Abstract It is well established that compaction bands (CBs), joints and faults are often present in the same rock volume in the Jurassic aeolian Aztec Sandstone, exposed in the Valley of Fire State Park, Nevada, USA. Because the permeability of CBs can be one or more orders of magnitude less than the matrix permeability, and joint permeability, depending on its aperture, can be several orders of magnitude greater than matrix permeability, the combined effect of these structures on subsurface flow can be complex and substantial. In this study, we investigate the effects of a variety of intersecting geological structures on fluid flow. This is accomplished by performing two- (2D) and three-dimensional (3D) permeability upscaling and waterflood simulations over areas/volumes populated by hydraulically interacting geological features. The regions considered are approximately the size of typical grid blocks used for reservoir or aquifer flow simulations, so the results are of practical interest. The systems studied include models with two sets of vertical CBs intersecting at various angles, an inclined CB set intersecting a vertical CB set, a joint set intersecting a CB set at various angles, and a small fault and its damage zone overprinting a CB set. Our numerical results quantify the impact of these composite structures on subsurface flow and show, for example, that the intersection angle of two sets of structures can have a considerable effect on the upscaled directional permeability. In addition, waterflood simulations demonstrate that the efficiency of oil recovery can be significantly impacted by the direction of flow relative to the orientation of intersecting geological structures.

Abstract Using an elastic dislocation model, we incorporate a historical earthquake catalog, mapped Marmara Sea fault traces, and fault slip distributions for the 1999 Izmit earthquake inferred from InSAR and GPS data to determine various stress change scenarios crucial for evaluating future earthquake potential in the eastern Marmara Sea. We have tested six plausible past rupture configurations arising from the uncertainty in the location of the western termination of 1999 Izmit earthquake rupture and the location of the 1963 Yalova earthquake rupture. Coulomb stresses calculated are increased on the Princes’ Islands, Çinarcik, and Armutlu fault segments in each case. In four of the six plausible configurations of previous ruptures, the Çinarcik fault receives the greatest average stress change. In one other configuration, the average stress increase on the Princes’ Islands fault is greatest. In another, the stress changes on the Çinarcik and Princes’ Islands faults are comparable. Moreover, we show that rupture initiating on either the Princes’ Islands or Armutlu faults would be favoured to propagate onto the Central Marmara, or Imrali fault, respectively, based on the favourable geometries of the respective fault intersections. Rupture initiating on the Çinarcik fault, however would be limited to a much shorter length based on its mapped western termination. Therefore, while the earthquake-induced stress changes may, in most cases, be greatest on the Çinarcik fault, an earthquake initiating on this fault segment may produce a shorter cumulative rupture compared to rupture initiated on the two other major eastern Marmara Sea fault segments. These results are encouraging for the use of geomechanical modelling tools in addressing uncertainties inherent in most geological and geophysical data applied to earthquake-related problems.

Abstract To refine flow models for sand-dominated fault rock, we present petrophysical data of host and fault rock samples from the eolian Aztec Sandstone, Valley of Fire State Park, Nevada, that has been deformed by strike-slip faults formed by progressive shearing along joint zones. The data include bulk mineralogy, porosity, permeability, grain-size distribution, and mercury-injection capillary pressure measurements of 40 host, fragmented, and fault rock samples. To investigate the impact of shear strain on fault zone properties, three sample localities with average shear strains of 28, 63, and 80 were investigated (25-160-m [82-525-ft] slip). No bulk mineralogical changes caused by fault zone cementation or mineral alteration were detected when comparing host and fault rock. Fault rock permeability is one to three orders of magnitude lower than median host rock permeability. Porosity reductions are less pronounced and show considerable overlap in values between the sample suites. Some fault rock samples appear to have dilated with respect to median host rock porosity. Median grain sizes for fault rock samples range from 3 to 51 mm, which is as much as two orders of magnitude reduction from host rock median grain sizes. There appears to be a lower limit of median grain size of 3 mm for fault rock samples irrespective of average fault shear strain. Fault rock capillary injection pressures range from one to almost two orders of magnitude higher than the host rock equivalent. For standard fluid properties, calculated maximum sealable hydrocarbon column heights range between 10 and 70 m (33 and 230 ft) of gas and 20-120 m (66-400 ft) of oil. These petrophysical data show that faults formed by shearing of joints in high-permeability, sand-prone systems will act as significant barriers to fluid flow during reservoir production and might be capable of sealing small to moderate hydrocarbon columns on an exploration timescale as well, assuming adequate continuity of the fault rock over large areas of the fault.

Joints are the most common result of brittle fracture of rock in the Earth’s crust. They control the physiography of many spectacular landforms and play an important role in the transport of fluids. In its first century, the Geological Society of America Bulletin has published a significant number of papers on joints and jointing. One hundred years ago, there were lively debates in the literature about the origin of joints, and detailed descriptions of joints near the turn of the century catalogued most geometric features that we recognize on joints today. In the 1920s, theories relating joint orientation to the tectonic stress field and to other geologic structures led to a proliferation of data on the strike and dip of joints in different regions. The gathering of orientation data dominated work on joints for the next 50 yr. In the 1960s, key papers re-established the need to document surface textures, determine age relations, and measure relative displacements across joints in order to interpret their origins. At about this time, fundamental relationships from the fields of continuum and fracture mechanics were first used to understand the process of jointing. In the past two decades, we have witnessed an effort to use field data to interpret the kinematics of jointing and to understand the initiation, propagation, interaction, and termination of joints. Theoretical methods have been developed to study the evolution of joint sets and the mechanical response of a jointed rock mass to tectonic loading. Although many interesting problems remain to be explored, a sound conceptual and theoretical framework is now available to guide research into the next century.

Thrust faults are typically discontinuous. Based on the relative positions of adjacent fault segments, the discontinuities along thrust faults can be classified into two major groups: along-strike and down-dip. Adjacent fault segments are linked by transfer structures such as secondary dip-slip faults, tear faults, folds, cleavages, and pull-apart openings. Duplex structures are compressional down-dip discontinuities associated with echelon thrust faults with relatively large overlaps. Duplex structures in general, and cleavage duplexes in particular, are analyzed by calculating stresses due to interacting echelon mode II cracks. The results indicate that there are significant increases in the maximum compressive stress, mean stress, and maximum shear stress at the stepover area. The orientations of the planes upon which the largest compressive stresses act in the model are approximately consistent with the orientations of the cleavage planes in a few duplexes described in the literature.

Abstract Stepovers are fundamental features along strike-slip faults of various lengths. Two types of stepover between strike-slip faults are considered in this paper: (1) along-strike stepovers that are due to en echelon arrangement of faults in map view, and (2) down-dip stepovers that are due to en echelon arrangement of faults in cross section. Along-strike stepovers produce pull-apart basins und push-up ranges depending on the sense of stepover. Down-dip stepovers of both senses may produce strike-slip faults in orientations different from the initial major strike-slip faults that are arranged en echelon. Some possible mechanisms that produce stepovers and control the sense of stepover are (1) bending of initially straight faults. (2) faulting within a weak zone oriented slightly off a local failure plane. (3) segmentation of faults to accommodate curved fault traces. (4) horizontal slip across pre-existing extensional fractures or dip-slip faults that have steps. (5) a change of physical parameters such as elastic moduli and pore pressure, and (6) stress field resulting from fault interaction.