Graduate Student Projects (Past)

Summary of key structural features identified and mapped in core.

Comparison of acoustic borehole image log (left) with mapped core (right).

Summary of fracture population mapped in acoustic borehole televiewer log (left) and in core (right). Modified tadpole plot shows the shows the variation in fracture attitude with depth. Note the core reveals far more fractures than are mapped in the image log and a greater diversity of fracture attitude.

A series of washes reveal serial cross sections of a thin frozen dike showing its upper tip and vertical, en echelon segmentation indicating later propagation.

Model of dike opening in a stratigraphy of variable density. Resulting variation in buoyancy pressure produces variable opening. At the upper tip, insufficient buoyancy pressure is available to open and invade the upper tip of the dike model. Top row depicts a map view, whereas bottom row indicates a cross section through the center of the dike. First column is the tractions imposed on the dike walls due to buoyancy pressure. Middle column is the resulting opening. Right column is the horizontal stress component normal to the dike.

Left: Topographic map of Mount Saint Helens and immediate surroundings with earthquakes hypocenters and magnitude from the Pacific Northwest Seismic Network. Right: Cross-section through Mount Saint Helens aligned with the Saint Helens Seismic Zone illustrating the distribution and timing of earthquakes.

Position of detailed structural maps illustrating the fracture and fault patterns in the contact metamorphic aureole around intrusive igneous rock.

Detailed maps of the contact metamorphic facies and fracture patterns around the intrusion.

Modified tadpole plot, fracture density, static temperature, and stereogram of fracture attitude in a borehole near Mount Saint Helens.

Summary of local stress state in the overlap between two en echelon faults for various spacing configurations. Left: Idealized overlap geometry. Middle: Summary of max coulomb stress in the region between the two simulated faults. Right: stress induced by slip on an example fault model.

Analysis of dilation history associated with repeated slip on natural fractures in core from well GEO N-2 in the flank of the Newberry Volcano. The pattern reveals two stages of dilation: (1) In the first stage, dilation correlates with primary grain size; (2) In the second stage, dilation is dictated by the linkage of early fractures and natural flaws. The first phase results in significant overall gain in small pores and potential storage for reservoir fluids whereas the second phase results in the largest pore sizes and thus potential increases in permeability.

Conceptual model of the non-linear evolution of secondary porosity as slip increases. Both the grain size of the rock and initial distribution of flaws (small fractures, vugs or vesicles, etc.) control porosity production at small slip. As slip progresses past a critical distance, asperity destruction, gouge production, and fluid-assisted re-mineralization govern a transition to porosity loss.

Detailed mapping of porosity structure in a multiply reactivated natural fracture from GEO N-2 core in the flank of the Newberry Volcano.

(a) Tracer analysis shows pre-stimulation isolation of well 27-15 and post-stimulation connection. (b) Simulated direction of SHmax due to slip on the Rhyolite Ridge fault to remote loading by Basin and Range Extension. In normal fault stress regimes it is assumed this coincides with the preferred strike of normal faults and joints. (c) Slip and (d) slip tendency (ration of shear to effective normal tractions) simulated on the Rhyolite Ridge fault. (e) Seismicity in the vicinity of the geothermal field (re-injection wells are blue, production wells are red, and the study site is a gray square).

Analysis of how clay in fault rock controls key fault properties. Increasing clay content reduces both the friction and permeability of fault rock. The relationship is non -linear; a clay fraction beyond a critical threshold is associated with a transition to the clay-end member properties. The properties and critical clay-fraction are mineral dependent. (This analysis was performed at the USGS rock mechanics laboratory as a summer research project by Mike separate from the main focus of his thesis work.)

(left) Visualization of stress concentration around a borehole can (middle left)) induce tensile or compressive failure of the borehole wall evident in acoustic (resistivity, and optical) image logs. (middle right) The shape and position of these structures same the stress state resolved on the borehole wall and is used to infer the local stress tensor. (right) In Coso and other sites we observe that the directions in which the principal stresses acts varies along the borehole relative to an overall mean. This variation occurs within a family of wavelengths and amplitudes whose relative contribution is assessed as a power spectral density.

Map of the preferred direction of SHmax +/- one standard deviation in geothermal fields in the west-central Basin and Range of Nevada. In normal faulting terrains, SHmax is generally expected to align with the strike of active normal faults that define the ranges and will be orthogonal to the overall extension direction expressed by the gradient in the GPS velocity vector field.

Conceptual model of how faults can incorporate clay-rich material into a fault zone crossing layered sandstone and shale.

A key issue in fault zone development is the source of fault rock, which can be a combination of mechanically incorporated host rock or authigenically precipitated minerals. Formation of new minerals is promoted by mechanical fracture and grain size reduction exposing new, larger surface area to reaction and simultaneously facilitating fluid flow. In the fault zone we can explore the relative contribution of these two mechanisms by comparing the mineralogy and chemistry of a sample of fault rock to the host rocks dragged past the sample location.

Observations of stress in Iceland from the world stress map.

Resolution of the stress tensor onto an arbitrarily oriented borehole surface. The cavity introduced by the borehole concentrates the stress which can cause the borehole wall to fail. The position and shape of these failure structures is used to infer the stress state (direction of principal stresses and their magnitudes) in the rock where they occur. (left) Illustration of the relationship between the borehole and stress tensor. (middle) Position of induced borehole failure structures on the deviated borehole surface as well as deviation data. (right) Model of SHmax azimuth as constrained by a vertical stress calculated from a model of rock density and a model of the unconfined compressive rock strength.

Porosity mapped in GEO N-2 core from the flank of the Newberry Volcano.

Fracture thickness and porosity observed in core from GEO N-2 in the flank of the Newberry Volcano, OR. Mapping of component structures is used to infer the relative accumulated slip and maturity of the fractures. Red indicates fractures with abundant quartz and calcite relative to phyllosilicates. Blue indicates fractures relatively enriched in phyllosilicates.

Visualization of the stress concentration along the sides and below the bottom of a borehole. Analysis performed in Poly3D in collaboration with IGEOSS.