Insights into rates of fracture growth and sealing from a model for quartz cementation in fractured sandstones

Abstract

A new model accounts for crystal growth patterns and internal textures in quartz cement in sandstone fractures, including massive sealing deposits, thin rinds or veneers that line open fracture surfaces, and bridge structures that span otherwise open fractures. High-resolution cathodoluminescence imaging of bridge structures and massive sealing deposits indicates that they form in association with repeated micron-scale fracturing of growing quartz crystals, whereas thin rinds do not. Model results indicate that the three morphology types develop in response to (1) the ratio of the rates of quartz growth to fracture opening and (2) the substantially faster growth rate that occurs on noneuhedral surfaces in certain crystallographic orientations compared to euhedral crystal faces. Rind morphologies develop when the fracture opening rate exceeds two times the fastest rate of quartz growth (along the c axis on noneuhedral surfaces) because growing crystals develop slow-growing euhedral faces. Massive sealing, on the other hand, develops where the net rate of fracture opening is less than twice the rate of quartz growth on euhedral faces because all quartz growth surfaces along the fracture wall seal the fracture between fracturing events. Bridge structures form at fracture opening rates that are intermediate between the massive sealing and rind cases and are associated with crystallographic orientations that allow growth to span the fracture between fracturing events. Subsequent fractures break the spanned crystal, introducing new, fastgrowing noneuhedral growth surfaces where quartz grows more rapidly compared to the euhedral faces of nonspanning crystals. As the ratio of fracture opening to quartz growth rate increases, the proportion of overgrowths that span the fracture decreases, and the range in c-axis orientations for these crystals comes progressively closer to perpendicular to the fracture wall until the maximum spanning limit is reached. Simulation results also reproduce "stretched crystal," "radiator structure," and "elongate blocky" textures in metamorphic quartz veins.The model replicates a well-characterized quartz bridge from the Cretaceous Travis Peak Formation as well as quartz cement abundances, internal textures, and morphologies in the sandstone host rock and fracture zone using the same kinetic parameters while honoring fl uid-inclusion and thermal-history constraints. The same fundamental driving forces, in both in the host rock and fracture system, are responsible for quartz cementation, with the only signifi cant difference within the fracture zone being the creation of new pore space as well as new noneuhedral surfaces for cases where overgrowths span fractures between fracturing events. Rates of fracture growth and sealing may be inferred from fracture cement textures using model results.

Modeling fracture cementation processes in calcite limestone: a phase-field study

“…The veins are open at all stages of time and the resulting crystals are euhedral in nature with flat facets and sharp corners, as depicted at t = 5985 in Fig. 4d, g. The texture resembles rind morphologies described in Lander and Laubach (2015). In general, the aforementioned simulated results for calcite cement growth in fractures, depending upon fracture opening rate and crystal orientations are in good agreement with natural and experimental results, presented previously for quartz cements in fractures (Lander and Laubach 2015).…”

“…1a) are characterized by parallel grain boundaries occurring as thin fibers of different lengths and high length-to-width ratios. Figure 1b, Lander and Laubach (2015)]. Here, euhedral crystal faces border some porosity within the partially open fracture (Fig.…”

“…During synkinematic cementation, a complicated interplay of the continued crack opening and crystal growth occurs. This interplay determines the morphological attributes of the resulting veins based on the crack opening rates (Mügge 1928;Urai et al 1991) and was shown to affect quartz-bearing veins (e.g., Laubach 1988;Lander and Laubach 2015), calcite veins (Hilgers et al 2001), and carbonate veins in dolostones (Gale et al 2010). Carbonate vein textures can also be subject to an interplay of synand postkinematic cementation (Ukar and Laubach 2016).…”

Predictability of properties of a fractured geothermal reservoir: the opportunities and limitations of an outcrop analogue study

“…the area underwent a multiphase deformation history, and is characterized by sedimentary rocks that show strong facies changes at a small scale. Given, however, a less complex setting, knowledge of original rock composition, thermal history, and fracture timing should improve comparison and prediction of cementation (Lander et al 2008;Lander and Laubach 2015) and thus allow mechanical rock properties to be better determined.…”

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…Such models are enticing because they have the potential to predict textures and morphologies that may be compared directly with data from natural samples and laboratory experiments and because they serve as a more rigorous basis for predicting a wide range of rock properties compared with the model types discussed above. To date, most grain/pore scale simulation studies either have involved imposing a predefined diagenetic state as a means to rigorously simulate the impact of diagenetic alteration on bulk rock properties (Bakke & Øren 1997;Øren & Bakke 2002Øren & Bakke , 2003Øren et al 2007;Jin et al 2012;Mousavi & Bryant 2013;Prodanovic et al 2013;van der Land et al 2013;Hosa & Wood 2017) or consider a subset of the diagenetic processes that affect reservoir quality (Abe & Mair 2005;Lander et al 2008a;Marketos & Bolton 2009;Gale et al 2010;Cheung et al 2013;Ankit et al 2015;Lander & Laubach 2015;Wendler et al 2016). Although the promise of models of this class is enormous, considerable work is needed to develop comprehensive modelling approaches that are capable of making accurate pre-drill predictions.…”