction mechanisms under different diagenetic facies and build suitable saturation evaluation models. Experimental data of tight gas sandstone from the Ordos Basin were analysed, including data from scanning electron microscopy, conventional core physical property analysis, core casting thin-section analysis, core mercury intrusion experimentation and rock electrical conductivity experimentation. Accordingly, the diagenetic minerals of the study block were examined, and the diagenetic facies were classified by the differences in the diagenetic properties across the study area. The reservoir was divided into three types of diagenetic facies: construction facies, cementation facies and destruction facies. On this basis, the conductivity characteristics and saturation models of different diagenetic facies within the study area were systematically discussed for the first time. A number of experiments showed that according to the type of diagenesis, the structure of the pores and the influence of the reservoir, a classification scheme for diagenetic facies (consisting of construction, cementation and destruction facies) can be established. According to the influence of the diagenesis of various diagenetic facies, theoretical pore structure models of the three diagenetic facies were established, in which the construction facies includes mainly dissolved feldspar pores and intergranular pores, the destruction facies includes clay residual intergranular pores and intergranular pores, and the cementation facies includes primarily residual intergranular pores. Based on these theoretical pore structure models, the construction facies was evaluated with a pore-connected vuggy conductivity model, the destruction facies was evaluated with a non-connected matrix pore conductivity model, and the cementation facies was evaluated with a residual intergranular pore conductivity model. Then, the rationality of each model and the effects of the parameters in each model on the final cementation exponent were analysed by simulation. The predicted cementation exponents of the diagenetic facies match the measured cementation exponents well and can guide the qualitative description of these characteristics in such reservoirs.

Geoelectric heterogeneities of the Kerch iron ore basin

The three-dimensional geoelectric model of the Earth’s crust and upper mantle of the Kerch Peninsula has been built for the first time based on the results of experimental observations of the Earth’s low-frequency electromagnetic field, carried out in 2007—2013 by the Institutes of the National Academy of Sciences of Ukraine. Its physical and geological interpretation and detailing of the near-surface part were carried out according to the data of the audiomagnetotelluric sounding method to study the deep structure of the Kerch iron ore basin. To the east of the Korsak-Feodosiya fault along the southern part of the Indolo-Kuban trough (in the north of the South Kerch and almost under the entire North Kerch zones), a low-resistance anomaly (ρ=1 Ohm∙m) was found at depths from 2.5 km to 12 km about 20 km wide. Its eastern part is located in the consolidated Earth’s crust and is galvanically connected with surface sedimentary strata, while the western part is completely in sedimentary deposits. The anomaly covers the territory of the Kerch iron ore basin and occurrences of mud volcanism. The characteristics of the upper part of the layered section of the Kerch Peninsula in the interval of the first hundreds of meters were obtained from the results of one-dimensional inversion of the audiomagnetotelluric sounding data (frequency range 8—4000 Hz). It is shown that the first 15 m of the section, corresponding to Quaternary deposits, have resistivity values up to 1 Ohm∙m. Below, in the Neogene sediments, the electrical resistance increases to values of 5 Ohm∙m and more. Both horizontally and vertically, the distribution of resistivity values has a variable character, manifesting as a thin-layered structure with low resistivity values. Possibly, such areas have a direct connection with the channel for transporting hummock material and gases. A connection is assumed between the low-resistivity thin-layered near-surface areas, a deep anomaly of electrical conductivity in the upper part of the Earth’s crust, and the likely high electrical conductivity of rocks at the depths of the upper mantle with iron ore deposits, as well as the manifestation of mud volcanism. The heterogeneity of the crustal and mantle highly conductive layers may indicate a high permeability of the contact zones for deep fluids.

Application of Data-Driven Discovery Machine Learning Algorithms in Predicting Geothermal Reservoir Temperature from Geophysical Resistivity Method

Geophysical methods including seismology, resistivity, gravity, magnetic and electromagnetic have been put in use for geothermal resource mapping at the Great Olkaria Geothermal field for decades. Reservoir temperature distribution and the electrical conductivity of rocks mainly depend on the same parameters such permeability, porosity, fluid salinity and temperature. This research focused on the integration of Olkaria Domes geothermal well testing temperature and geophysical Electromagnetic resistivity data with the aim of establishing an alternative estimation method for temperature of the reservoir through machine learning Analytics. To achieve this, Data-Driven Discovery Predictive Model Algorithm was built using Python programming language on Anaconda framework. The open-source web based application Jupyter Notebook for coding and visualization was used. Decision Tree Regression, Adaptive Booster Regression, Support Vector Regression and Random Forest Regression were used. The model performance was evaluated using R-Score and Mean Absolute Error metrics. Based on these performance score, the best performing model was suggested to predict subsurface temperature from resistivity. Training the model using the DTR algorithm approach provides superior outputs with R2 of 0.81 and lowest MAE of 29.8. The DTR algorithm could be implemented in determination of subsurface Temperature from resistivity in high temperature hydrothermal fields.

A novel saturation calculation model of fractured-vuggy carbonate reservoir via multiscale pore networks: a case study from Sichuan Basin, China

Abstract Existing saturation models cannot effectively describe the specific fractured-vuggy carbonate reservoir in area A of the Sichuan Basin, southwestern China. This reservoir has got a wide pore size distribution, strong heterogeneity, high gas saturation and complex electrical conductivity mechanism. Hence, the present study attempted to establish a new saturation calculation equation for this carbonate reservoir based on the microscopic conductivity mechanism of the rock. Here, we first used the multiscale computed tomography (CT) scanning method to build multiscale digital rocks. Subsequently, we applied the maximum sphere algorithm to extract the pore space structure and constructed the multiscale pore network models. By using the cross-scale fusion method, four different pore configurations were determined. Then, the percolation theory was implemented to simulate the conductivity mechanism of the constructed pore network models. As a result, the fluid distribution characteristics and the resistivity variation trends of the different pore structures were obtained. The simulation results showed that the fracture system of the studied reservoir had a much greater effect than the vug system on the carbonate rock's electrical conductivity, and the conductivity was closely related to the fluid distribution. In addition, based on the simulation results, a new conductivity model was proposed that incorporates the coupling phenomenon of pores, vugs and fractures; and also a new saturation calculation equation for triple-porosity media was established. The observations indicated that the field application of the proposed equation had an acceptable performance with an error value of less than 2.56%. The results from the present study provide new insights into the evolution of electrical properties in triple-porosity carbonate systems.

Simulation Monitoring for Rainfall Infiltration in Soil Based on High Density Electrical Method

Rainfall infiltration is a porous medium flow problem with variable saturation. Based on the theoretical analysis of the flow field, electrical conductivity of rocks, the electrical field, the paper simulates the coupling relationship between the water saturation in soil and the apparent resistivity distribution. It combines the Richards equation, the Archie formula and the Laplace equation. The experiment simulates the potential field data by the Wenner setting in electrical exploration on a two-layer geologic model with continuous rainfall during 5 days, which shows that the effective saturation in soil is increasing with the rainfall time, while the apparent resistivity is decreasing. This can provide a theoretical basis for the analyzing the rainfall infiltration and porosity of the soil by using high-density electrical method in the future.

Smectite quantification in hydrothermally altered volcanic rocks

In volcanic environments, the presence of smectite may indicate recent hydrothermal circulations. Smectite is also responsible for enhanced rock electrical conductivity, as well as mechanical weakening. Therefore, quantifying smectite is important in geothermal exploration. Smectite identification requires X-ray diffraction (XRD) but quantification based on XRD is time-consuming and not always accurate. In the present study, we investigate the use of an optimized unbuffered Cation Exchange Capacity (CEC) determination, by back-titration of the Copper-triethylenetetramine(II) “Cu-trien” molecule, to quantify the smectite content of altered volcanic rock samples. We establish that a satisfying trade-off between the instrument uncertainty and an independant systematic error is theoretically reached for a fraction of reactants consumed of about 30% at the end of the exchange reaction. We suggest a modification to classical protocols to fall in that range. Finally, we show that optimized CEC determination by Cu-trien are a direct measure of the smectite weight fraction in altered volcanic samples, with an average CEC of pure smectite of 90 ± 5 meq/100 g.

Electrical conductivity of Icelandic deep geothermal reservoirs up to supercritical conditions: Insight from laboratory experiments

Electrical resistivity is extensively used in geothermal systems to accurately determine the existing conditions of the reservoirs at depth. Up to this point, technical challenges related to pore fluid confinement made difficult to measure electrical conductivity at temperatures and pressures representative of very deep geothermal reservoirs. In this study, we are overcoming these limitations thanks to a new electrical resistivity cell designed to fit into a high temperature gas medium apparatus. This allows us to perform resistivity measurement at temperatures up to 700 °C and at effective pressures up to 100 MPa (i.e. a confining pressure of 130 MPa and an equilibrium pore pressure of 30 MPa) using cm-scale plugs. Rock samples originate from five boreholes located in the Icelandic geothermal fields of Reykjanes (RN-17B/Hyaloclastite, RN-19/RN-30/dolerites) and Hengill (NJ-17/basalt and NJ-17B/Hyaloclastite). These samples were selected for their high degree of hydrothermal alteration in the epidote and amphibole facies (i.e. temperature of 250 °C and 400 °C respectively), and their wide range of porosities (from 3% to 20%). To determine the effects of surface, mineral and electrolytic conductions on bulk electrical conduction, experiments were performed under dry and saturated conditions using three different fluid salinities.At temperatures ranging from 25 to ~350 °C, electrical conductivity in all our experiments increases as a result of both increasing surface and electrolytic conduction. Then, under supercritical conditions, i.e. temperature from 374 °C to 600 °C, electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction. At higher temperatures (500 °C–700 °C), the rock conductivities lie within the range of dry rock electrical conductivity values, suggesting that mineral conduction controls the bulk conductivity with ferromagnesian minerals acting as principal contributors of mineral conduction. Amphibole-rich samples show an irreversible increase in conductivity at temperature above 500 °C–600 °C, which can be attributed to amphibole dehydration. Comparison of these laboratory data to magnetotelluric soundings and downhole temperatures obtained beneath several geothermal areas indicate a good agreement between laboratory and large-scale surveys. Our results provide a general trend that helps interpreting electrical conductivity surveys in the Icelandic crust.

Modelling the low-frequency electrical properties of pyrite-bearing reservoir sandstones

Marine controlled source electromagnetic sounding has developed rapidly in recent years to complement conventional seismic method for better discrimination of pore fluid. To obtain quantitative and reliable interpretation of the CSEM data, a robust rock physics model is vital to link the bulk rock electrical conductivity to the electrical properties and microstructure of the rock constituents, especially when some of the constituents exhibit extreme electrical behaviours (e.g., pyrite, a common mineral associated with reservoir rocks). Based on the multi-phase incremental model validated on published experimentally measured electrical conductivity of pyrite-bearing sandstones, the effects of key parameters, i.e., pyrite content, porosity, pyrite conductivity, grain aspect ratio and water saturation on the electrical conductivity of pyrite-bearing sandstones and their corresponding CSEM responses were comprehensively studied. The results are expected to assist in the CSEM data interpretation when a pyrite-bearing sandstone reservoir is encountered in the future.

Geophysical assessment of migration and storage conditions of fluids in subduction zones

By enhancing mass transfer and energy release, the cycle of volatiles and melt is a major component of subduction. Investigating this fluid cycle is therefore critical to understand the past and current activity of subduction zones. Fluids can significantly affect rock electrical conductivity and elastic parameters that are measured using electromagnetic and seismic methods, respectively. This letter emphasizes how these geophysical methods complement each other to provide information about the storage of fluids in subduction systems. By compiling electromagnetic and seismic results from various subduction zones, a possible correlation between electrical conductivity and seismic wave attenuation anomalies in the mantle wedge is observed, consistent with fluid accumulation. A possible relationship between geophysical properties and the slab age is also suggested, whereas no significant trend is observed between electrical conductivity or seismic wave attenuation and estimates of water flux in the mantle wedge. These field-based relationships require further constrains, emphasizing the need for new measurements in the laboratory.

Graphite and quartz petrofabrics: Examples from the Ediacaran black quartzites of the Ossa-Morena Zone (SW Iberia)

Abstract We study with the Electron Back-Scattered Diffraction (EBSD) technique the fabric of metamorphosed and ductilely deformed phtanites or graphitic cherts, a common lithotype in Ediacaran supracrustals of the west European Cadomian orogen. Currently they are black quartzites with a planolinear tectonite structure. Microscopically they present a sub-mm-scale alternation of coarse- and fine-grained, dynamically recrystallized quartz bands. We attribute intracrystalline plasticity partitioning to variations in graphite inclusion concentration constraining quartz grain boundary mobility during dynamic recrystallization under non-coaxial strain regimes and moderate to elevated temperatures (400–650 °C). Lattice-preferred orientations of quartz [c] and axes are geometrically related to the external reference frame provided by foliations and lineations. We also identify the involvement of (0001) , {r} , {m} , and {m}[c] quartz intracrystalline slip systems. Deformation modes operated simultaneously and under identical temperatures in interleaved parallel domains mm- to cm-thick in adjacent coarse- and fine-grained bands. Medium- to high-T plasticity is congruent with the syntectonic temperature gradients associated with the amphibolite-facies metamorphism of the country rocks. We also present the first specific study published so far on natural deformation graphite lattice-preferred orientation. Graphite inclusions (as well as those of mica) exhibit mineral shape fabrics that suggest operation of (0001) slip. However, EBSD measurements also record fabrics suggestive of {m} slip. In spite of a rather small volumetric proportion, graphite spatial organization at increased shear strains facilitated ductile deformation. If a graphite network is established in the rock, it can potentially increase rock electrical conductivity, thus accounting for mid and lower crust anomalous electric conductivity.

Electrical conductivity of amphibole-bearing rocks: influence of dehydration

We investigated the electrical conductivity of amphibole-bearing rocks under the conditions of the middle to lower crust. Alternating current measurements were performed in the frequency range of 10–106 Hz in a cubic-anvil high-pressure apparatus at 0.5–1.0 GPa and 373–873 K. The electrical conductivity of these rocks is weakly temperature dependent below ~800 K with modest anisotropy and relatively low conductivity (~5 × 10−3 S/m at ~750 K with the activation enthalpy of 64–67 kJ/mol). However, the electrical conductivity starts to increase with temperature more rapidly above ~800 K (activation enthalpy of 320–380 kJ/mol). The infrared spectroscopy observations indicate that dehydration occurs in this high temperature regime. The observed high activation enthalpy and the reproducibility suggest that the enhanced conductivity is not due to the direct effect caused by the generation of conductive fluids. Dehydration of amphibole is associated with the oxidation of iron (from ferrous to ferric), and we suggest that the increased conductivity associated with dehydration is caused by oxidation. This effect may explain high electrical conductivity observed in some regions of the continental crust.

Electrical Conductivity of Rocks and Dominant Charge Carriers: The Paradox of Thermally Activated Positive Holes

In this paper we have focused on fundamental processes that are important for understanding the electrical properties of materials, both single crystal minerals and igneous rocks, both laboratory-grown and from natural environments. The prevailing view in the geophysics community is that the electrical conductivity structure of the Earth’s continental crust over the 5-35 km depth range can best be understood by assuming the presence of intergranular fluids and/or intergranular carbon films. Based on studies of melt-grown MgO, magma-derived sanidine and anorthosite feldspar and upper mantle olivine single crystal we present evidence for the presence of electronic charge carriers, the importance of which has been largely ignored. These charge carriers derive from peroxy defects, which are introduced during cooling, under non-equilibrium conditions, through a redox conversion of pairs of solute OH- arising from the solid state dissolution of H2O. It can be shown that, during reheating, the peroxy defects become thermally activated in a 2-step process. Step 2 leads to the release of defect electrons in the oxygen anion sub lattice. Known as positive holes and symbolized by h•, these electronic charge carriers are associated with energy states at the upper edge of the valence band. They are highly mobile. Chemically equivalent to O– in a matrix of O2– they are highly oxidizing. However, though metastable, the h• can exist in minerals, which crystallized in highly reduced environments. The h• appear to control the electrical conductivity of crustal rocks over much of the 5-35 km depth range. We make the extraordinary and seemingly paradoxial claim that MgO crystals, grown from the melt under the viciously reducing conditions of a carbon arc fusion furnace, contain peroxy defects in their crystal structure, hence oxygen in the valence state 1–. When the peroxy defects break up, they release positive hole charge carriers, formally defect electron in the oxygen anion sublattice, equivalent to O– in a matrix of O2–.These positive holes have two outstanding properties: they are highly mobile and highly oxidizing.

Variations in electrical conductivity of rocks above metamorphic conditions

This study of the electrical conductivity of metamorphic rocks as a function of temperature highlights the importance of conductivity variations associated with dehydration. We observed conductivity changes for anhydrous rocks and hydrous amphibolites up to 1100 K at 0.5 GPa. Our experimental results revealed that dehydration takes place and that there is a non-linear variation above metamorphic temperatures. To evaluate the observed changes in conductivity after dehydration, numerical calculations were conducted. The calculation results showed that the bulk conductivity of rock plus pure water cannot account for the conductivity values of reacted rocks if the crust dehydrates. However, the data for 0.1 m NaCl solutions are consistent with those for amphibolites between 530 and 1100 K. In this temperature range, the estimated volume fractions of dewatered saline solutions vary between 0.04 and 0.36 vol.%. These results show that although dehydration is insignificant, and only small amounts of high-salinity fluids are formed in the crust, the total conductivity of amphibolites is high after dehydration.

Methodology of the indirect temperature estimation basing on magnetotelluric data: Northern Tien Shan case study

Abstract Electrical conductivity of rocks being closely related to their temperature could serve as a proxy parameter to be used for indirect temperature estimation from the surface electromagnetic (EM) data. Studies are carried out aimed at estimating the feasibility of indirect temperature estimation in the geologically complicated areas of the Earth crust from the electromagnetic data collected at the surface. Basing on the neuronet analysis of magnetotelluric (MT) and temperature data measured at the Bishkek geodynamical testing ground in the northern Tien Shan, optimal methodologies for calibration and application of indirect electromagnetic geothermometer are developed. It is shown that the temperature estimation my means of the EM geothermometer calibrated by 6–8 temperature logs results in 12% average relative error (instead of 30% achieved using only temperature logs). The availability of prior geological information about the region under study and preliminary analysis of the local heterogeneities' indicators determined from the available MT data make it possible rejection of inappropriate site locations that may, in turn, decrease average error to only 11%. The results of the electromagnetic temperature extrapolation in depth indicate that the extrapolation accuracy essentially depends on the ratio between the well length and the extrapolation depth. In particular, in extrapolation to a depth twice as large as the well depth the relative error is 5–6%, and in case of its threefold excess the error is around 20%. This result makes it possible to increase significantly the deepness of indirect temperature estimation in the Earth interior (in particular, for geothermal exploration) based on the available temperature logs.

Simulation of triaxial induction measurements in dipping, invaded, and anisotropic formations using a Fourier series expansion in a nonorthogonal system of coordinates and a self-adaptive hp finite-element method

Borehole triaxial induction instruments were designed to diagnose and measure rock electrical conductivity parallel and perpendicular to the bedding plane. Experience has shown that the interpretation of triaxial induction measurements often requires numerical modeling for a proper diagnosis of rock electrical conductivity anisotropy in the presence of geometric effects such as dipping wells, layer boundaries, and invasion. We introduce a new algorithm to simulate triaxial induction measurements that combines a Fourier series expansion in a nonorthogonal system of coordinates with a 2D goal-oriented, self-adaptive, high-order [Formula: see text] finite-element method. This procedure enables the accurate and reliable simulation of triaxial induction measurements across reservoir rock formations with extreme contrasts of electrical conductivity while reducing the 3D computational complexity associated with deviated wells. Numerical results indicate that borehole dip effects on triaxial induction measurements are larger than on standard coaxial induction measurements. The sensitivity of triaxial induction measurements to transversely isotropic rock formations decreases with increasing dip angle.

Using helicopter electromagnetic data to predict groundwater quality in fractured crystalline bedrock in a semi-arid region, Northeast Brazil

Geostatistical modeling, using airborne and borehole electromagnetic data, was used to estimate electrical conductivity in groundwater within fractured paragneisses and migmatites in a semi-arid climate in northeastern Brazil. Despite the geologic heterogeneity of crystalline aquifers, the use of high resolution helicopter electromagnetic (HEM) data enabled the characterization of groundwater electrical conductivity where data from drilled wells were insufficient. The tacit assumption is that HEM measurements can be used to relate rock electrical conductivity to groundwater electrical conductivity. In this study, the HEM data were used as an external drift variable in non-stationary estimation and stochastic simulation to identify the variability of groundwater electrical conductivity. Validation tests, comparing predicted values for groundwater conductivity with measurements in new wells, confirmed the success of these models in locating fresh groundwater sources in crystalline bedrock.

ESTIMATION OF AN EQUIVALENT SHALINESS PARAMETER USING RESISTIVITY AND SPECTROSCOPY LOGS: A TOTAL EXPANSIBLE CLAY APPROACH

We develop a method for estimating the degree of saturation in shaly sand formations based on an equivalent shaliness parameter. In shaly sands, the measured electrical conductivity is influenced by the clay content because of the charge deficit associated with the clay particles. This deficit is offset by counter‐ions in a double layer whose contribution to total charge transfer is dominated by the total expansible clays. We derive an approximation for clay content and cation exchange capacity (CEC) from a linear relationship between the CEC and the total expansible clay (TEC) content of a reservoir rock. By combining mineralogy from quantitative X‐ray diffraction (XRD) with wet chemistry CEC data, an expression for the counter‐ions in the double layer as a function of the total expansible clays is derived. The analytical expression obtained is coupled with well‐log data and compared to existing models. For determining surface conductivity, the new methodology eliminates the need for performing core measurements, subjective selection of CEC values from existing data, or the use of log‐based volume‐porosity translations. We show that existing popular industry models, and effective medium and electrochemical models of rock electrical conductivity can be adapted to use the new methodology. The results demonstrate a strong correlation between the estimated and measured bulk conductivity, and also resolve the scatter in estimated bulk conductivity often observed under low pore‐water salinity conditions, where the ratio of surface to electrolyte conductivity approaches unity. The methodology has been applied to water saturation estimation in two Australian petroleum wells, Yolla‐4 and Cliff Head‐4. The results demonstrate that this technique provides better estimates of water saturation in both low conductivity contrast pays and tight pays compared to existing approaches.

地殻・マントル物質の地震波速度と電気伝導度：沈み込み帯の水を探る

地殻・マントル物質の地震波速度と電気伝導度：沈み込み帯の水を探る

Carbon deposition during brittle rock deformation: Changes in electrical properties of fault zones and potential geoelectric phenomena during earthquakes

To investigate potential mechanisms for geoelectric phenomena accompanying earthquakes, we have deformed hollow cylinders of Sioux quartzite to failure in the presence of carbonaceous pore fluids and investigated the resulting changes in electrical conductivity and carbon distribution. Samples were loaded at room temperature or 400°C by a hydrostatic pressure at their outer diameter, increasing pressure at a constant rate to ∼290 MPa. Pore fluids consisted of pure CO, CO2, and CH4 and a 1:1 mixture of CO2 and CH4, each with pore pressures of 2.0 to 4.1 MPa. Failure occurred by the formation of Mode II shear fractures transecting the hollow cylinder walls. Radial resistivities of the cylinders fell to 2.9 to 3.1 MΩ‐m for CO tests and 15.2 to 16.5 MΩ‐m for CO2:CH4 tests, compared with >23 MΩ‐m for dry undeformed cylinders. Carbonaceous fluids had no discernable influence on rock strength. On the basis of mapping using electron microprobe techniques, carbon occurs preferentially as quasi‐continuous films on newly formed fracture surfaces, but these films are absent from preexisting surfaces in those same experiments. The observations support the hypothesis that electrical conductivity of rocks is enhanced by the deposition of carbon on fracture surfaces and imply that electrical properties may change in direct response to brittle deformation. They also suggest that the carbon films formed nearly instantaneously as the cracks formed. Carbon film deposition may accompany the development of microfracture arrays prior to and during fault rupture and thus may be capable of explaining precursory and coseismic geoelectric phenomena.