Browsing by Subject "Fracture growth"

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A fluid inclusion and cathodoluminescence approach to reconstruct fracture growth in the Triassic-Jurassic La Boca Formation, Northeastern Mexico
(2011-12) Kaylor, Autumn Leigh; Laubach, Stephen E. (Stephen Ernest), 1955-; Eichhubl, Peter; Catlos, Elizabeth; Ketcham, Richard
Opening-mode fracture shapes are typically the result of brittle deformation and proportional growth in fracture height, length, and width. Based on the typical fracture shape, it is assumed that fracture tips are free to propagate in all directions. Some natural rock fractures have been shown to form as a result of slow non-elastic deformation processes. Such fractures may propagate to a finite length or height and accommodate further growth by aperture widening only. To determine the growth conditions of a fracture in the Triassic-Jurassic La Boca Formation of northeastern Mexico and to test fracture growth models, I combined fluid inclusion microthermometry and SEM-based cathodoluminescence cement texture analysis to determine the relative timing of fracture cement precipitation and related fracture opening for five samples collected along its trace. Fracture growth initiated at a minimum age of 70 Ma as two separate fractures with branching fracture tips that coalesced to a single continuous fracture under prograde burial conditions at a minimum age of 54 Ma. At this stage, fracture growth was accommodated by both propagation (i.e. increase in trace length) and by an increase in aperture during maximum burial and early exhumation. Samples collected at the fracture tips recorded temperatures reflecting fracture opening starting with maximum burial at a minimum age of 48 Ma at one tip and of 38 Ma at the other tip. Synkinematic fluid inclusions in crack-seal cement track continued fracture opening close to the fracture tips without a concurrent increase in trace length after 38 Ma until about 21 Ma. I attribute the observed change in fracture growth mechanism to a change in material response. The stage in aperture increase without propagation corresponds to an increase in elastic compliance or in non-elastic flow properties. Non-elastic flow can be attributed to solution-precipitation creep of the host rock. Dissolution of host quartz grains and subsequent quartz precipitation is consistent with the abundance of quartz fracture cement formed during exhumation. Cement textures from fractures in the La Boca Formation mimic those found in subsurface core, which allows application of the results to a variety of geologic environments.
Fracture Growth Kinematics in Tight Sandstone Reservoirs
(2014-08) Alzayer, Yaser Abdullah; Eichhubl, Peter
Opening-mode fractures—joints and veins—are widespread structures in sedimentary rocks even in slightly deformed and flat-lying sequences. Understanding the growth and connectivity of fractures in low permeability sandstone reservoirs is essential for optimal hydrocarbon exploitation. In a linear elastic fracture mechanics framework, it is generally assumed that fractures widen in aperture while they propagate in length or height. However, it is also conceivable that a phase of proportional aperture to length or height growth is followed by a phase of aperture growth with relatively slow or arrested tip propagation. Slow propagation relative to aperture opening can occur by non-elastic deformation processes or if the material elastic properties change over time. Fracture propagation in length or height can be halted by material strength heterogeneities. To test for concurrent length versus aperture growth of these fractures, I reconstructed the crack-seal opening history for multiple cement bridges sampled at different distances from the tip of three opening-mode fractures in Travis Peak Sandstone of the SFOT-1 well, East Texas. Crack-seal cement bridges have been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement that bridges the fractures. Crack-seal cement textures were imaged using a scanning electron microscope with a cathodoluminescence detector, and the number and thickness of crack-seal cement increments determined. Trends in crack-seal increments number and thickness are consistent with fast initial fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Cumulative fracture opening displacement based on palinspastic reconstruction of two cement bridges was compared to analytical solutions for a stationary and a propagating fracture aperture as a function of position relative to the fracture tip in an elastic medium. Based on this comparison, I conclude that the crack-seal cement record reflects largely the phase of dominant aperture growth and subcritical fracture propagation under constant loading stress.