Multi-scale Coupled Processes Modeling of Fractures as Porous, Interfacial and Granular Systems from Rock Images with the Numerical Manifold Method

The greatest challenges of rigorously modeling coupled hydro-mechanical processes in fractured rocks at different scales are associated with computational geometry. In addition, selections of continuous or discontinuous models, physical laws, and coupling priorities at different scales based on different geometric features determine the applicability of a numerical model for a certain type of problem. In this study, we present our multi-scale modeling capabilities that have been developed based on the numerical manifold method for analyzing coupled hydro-mechanical processes in fractured rocks. Based on their geometric features, the fractures are modeled as continua—finite-thickness porous zones, and discontinua—discontinuous interfaces and microscale asperities and granular systems. Different governing equations, physical laws, coupling priorities, and approaches for addressing fracture intersections and shearing are then applied to describe these. We applied these models to simulate coupled processes in fractured rocks using realistic geometry obtained from rock images at different scales. We first calculated shearing of a single fracture with different models and demonstrated the impacts of asperities on shearing. We then applied the continuous and discontinuous models to simulate a network of rough fractures, demonstrating that contact dynamics contribute significantly to the geometric, multi-physical evolution of systems where rough fractures are not mineral filled. For a discrete fracture network, our coupled processes modeling demonstrates that shearing of the discrete fractures can have a major impact on stress and pore pressure distribution. Lastly, we applied the discontinuous granular model to simulate evolution of a complex granular system with a deformation band, demonstrating that the deformation band can dominate contact dynamics, the structural and the stress evolution of the granular system.

Along-strike variations in protothrust zone characteristics at the Nankai Trough subduction margin

Significant along-strike changes in the protothrust zone at the toe of the Nankai Trough accretionary prism were imaged in new high-resolution seismic reflection data. The width of the protothrust zone varies greatly along strike; two spatially discrete segments have a wide protothrust zone (∼3.3–7.8 km, ∼50–110 protothrusts), and two segments have almost no protothrust zone (∼0.5–2.8 km, <20 protothrusts). The widest protothrust zone occurs in the region with the widest and thickest sediment wedge and subducting turbidite package, both of which are influenced by basement topography. The trench wedge size and lithology, the lithology of the subducting section, and the basement topography all influence the rate of consolidation in the trench wedge, which we hypothesize is an important control over the presence and width of the protothrust zone. We conclude that protothrusts are fractures that form from shear surfaces in deformation band clusters as the trench fill sediment is consolidated. Strain localization occurs at sites with a high density of protothrusts, which become the probable locations of future frontal thrust propagation. The frontal thrust may propagate forward with a lower buildup of strain where it is adjacent to a wide protothrust zone than at areas with a narrow or no protothrust zone. This is reflected in the accretionary prism geometry, where wide protothrust zones occur adjacent to fault-propagation folds with shallow prism toe surface slopes.

Analysis of deformation bands associated with the Trachyte Mesa intrusion, Henry Mountains, Utah: implications for reservoir connectivity and fluid flow around sill intrusions

Shallow-level igneous intrusions are a common feature of many sedimentary basins, and there is increased recognition of the syn-emplacement deformation structures in the host rock that help to accommodate this magma addition. However, the sub-seismic structure and reservoir-scale implications of igneous intrusions remain poorly understood. The Trachyte Mesa intrusion is a small (∼1.5 km2), NE–SW trending satellite intrusion to the Oligocene-age Mount Hillers intrusive complex in the Henry Mountains, Utah. It is emplaced within the highly porous, aeolian Entrada Sandstone Formation (Jurassic), producing a network of conjugate sets of NE–SW striking deformation bands trending parallel to the intrusion margins. The network was characterized by defining a series of nodes and branches, from which the topology, frequency, intensity, spacing, characteristic length, and dimensionless intensity of the deformation band traces and branches were determined. These quantitative geometric and topological measures were supplemented by petrological, porosity and microstructural analyses. Results show a marked increase in deformation band intensity and significant porosity reduction with increasing proximity to the intrusion. The deformation bands are likely to impede fluid flow, forming barriers and baffles within the Entrada reservoir unit. A corresponding increase in Y- and X-nodes highlights the significant increase in deformation band connectivity, which in turn will significantly reduce the permeability of the sandstone. This study indicates that fluid flow in deformed host rocks around igneous bodies may vary significantly from that in the undeformed host rock. A better understanding of the variability of deformation structures, and their association with intrusion geometry, will have important implications for industries where fluid flow within naturally fractured reservoirs adds value (e.g. hydrocarbon reservoir deliverability, hydrology, geothermal energy and carbon sequestration).

Analysis of deformation bands associated with the Trachyte Mesa intrusion, Henry Mountains, Utah: implications for reservoir connectivity and fluid flow around sill intrusions

Shallow-level igneous intrusions are a common feature of many sedimentary basins, and there is increased recognition of the syn-emplacement deformation structures in the host rock that help to accommodate this magma addition. However, the sub-seismic structure and reservoir-scale implications of igneous intrusions remain poorly understood. The Trachyte Mesa intrusion is a small (~ 1.5 km2), NE–SW trending satellite intrusion to the Oligocene-age Mount Hillers intrusive complex in the Henry Mountains, Utah. It is emplaced within the highly porous, aeolian Entrada Sandstone Formation (Jurassic), producing a network of conjugate sets of NE–SW striking deformation bands trending parallel to the intrusion margins. The network was characterized by defining a series of nodes and branches, from which the topology, frequency, intensity, spacing, characteristic length, and dimensionless intensity of the deformation band traces and branches were determined. These quantitative geometric and topological measures were supplemented by petrological, porosity and microstructural analyses. Results show a marked increase in deformation band intensity and significant porosity reduction with increasing proximity to the intrusion. The deformation bands are likely to impede fluid flow, forming barriers and baffles within the Entrada reservoir unit. A corresponding increase in Y- and X- nodes highlights the significant increase in deformation band connectivity, which in turn will significantly reduce the permeability of the sandstone. This study indicates that fluid flow in deformed host rocks around igneous bodies may vary significantly from that in the undeformed host rock. A better understanding of the variability of deformation structures, and their association with intrusion geometry, will have important implications for industries where fluid flow within naturally fractured reservoirs adds value (e.g. hydrocarbon reservoir deliverability, hydrology, geothermal energy and carbon sequestration).

Tectonic evolution of the seismically active western continental margin of the Indian plate: Implications for kinematic history and fluid flow

<p>The deformation history along the E-W trending Kachchh rift basin at the western continental margin of the Indian plate located in the state of Gujarat, India, has been controlled by activation of NW-SE, NE-SW and E-W trending, 0.25–50 km long oblique-slip and dip-slip faults.</p><p>The study is an attempt to establish the kinematic framework along sub-parallel, NW-SE striking group of intra-uplift, striated, high-angle reverse faults, consisting of, Vigodi Fault (VF) and its bifurcation – West Vigodi Fault (WVF), Gugriana Fault (GUF) and its bifurcation – Khirasra fault (KHIF) from the western part of the Kachchh basin in the northern part of Gujarat state in western India. They meet the E-W trending master faults – the Kachchh Mainland Fault (KMF) to the north and the Katrol Hill Fault (KHF) to the south at an acute angle.</p><p>Fault-slip data consisting of fault plane and slickenside attitudes along with other kinematic indicators were recorded along the faults at 69 structural stations. A total of 1258 fault-slip data were used to carry out paleostress analysis using Win-Tensor (v.5.8.8) and T-Tecto Studio X5 by executing the Right Dihedral Method.</p><p>The NW-SE trending fault system exposes highly porous and permeable deformed sandstones belonging to the Jhuran and Bhuj Formation. The pure compaction bands, cataclastic deformation band clusters, slipped deformation bands and deformation band faults are documented. These tabular structures are densely populated in the fault damage zones of VF, WVF, GUF and KHIF. The field observations related to fluid flow conduits are discussed. We also present the field characteristics and petrographic evidences of chemical bleaching caused by fluid-rock interaction found in the Bhuj and the Jhuran sandstones. The change in the coloration pattern of deformation bands in comparison with the host rock color, presence of iron concretions, iron rinds and liesegang rings are important records of the diagenetic control over the fluid flow. The study is an attempt to the link the tectonic activity and simultaneous chemical reactions that affect the fluid flow transport.</p><p>We attribute the deformation history in the western continental margin of the Indian plate has been dominantly controlled by intraplate compressional stresses induced by anticlockwise rotation and collision of the Indian plate with the Eurasian plate at ~55 Ma. This correlates well with the Kachchh basin where rifting aborted during the Late Cretaceous, accommodated syn-rifting extensional component in the intra-uplift VF, GUF and KHIF. It has then undergone inversion phase due to onset of compressive stresses during the Post-Deccan Trap time up to the present. The NW-SE trending intra-uplift faults reactivated multiple times and generated deformation bands having high porosity contrast with the host Bhuj sandstone.</p>