Fossil overpressures compartments? A case study from the Eifel area and some general aspects

Fluid overpressures are well known from hydrocarbon exploration in many sedimentary basins. They can reach almost lithostatic values, and may cause the fracturing of rock. Fracturing allows the discharge of fluid overpressure, and fluid flows along a hydraulic gradient towards a low pressure reservoir. Different mechanisms may cause the precipitation from the fluid, such as a fluid pressure drop, a variation of temperature at the low pressure reservoir, or a different rock type inducing different Eh-pH conditions. Such precipitates in fractures are called veins, which often display paleo-fluid overpressures in rocks. In this study, we present some results from Devonian clastic sedimentary rocks of the Eifel area. Results are compared with other sedimentary basins to highlight some general aspects.conferenc

Clay smear: Review of mechanisms and applications

AbstractClay smear is a collection of fault processes and resulting fault structures that form when normal faults deform layered sedimentary sections. These elusive structures have attracted deep interest from researchers interested in subsurface fluid flow, particularly in the oil and gas industry. In the four decades since the association between clay-smear structures and oil and gas accumulations was introduced, there has been extensive research into the fault processes that create clay smear and the resulting effects of that clay smear on fluid flow. We undertake a critical review of the literature associated with outcrop studies, laboratory and numerical modeling, and subsurface field studies of clay smear and propose a comprehensive summary that encompasses all of these elements. Important fault processes that contribute to clay smear are defined in the context of the ratio of rock strength and in situ effective stresses, the geometric evolution of fault systems, and the composition of the faulted section. We find that although there has been progress in all avenues pursued, progress has been uneven, and the processes that disrupt clay smears are mostly overlooked. We highlight those research areas that we think will yield the greatest benefit and suggest that taking these emerging results within a more process-based framework presented here will lead to a new generation of clay smear models

Kinematics of Crystal Growth in Single‐Seal Syntaxial Veins in Limestone ‐ A Phase‐Field Study

Building on recent developments in phase-field modeling of structural diagenesis, we present an analysis of single-seal syntaxial calcite vein microstructure in a variety of limestones. We focus on the effects of fracture aperture, intergranular versus transgranular fracturing, crystal habit and the presence of second phases in the host rock, to systematically investigate a simplified set of models covering the main classes of limestone in 2D. We incorporate the kinematic process of growth competition between differently oriented crystals, growth rate anisotropy between rough and faceted crystal surfaces and different growth rates on intergranular to transgranular fractures. Results show that within the considered parameter space we can reproduce a wide range of vein microstructures in limestone known in nature, such as stretched crystals, wide-blocky veins, and elongated crystals. We identify five archetypes of vein microstructures in limestones, which are diagnostic for different kinematics and evolution of transport processes and illustrate the effect of key parameters in microstructure maps. We show how syntaxial veins with median line form after intergranular fracturing, while stretched crystals indicate transgranular fracturing. Intergranular fracturing leads to stronger growth competition and more prominent CPO in syntaxial veins. Our results can be extended to 3D to include multiple crack-seal events, pore-space cementation and simulation of fluid flow, providing a generic platform for modeling structural diagenesis in limestones

Incomplete Crack Sealing Causes Localization of Fracturing in Hydrothermal Quartz Veins

Cyclic microfracturing and epitaxial crystal growth have long been recognized in crack‐seal veins, but an understanding of a single crack‐seal cycle is still missing. Here we present a phase‐field model that includes both fracture mechanics of crack propagation, and epitaxial crystal growth on the fracture walls, repeating this cycle multiple times in a polycrystalline, microporous quartz rock. Our simulations have two end members: If a vein completely seals, it is stronger than the host rock, cracking is delocalized, forming many single‐seal microveins. Incomplete sealing makes the vein weaker than the host rock and localizes the new fracture inside the vein, leading to multi‐crack‐seal. We suggest that the sealing degree is a key parameter in hydrothermal systems and multi‐crack‐seal veins are long‐lived, microporous sites of mechanical weakness. We generalize the phase‐field approach to conduct probabilistic simulations in between these two types, and show how systems of microveins and multi‐crack‐seal veins emerge.Plain Language Summary: Fluids in the Earth's crust can alter permeability and porosity, precipitate and dissolve minerals, transport material and interact with deformation. This affects the transport and mechanical properties of the rock system and in turn has consequences for example, in subsurface engineering applications. In this work we simulate the processes of fracturing and crystal growth on grain scale in a microporous rock structure and show how different crystal structures form. The basic steps of a crack‐seal process and how fracturing and sealing interact are explored. Our results show that if a fracture completely seals a new crack will form in the host rock and many thin microveins form. In contrast, an incomplete sealing makes the vein weaker than the host rock and leads to a new cracking inside the vein, which enlarges the existing structure with each cycle. This implies that the degree of sealing is the cause of this division, where crack‐seal veins are microporous sites of mechanical weakness. Additionally, we perform probabilistic simulations which show how many single‐seal microveins form side‐by‐side with a few multi‐crack‐seal veins. Our studies provide valuable insight in structure‐property linkages and enable a better prediction of fracture‐sealing.Key Points: Systematic phase‐field study captures elementary steps of the crack‐seal process at grain scale. Incomplete sealing makes a vein weaker than the host rock and localizes a new fracture inside the vein which leads to multi‐crack‐seal. Probabilistic simulations show how systems of many microveins and a few thick crack‐seal veins form side by side.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659https://doi.org/10.5281/zenodo.633765