Theoretical framework for Great Plains fracture generation - Version 2
Draft (feedback welcome, still very much in revision)
Harmon D. Maher Jr., University of Nebraska at Omaha
Introduction: This exercise is driven by the multiple working hypotheses. The question is - what are the various geologic events (triggers) and associated mechanisms that can produce or influence fracture patterns and the history of their development in the Great Plains region? The term fracture is meant to be broadly inclusive - including joints, veins, clastic dikes, and faults. The first three are tensile features, often known as mode 1 failure, whereas faults are shear fractures (which have sliding and tearing modes). Some emphasis is put on tensile fractures in this document since joint sets are so ubiquitous in the Great Plains.
Work from the last decade has brought to light a new suite of brittle failure modes known as deformation bands ( e.g. Fossen et al. 2007, Scholz & Siddharthan 2005 ) that occur in materials with relatively high porosity, such as sandstones and some pyroclastics. Different types of deformation bands are characterized by dilation, by shear, and by compaction, with the existence of hybrids. The primary difference is that tensile and shear fractures are considered surfaces, where as a deformation band is a planar feature with width (and hence has a volume). A related difference is in the tip propagation mechanics. With a deformation band typical linear elastic mechanics do not apply, and instead plastic yielding occurs. It is argued that in some cases deformation bands are precursors to joints and faults. Recent experimental work by Jocund (2011) distinquishes between mode 1 fractures and dilatancy bands, the latter of which is characterized by plumose morphology. Interestingly, Jocund et al. (2011) also found that dilatancy bands can form even with a slightly compressive sigma three. Another important difference between fracturing in porous materials is that typical Mohr Coulomb failure criteria that apply to crystalline materials do not apply. For the time being deformation bands are not considered in this document, but a separate but parallel document is being developed to describe these.
It is important to recognize that the geologic triggering events explored herein are not mutually exclusive and that some temporal and causal linkages are likely between them. It is also certain that the diverse array of fracture systems in the Great Plains are polygenetic. A focus of this document is on establishing criteria that will help generation mechanisms and formation times to be assigned to different fracture sets identified in the Great Plains Fracture Study, and also to better understanding these fractures in a coherent fashion.
Loading paths and geologic triggers: From a mechanical point it makes sense to consider the loading paths that lead to failure (Engelger 1985; Engelder & ) as a classification criteria. These loading paths can then be applied to different failure criteria. When trying to understand fractures from a historical and contextual perspective it makes sense to consider geologic events that trigger the fracture formation (e.g. such as differential compaction0. A more sophisticated understanding can be developed by considering both, which this document attempts to do.
Using the Mohr diagram and failure envelopes as an initial guide, three end-member ways to take a stable stress state and have it evolve so that it intersects the failure envelope (with a tensile cutoff included) is to: a) increase deviatoric stress, b) shift the stress circle to the left, c) or shift the Mohr envelope. Naturally, these can be done in tandem. A variety of processes including changing fluid pressures, changing rock stresses, changing elastic material moduli, can produce different loading paths. Figure 1 attempts to capture these possibilities.
The critical loading path can be visualized of as the evolution of the Mohr circle so that it intersects the failure envelope. Therefore, a significant part of modeling loading paths is the form/position of the failure envelope. Failure criteria can be sensitive to loading and environmental conditions. For example, sub-critical fracture growth is where fractures grow more slowly and at differential stress lower than otherwise due to interaction between fluid in the cracks, the rock and the stresses at the tip crack. A fracture tip corrosion and weakening can occur, allowing the fracture to propagate, but since it involves chemical processes (such as diffusion) this process is time dependent. Surface features on tensile fractures are suggested to inform as to the nature of this sub-critical fracture propagation. Jocund et al. (2011) suggest that loading conditions contribute to whether a true mode 1 feature or a dilation band forms. One approach may be to consider that different failure criteria exist for time independent and time dependent failure criteria. One way of thinking of this is that the failure envelope moves and reshapes as a function of the stress evolution and geologic history.
Fig. 1: Classification scheme for fractures(tensile and shear) based on possible loading paths, with associated possible geologic events that can theoretically produce that loading path and lead to failure in blue. Some of these loading paths require very specific circumstances and rarely occur, while others are much more common.
Increasing pore pressure and decreasing the effective stress (shifting the Mohr circle to the left) is argued to be a critical process in many loading paths to failure. A question that then arises is what geologic event caused the increase in pore pressure and hence the fracturing? Pore pressure can be greatly increased in confined situations by heating the water, by chemical reactions that produce water (think of baking soda and vinegar in simple play rockets), and by new stresses. In turn a variety of geologic processes can cause the heating, or reactions or new loads that increase the pore pressure. Fig. 1 attempts to capture possibilities, some of which are much more likely than others. The classification structure for fractures used in this document can be thought of as intersection matrix with hierarchal loading path classification on the one side, and geologic trigger events on the other (e.g. thermal aquapressurization intersecting with intrusion). An Excel sheet that accompanies this document expands on Figure 1 by listing traits for each of 30 some odd fracture. The hope is that an assemblage of traits is distinctive enough to allow confident assignment to a particular fracture class.
It is important to realize that a geologic event may influence loading paths in multiple ways. For example, a change in the tectonic stress field can both increase the differential stress and increase the pore pressure in confined units. Burial induces a new vertical load, can increase pore pressure in confined systems, can induce diagenesis and produce syneresis, in addition to changing material properties such as PoissonÕs ratio, and cause slow heating as the geothermal gradient equilibrates with the new sediment. The fracture event itself can change fluid flow paths, reduce pore pressures, changes bulk material properties. These combinations, linkages and feedback guarantee a great diversity of fracture sets and histories. The utility of the classification here is possibly in providing components from which to build a more realistic geologic model for a given fracture set.
The diagram above uses loading paths as a primary consideration. However, most fracture studies occur in a a distinct geologic context. It is therefore more efficient to consider different possible geologic trigger events and the various loading paths they may produce, which is the approach taken below. The following geologic triggers are considered in distinct sections (as individual pdfs), along with sections on potentially diagnostic traits and timing considerations.
I. Impact Related.