Chalk Samples Research Articles

Prędkość ultradźwiękowa analizy wpływu zawartości wody na kredę fosfatyczną

Water content may have a crucial effect on the petrophysical and mechanical properties of high porosity rocks such as chalk. Consequently, the water fluctuations may significantly influence the behaviour of engineering structures that are constructed in chalks. It is the aim of this study to illustrate how the elastic and strength properties of chalk can evolve considering water content variation using P-wave velocity and analytical analyses. To study the influence of water on the ultrasonic compressional wave velocity (VP), measurements of travel time of three samples subjected to different water content were performed. The Uniaxial Compressive Strength (UCS) in dry and saturated chalk specimens was also estimated. To clarify the possible influence of lithology on the physicomechanical properties of rocks, three samples of phosphatic chalk were studied in thin sections. Automated mineral analysis and textural imaging of the samples were performed using an FEI QEMSCAN®. The analyses of the Ultrasonic tests reveal that changes in the water content are associated with variations in the P-wave velocity. Based on P-wave velocity changes as a function of water content, analytical models have been used to predict the elastic and strength properties. The data indicate that the presence of water significantly reduced the elastic and strength parameters of the chalk, a result that is in agreement with the UCS laboratory tests. The derived equations can be used for predicting the elastic and strength properties of high-porous chalk from the P-wave velocity as a function of water content. This approach may avoid the necessity for time-consuming laboratory testing.

Diffusion in Porous Rock Is Anomalous.

Molecular diffusion of chemical species in subsurface environments─rock formations, soil sediments, marine, river, and lake sediments─plays a critical role in a variety of dynamic processes, many of which affect water chemistry. We investigate and demonstrate the occurrence of anomalous (non-Fickian) diffusion behavior, distinct from classically assumed Fickian diffusion. We measured molecular diffusion through a series of five chalk and dolomite rock samples over a period of about two months. We demonstrate that in all cases, diffusion behavior is significantly different than Fickian. We then analyze the results using a continuous time random walk framework that can describe anomalous diffusion in heterogeneous porous materials such as rock. This methodology shows extreme long-time tailing of tracer advance as compared to conventional Fickian diffusion processes. The finding that distinct anomalous diffusion occurs ubiquitously implies that diffusion-driven processes in subsurface zones should be analyzed using tools that account for non-Fickian diffusion.

Influence of soil salinity on the bearing capacity of the frozen wall

The article describes the results of laboratory studies on the unfrozen water content and ultimate long-term strength of frozen clay and chalk samples in the temperature range from −10 to −25 °C. The soil samples contained dissolved salt in the pore space, with three types of salts (NaCl, KCl, and CaCl2) being considered. The findings indicate that the influence of the content and type of dissolved salt on the ultimate long-term strength of soils is realized indirectly through the unfrozen water content. In this case, the soil freezing characteristic curve in the region of negative temperatures significantly depends on both the type of dissolved salt and its quantity. The experimental data obtained were used to parameterize the model and calculate the maximum bearing capacity of the frozen wall (FW) in the presence of dissolved salts in the volume of frozen soils. It has been demonstrated that the decrease in the maximum FW bearing capacity is substantial with the appearance of dissolved salt in the pore space of the soils. This decrease is at-tributed to the combined effects of two factors: 1) a reduction in the FW thickness and 2) a decrease in the frozen soil strength due to an increase in the amount of unfrozen water content in the soil pore space. The second factor is deemed more significant.

Diammonium Phosphate Treatment for Sustained Hydraulic/Acid Fracture Conductivity in Chalk and Limestone Formations

Summary The sustained conductivity of hydraulic/acid fractures is crucial for the continuous and effective production of hydrocarbons. However, hydraulic fractures in soft carbonate formations often experience a reduction in conductivity due to rock deformation and creeping under in-situ stresses. One approach to resolve this issue is to stiffen the fracture surfaces using a consolidating agent. This study examines the application of diammonium hydrogen phosphate (DAP) as an additive to improve the hydraulic/acid fracture conductivity in Indiana limestone and Austin chalk slabs. Initially, flat slabs of Indiana limestone and Austin chalk were subjected to acidization using 15% and 10% hydrochloric acid (HCl) for 10 minutes and 5 minutes, respectively. The resulting surface texture changes were measured using a profilometer. Subsequently, half of the samples underwent treatment in 1 M DAP under 1,000 psi and 75°C for 72 hours. The surface stiffness of the samples was evaluated using nondestructive impulse hammering before and after acid injection and DAP treatment, while hydraulic/acid fracture conductivity was determined using an API conductivity setup. Following acid injection, the experimental results demonstrate that the stiffness of all the samples was diminished to different degrees. However, intact Indiana limestone samples exhibited increased stiffness after treatment with DAP. Acidized Indiana limestone samples showed partial restoration of their original stiffness with DAP treatment. In contrast, DAP treatment fully restored the stiffness of Austin chalk samples and further enhanced it, resulting in a twofold to fivefold increase. The increased stiffness observed in the treated samples had a direct impact on short- and long-term conductivity. Treated Indiana limestone fractures exhibited double the conductivity of untreated fractures. The improvement in Austin chalk conductivity was even more significant after DAP treatment, resulting in seven to eight times higher conductivity in the treated hydraulic/acid fractures. It is worth noting that there was little to no correlation between surface roughness and conductivity, highlighting the significant influence of fracture stiffness on conductivity.

Enhancing Fracture Conductivity in Soft Chalk Formations With Diammonium Phosphate Treatment: A Study at High Temperature, Pressure, and Stresses

Summary This study aims to address the problem of fracture hydraulic conductivity decline in soft formations using a diammonium hydrogen phosphate (DAP) solution. A naturally weak carbonate, Austin chalk was chosen as an ideal specimen. Flat chalk samples with reduced elastic modulus and roughness were evaluated before and after aging with 1 M DAP for 72 hours at 75°C and 1,000 psi. The fracture gas conductivity of DAP-aged and untreated samples was measured at various flow rates and stresses while recording sample compaction using linear variable differential transformers (LVDTs). The study found that DAP aging increased the reduced elastic modulus of chalk specimens up to 330% of the original value, improving their resistance to deformation and failure under stress by 200 psi. The hydraulic conductivity of DAP-aged samples was at least twice that of untreated samples, with an extended hydraulic fracture conductivity seven times higher than that of the untreated ones. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis revealed that DAP reacted with the chalk to form hydroxyapatite (HAP), which binds the calcite grains, yielding a stiffer, more deformation-resisting rock surface. Overall, the study demonstrates the potential of chemically enhancing and extending the fracture hydraulic conductivity of weak carbonates using DAP.

Role of sulfate ion on wettability alteration and oil mobilization in chalk reservoirs during modified salinity waterflooding

It has often been reported that an increase in sulfate has a positive effect on altering wettability and mobilizing oil during modified salinity water flooding in carbonate reservoirs. However, this understanding is mostly based on experimental studies using outcrop rock material. Therefore, this study aims to reassess the role of sulfate on altering wettability and recovery performance during modified salinity water injection in oil-saturated porous media of chalk reservoirs using real reservoir material. We compare the results with outcrop samples to evaluate the effects of sulfate and rock material on wettability alteration and oil mobilization. We conduct dynamic contact angle measurements and coreflooding experiments in reservoir conditions. Our findings contradict the previous observations, as we discovered that decreasing the sulfate concentration in the displacing fluid causes a shift in the reservoir chalk's wettability to a more water-wet condition, which significantly improves oil mobilization. We also establish that outcrop chalk samples, typically used in laboratory experiments, cannot be used as reliable analogues for reservoir chalk samples with the same geological age. Finally, we use direct pore scale simulations to demonstrate the impact of observed wettability alteration on oil mobilization. Our simulation results align with our experimental findings, indicating that wettability alteration is the primary mechanism in our experiments, as fluid-fluid interactions were ignored, and flow was mainly controlled by the medium's wettability state.

Pore connectivity influences mass transport in natural rocks: Pore structure, gas diffusion and batch sorption studies

Six rocks (one granodiorite, one limestone, two chalks, one mudstone, and one dolostone) with different extents of heterogeneity at six different particle sizes (from 75 to 8000 μm) were studied to describe the effects of pore connectivity on mass transport. The methods applied were (i) porosity measurement of granular rocks, (ii) analyses of gas-phase diffusive transport in a bed of packed particles, along with a solid quartz method at these six particle sizes being developed to identify the contribution of intraparticle diffusion, and (iii) batch sorption tests of multiple ions (anions and cations) with subsequent analyses of inductively coupled plasma-mass spectrometry. Granular porosity measurement results reveal that with decreasing particle sizes, the effective porosities for the “heterogenous” group of rocks (Grimsel granodiorite and Edwards limestone) increase, whereas the porosities of another “homogeneous” group (two Israel chalk samples, Japan mudstone, and Wyoming dolostone) remain constant. Gas diffusion results show that the intraparticle gas diffusion coefficient among these two sample groups, varying in the magnitude of 10-8 to 10-6 m2/s, are not directly correlated to the porosity differences. Moreover, the batch sorption work displays a different affinity of rocks for various tracers. For Grimsel granodiorite, Japan mudstone, and Wyoming dolostone, the adsorption capacity of Sm3+ and Eu3+ increases as the particle size decreases. In general, this integrated research of grain size distribution, granular rock porosity, intraparticle diffusivity, and ionic sorption capacity gives insights into the pore connectivity effect on both physical and chemical transport behaviors for different lithologies and/or different particle sizes.

Barite and gypsum precipitation in chalk: A numerical simulation approach revealing the coupled impact of physical and chemical heterogeneities in porous media

In studies relevant to the contaminant transport in geosphere, the evolution of local physicochemical processes imparts potential challenges in safety and critical assessment issues. The transport of such contaminants in a rock pore structure could occur under the advective or diffusive regime. Such transport over the spatial and temporal scale depends upon the coupling between physical properties of the pore structure (permeability, pore size distribution, porosity) and chemical properties of the dissolved ionic species (pH, ionic strength, and surface interactions). The nature of such interactions leads to the generation of local imbalances in pore solution and thereby enhances the dissolution and precipitation of minerals. To predict the occurrence of such processes over large spatial and temporal scales, it is imperative to demonstrate how these physical and chemical processes interact within the heterogeneous porous media. In this context, numerical simulations were conducted to model the results of experimental dataset investigating barite and gypsum precipitation in chalk. In the experimental approach, the selected two minerals are end-members of the sulfate-alkali family and exhibit large differences in the kinetic rate and solubility, and the reference chalk sample resembles heterogeneity in its pore structure. The numerical simulations revealed that at the 1D scale, it was possible to model the overall experimental observations at the boundary monitoring points such as chemistry evolution in reservoirs, porosity loss during barite and gypsum precipitation, the total amount of barite and gypsum formation in clogging zone and the threshold saturation index to initiate precipitation. However, these simulations could not validate the experimentally observed impact of barite and gypsum clogging on the changes in water tracer transport. The underlying reason for such behavior was the formation of a unique clogging zone due to barite and gypsum precipitation. We showed that the 2D simulations incorporating spatial heterogeneity were able to reproduce the observed precipitation patterns, for both of the barite and gypsum experiments. Through 1D and 2D numerical results, we demonstrate the capability of a reactive transport model to validate the experimentally observed barite and gypsum precipitation behavior in chalk samples. Furthermore, it also highlights the interplays between the physicochemical heterogeneities, mineral reactivity and the transport rates that are responsible for the distinct precipitation pattern of the barite and gypsum in chalk samples, and consequently their impacts on transport properties.

Role of mineralogical, structural and hydrodynamic rock properties in conduits formation in three distinct carbonate rock types

Adopting sustainable strategies to manage water resources of karst reservoirs requires to understand the carbonate rock reactivity that is responsible for the formation of these reservoirs, as well as the role of the rock mineralogy in the conduit formation. To this end, three carbonate rocks (chalk, crinoidal limestone and dolomite) with different mineralogy and internal structure are submitted to laboratory dissolution experiments by injecting an acid solution under atmospheric conditions and various hydrodynamic conditions with a homemade experimental device. The core samples are characterized by petrophysical investigations with laboratory and imagery techniques before and after the experiments, and the changes in chemical and hydraulic properties are recorded during the experiment. The resulting carbonate dissolution leads to the formation of preferential conduits and the increase in porosity and permeability of the samples. For the three rock types, the dissolution rate mainly depends on the mineral composition, the flow conditions and the initial structural properties. In addition, we observe that (i) for each rock type, the lower Péclet conditions applied to the samples, the lower global dissolution rate, (ii) for each hydrodynamic condition, chalk and crinoidal limestone have the highest and smallest dissolution rates, respectively, (iii) for a similar amount of injected acid, the dissolution is lower in dolomite than in calcite, and (iv) high rock heterogeneities in the chalk samples are responsible for high renewal rates that induce high dissolution rates.

An evaluation of viscous deformation of chalk on wellbore stability

Wellbore instability has become an increasing concern for horizontal wells, especially with the move towards using drilling innovations that the entire lateral sections are remained without casing during the lifetime of the reservoir and after plug and abandonment operations. We aim to understand the long-term effect of viscous deformation on wellbore stability for horizontal wells through advanced laboratory experiments and viscoplastic simulations. Two-stage single lateral hole tests were conducted on chalk samples, where a horizontal wellbore was drilled laterally at the center of a cylindrical specimen and loaded under triaxial condition with a constant stress ratio. The chalk samples were imaged at two stages in the creep phase using X-ray computed tomography CT(scanning) to precisely observe the breakout development of the damaged zones formed around the borehole. The results from the experiments and CT analysis confirmed model predictions. Results presented in this work clearly demonstrate that viscous deformation can trigger failure modes and induce breakout development around the wellbore in the creep phase.

Measurement of the hydraulic properties of chalk using centrifuge permeameter; the study of chalk hydraulic properties under accelerated gravitational force

In this study, a geotechnical centrifuge has been used to study the unsaturated hydraulic properties of rock samples from the chalk aquifer, SE England. This method allows rapid measurement of hydraulic properties in a controlled environment, in contrast to previous studies on the chalk unsaturated zone, which required either an extended period (years) of data monitoring in the field or extended experimental periods (weeks) in the laboratory. Three types of specially built sensors were used to monitor water flow through chalk samples: a water pressure transducer to measure matric potential, frequency domain reflectometry probes to measure the volumetric water content and pressure transducers to measure the volume of water passing through the sample. Chalk samples were tested during wetting and draining processes to understand any hysteresis occurring during periodic recharge of the aquifer. Before undertaking physical tests of chalk samples in the centrifuge, a theoretical model of chalk hydraulic behaviour under centrifugal force was developed. This model was used to define and justify the instrumentation plan of the physical model and predict the shape of the soil moisture characteristic (SMC) curve and unsaturated hydraulic conductivity ( K u ) function. The results were then evaluated and compared with the experimental results. These results show that chalk samples can be successfully tested under centrifuge conditions and hydraulic properties can be measured, including soil moisture characteristic curves and hydraulic conductivity. Supplementary material: Calculation spread sheets are available at https://doi.org/10.17033/DATA.00000283

Impact of hydro-chemical conditions on structural and hydro-mechanical properties of chalk samples during dissolution experiments

The importance of karst reservoirs for water resources, and their complexity in terms of structural properties and hydraulic responses, require a better understanding of the formation and location of conduits in these systems, in particular for chalk reservoirs. For this purpose, we conduct laboratory experiments of acid solutions injection in chalk core samples using a homemade experimental device. Three different flow rates and two acid concentrations are applied on six samples at atmospheric conditions with Peclet numbers ranging from 0.1 to 1.7 and Damköhler numbers from 10−6 to 10−3. Hydraulic and chemical measurements are carried out during the experiments, while petrophysical characterization is conducted on the samples before and after the reactive percolation experiments. The analysis of these data shows an increase in permeability of the rock samples due to calcite dissolution, which is associated with the creation of preferential conduits. We also observe that the initial structural properties control the dissolution regime with the following main conclusions. For homogeneous systems, the overall dissolution rate and percolation time are directly linked to the considered flow rate and injected acid concentration. For heterogeneous systems, (i) pre-existing large-scale structural heterogeneities control the location of newly created paths while reducing the value of the dissolution rate, and (ii) micro-scale heterogeneities result in highly heterogeneous general structures, which are responsible for channels formation while applying low Damköhler numbers.

Comparison of machine learning techniques for predicting porosity of chalk

Precise and fast estimation of porosity is a vital element of reservoir characterization. A new technology for fast and reliable porosity prediction of chalk samples is presented by applying machine learning methods and X-ray fluorescence (XRF) elemental analysis. Input parameters of prediction models are based on rapid and accurate elemental analysis of chalk samples obtained from Hand-held X-ray fluorescence (HH-XRF) measurements. The intelligent models, including Random Forest (RF), Multilayer perceptron (MLP), Random Forest integrated by Genetic Algorithm (GA-RF) and Multilayer Perceptron integrated by Genetic Algorithm (GA-MLP), are trained and tested based on samples consisting of outcrop chalk samples from Rørdal and Stevns Klint (ST) and core samples from Ekofisk Formation in the North Sea. Results are evaluated by sustainability index (SI), determination coefficient (R2), correlation coefficient (CC), and Willmott's Index of agreement (WI). Results indicate that the combination of GA-RF intelligent method with XRF elemental analysis successfully provides an accurate model by 0.99, 0.02, 0.995 and 0.99 respectively for CC, SI, WI and R2, respectively.

A pilot study investigating the rapid estimation of intact dry density of chalk using low impact energy hardness measurements taken with an Equotip Leeb hardness tester

Intact dry density (IDD) is a key parameter for characterizing the rock mass behaviour of chalk and is often the main classification property reported for engineering design. The role of IDD in coastal chalk settings has been discussed in the literature as an important rock control for coastal cliff recession behaviour. This study demonstrates a tool that can reliably estimate the IDD of chalk samples. Originally developed for testing sheet steel materials, the Equotip has been shown to be adaptable for geological applications and the results of this study show that the Equotip can be used to derive a surface hardness value of dry chalk samples that has a positive correlation with IDD. Furthermore, the results show that the device is sensitive to the anisotropy of chalk and that testing undertaken in the diametral orientation gives more accurate correlations. The results of this study also show that the Equotip is sensitive to environmental setting and the effects of weathering, giving a separate correlation for samples collected along the Sussex coastline from those obtained inland. A statistical assessment of the results shows that a characteristic hardness within an acceptable margin of error can be achieved from an optimal number of single impacts.

X-ray in-line holography and holotomography at the NanoMAX beamline.

Coherent X-ray imaging techniques, such as in-line holography, exploit the high brilliance provided by diffraction-limited storage rings to perform imaging sensitive to the electron density through contrast due to the phase shift, rather than conventional attenuation contrast. Thus, coherent X-ray imaging techniques enable high-sensitivity and low-dose imaging, especially for low-atomic-number (Z) chemical elements and materials with similar attenuation contrast. Here, the first implementation of in-line holography at the NanoMAX beamline is presented, which benefits from the exceptional focusing capabilities and the high brilliance provided by MAX IV, the first operational diffraction-limited storage ring up to approximately 300 eV. It is demonstrated that in-line holography at NanoMAX can provide 2D diffraction-limited images, where the achievable resolution is only limited by the 70 nm focal spot at 13 keV X-ray energy. Also, the 3D capabilities of this instrument are demonstrated by performing holotomography on a chalk sample at a mesoscale resolution of around 155 nm. It is foreseen that in-line holography will broaden the spectra of capabilities of MAX IV by providing fast 2D and 3D electron density images from mesoscale down to nanoscale resolution.

Impact of acid wormhole on the mechanical properties of chalk, limestone, and dolomite: Experimental and modeling studies

Matrix acidizing is a widely applied stimulation process in carbonate reservoirs to remove damaged parts of the formation and enhance wellbore productivity. Few studies considered the impact of acid on the deterioration of mechanical rock properties considering different carbonate types. Assessment of the wellbore stability and the sustainability of the wormhole structure depends on the mechanical rock properties after acidizing. In this study, three rock lithologies (chalk, limestone, and dolomite) were treated with acid at different injection rates to establish acid efficiency curves. The acid was injected in a coreflooding setup until wormhole breakthrough, indicated by zero pressure drop across the sample. The rock petrophysical (i.e., porosity, permeability) and mechanical properties were measured before and after the treatment. XRD was used to estimate the mineralogical compositions of each rock type. Then, the impulse hammer technique was implemented to measure the rock surface hardness at ambient pressure. Also, acoustic measurements were used to quantify the dynamic Young’s modulus and Poisson ratio at high confining pressures. To estimate the deformation in the samples with wormholes under varying confining stress, a finite element model was constructed. Results indicate that the acid treatment significantly reduces the rock surface hardness (by 47 to 77 %), while Young’s modulus decreases by 18 to 33 % at ambient stress. The treated rock samples showed higher Young’s modulus under high confining stress. Chalk samples were the most impacted by the treatment, while dolomite specimens were the least. The finite element numerical model indicates that rock deformation is more significant in chalk samples, especially at high confining stress that acts perpendicular to the wormhole.

Analysis of red chalk drawings from the workshop of Giovanni Battista Piranesi using fiber optics reflectance spectroscopy

The viability of fiber optics reflectance spectroscopy (FORS) for the differentiation of red chalk drawing media was investigated, focusing on the group of drawings from the workshop of Giovanni Battista Piranesi (1720–1778) at the Staatliche Kunsthalle Karlsruhe, Germany. The evaluation of spectra was supported by principal component analysis (PCA). The method was tested on mock-up drawings made with recently acquired natural and synthetic red chalks of known origin. It was possible to sort these mock-up drawings according to chalk type and application technique. The compositional differences of these reference chalks were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Subsequent FORS analysis of selected original drawings revealed the existence of several closely grouped clusters, implying similarities on the basis of the underlying spectral features among the historical red chalks used in Rome. These similarities distinguished the historical drawings from the red chalk mock-up drawings, except for the drawings made with red chalk samples from the area near the town of Theley, Germany, which were shown to bear close similarities to those in the cluster of historical samples.

A green and efficient acid system for carbonate reservoir stimulation

Matrix acidizing is a well stimulation method that is applied to enhance productivity. The success of a stimulation job depends mainly on the efficiency of the acid systems that are employed. This study investigated a new and efficient acid system that is biodegradable, not corrosive, and safe to handle. The system efficiency was compared to that of HCl and chelating agents. Acid coreflooding has been applied to chalk, dolomite, and limestone carbonate rock samples to measure the efficiency in terms of injected pore volume to breakthrough (PVBT). The rock samples were characterized in terms of porosity, permeability, composition using X-ray diffraction (XRD), and pore distribution using nuclear magnetic resonance (NMR). The effluent was collected at different times and analyzed using induced coupled plasma (ICP). Medical computed tomography (MCT) was used to visualize the interior of the rock samples after acidizing. The new acid system was more efficient in terms of acid spending than straight HCl acid and chelating agents in Indiana limestone. At the optimum injection rate, chelating agents had an optimum PVBT value of about 15 compared to 0.85 for plain HCl acid. Using the new acid system resulted in an optimum PVBT value of only 0.41, which is less than half of HCl acid. A numerical wormhole simulator was implemented to match the experimental PVBT and assess the reactivity of the acid system.

Correlative nano-spectroscopic imaging of heterogeneity in migrated petroleum in unconventional reservoir pores

Low permeability unconventional oil and gas resources may experience formation damage due to the emplacement of viscous petroleum in narrow pore throats during petroleum migration and production. The composition of such pore-clogging molecules remains under-characterized, as standard analytical techniques in organic geochemistry either lack the desired nano-scale spatial resolution with simultaneous chemical specificity, or require extraction and therefore averaging of the suite of compounds that make up bulk crude oil. A new correlative imaging technique with high spatial resolution is demonstrated for migrated residual petroleum in nanopores of Niobrara chalk samples. Imaging combines Infrared scattering-Scanning Near-field Optical Microscopy (IR s-SNOM) with nano-mechanical atomic force microscopy to map both chemical and mechanical properties at ~30 nm spatial resolution. These correlative images demonstrate that heterogeneity exists within emplaced petroleum in situ on the scale of hundreds of nanometers, visualized as clusters of petroleum having distinct adhesive forces. The chemical composition of these clusters remains ambiguous, but their geometry and mechanical properties may imply that they are phase-separated asphaltene preferentially associated with petroleum-attractive nano-domains on calcite pore walls. Despite outstanding challenges, correlative IR s-SNOM and nano-mechanical imaging offer a promising and novel analytical approach to understanding fluid behavior in nanoporous reservoir rocks.

Mineralogy and geochemistry of reservoir and non-reservoir chalk from the Norwegian continental shelf

A first and detailed study of the geochemistry and mineralogy characterizing the North Sea reservoir and non-reservoir chalk is provided in this work. The study is based on 185 cores from exploration and development wells in the North Sea. The cores related to reservoir development have different flooding status – unflooded or waterflooded at various temperatures – and are directly sampled from the Ekofisk field. Optical petrography shows a micritic carbonate matrix, with grains represented by various microfossils such as foraminifers and sponge spicules. Scanning electron microscopy (SEM) reveals post-depositional calcite precipitation and cementation. Dolomite is found only in the reservoir samples, but it is discussed as a diagenetic feature, unrelated to the hydrocarbon content or EOR exposure. The non-carbonate minerals observed with BSE-SEM and XRD include mostly quartz but also smectite, illite, kaolinite, mica, and pyrite. The abundance of clastic input varies, and there is a clear decrease in porosity stratigraphically downwards, with stronger cementation and higher compaction. δ 13 C reflects primary trends for Upper Cretaceous stages while δ 18 O in all samples is lower than the secular global isotopic values for this period. However, the δ 18 O values are not sufficiently low to imply a strong diagenetic overprint, but rather suggest the influence of a secondary fluid. This fluid cannot be a hydrocarbon-rich one, nor EOR fluids, as non-reservoir samples, as well as flooded and unflooded reservoir samples show very similar stable isotope values. • Mineralogical and geochemical characterization of broad North Sea chalk sample set. • Comparison between flooded and unflooded reservoir chalk and non-reservoir chalk. • The diagenetic overprint is low in all lithotypes. • The presence of hydrocarbons is not responsible for geochemical alterations. • The geochemical effect of EOR flooding of reservoir chalk is not obvious.