Vein microstructures contain a wealth of information on coupled chemical and mechanical processes of fracturing, fluid transport, and crystal growth. Numerical simulations have been used for exploring the factors controlling the development of vein microstructures; however, they have not been quantitatively validated against natural veins. Here we combined phase-field modeling with microtextural analysis of previously unexplained wide-blocky calcite veins in natural limestone and of the fresh fracture surface in this limestone. Results show that the wide-blocky vein textures can only be reproduced if ~10%–20% of crystals grow faster than the rest. This fraction corresponds to the amount of transgranularly broken grains that were observed on the experimental fracture surfaces, which are dominantly intergranular. We hypothesize that transgranular fractures allow faster growth of vein minerals due to the lack of clay coatings and other nucleation discontinuities that are common along intergranular cracks. Our simulation results show remarkable similarity to the natural veins and reproduce the nonlinear relationship between vein crystal width and vein aperture. This allows accurate simulations of crystal growth processes and related permeability evolution in fractured rocks.

We developed a generalized multiphase-field modeling framework for addressing the problem of brittle fracture propagation in quartz sandstones at microscopic length scale. Within this numerical approach, the grain boundaries and crack surfaces are modeled as diffuse interfaces. The two novel aspects of the model are the formulations of (I) anisotropic crack resistance in order to account for preferential cleavage planes within each randomly oriented quartz grain and (II) reduced interfacial crack resistance for incorporating lower fracture toughness along the grain boundaries that might result in intergranular crack propagation. The presented model is capable of simulating the competition between inter- and transgranular modes of fracturing based on the nature of grain boundaries, while exhibiting preferred fracturing directions within each grain. In the full parameter space, the model can serve as a powerful tool to investigate the complicated fracturing processes in heterogeneous polycrystalline rocks comprising of grains of distinct elastic properties, cleavage planes, and grain boundary attributes. We demonstrate the performance of the model through the representative numerical examples.

Liassic limestones at the Somerset coast (UK) contain dense arrays of calcite microveins with a common but poorly understood microstructure, characterized by laterally wide crystals that form bridges across the vein. This paper investigates the formation mechanisms and evolution of these wide-blocky vein microstructures by a combination of high-resolution analytical methods (ViP microscopy, optical CL and SEM techniques (EDS, BSE, CL and EBSD)), laboratory experiments and phase-field modelling.Results indicate that the studied veins formed in open, fluid-filled fractures, each in a single opening and sealing episode. As shown by optical and EBSD images, vein crystals grow epitaxially on wall-rock grains and we hypothesize that their growth rates differ depending on whether crystals are substrated on wall-rock grains that are fractured intergranularly (slow) or transgranularly (fast). Phase-field models support this hypothesis, showing that wide-blocky crystals only form in cases with significant growth rate differences that are dependent on the type of seed grain.This provides strong evidence for “extreme growth competition”, a process, which we propose controls vein filling in many micritic carbonate reservoirs, as well as demonstrating that the characteristics of the fracture wall can affect filling processes in syntaxial veins.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5172371