Controls on fracture distributions within regional, km-scale folds
Sparks, Travis A.
Fractures are ubiquitous features in regions of folded and uplifted rocks. Their occurrence is relevant to hydrocarbon reservoir and water aquifer studies, as well as seismic hazard analysis. This study utilizes detailed fracture density mapping to develop a quantitative model to predict fracture densities in regional folds. Along with fracture density mapping, fracture orientations were analyzed and correlated to folding. The models account for multiple variables including spatial location within the fold, lithotypes, bedding thickness and fault locations. Naturally occurring fractures play a fundamental role in increasing permeability whether it be in a hydrocarbon reservoir or in an aquifer. The morphology and density of these structures controls how effectively fluids can move through the subsurface and the rate at which they can be extracted or injected. Hydraulic fracturing, or “fracking” in the modern vernacular, is a technique used to extract hydrocarbons by injecting over-pressured fluids into the subsurface thus enabling existing or new fractures to form, allowing the release and migration of hydrocarbons into a well bore. Such a process is also used to re-inject waste water into the subsurface. Critical data that can help in evaluating the nature of permeability pathways in the subsurface include detailed analysis of the spatial distribution and characteristics of fractures as observed in natural outcrops. The Arbuckle Mountains, in south-central Oklahoma, provide an excellent natural laboratory to study the spatial and temporal evolution of fractures and faults over a varying range of scales from kilometers to micrometers. The majority of fractures within the Arbuckle anticline can be correlated to folding. Thus the validity of the relationship between the two is enhanced. The Arbuckle Mountains allowed for a wide range of fracture distribution analysis to be integrated within regional folds. These analyses include fracture orientation analysis, detail fracture density mappings, and further analysis on the controls of fracture distributions themselves. Data was collected from 27 outcrops in the Arbuckle anticline including: (a) fracture density, (b) fracture orientations, (c) spatial location of exposure within fold, (d) rock brittleness, (e) bedding thickness, and (f) fault zone locations. From analysis of exposures within the Arbuckle anticline, the developed relationships between fracture distributions and regional folds indicate: (i) Controls on the distribution of fractures within regional folds are seen with the Arbuckle anticline as: (a) spatial distribution of the fracture data set (b) timing of fracture during folding creation (c) the lithologies which the fracture is created in (d) the bedding thickness of those lithologies (e) locality of fault zones near the fracture exposure. Other finding from analysis include: (ii) controls on fracture orientation differ from controls on the fracture density at a given exposure. (iii) Fracture densities and orientations can be modelled and predicted with regional folds.