Modeling quartz cementation provides a means to understand the evolution of fracture characterist... more Modeling quartz cementation provides a means to understand the evolution of fracture characteristics when used in concert with basin and geomechanical models. Previous diagenetic models, however, do not consider the impact that fracturing has on quartz cementation. We have developed a 2D quartz cementation model that explicitly considers the influence of fractures on geometry of nucleation surface area in intergranular and fracture pores. The modeled surface area evolves in response to both the cementation process (which tends to reduce the surface area) and the fracture process (which tends to increase the surface area). The model also accounts for the anisotropy in precipitation rate with crystallographic orientation. For quartz cementation in trans-granular fractures this anisotropy significantly affects the extent to which overgrowths can bridge fracture apertures.
... Opening-mode fractures have an important influence over fluid flow in many dolostones (eg, [M... more ... Opening-mode fractures have an important influence over fluid flow in many dolostones (eg, [Montañez, 1997] , [Antonellini and Mollema, 2000] , [Gale et al., 2004] , [Philip et al., 2005] and [Kosa and Hunt, 2006] ). ... 1) ( [Gale et al., 2004] and [Gale and Gomez, 2007] ). ...
Geological Society, London, Special Publications, 2012
A correlation is demonstrated between the presence of crack-seal texture and power-law kinematic ... more A correlation is demonstrated between the presence of crack-seal texture and power-law kinematic aperture-size (width) distributions among opening-mode fractures in rocks of dominantly carbonate mineralogy. Crack-seal opening increments (opening-displacement increment sizes or ‘gaps’) within individual fractures follow narrow normal or log-normal size distributions, suggesting that fracture widening accumulates in characteristic (usually micrometre-scale) size increments. The scale invariance in overall fracture width distributions present in some fracture sets most likely arises from grouping of these increments (localization) to form larger fractures (millimetre- to centimetre-scale widths). Such localization could be a consequence of the tendency for larger, less cemented fractures to break preferentially during subsequent deformation. Cement accumulation patterns thus provide a mechanism for positive feedback whereby large-fracture growth exceeds small-fracture growth. Using characteristically sized growth increments, a fracture growth model accurately simulates fracture arrays having power-law fracture-width distributions. Model parameters can be altered to produce characteristic-width fracture size distributions. The results have implications for how fracture porosity and permeability evolve in carbonate reservoirs.
Power-law variation of aperture size with cumulative frequency has been documented in vein arrays... more Power-law variation of aperture size with cumulative frequency has been documented in vein arrays, but such patterns have not been conclusively demonstrated from open or incompletely mineralized opening-mode fractures (joints) in otherwise-undeformed sedimentary rocks. We used subhorizontal core from the nearly flat- lying Cretaceous Cozzette Sandstone, Piceance Basin, Colorado, to document fracture aperture sizes over five orders of magnitude. We measured microfractures (0.0004-0.1164 mm in aperture) along a 276-mm-long scanline using scanning electron microscope-based cathodoluminescence; we measured macrofractures (0.5- 2.15 mm in aperture) in 35 m of approximately horizontal core cut normal to fracture strike. Microfractures are typically filled with quartz. Macrofractures are mostly open and resemble non-mineralized joints, except for thin veneers of quartz cement lining their walls. Micro- and macrofractures share both a common orientation and the same timing with respect to diagenetic sequence, only differing in size and the degree to which they are filled with quartz cement. Power-law scaling equations were derived by fitting trendlines to aperture vs. cumulative frequency data for the microfractures. These equations successfully predicted the cumulative frequencies of the macrofractures, accurate to within a factor of four in each test and within a factor of two in 75 percent of tests. Our results show that tectonic deformation is not prerequisite for power-law scaling of fractures, but instead suggest that scaling emerges from fracture interaction during propagation.
ABSTRACT Natural fractures in tight sandstone and shale reservoirs are characterized by partial t... more ABSTRACT Natural fractures in tight sandstone and shale reservoirs are characterized by partial to complete cementation. In all tight-gas sandstone reservoirs and suitable outcrop reservoir analogs, fractures frequently contain crack-seal quartz and carbonate cement that formed during incremental fracture opening. These synkinematic cements may be followed by blocky postkinematic cement occluding any residual fracture porosity. Fluid inclusion microthermometry combined with Raman analyses demonstrate that synkinematic cement formed under conditions close to maximum burial and incipient exhumation under elevated pore fluid pressures and over time spans of 10-50 m.y.. Fracture opening rates, integrated over the kinematic fracture aperture, are on the order of 10 microm/m.y. Based on the textural evidence of synkinematic cement growth, in combination with kinetic models of quartz cementation, we infer that these rates are comparable to rates of dissolution-precipitation reactions in the host rock, and of mass transfer between host rock and fracture. It is thus suggested that dissolution-precipitation creep is a dominant deformation mechanism allowing accommodation of permanent fracture strain under these deep-burial, diagenetically reactive conditions. Synkinematic mineral reactions in the host rock and precipitation of fracture lining cement guarantee that partially cemented natural fractures remain propped open and thus conductive under production conditions. However, cement linings and bridges can inhibit flow between micro-porous host rock and residual fracture porosity resulting in flow barriers. Complex pore geometry in partially cemented fractures may impede multiphase fracture flow and production. In shale, the interface between host rock and fracture cement is frequently mechanically weak potentially allowing fracture reactivation during well completion. Such artificially reactivated fractures may thus increase flow of production fluids even in formation containing otherwise sealed natural fractures.
Modeling quartz cementation provides a means to understand the evolution of fracture characterist... more Modeling quartz cementation provides a means to understand the evolution of fracture characteristics when used in concert with basin and geomechanical models. Previous diagenetic models, however, do not consider the impact that fracturing has on quartz cementation. We have developed a 2D quartz cementation model that explicitly considers the influence of fractures on geometry of nucleation surface area in intergranular and fracture pores. The modeled surface area evolves in response to both the cementation process (which tends to reduce the surface area) and the fracture process (which tends to increase the surface area). The model also accounts for the anisotropy in precipitation rate with crystallographic orientation. For quartz cementation in trans-granular fractures this anisotropy significantly affects the extent to which overgrowths can bridge fracture apertures.
... Opening-mode fractures have an important influence over fluid flow in many dolostones (eg, [M... more ... Opening-mode fractures have an important influence over fluid flow in many dolostones (eg, [Montañez, 1997] , [Antonellini and Mollema, 2000] , [Gale et al., 2004] , [Philip et al., 2005] and [Kosa and Hunt, 2006] ). ... 1) ( [Gale et al., 2004] and [Gale and Gomez, 2007] ). ...
Geological Society, London, Special Publications, 2012
A correlation is demonstrated between the presence of crack-seal texture and power-law kinematic ... more A correlation is demonstrated between the presence of crack-seal texture and power-law kinematic aperture-size (width) distributions among opening-mode fractures in rocks of dominantly carbonate mineralogy. Crack-seal opening increments (opening-displacement increment sizes or ‘gaps’) within individual fractures follow narrow normal or log-normal size distributions, suggesting that fracture widening accumulates in characteristic (usually micrometre-scale) size increments. The scale invariance in overall fracture width distributions present in some fracture sets most likely arises from grouping of these increments (localization) to form larger fractures (millimetre- to centimetre-scale widths). Such localization could be a consequence of the tendency for larger, less cemented fractures to break preferentially during subsequent deformation. Cement accumulation patterns thus provide a mechanism for positive feedback whereby large-fracture growth exceeds small-fracture growth. Using characteristically sized growth increments, a fracture growth model accurately simulates fracture arrays having power-law fracture-width distributions. Model parameters can be altered to produce characteristic-width fracture size distributions. The results have implications for how fracture porosity and permeability evolve in carbonate reservoirs.
Power-law variation of aperture size with cumulative frequency has been documented in vein arrays... more Power-law variation of aperture size with cumulative frequency has been documented in vein arrays, but such patterns have not been conclusively demonstrated from open or incompletely mineralized opening-mode fractures (joints) in otherwise-undeformed sedimentary rocks. We used subhorizontal core from the nearly flat- lying Cretaceous Cozzette Sandstone, Piceance Basin, Colorado, to document fracture aperture sizes over five orders of magnitude. We measured microfractures (0.0004-0.1164 mm in aperture) along a 276-mm-long scanline using scanning electron microscope-based cathodoluminescence; we measured macrofractures (0.5- 2.15 mm in aperture) in 35 m of approximately horizontal core cut normal to fracture strike. Microfractures are typically filled with quartz. Macrofractures are mostly open and resemble non-mineralized joints, except for thin veneers of quartz cement lining their walls. Micro- and macrofractures share both a common orientation and the same timing with respect to diagenetic sequence, only differing in size and the degree to which they are filled with quartz cement. Power-law scaling equations were derived by fitting trendlines to aperture vs. cumulative frequency data for the microfractures. These equations successfully predicted the cumulative frequencies of the macrofractures, accurate to within a factor of four in each test and within a factor of two in 75 percent of tests. Our results show that tectonic deformation is not prerequisite for power-law scaling of fractures, but instead suggest that scaling emerges from fracture interaction during propagation.
ABSTRACT Natural fractures in tight sandstone and shale reservoirs are characterized by partial t... more ABSTRACT Natural fractures in tight sandstone and shale reservoirs are characterized by partial to complete cementation. In all tight-gas sandstone reservoirs and suitable outcrop reservoir analogs, fractures frequently contain crack-seal quartz and carbonate cement that formed during incremental fracture opening. These synkinematic cements may be followed by blocky postkinematic cement occluding any residual fracture porosity. Fluid inclusion microthermometry combined with Raman analyses demonstrate that synkinematic cement formed under conditions close to maximum burial and incipient exhumation under elevated pore fluid pressures and over time spans of 10-50 m.y.. Fracture opening rates, integrated over the kinematic fracture aperture, are on the order of 10 microm/m.y. Based on the textural evidence of synkinematic cement growth, in combination with kinetic models of quartz cementation, we infer that these rates are comparable to rates of dissolution-precipitation reactions in the host rock, and of mass transfer between host rock and fracture. It is thus suggested that dissolution-precipitation creep is a dominant deformation mechanism allowing accommodation of permanent fracture strain under these deep-burial, diagenetically reactive conditions. Synkinematic mineral reactions in the host rock and precipitation of fracture lining cement guarantee that partially cemented natural fractures remain propped open and thus conductive under production conditions. However, cement linings and bridges can inhibit flow between micro-porous host rock and residual fracture porosity resulting in flow barriers. Complex pore geometry in partially cemented fractures may impede multiphase fracture flow and production. In shale, the interface between host rock and fracture cement is frequently mechanically weak potentially allowing fracture reactivation during well completion. Such artificially reactivated fractures may thus increase flow of production fluids even in formation containing otherwise sealed natural fractures.
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