Abstract
In fold-and-thrust belts rocks undergo deformation as fold geometries evolve. Deformation may be accommodated by brittle fracturing, which can vary depending on structural position. We use 2D forward modelling and 3D restorations to determine strain distributions throughout folds of the Achnashellach Culmination, Moine Thrust Belt, NW Scotland. Fracture data is taken from the Torridon Group; a thick, coarse grained fluviatile sandstone deposited during the Proterozoic. Modelling infers a correlation between strain and simple curvature; we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt.In high curvature regions, such as forelimbs, fracture intensities are high and fractures are short and oriented parallel to fold hinges. In low curvature regions fractures have variable intensities and are longer. Fracture orientations in these regions are scattered and vary over short distances. These variations do not relate to strain; data suggests lithology may influence fracturing. The strain history of fold structures also influences fracturing; structures with longer deformation histories exhibit consistent fracture attributes due to moderate-high strain during folding, despite present day low curvature. This is in contrast to younger folds with similar curvatures but shorter deformation histories. We suggest in high strain regions fracturing is influenced by structural controls, whereas in low strain regions lithology becomes more important in influencing fracturing.
Highlights
Fractures in fold and thrust belts are often thought to form synchronously with folding and fracture pattern variations may be expected to relate to structural position on the fold
We suggest that in deformed regions such as folds, lithology may still influence fracture intensity but structural position becomes increasingly important as strain increases
Lithology may play a role in influencing fracture development, and the importance of lithology varies depending on structural position
Summary
Fractures in fold and thrust belts are often thought to form synchronously with folding and fracture pattern variations may be expected to relate to structural position on the fold. Understanding the mode of fold formation and strain history is critical for prediction of fracture attribute variations, which can be used in a range of applications, including fractured reservoir exploration, carbon capture and storage, aquifer characterisation and civil and mining engineering. Many studies have been conducted to investigate how fracture attributes vary in carbonate thrust belts for hydrocarbon exploration, such as in the Zagros foldand-thrust belt of Iran (McQuillan, 1973, 1974; Wennberg et al, 2006; Wennberg et al, 2007; Awdal et al, 2013), the Italian Apennines (Storti and Salvini, 2001), the Rocky Mountains of the USA and Canada (Ghosh and Mitra, 2009; Barbier et al, 2012) and the Northeastern Brooks Range, Alaska (Hanks et al, 1997). This paper contributes to the limited studies on fracturing in sandstone thrust belts, using the Torridon Group of the Moine Thrust Belt, NW Scotland as an analogue for a tight fractured sandstone reservoir in a fold and thrust belt
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