In this work we focus on further understanding reactive transport in carbonate rocks, in particular limestones characterized by a bimodal pore size distribution. To this end, we performed injection experiments with CO2-saturated water on a sample of Euville limestone and monitored the experiments with a medical CT scanner. Microscanner imaging was performed before and after alteration. Experiments showed that permeability increased by nearly two decades due to the alteration process. This increase could be attributed to the formation of a preferential dissolution path visualized on the CT images. Microscanner images show that preferential dissolution areas are characterized by the presence of numerous enlarged macropores. The preferential dissolution path created therefore retains a porous structure and does not correspond to a wormhole-type channel. To provide further knowledge of the small-scale physics of reactive transport, we performed Lattice-Boltzmann simulations of flow in a numerically generated model 2D porous medium having geometrical and topological features designed to approach Euville limestone. We showed that the fluid velocity increased in nearly percolating paths of macropores. Considering the experiments, this means that the CO2-saturated water starts to enter high-velocity zones earlier than low-velocity zones, inducing an earlier onset of the alteration process and a more pronounced local dissolution. However, numerical results showed that the alteration of non-connected macropores leads to an increase of permeability much smaller than the experimentally observed one. To explain this fact we used effective medium modelling that permits predicting the variation in permeability as a function of the fraction of macropores and consequently as a function of alteration. It proved that as long as there is no alteration-induced percolating path consisting of macropores, the increase in permeability is relatively low as shown by the Lattice-Boltzmann simulations. An increase in permeability of several orders of magnitude is only observed when the macroporosity is close to the percolation threshold. This fact is in accordance with the experimentally observed results.

Outcrop analogues play a key role in the characterization of subsurface carbonate platforms. The lack of well data and relevant outcrop analogues can result in the misinterpretation of seismic data. To address this issue, we apply an integrated workflow based on sedimentology, geophysics and petrophysics on outcrop analogues present onshore Lebanon, to constrain the carbonate platform's properties on onshore seismic data. A thorough sedimentary description is completed for a 400‐m‐thick Cenomanian–Turonian carbonate platform located in Kfarhelda, northern Lebanon. P‐wave velocity is acquired directly on the outcrop, and the petrophysical properties are measured on 44 samples. A 1D synthetic seismogram is computed with Ricker wavelet 25 Hz resembling seismic resolution. The resulting reflectors are mainly (1) high amplitude reflectors at the limit between two facies with contrasting physical properties enhanced by diagenesis, (2) moderate amplitude reflectors corresponding to stratigraphic limits at the transition between facies and (3) very low amplitude reflectors in karstified units. The integration of outcrop and seismic data is based on the generation of the synthetic seismogram to identify the geological origin of reflectors. The best fit between the synthetic seismic and seismic profile is used to interpret a seismic facies representing bedded limestones of Sannine and Maameltain formations (Cenomanian–Turonian). Two other distinctive reflectors are identified at the boundary of the Marly Limestone Zone, and the Channel facies unit characterized by bioclastic packstone to floatstone. This study highlights the importance of using outcrop analogues to identify the seismic signal of stratigraphic sequences and improve the interpretation of onshore seismic data.

Abstract On the Island of Samos (East Aegean region, Greece), two sedimentary basins are filled by thick continental series dated to the Late Miocene to Early Pliocene. A multidisciplinary study has been performed including (1) the definition of 21 sedimentary facies, (2) a review of the biological components and (3) carbon, oxygen and strontium stable isotope analyses. The succession is characterised by various depositional settings and hydrochemical compositions. Five main stages of basin evolution have been identified: (1) The Late Serravallian is marked by the development of alluvial fans and fan delta; (2) during the Lower Tortonian, isolated shallow lakes with variable salinity, from fresh to brackish, developed under warm and relatively humid conditions; (3) the Middle to Upper Tortonian is marked by the development of a large and deep lake with saline and alkaline waters, under colder and drier conditions; (4) the Latest Tortonian to Messinian period is represented by an ephemeral alluvial system, developed under a dry climate; (5) during the Zanclean, a palustrine and paludal wetland system, dominated by tufa carbonates, developed under moderately humid conditions. This succession is of particular interest for the reconstruction of the palaeoenvironmental evolution of the transition zone between the Mediterranean domain, and the Paratethys and circum‐Paratethys areas. The geochemical data and the presence of flora (diatoms) and fauna (gastropods) of marine affinity suggest transient ingressions of marine‐related water or groundwater inflows as early as the Lower Tortonian. The Samos succession records the complex interaction between the regional geodynamics and climate. The extensional regime of the Eastern Aegean zone generates subsidence, interrupted in the mid‐Tortonian (9 Ma) by a brief compressive event and a major exposure of the basins. Furthermore, the Late Miocene progressive aridification, followed by a change to a more humid climate (Pliocene) is also a major driver of the sedimentation.

Continental carbonates are characterized by multi-scale heterogeneous porous networks, making the geological interpretation of seismic imaging difficult. We investigate two sedimentary sections exhibiting a similar facies succession, combining geological characterizations and multi-scale acoustic measurements. Based on outcrop investigation and petrographic description, we define nine sedimentary facies displaying contrasted early diagenetic evolutions. According to the vertical facies variations, we develop a depositional model corresponding to a low gradient valley fed by freshwaters, subdivided into three main domains (alluvial plain, palustrine and paludal). To understand the acoustic properties of the studied sedimentary rocks while remaining representative of their multi-scale heterogeneity, we acquire acoustic measurements at two different scales: i) at log-scale, directly on the outcrop surfaces using a frequency of 250 kHz; and ii) at plug-scale as usually done in laboratory using a frequency of 500 kHz. Based on these multi-scale geophysical acquisitions, we link in-situ P-wave velocities with the different sedimentary facies while characterizing centimeter-scale Representative Elementary Volumes (REVs). Conversely, based on laboratory measurements and thin-section petrography, we define relationships between P-wave velocity, porosity, facies, and diagenesis, corresponding to millimeter-scale REVs. Using both in-situ P-wave velocity measurements and plug densities, we construct 1-D synthetic seismograms showing meter-scale seismic reflectors equivalent to crosswell seismic frequency ranges. This approach shows the following: i) high-amplitude seismic reflectors fit with facies changes associated to diagenetic contrasts (e.g. cemented versus uncemented carbonates); ii) reflection free-zones match with a succession of facies changes affected by diagenetic homogenization (e.g. intensely to pervasively recrystallized and cemented carbonates). Our work highlights the importance of relating an extensive geological description of carbonates (facies, depositional model, diagenesis) together with multi-scale acoustic measurements and synthetic seismic modelling to predict the high-resolution heterogeneities of subsurface reservoirs.

In the last decades, geological degassing in shallow marine environments has been recognized as a significant contributor to atmospheric methane, hence to the global‐warming potential. Especially in shallow water environments, a proper assessment of the distribution, quantification and migration pathways of methane within the sediments is fundamental to help forecasting the amount that could leak and eventually reach the atmosphere. Traditionally, velocity anomalies from seismic data are the ones used to assess the occurrence of gas and its concentration. However, in shallow‐waters (<30 m), the post‐critical conditions make the near‐surface velocity estimation from P‐wave reflections extremely challenging, requiring an integrated approach. Here, we propose an original joint analysis of seismic data and geophysical logs, together with information from drilling reports, with the aim of characterizing and quantifying the gas along two crossing multichannel seismic profiles in the Northern Adriatic Sea, a very shallow marine basin where methane occurrence within the sedimentary succession is widespread. We estimated the gas distribution from resistivity anomalies, which are correlated with the seismic response associated with the presence of gas through the signal frequency content. Our results show a different concentration pattern in the two seismic profiles, revealing that gas is both diffuse ad concentrated in local accumulations, in agreement with the gas‐related features already identified on the seismic data. Gas concentration appears to be locally associated to the tectonic features identified in the area, indicating that faults act as preferential conduits for gas migration, locally reaching the seafloor and seeping in the water column.

Carbonate formations are characterized by multiscale heterogeneities that control their acoustic response and flow properties. At the laboratory scale, carbonate rocks do not indicate a strong correlation between P- and S-wave velocities and porosity. The velocity disparities between carbonates of similar mineralogy and porosity result from different microstructures derived from their sedimentary facies and subsequent diagenetic transformations. The still-discussed applicability of Biot-Gassmann’s equation for fluid substitution in carbonate rocks remains another key issue. We have developed an integrated experimental workflow that allows a consistent checking of the applicability of Biot-Gassmann’s equation and provides key geologic and microstructural information to understand the petroacoustic signature of carbonate rocks. The defined approach is based on the phase-velocity measurements performed in liquid-saturated conditions using polar and nonpolar fluids. It allows the identification of the whole set of parameters required by Biot-Gassmann’s equation including the bulk modulus of the solid matrix. This approach is implemented on samples representative of two different carbonate formations deposited in lacustrine and marine environments, respectively. The obtained results demonstrate the applicability of Biot-Gassmann’s equation for the two studied carbonate families and indicate the link between their petroacoustic signature and diagenetic history.

Carbonate formations are highly heterogeneous, and the velocity- porosity relationships are controlled by various microstructural parameters, such as the types of pores and their distribution. Because diagenesis is responsible for important changes in the microstructure of carbonate rocks, we have extended the standard effective medium approach to model the impact of diagenesis on the carbonate elastic properties through a step-by-step effective medium modeling. Two different carbonate rocks deposited, respectively, in lacustrine and marine environments are considered in this study. The first key step is the characterization of the diagenesis, which affected the two studied carbonate sample sets. Effective medium models integrating all of the geologic information accessible from petrographic analysis are then built. The evolution of the microstructural parameters during diagenesis is thoroughly constrained based on an extensive experimental data set, including X-ray diffraction analysis, different porosimetry methods, and ultrasonic velocity measurements. A new theoretical approach including two sources of compliance is developed to model the specific behavior of carbonates. A compliant interface is introduced around the main carbonate grains to represent grain contacts and the different pore scales are taken into account through multiscale modeling. Finally, direct calculations with the model provide elastic wave velocities representative of the different diagenetic stages. An extrapolation to permeability evolution is also introduced. This approach allows the identification of the acoustic signature of specific diagenetic events, such as dolomitization, dissolution, or cementation, and the assessment of their impact on the elastic properties of carbonates.

Linking ultrasonic measurements made on samples, with sonic logs and seismic subsurface data, is a key challenge for the understanding of carbonate reservoirs. To deal with this problem, we investigate the elastic properties of dry lacustrine carbonates. At one study site, we perform a seismic refraction survey (100 Hz), as well as “sonic” (54 kHz) and ultrasonic (250 kHz) measurements directly on outcrop and ultrasonic measurements on samples (500 kHz). By comparing the median of each data set, we show that the P wave velocity decreases from laboratory to seismic scale. Nevertheless, the median of the sonic measurements acquired on outcrop surfaces seems to fit with the seismic data, meaning that sonic acquisition may be representative of seismic scale. To explain the variations due to upscaling, we relate the concept of representative elementary volume with the wavelength of each scale of study. Indeed, with upscaling, the wavelength varies from millimetric to pluri‐metric. This change of scale allows us to conclude that the behavior of P wave velocity is due to different geological features (matrix porosity, cracks, and fractures) related to the different wavelengths used. Based on effective medium theory, we quantify the pore aspect ratio at sample scale and the crack/fracture density at outcrop and seismic scales using a multiscale representative elementary volume concept. Results show that the matrix porosity that controls the ultrasonic P wave velocities is progressively lost with upscaling, implying that crack and fracture porosity impacts sonic and seismic P wave velocities, a result of paramount importance for seismic interpretation based on deterministic approaches.

For the subsurface characterization of carbonates, linking physical properties (e.g. porosity and seismic reflectors) with their geological significance (e.g. sedimentary facies and diagenesis) is of primary importance. To address this issue, we study the lacustrine and palustrine carbonates on Samos Island through a geological and geophysical characterization of a sedimentary succession. The microstructures of the samples are described, and the samples' physical properties are measured (porosity, P-wave velocity and density). The results show that the identification of only the primary (i.e. sedimentary) microstructure is not sufficient for explaining the huge variations in porosity and P-wave velocity. Hence, we highlight two early diagenetic processes that strongly impact the microstructures and control the physical properties: (i) neomorphism occludes porosity and increases the P-wave velocity of mud- and grain-supported microstructures, which implies a mineralogical stabilization of the grains; (ii) conversely, the dissolution process creates porosity and decreases the P-wave velocity of grain-supported microstructures if the mineralogical composition of the grains is not previously stabilized. These two diagenetic processes thus depend on the primary microstructures and mineralogy of the sediments. This work aims to explain the variations in porosity and P-wave velocity for each defined primary microstructure. A 1-D seismogram is then built to highlight seismic reflectors with a metre-scale resolution. These reflectors are associated with several geological contrasts. Hard kicks (positive amplitude reflectors) match well with exposure surfaces related to palaeosols. They correspond to contrasts between non-modified primary microstructures and highly neomorphosed microstructures. Conversely, soft kicks (negative amplitude reflectors) are linked with diagenetic contrasts (e.g. neomorphosed microstructures versus non-modified primary microstructures) and sedimentary contrasts that can be overprinted by diagenesis (e.g. neomorphosed mud-supported microstructures versus dissolved grain-supported microstructures). This study highlights that high-resolution seismic reflectors of lacustrine and palustrine carbonates are strongly related to the spatial contrasts of primary microstructures overprinted by early diagenesis.

Summary This study aims to assess the mechanical properties of the mineral and organic phases of source rocks in order to provide consistent input data for effective medium modeling. To this end, nanoindentation measurements are combined with SEM-EDS analysis to match the measured mechanical properties to unequivocally identified mineral and organic phases. The defined workflow consists of four key steps: 1) preparation and characterization of the surface to be analyzed; 2) mechanical characterization through nanoindentation measurements; 3) localization of residual nanoindentation imprints and chemical characterization of indented phases through SEM-EDS analysis; 4) withdrawal of multiphase indentations and one-to-one association of mechanical properties to identified phases. The results obtained on a sample from the Montney formation (Western Canadian sedimentary basin) are presented to illustrate the key contribution of the positive identification of the indented phases through SEM-EDS analysis. The measured indentation moduli are consistent with nanoindentation data and elastic properties from the literature.

The interpretation of five 2D PSTM seismic reflection sections (14 s TWT) covering the northern Levant Basin revealed a total of 10 horizons, among which, one is interpreted as an interface that may represent the Moho. The interpretation of seismic packages and their bounding surfaces as well as the seismic facies analysis were constrained by published 2D seismic interpretations of the northern Lebanese offshore. A total of nine seismic packages are identified in the basin with ages varying from the Mid Jurassic to the Quaternary. The filling of the basin is made up of thick Cenozoic and Mesozoic strata deposited above rifted Triassic - Early Jurassic interval. The sediments are deposited in deep water mixed-settings resulting from high-stand systems (various types of carbonate platforms) and low-stand systems (siliciclastic and carbonate deep-water turbidite complexes). Carbonate and siliciclastic systems are sealed by 1-1.5 km of evaporites, and underlie Plio-Quaternary hemipelagic and pelagic sediments intercalated by turbiditic sheets. The time horizons were converted into depth using two methods; the first one is based on stacking velocities and the second one on velocities resulting from refraction data. 2D crustal modeling was achieved by integrating free-air gravity anomaly, geoid heights and topography data on the five interpreted PSTM seismic lines. The models representing five sections across the northern Levant Basin, show a progressively attenuated crystalline crust in an EW direction (away from the basin's eastern margin). The crystalline crust is best interpreted as a strongly thinned continental crust under the Levant Basin, represented by two distinct components, an upper and a lower continental crust. The Moho appears to be situated between 20 and 23 km in the central and southern Lebanese offshore. Estimated surface heatflow in the basin is around 40 mW/m(2), which is lower than reported values for the onshore and the margin. These differences in heatflow values between the offshore, the margin and the onshore have an important impact on hydrocarbon maturation and assessment of potential petroleum systems.

Summary Pre-salt carbonate reservoirs are a challenge for the exploration and production. Using outcrop analogues provides key information for a better characterization of the complex architectures of these deep reserves. The aim of this study is to better understand heterogeneity of an outcrop analogue of an Atlantic pre-salt carbonate reservoir: the Green River Formation (Wyoming, USA). The Laney Member of the Green River Fm corresponds to marginal lacustrine carbonates deposits, and its architecture is complex: it is organized in patches of reef-like morphologies made up of microbialites, with intercalated breccia and shales. The internal organization of microbialites consists in various nested scales of heterogeneities. In order to describe reservoir properties and facies distribution in these complex architectures, an integrated approach is built using sedimentological, petroacoustical and petrophysical data acquired at different spatial and frequency scales. From this exhaustive dataset, electrofacies are determined from gamma ray and sonic and then compared with sedimentological facies. Finally, using measurements on core samples, dynamic properties are introduced into the classification in order to obtain rock-types. This integrated workflow is a tool for better understanding microbial dominated lacustrine system. It allows to identify main heterogeneities, but shows also some limits.

In case of subsurface characterization of continental carbonates, linking seismic, sonic logs and ultrasonic measurements on samples is challenging. Indeed, one of the main issue to tackle is the high petrophysical heterogeneity and the different factors that drive this heterogeneity in these continental carbonates. This work focuses on Miocene continental carbonates of the East Aegean (Samos Island) which present small continental basins with lacustrine and palustrine carbonate deposits.

Due to their complex structure and the difficulty of collecting data, the hydrogeology of basaltic islands remains misunderstood, and the Galapagos islands are not an exception. Geophysics allows the possibility to describe the subsurface of these islands and to quantify the hydrodynamical properties of its ground layers, which can be useful to build robust hydrogeological models. In this paper, we present seismic refraction data acquired on Santa Cruz and San Cristobal, the two main inhabited islands of Galapagos. We investigated sites with several hydrogeological contexts, located at different altitudes and at different distances to the coast. At each site, a 2D P-wave velocity profile is built, highlighting unsaturated and saturated volcanic layers. At the coastal sites, seawater intrusion is identified and basal aquifer is characterized in terms of variations in compressional sound wave velocities, according to saturation state. At highlands sites, the limits between soils and lava flows are identified. On San Cristobal Island, the 2D velocity profile obtained on a mid-slope site (altitude 150m), indicates the presence of a near surface freshwater aquifer, which is in agreement with previous geophysical studies and the hydrogeological conceptual model developed for this island. The originality of our paper is the use of velocity data to compute field porosity based on poroelasticity theory and the Biot-Gassmann equations. Given that porosity is a key parameter in quantitative hydrogeological models, it is a step forward to a better understanding of shallow fluid flows within a complex structure, such as Galapagos volcanoes.

The characterization, modeling and production of many carbonate reservoirs pose significant challenges due to multiscale geological heterogeneities resulting from the combination of sedimentary and diagenetic processes. This paper proposes an integrated geological, petrophysical and petroacoustic workflow designed to derive correlations between the petrophysical and petroacoustic signature of key reservoir facies and their microstructure. The ultimate objective of this workflow is to provide a reservoir model populated with petrophysical and petroacoustic properties consistent with geological description. First results obtained on a well-preserved Oligo-Miocene succession of mixed carbonate-siliciclastic facies outcropping in the South-East of France have been used as a guiding line to discuss the successive workflow steps and their interrelations.

The accurate inference of reservoir properties such as porosity and permeability is crucial in reservoir characterization for oil and gas exploration and production as well as for other geologic applications. In most cases, direct measurements of those properties are done in wells that provide high vertical resolution but limited lateral coverage. To fill this gap, geophysical methods can often offer data with dense 3D coverage that can serve as proxy for the variable of interest. All the information available can then be integrated using multivariate geostatistical methods to provide stochastic or deterministic estimate of the reservoir properties. Our objective is to generate multiple scenarios of porosity at different scales, considering four formations of the Fort Worth Basin altogether and then restricting the process to the Marble Falls limestones. Under the hypothesis that a statistical relation between 3D seismic attributes and porosity can be inferred from well logs, a Bayesian sequential simulation (BSS) framework proved to be an efficient approach to infer reservoir porosity from an acoustic impedance cube. However, previous BBS approaches only took two variables upscaled at the resolution of the seismic data, which is not suitable for thin-bed reservoirs. We have developed three modified BSS algorithms that better adapt the BSS approach for unconventional reservoir petrophysical properties estimation from deterministic prestack seismic inversion. A methodology that includes a stochastic downscaling procedure is built and one that integrates two secondary downscaled constraints to the porosity estimation process. Results suggest that when working at resolution higher than surface seismic, it is better to execute the workflow for each geologic formation separately.

Source rocks are now both regarded as essential elements of petroleum systems and hydrocarbon prospects. Successful exploration and production of such unconventional reservoirs implies an understanding of their poromechanical behavior. The large variability of source rocks prevents any straightforward generalization of the observations. In this work, we seek to understand the link between the acoustic properties of source rocks and their microstructure characteristics.

In the context of Cigeo facility project (industrial center for geological disposal), the 130 m thick Callovo-Oxfordian (COX) clay-rock formation has been thoroughly studied. A 250 km area was investigated first, where well logging and coring were performed on many boreholes, then a restricted 30 km area (called ZIRA: Zone of interest for further surveying), where a 3D high resolution seismic survey was carried out. The COX formation is a clay-rich layer located approximatively at a depth of 500 m and having key properties such as very low hydraulic conductivity, low diffusion coefficient, high self-sealing capacity and strong sorption capacity for radionuclides. From the bottom to the top of the COX, the layer is divided into five major petrophysical units: three argillaceous units (UA1, UA2, UA3), one transition unit (UT), and one silto-carbonated unit (USC). The UA2-UA3 units represent the mostly clayey levels of the formation where the deep disposal facilities will be built. From a mechanical behavior, previous studies show that these units have good properties to ensure construction and operation of a geological waste disposal. Here we propose to go further on this topic by evaluating the 3D distribution of static and dynamic mechanical moduli of Callovo-Oxfordian argillites in the ZIRA.

Summary Rock Physics Templates (RPT) have been introduced to help non-experts in rock physics for lithology and pore fluid interpretation of sonic log data and elastic inversion results. Here we propose an extension of the approach in order to deal with anisotropy. For this we start with the classical RPT chart, namely crossplot of acoustic impedance Zp and P-wave over S-wave velocity ratio. A third dimension, here an anisotropy coefficient, is reported as isolines. On this initial 3D RPT chart we superimpose as color-coded points interpretation parameters, such as porosity or shaliness. We use this representation for studying chemically compacted shales, idealized as thinly stratified random mix of transversely–isotropic smectite and isotropic illite, our purpose being more to illustrate the value of the representation than to demonstrate relevancy of the model. The interpretation parameters are illite proportion Villite and the Orientation Distribution Function coefficients W200 and W400 of the clay minerals in smectite. Among other results, it clearly appears from the charts, as expected, that increasing illitization tends to stiffen the rock, and as a consequence to increase Zp. Increasing alignment of the clay minerals (mainly by increasing W200) surprisingly tends to decrease the P-wave time processing anisotropy parameter Eta.

ABSTRACTThe technical and economic success of a CO2 geological storage project requires the preservation of the site injectivity and integrity properties over its lifetime. Unlike conventional hydrocarbon gas injection, CO2 injection may imply geochemical reactions between acidified pore fluids and target reservoir formations, leading to modifications of their poromechanical properties. To date, the chemical effects on the host rock mechanical behaviour are not satisfactorily taken into account in site‐scale numerical models of CO2 injection, mainly due to a lack of quantitative data. The present experimental work aims at characterizing the evolution of carbonate poromechanical properties induced by acid alteration. Unlike standard experimental approaches, the implemented alteration method induces a homogeneous dissolution pattern, which ensures reliable poromechanical measurements on altered samples. These well‐controlled alteration conditions allow a proper interpretation of the test results through the macroscopic continuous approach of poromechanics. Petrophysical, geomechanical, and petroacoustic properties of outcrop carbonate samples have been measured for different levels of alteration to mimic long‐term exposure to reactive brine. The obtained experimental data show clear trends of chemically induced mechanical weakening. Nuclear magnetic resonance measurements and microscanner imaging performed before and after alteration have provided complementary insights into the alteration effects at the microscopic scale.