Quartz c-axis orientation patterns in fracture cement as a measure of fracture opening rate and a validation tool for fracture pattern models

Abstract

We evaluate a published model for crystal growth patterns in quartz cement in sandstone fractures by comparing crystal fracture-spanning predictions to quartz c-axis orientation distributions measured by electron backscatter diffraction (EBSD) of spanning quartz deposits. Samples from eight subvertical opening-mode fractures in four sandstone formations, the Jurassic-Cretaceous Nikanassin Formation, northwestern Alberta Foothills (Canada), Cretaceous Mesaverde Group (USA; Cozzette Sandstone Member of the Iles Formation), Piceance Basin, Colorado (USA), and upper Jurassic-lower Cretaceous Cotton Valley Group (Taylor sandstone) and overlying Travis Peak Formation, east Texas, have similar quartzose composition and grain size but contain fractures with different temperature histories and opening rates based on fluid inclusion assemblages and burial history. Spherical statistical analysis shows that, in agreement with model predictions, bridging crystals have a preferred orientation with c-axis orientations at a high angle to fracture walls. The second form of validation is for spanning potential that depends on the size of cut substrate grains. Using measured cut substrate grain sizes and c-axis orientations of spanning bridges, we calculated the required orientation for the smallest cut grain to span the maximum gap size and the required orientation of the crystal with the least spanning potential to form overgrowths that span across maximum measured gap sizes. We find that within a 10° error all spanning crystals conform to model predictions. Using crystals with the lowest spanning potential based on crystallographic orientation (c-axis parallel to fracture wall) and a temperature range for fracture opening measured from fluid inclusion assemblages, we calculate maximum fracture opening rates that allow crystals to span. These rates are comparable to those derived independently from fracture temperature histories based on burial history and multiple sequential fluid inclusion assemblages. Results support the R. Lander and S. Laubach model, which predicts that for quartz deposited synchronously with fracture opening, spanning potential, or likelihood of quartz deposits that are thick enough to span between fracture walls, depends on temperature history, fracture opening rate, size of opening increments, and size, mineralogy, and crystallographic orientation of substrates in the fracture wall (transected grains). Results suggest that EBSD maps, which can be more rapidly acquired than measurement of tens to hundreds of fluid inclusion assemblages, can provide a useful measure of relative opening rates within populations of quartz-filled fractures formed under sedimentary basin conditions. Such data are useful for evaluating fracture pattern development models.

Three‐Dimensional Phase‐Field Investigation of Pore Space Cementation and Permeability in Quartz Sandstone

“…Further, after carefully analyzing different stages of unitaxial growth for c / a = 3.0 as shown in Figure c, we deduce that during initial stages, the growth competition is not intense but temporally increases as evident in the later stages from the overgrowth of less misoriented (greenish) grains over the highly misoriented (reddish and bluish) ones, eventually resulting in the formation of well‐defined quartz bridges. These results are in qualitative agreement with experimentally synthesized quartz veins (Okamoto & Sekine, ) as well as natural samples (Ukar et al, ), both of whom reported that bridging crystals were commonly oriented subnormal to the vein wall. Based on inferences drawn from the numerical studies and correlations with previous findings, it is reasonable to choose c / a = 3.0 for the 3‐D phase‐field modeling of quartz cementation in sandstones, presented in forthcoming sections.…”

Quartz Vein Formation and Deformation during Porphyry Cu Deposit Formation: A Microstructural and Geochemical Analysis of the Butte, Montana, Ore Deposit

“…In the process of geometric selection, an initial nucleation along a substrate creates a layer of randomly oriented crystals nucleated on the fracture surface (Figure 10(a), step 1) [60,61]. Crystals that happen to be oriented with c-axes perpendicular to the fracture surface outgrow the others because their tips protrude into the cavity where the supply of silica from the fluid is most readily available (Figure 10(a), step 2), resulting in a layer of large, subparallel crystals (Figure 10(a), step 3) [62,63]. This creates cockscomb quartz, which can form from fluids at low degrees of quartz supersaturation that undergo either slow or mild fluctuations in growth conditions such as pressure or temperature [59,64,65].…”

The Role of Chemistry in Fracture Pattern Development and Opportunities to Advance Interpretations of Geological Materials

“…The literature on these measurements is extensive (e.g., Aydin, ; Dershowitz et al, ; Dershowitz & Herda, ; Iñigo et al, ; La Pointe & Hudson, ; Laubach & Ward, ; Marrett et al, ; Narr et al, ; Sanderson & Nixon, ; Tavani et al, ; Ukar et al, ; Wennberg et al, ).…”