Scaling carbon sequestration phenomena to the earth's surface

We present a computational framework for fast monitoring of fault stability and ground deformation in multiphase geomechanics and demonstrate its efficacy for a carbon sequestration--enhanced oil recovery case study. The staggered solution algorithm for the coupled problem is augmented with a feature that allows for the flow and geomechanics sub-problems to be solved on different unstructured tetrahedral grids. For the field scale problem, the geomechanics grid goes all the way to the free surface while the flow grid is truncated at a depth above which the layers are impermeable. This framework avoids the unnecessary computational burden associated with equilibrating the initial pressure solution in the overburden, allows for a study of the critical interaction between overburden and faults, allows for fast renditions of ground deformation, and allows a choice of resolution for the flow and geomechanics grids independently to capture disparate length scales of the underlying physics.

Pre-folding fracturing in a foredeep environment: insights from the Carseolani Mountains (central Apennines, Italy)

Bedding-perpendicular joints striking parallel (longitudinal) and perpendicular (transverse) to both the axis of the hosting anticline and the trend of the foredeep-belt system are widely recognized in fold-and-thrust belts. Their occurrence has been commonly attributed to folding-related processes, such as syn-folding outer-arc extension, although they can also be consistent with a pre-folding foredeep-related fracturing stage. Here we report the pre-folding fracture pattern affecting the Pietrasecca Anticline, in the central Apennines (Italy), resolved by a detailed field structural analysis. Field observations, scan-lines and interpretation of virtual outcrops were used to study the intensity, distribution and the orientations of fracture pattern along the anticline. The fracture pattern of the Pietrasecca Anticline consists of longitudinal and transverse joints, oriented approximately perpendicular to bedding, and of a pre-folding longitudinal pressure-solution cleavage set, which is oblique to bedding regardless of the bedding dip. Cross-cutting relationships show that joints predated the development of the pressure-solution cleavage. Furthermore, joint intensity does not relate to the structural position along the anticline. Taken together, these observations suggest that jointing occurred in a foredeep environment before the Pietrasecca Anticline growth. Our work further demonstrates that joints striking parallel and orthogonal to the main fold axis do not necessarily represent syn-folding deformation structures.

Surface microstructures developed on polished quartz crystals embedded in wet quartz sand compacted under hydrothermal conditions

Intergranular pressure solution plays a key role as a deformation mechanism during diagenesis and in fault sealing and healing. Here, we present microstructural observations following experiments conducted on quartz aggregates under conditions known to favor pressure solution. We conducted two long term experiments in which a quartz crystal with polished faces of known crystallographic orientation was embedded in a matrix of randomly oriented quartz sand grains. For about two months an effective axial stress of 15 MPa was applied in one experiment, and an effective confining pressure of 28 MPa in the second. Loading occurred at 350 °C in the presence of a silica-saturated aqueous solution. In the first experiment, quartz sand grains in contact with polished quartz prism ($$\overline10{1 }0$$ 1 ¯ 010 ) faces became ubiquitously truncated against these faces, without indenting or pitting them. By contrast, numerous sand-grain-shaped pits formed in polished pyramidal ($$17\overline{6 }3$$ 17 6 ¯ 3 ) and ($$\overline{4 }134$$ 4 ¯ 134 ) crystal faces in the second experiment. In addition, four-leaved and (in some cases) three-leafed clover-shaped zones of precipitation formed on these prism faces, in a consistent orientation and pattern around individual pits. The microstructures observed in both experiments were interpreted as evidence for the operation of intergranular pressure solution. The dependence of the observed indentation/truncation microstructures on crystal face orientation can be explained by crystallographic control of stress-induced quartz dissolution kinetics, in line with previously published experimental and petrographic data, or possibly by an effect of contact orientation on the stress-induced driving force for pressure solution. This should be investigated in future experiments, providing data and microstructures which enable further mechanism-based analysis of deformation by pressure solution and the effect of crystallographic control on its kinetics in quartz-rich sands and sandstones.

What process causes the slowdown of pressure solution creep

The slowdown of pressure solution creep has been thought to be caused by stress redistribution. This study presents a fresh view towards this creep behaviour. Basically, two rate-limiting mechanisms come into play amid pressure solution creep: (1) stress redistribution across expanding inter-granular contacts and (2) solute accumulation in the water film. Because non-hydrostatic dissolution occurs under open system conditions, solute accumulation in the water film is constrained by the ensuing solute transport process. Relying on the matter exchange across the contact surface boundary, the active processes in the voids, e.g., solute migration and deposition, affect pressure solution creep. Based upon the above, we sum up two requirements that have to be met for achieving chemical compaction equilibrium: (1) the Gibbs free energy of reaction, i.e., the driving force of non-hydrostatic dissolution process, gets depleted and (2) the concentration gradient between the water film and surrounding pore water vanishes. Highlights The slowdown of pressure solution creep is a combined result of stress migration across contacts and solute accumulation in the water film. Matter exchange with the surroundings inhibits solute accumulation in the water film. This article identifies two prerequisites that need to be fulfilled for achieving chemical compaction equilibrium.

Stylolite in Upper Cretaceous Carbonate Reservoirs from Northwestern Iraq

Stylolites are commonly observed in the carbonate reservoirs in various oilfield of Iraq including those of upper Cretaceous successions from northwestern Iraq, where they are characterized by stylolite-rich zones in the Cenomanian-early Turonian Gir Bir Formation and to a lesser extent in the Turonian-Santonian Wajna and early Campanian Mushorah formations respectively. The observed stylolites are either large to be identified in the core samples or smaller ones that are well observed in the thin sections and are characterized by variations in amplitude, morphology and accumulated insoluble residues. The recorded stylolites are classified as hummocky, irregular, low and high-amplitudes peaks, and irregular anastomosing stylolites. Stylolites affect the porosity permeability and thickness reduction compaction as the main chemical compaction (pressure solution) that reduce porosity. Whereas, in other places, the stylolites act as seals and stop the upward movement of hydrocarbons. This is also seen for mineralization processes such as silicification that ended near the stylolite surfaces.

Proposed Model for Shale Compaction Kinetics

Shales are the most abundant class of sedimentary rocks, distinguished by being very fine-grained, clayey, and compressible. Their physical and chemical properties are important in widely different enterprises such as civil engineering, ceramics, and petroleum exploration. One characteristic, which is studied here, is a systematic reduction of porosity with depth of burial. This is due increases in grain-to-grain stress and temperature. Vertical stress in sediments is given by the overburden less the pore fluid pressure, σ, divided by the fraction of the horizontal area which is the supporting matrix, (1−φ), where φ is the porosity. It is proposed that the fractional reduction of this ratio, Λ, with time is given by the product of φ4m/3, (1−φ)4n/3, and one or more Arrhenius functions Aexp(−E/RT) with m and n close to 1. This proposal is tested for shale sections in six wells from around the world for which porosity-depth data are available. Good agreement is obtained above 30–40 °C and fractional porosities less than 0.5. Single activation energies for each well are obtained in the range 15–33 kJ/mole, close to the approximate pressure solution of quartz, 24 kJ/mol. Values of m and n are in the range 1 to 0.8, indicating nearly fractal water-wet pore-to-matrix interfaces at pressure solution locations. Results are independent of over- or under-pressure of pore water. This model attempts to explain shale compaction quantitatively. For the petoleum industry, given porosity-depth data for uneroded sections and accurate activation energy, E, paleo-geothermal-gradient can be inferred and from that organic maturity, indicating better drilling prospects.

Elastic strains of quartz inclusions and microstructures from pressure solution in garnet reveal orientation and low magnitude of differential stress during subduction metamorphism

<p>Determining the stress state during metamorphism is a key challenge in metamorphic petrology as the effect of differential stress on metamorphic reactions is currently debated. Conventional piezometry generally gives stresses that correspond to overprinting deformation rather than to mineral growth of high-grade metamorphism, so an alternative approach is required. Garnetite lenses from the ultrahigh-pressure, low-temperature metamorphic Lago di Cignana unit (Western Alps, Italy) record compaction by a high degree of mineral dissolution in the fluid-rich environment of a cold subduction zone. This work combines microstructural analysis of deformed garnet with elastic strains of quartz inclusions to study the stresses in these metasedimentary rocks.</p><p>Garnet exhibits abundant evidence for incongruent pressure solution (IPS), most notably as truncated zones that mismatch across grain boundaries, interlocking structures, and shape-preferred orientation (SPO). The gap in garnet compositions represented by overgrown truncated zonation corresponds to undeformed garnet with inclusions of quartz and coesite, indicating that IPS operated during prograde to peak metamorphism. The distribution of aspect ratios in the garnet grain population suggests that pressure solution preferentially affected smaller grains. SPO analysis of many subregions across a garnetite sample reveals a complex distribution, however the local SPO is consistent with the stress orientation expected for local microstructures such as layering, garnet stacks, or fine-grained internal fluid pathways. Locally, two different preferential orientations are observed, interpreted as the result of two subsequent deformation stages under different stress configurations.</p><p>Quartz inclusions in prograde euhedral garnet, grown on the outer margin of coevally deformed garnetite, were analysed with Raman spectroscopy. Elastic strains obtained for these inclusions are in agreement with predicted strains for entrapment along the prograde <em>P</em>-<em>T</em> path for the Lago di Cignana unit (~1.5–2.0 GPa; ~450–500 °C), whereas significant differential stress during entrapment is expected to result in deviating strain components.</p><p>By combining microstructural analysis of garnet with elastic-strain analysis of quartz inclusions, stress orientations obtained from deformed garnet are combined with the stress magnitude for coeval garnet growth. The results indicate that the garnetite lenses were deformed and metamorphosed under low differential stress of variable orientation during subduction. These results are in agreement with a system where garnet is wet by a fluid phase that allows IPS.</p><p> </p><p>Acknowledgements: This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro)</p>

Development of an intra-carbonate detachment during thrusting: The variable influence of pressure solution on deformation style, Khao Khwang Fold and Thrust Belt, Thailand

Classic detachment zones in fold and thrust belts are generally defined by a weak lithology (typically salt or shale), often accompanied by high over-pressures. This study describes an atypical detachment that occurs entirely within a relatively strong Permian carbonate lithology, deformed during the Triassic Indosinian orogeny in Thailand under late diagenetic-anchimetamorphic conditions. The key differences between stratigraphic members that led to development of a detachment zone are bedding spacing and clay content. The lower, older, unit is the Khao Yai Member (KYM), which is a dark-gray to black, well-bedded, clay-rich limestone. The upper unit, the Na Phra Lan Member (NPM), comprises more massive, medium- to light-gray, commonly recrystallized limestones and marble. The KYM displays much tighter to even isoclinal, shorter-wavelength folds than the NPM. Pressure solution played a dominant role throughout the structural development—first forming early diagenetic bedding; later tectonic pressure solution preferentially followed this bedding instead of forming axial planar cleavage. The detachment zone between the two members is transitional over tens of meters. Moving up-section, tight to isoclinal folds with steeply inclined axial surfaces are replaced by folds with low-angle axial planes, thrusts, and thrust wedging, bed-parallel shearing, and by pressure solution along bedding-parallel seams (that reduce fold amplitude). In outcrops 100–300 m long, reduction of line-length shortening on folds from >50% to <10% shortening upwards indicates that deformation in the NPM is being accommodated differently from the KYM, probably predominantly by shortening on longer wavelength and/or spacing folds and thrusts, given the low amount of strain observed within the NPM, which excludes widespread layer-parallel thickening