Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling

Though the rheology of kilometre-scale polymineralic rock units is crucial for reliable large-scale, geotectonic models, this information is difficult to obtain. In geotectonic models, a layer is defined as an entity at the kilometre scale, even though it is heterogeneous at the millimetre to metre scale. Here, we use the shape characteristics of the boundaries between rock units to derive the relative bulk viscosity of those units at the kilometre scale. We examine the shape of a vertically oriented ultramafic, harzburgitic-lherzolitic unit, which developed a kilometre-scale pinch and swell structure at mid-crustal conditions (~ 600 °C, ~ 8.5 kbar), in the Anita Shear Zone, New Zealand. The ultramafic layer is embedded between a typical polymineralic paragneiss to the west, and a feldspar-quartz-hornblende orthogneiss, to the east. Notably, the boundaries on either side of the ultramafic layer give the ultramafics an asymmetric shape. Microstructural analysis shows that deformation was dominated by dislocation creep (n = 3). Based on the inferred rheological behaviour from the field, a series of numerical simulations are performed. Relative and absolute values are derived for bulk viscosity of the rock units by comparing boundary tortuosity difference measured on the field example and the numerical series. Our analysis shows that during deformation at mid-crustal conditions, paragneisses can be ~ 30 times less viscous than an ultramafic unit, whereas orthogneisses have intermediate viscosity, ~ 3 times greater than the paragneisses. If we assume a strain rate of 10⁻ ¹⁴ s⁻ ¹ the ultramafic, orthogneiss and paragneiss have syn-deformational viscosities of 3 × 10²², 2.3 × 10²¹ and 9.4 × 10²⁰ Pa s, respectively. Our study shows pinch and swell structures are useful as a gauge to assess relative bulk viscosity of rock units based on shape characteristics at the kilometre scale and in non-Newtonian flow regimes, even where heterogeneity occurs within the units at the outcrop scale

Fractures and faults in tight gas sandstones : a study using laboratory and field data

In low permeability, tight gas sandstone reservoirs, an understanding of fracture systems is important in hydrocarbon exploration and production, because fracture networks affect the fluid flow properties in such reservoirs. Rocks can deform either in a ductile or brittle way depending on the rock mechanical properties and the stress condition. To better understand the fluid flow characteristics of a fault system within a reservoir, the knowledge of the mechanical properties and the stress condition of the reservoir and the geometry of the fracture networks, both in seismic-scale and micro-scale, is important. This thesis presents a multidisciplinary and multi-scale analysis of the rock mechanical properties in sandstones relevant to tight reservoirs.Fractures and the geometry of fault damage zones were studied in two normal faults in Moab, Utah. The Courthouse Junction fault, which is a branch of the Moab Fault (with a throw of about 80 m) is characterised by cataclastic deformation bands and slip planes and minor fluid-flow alteration. In the cataclastic bands, grain and pore sizes range from about 1 to 0.1 µm in diameter, about two orders of magnitude smaller than those of the host rock; so significantly reducing the permeability of this fault zone. The other fault is located at the Klondike Bluffs area (the maximum throw of about 10 m) which is characterised by minor cataclasis, strong diagenesis and dislocation or disaggregation deformation bands. In the deformation bands, the grains are not crushed and the average pore size is nearly double, which increases the permeability within the fractures. As a result of past fluid flow, calcite now fully fills the fractures, so rendering them as impermeable barriers. In general, the fracture density decreases logarithmically outwards from the fault core; however, irregularities tend to often disrupt this tendency. Peaks, i.e. increases in deformation band density, are not always related to faults. The orientations of the deformation bands are sub-parallel and their dip varies between 70° and 90°. The width of the damage zone in the footwall at Klondike Bluffs is about 150 m and at the Courthouse Junction this varies from 200 to >300 m. In the hanging wall at Klondike Bluffs, the damage zone ranges from 180 to 300 m. Along the faults, the width and the deformation band distribution change significantly; the range of dip and dip-direction varies moderately, while the fracture characteristics remain constant.The microstructure of fractures in North Sea Rotliegend Sandstone core samples and in the Moab field samples was analysed. The results show that the characteristics of the fractures and the host rocks of both the field study samples and the North Sea core samples are similar. In conclusion, the Moab sandstones may provide a good analogue to that of these North Sea sandstones.The second aim of the study was to analyse the relationship between the rock mechanical properties, log properties and the brittleness of rocks. The relationship between unconfined compressive strength (UCS), Young’s modulus and wireline well logs (i.e. acoustic velocity, density, resistivity, natural gamma-ray, spectral gamma-ray and neutron-porosity) was studied in North Sea Lower Germanic Triassic Sandstone (depths range 2700 to 4050 m) and Rotliegend Sandstone (depths range 3900 to 4900 m). A multivariate regression method was used to calculate the empirical correlation equations. In the Triassic sandstone, acoustic velocity has a much weaker dependence on velocity than it has in the Rotliegend sandstone. Multivariate regressions using more prediction variables provided better-fit correlation equations. A significant increase was observed in the goodness of regression using spectral gamma logs. The highest squared regression coefficient was attained as a result of a UCS-log multivariate regression for Rotliegend samples: R² = 0.84 using spectral gamma logs, and for Triassic samples R² = 0.55 when using a cumulative gamma log (due to the unavailability of spectral gamma logs). This same tendency was found in the results of regressions made for Young’s moduli and log properties. Strong dependency was exhibited between UCS and Young’s moduli (R² = 0.9) in the Rotliegend samples; however, dependency was much lower in the Triassic samples (R² = 0.46).Based on the brittleness index approach of Ingram and Urai (1999) and Hoogerduijn-Strating and Urai (2003), a new brittleness index equation has been developed in which stress conditions and UCS are considered. The derived UCS-log correlation equations were used to calculate brittleness logs in the wells from where the rock samples originate. By applying the calculated BRI logs to characterise the brittle, less brittle and ductile formations or intervals of North Sea sandstones were identified and provided good examples for the application of this BRI concept.The results of my work can provide a better understanding of the properties of faults and fractures, together with hydrocarbon migration through tight sandstone reservoirs, and may be applied to improve the seismic interpretation of faults

Fractures and faults in tight gas sandstones : a study using laboratory and field data

In low permeability, tight gas sandstone reservoirs, an understanding of fracture systems is important in hydrocarbon exploration and production, because fracture networks affect the fluid flow properties in such reservoirs. Rocks can deform either in a ductile or brittle way depending on the rock mechanical properties and the stress condition. To better understand the fluid flow characteristics of a fault system within a reservoir, the knowledge of the mechanical properties and the stress condition of the reservoir and the geometry of the fracture networks, both in seismic-scale and micro-scale, is important. This thesis presents a multidisciplinary and multi-scale analysis of the rock mechanical properties in sandstones relevant to tight reservoirs.Fractures and the geometry of fault damage zones were studied in two normal faults in Moab, Utah. The Courthouse Junction fault, which is a branch of the Moab Fault (with a throw of about 80 m) is characterised by cataclastic deformation bands and slip planes and minor fluid-flow alteration. In the cataclastic bands, grain and pore sizes range from about 1 to 0.1 µm in diameter, about two orders of magnitude smaller than those of the host rock; so significantly reducing the permeability of this fault zone. The other fault is located at the Klondike Bluffs area (the maximum throw of about 10 m) which is characterised by minor cataclasis, strong diagenesis and dislocation or disaggregation deformation bands. In the deformation bands, the grains are not crushed and the average pore size is nearly double, which increases the permeability within the fractures. As a result of past fluid flow, calcite now fully fills the fractures, so rendering them as impermeable barriers. In general, the fracture density decreases logarithmically outwards from the fault core; however, irregularities tend to often disrupt this tendency. Peaks, i.e. increases in deformation band density, are not always related to faults. The orientations of the deformation bands are sub-parallel and their dip varies between 70° and 90°. The width of the damage zone in the footwall at Klondike Bluffs is about 150 m and at the Courthouse Junction this varies from 200 to >300 m. In the hanging wall at Klondike Bluffs, the damage zone ranges from 180 to 300 m. Along the faults, the width and the deformation band distribution change significantly; the range of dip and dip-direction varies moderately, while the fracture characteristics remain constant.The microstructure of fractures in North Sea Rotliegend Sandstone core samples and in the Moab field samples was analysed. The results show that the characteristics of the fractures and the host rocks of both the field study samples and the North Sea core samples are similar. In conclusion, the Moab sandstones may provide a good analogue to that of these North Sea sandstones.The second aim of the study was to analyse the relationship between the rock mechanical properties, log properties and the brittleness of rocks. The relationship between unconfined compressive strength (UCS), Young’s modulus and wireline well logs (i.e. acoustic velocity, density, resistivity, natural gamma-ray, spectral gamma-ray and neutron-porosity) was studied in North Sea Lower Germanic Triassic Sandstone (depths range 2700 to 4050 m) and Rotliegend Sandstone (depths range 3900 to 4900 m). A multivariate regression method was used to calculate the empirical correlation equations. In the Triassic sandstone, acoustic velocity has a much weaker dependence on velocity than it has in the Rotliegend sandstone. Multivariate regressions using more prediction variables provided better-fit correlation equations. A significant increase was observed in the goodness of regression using spectral gamma logs. The highest squared regression coefficient was attained as a result of a UCS-log multivariate regression for Rotliegend samples: R² = 0.84 using spectral gamma logs, and for Triassic samples R² = 0.55 when using a cumulative gamma log (due to the unavailability of spectral gamma logs). This same tendency was found in the results of regressions made for Young’s moduli and log properties. Strong dependency was exhibited between UCS and Young’s moduli (R² = 0.9) in the Rotliegend samples; however, dependency was much lower in the Triassic samples (R² = 0.46).Based on the brittleness index approach of Ingram and Urai (1999) and Hoogerduijn-Strating and Urai (2003), a new brittleness index equation has been developed in which stress conditions and UCS are considered. The derived UCS-log correlation equations were used to calculate brittleness logs in the wells from where the rock samples originate. By applying the calculated BRI logs to characterise the brittle, less brittle and ductile formations or intervals of North Sea sandstones were identified and provided good examples for the application of this BRI concept.The results of my work can provide a better understanding of the properties of faults and fractures, together with hydrocarbon migration through tight sandstone reservoirs, and may be applied to improve the seismic interpretation of faults

Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha, Aravalli–Delhi Fold Belt, NW India

In the present study we investigate the microstructural development in mullite, quartz and garnet in an anatectic migmatite hosted within a Grenvillian-age shear zone in the Aravalli–Delhi Fold Belt. The migmatite exhibits three main deformation structures and fabrics (S1, S2, S3). Elongated garnet porphyroblasts are aligned parallel to the metatexite S2 layers and contain crenulation hinges defined by biotite–sillimanite–mullite–quartz (with S1 axial planar foliation). Microstructural evidence and phase equilibrium relations establish the garnet as a peritectic phase of incongruent melting by breakdown of biotite, sillimanite ± mullite and quartz at peak P–T of ~ 8 kbar, 730 °C along a tight-loop, clockwise P–T path. Monazite dating establishes that the partial melting occurred between ~ 1000 and 870 Ma. The absence of subgrains and systematic crystal lattice distortions in these garnets despite their elongation suggests growth pseudomorphing pre-existing 3-D networks of S1 biotite aggregates rather than high-temperature crystal plastic deformation which is noted in the S1 quartz grains that exhibit strong crystallographic preferred orientation (CPO), undulatory extinction and subgrains. Mode-I fractures in these garnet porphyroblasts induced by high melt pressure during late stage of partial melt crystallization are filled by retrograde biotite–sillimanite. Weak CPO and non-systematic crystal lattice distortions in the coarse quartz grains within the S2 leucosome domains indicate these crystallized during melt solidification without later crystal plastic deformation overprint. In the later stages of deformation (D3), strain was mostly accommodated in the mullite–biotite–sillimanite-rich restite domains forming S3 which warps around garnet and leucosome domains; consequently, fine-grained S3 quartz does not exhibit strong CPOs