Ben Laurich

The Jabal Akhdar dome in the Oman Mountains: Evolution of a dynamic fracture system

The Mesozoic succession of the Jabal Akhdar dome in the Oman Mountains hosts complex networks of fractures and veins in carbonates, which are a clear example of dynamic fracture opening and sealing in a highly overpressured system. The area underwent several tectonic events during the Late Cretaceous and Cenozoic, including the obduction of the Samail ophiolite and Hawasina nappes, followed by uplift and compression due to the Arabia-Eurasia convergence. This study presents the results of an extensive tectonic survey, and correlates subseismic-scale structures in Jabal Akhdar (faults, fractures, veins and stylolites) with the main tectonic events in the Northeastern Arabian plate. As some of the studied formations host large oil reserves in neighboring areas, determining the relative timing of these events in the exhumed rocks is important to understand hydrocarbon distribution and fracture patterns in these reservoirs. The formation of early veins and stylolites in the Oman Mountains is followed by top-to-the-South layer-parallel shearing that may be associated with the obduction of the Samail and Hawasina nappes. This compressional tectonic event is followed by normal (dip-slip) to oblique-slip faults and veins. Top-to-the-Northeast layer-parallel shearing, which corresponds to the first stage of exhumation of the autochthonous rocks offsets these structures. Our new data indicate that this first phase of events is overprinted by complex strike-slip networks of veins and fractures, as well as by the reactivation and onset of seismic-scale faults. Strike-slip structures belong to three distinct events. The first one (NW-SE-oriented compression) is probably associated with the oblique collision of the Indian plate against the Arabian platform during the Late Campanian to the Mid Eocene. The second event (E-W-oriented compression) is likely to have been formed during the Late Oligocene-Middle Miocene during uplift. The last event (NE-SW-oriented compression) probably took place during the Miocene-Pliocene. Structures of the first two strike-slip events have the same orientation as seismic-scale faults observed in the subsurface of Oman and Abu Dhabi. In addition, increasing vein intensity towards the top of the autochthonous formations in the Oman mountains, as well as the small angle between conjugate vein sets, indicate that high fluid pressures that are thought to be present during strike-slip deformation.

Electrical and fluid flow properties of carbonate microporosity types from multiscale digital image analysis and mercury injection

Electrical and fluid flow properties of porous media are directly related to the morphology of pores and the connectivity of the pore network. Both are closely linked to the amount and type of intrinsic microporosity in carbonate rocks, which is not resolved by conventional techniques. Broad-ion-beam (BIB) milling produces high-quality true-two-dimensional cross sections for scanning electron microscopy (SEM) and enables accurate quantification of carbonate microporosity for the first time. The combination of BIB-SEM mosaics with optical micrographs yields a multiscale digital image analysis (MsDIA) spanning six orders of magnitude. In this paper, the pore structures of 12 different carbonate rock samples from various rock types are quantified using MsDIA. Mercury injection capillary pressure measurements are used to assess pore-throat properties. The quantified pore-structure parameters are correlated with plug measurements of electrical resistivity and permeability. Results indicate that petrophysical properties are closely linked to the type of microporosity, which is distinctive for a certain rock type. Rock types with crystalline microporosity, such as mudstone and dolomite, generally show good connectivity, in which the size of the pore-network determines if the rock favors either hydraulic or electric flow. Rock types with intercement or micromoldic microporosity, such as bindstone and travertine, show variations in connectivity due to layering and moldic micropores of biological origin. Furthermore, pore-size distributions (PSD) follow a power law in all samples, despite their depositional and diagenetic differences. The slope of the PSD correlates with the electric properties, in which samples with a steeper slope show lower cementation factors. The linearity of the power law distribution enables predictions of pore populations outside the investigated length scales.

The effect of microstructural heterogeneity on pore size distribution and permeability in Opalinus Clay (Mont Terri, Switzerland): insights from an integrated study of laboratory fluid flow and pore morphology from BIB-SEM images

Abstract Opalinus Clay (OPA) is considered as a potential host rock for the deep geological disposal of radioactive waste. One key parameter in long-term storage prediction is permeability. In this study we investigated microstructural controls on permeability for the different facies of OPA. Permeability and porosity were determined under controlled pressure conditions. In addition, the pore space was investigated by SEM, using high-quality surfaces prepared by broad ion beam (BIB) milling. Water permeability coefficients range from 1.6×10−21 to 5.6×10−20 m2; He-pycnometer porosities range between approximately 21 and 12%. The sample with the highest He porosity (shaly facies) is characterized by the lowest permeability, and vice versa (carbonate-rich sandy facies). This inverse behaviour deviates from the generally reported trend of increasing permeability with increasing porosity, indicating that parameters other than porosity affect permeability. Visible porosities from SEM images revealed that 67–95% of the total porosity resides within pores smaller than the SEM detection limit. Pore sizes follow a power-law distribution, with characteristic power-law exponents (D) differing greatly between the facies. The carbonate-rich sandy facies contains a network of much larger pores (D(shaly)≈2.4; D(carbonate-rich) c. 2.0), because of the presence of load-supporting sand grains that locally prevent clay compaction, and are responsible for a higher permeability.

Investigation of microstructures in naturally and experimentally deformed reference clay rocks using innovative methods in scanning electron microscopy

The application of ion-beam milling techniques to clays allows investigation of the porosity at nm resolution using scanning electron microscopy (SEM). Imaging of pores by SEM of surfaces prepared by broad ion beam (BIB) gives both qualitative and quantitative insights into the porosity and mineral fabrics in 2D representative cross-sections. The combination of cryogenic techniques with ion-beam milling preparation (BIB and FIB, focused ion beam) allows the study of pore fluids in preserved clay-rich samples. Characterization of the pore network is achieved, first, using X-ray computed tomography to provide insights into the largest pore bodies only, which are generally not connected at the resolution achieved. Secondly, access to 3D pore connectivity is achieved by FIB-SEM tomography and the results are compared with 2D porosity analysis (BIBSEM) and correlated with bulk porosity measurements (e.g. mercury injection porosimetry, MIP). Effective pore connectivity was investigated with an analog of MIP based on Wood’s metal (WM), which is solid at room temperature and allows microstructural investigation of WM-filled pores with BIB-SEM after injection. Combination of these microstructural investigations at scales of ,1 mm with conventional stressstrain data, and strain localization characterized by strain-fields measurement (DIC – digital image correlation) on the same sample offers a unique opportunity to answer the fundamental questions: (1) when, (2) where, and (3) how the sample was deformed in the laboratory. All the methods above were combined to study the microstructures in naturally and experimentally deformed argillites. Preliminary results are promising and leading toward better understanding of the deformation behavior displayed by argillites in the transition between rocks and soils.

Lessons Learned from Electron Microscopy of Deformed Opalinus Clay

Using a combined approach of ion-beam milling and electron microscopy, we observe, describe and quantify the microstructure of naturally and synthetically deformed Opalinus Clay (OPA) and deduce its microstructural evolution and underlying deformation mechanisms. The investigated samples derive from the so-called Main Fault, a 10 m offset fold-bend thrust fault crossing the Mont Terri Rock Laboratory in the Swiss Jura Mountains. The samples are slightly overconsolidated, experienced a burial depth of 1350 m and a maximum temperature of 55 °C. Most impact on strain is attributed to frictional sliding and rigid body rotation. However, trans-granular fracturing, dissolution-precipitation of calcite, clay particle neoformation and grain deformation by intracrystalline plasticity have a significant contribution to the fabric evolution. The long-term in-situ deformation behavior of OPA is inferred to be more viscous than measured at laboratory conditions.

Overview of Integrated Microstructural Studies to Better Understand Mechanical and Fluid Flow Properties of Fine Grained

A comprehensive understanding of mechanical and fluid flow properties in fine-grained geo-materials like shales requires imaging the microstructure at a range of scales as the microstructures are small and the samples are heterogeneous. Broad Ion Beam (BIB) milling followed by Scanning Electron Microscopy (SEM) imaging provides access to these microstructures at nanometre resolution and large cross sections of up to several mm². Applying BIB-SEM on naturally and experimentally deformed or Wood’s Metal injected samples enables to resolve the related deformation processes and to image the pore connectivity, respectively. The pore fluid distribution, however, can be resolved by integrating BIB-SEM with cryogenic techniques. Examples of such integrated studies indicate calcite in shear fractures in Opalinus Clay and high connectivity in a fine-grained Boom Clay sample.