R. J. Knipe

Juxtaposition and Seal Diagrams to Help Analyze Fault Seals in Hydrocarbon Reservoirs

A new set of diagrams aids in analyzing fault juxtaposition and sealing. The diagrams are based on the interaction of rock lithology and the fault displacement (throw) magnitude to control juxtapositions and fault seal types. The advantages of the diagrams are that they allow an evaluation of a fault seal without the need for detailed three-dimensional mapping of stratigraphic horizons and fault planes, and can be used to contour permeability, sealing capacity, and transmissibility of fault zones. These diagrams may be used to rapidly identify the critical fault throw and juxtapositions that require mapping to identify compartments in hydrocarbon reservoirs.

Glossary of normal faults

Increased interest in normal faults and extended terranes has led to the development of an increasingly complex terminology. The most important terms are defined in this paper, with original references being given wherever possible, along with examples of current usage.

Fault sealing processes in siliciclastic sediments

Abstract The microstructure and petrophysical properties of fault rocks from siliciclastic hydrocarbon reservoirs of the North Sea are closely related to the effective stress, temperature and sediment composition at the time of deformation, as well as their post-deformation stress and temperature history. Low permeability fault rocks may develop due to a combination of processes including: the deformation induced mixing of heterogeneously distributed fine-grained material (principally clays) with framework grains, pressure solution, cataclasis, clay smear, and cementation. Fault rocks can be classified into various types (disaggregation zones, phyllosilicate-framework fault rocks, cataclasites, clay smears, and cemented faults/fractures) based upon their clay and cement content as well as the amount of cataclasis experienced. In the absence of extensive cementation, the distribution of fault rock types along a fault plane can often be predicted from a detailed knowledge of the reservoir sedimentology. The permeability of fault rocks can vary by over six orders of magnitude, depending on the extent to which the porosity reduction processes have operated. Utilizing the strong link between the petrophysical properties of fault rocks and their geohistory allows the risks associated with fault seal evaluation to be reduced.

Empirical estimation of fault rock properties

Abstract Faults in clastic sequences are often significant barriers to single-phase fluid flow and can act as absolute barriers to the flow of non-wetting phases over geological time. Knowledge of the fault rock flow properties, as well as the width of the fault zone is required in order to conduct fluid flow simulations in faulted reservoirs. In this paper we present an equation for estimating fault zone thickness from fault throw based on outcrop data from Sinai and Northumberland. These data show that the throw/thickness relationship is dependent on lithology, and can be related to the clay content of the fault zone. The permeability and threshold pressures of fault rocks are dependent on factors such as the mineralogical composition of the faulted rock, the effective stress conditions and the time-temperature history of the reservoir prior to, during and following deformation. A strong power law relationship is established between threshold pressure and permeability, which is insensitive to the faulting mechanisms. The permeability and the threshold pressures of both the host rocks and the fault rocks can be represented by functions which are dependent on the clay content and the maximum burial depth (i.e. time-temperature history), whereas for the fault rocks the depth (i.e. effective stress conditions) at the time of deformation also needs to be taken into account. The database from which these empirical relationships were derived contains core measurements from faults and their associated host rocks in siliciclastic sequences from the North Sea. Many types of fault rock are contained within the database (disaggregation zones, cataclastic faults, phyllosilicate-framework faults and clay smears) and these have experienced a wide range in their maximum burial depths (2000–4500 m). In reservoir simulation the sealing effect of the faults can be represented as transmissibility modifiers for each grid cell, calculated from knowledge of fault rock permeability, the width of the fault zone, the grid block permeabilities and the geometry of the simulation grid. We have applied the technique to a number of North Sea reservoirs, using the new equation for calcu- lating fault rock permeability. However, even if the new equation produced lower permeabilities than previously published relationships, in all cases the transmissibility modifiers generated by this technique proved consistently too high (1–2 orders of magnitude) in order to produce good history matches. In order to further improve the model, and to get better history match, we think that it is important to include capillary effects, relative transmissibility multipliers, the new equation for calculating fault zone width and to better constrain the clay content of the fault zone. However, better methods are still required for capturing complex fault geometries in the reservoir model.

The interaction of deformation and metamorphism in slates

Abstract The distribution, microchemistry and internal structure of phases present in a slate are reported and used as a basis for a discussion of the interaction between deformation and metamorphism in slates. The study reveals details of the interactions between mechanical and chemical processes involved in slaty cleavage evolution. Evidence for the involvement of mechanical rotation, solution and crystallisation, recrystallisation and metamorphic reactions is presented and amalgamated into a model of cleavage development. The model involves a synchronous variation in the type, location and rate of cleavage evolution processes and a change in the importance of particular processes during the total history. The early stages of cleavage development are dominated by mechanical rotation associated with crenulation and possibly accompanied by grain boundary sliding and solution processes. Later stages of cleavage evolution are more heterogeneous and occur by a complex interaction of deformation and metamorphic growth processes which are concentrated at the interfaces between domains generated earlier in the strain history. The variations in grain chemistry and orientation together with changes in the strain rate (or stress level) are suggested to control the different deformation behaviour of different grains along these strain accommodation zones and to dictate whether grains bend, kink, dissolve, recrystallise or react and undergo growth. Evidence for the involvement of reactions, ranging from ionic exchange to crystallisation from solution, and solid state reordering is presented and the influence of deformation on the location and rates of these reactions discussed. Approximately ten potential reactions are involved in the cleavage evolution of the slate studied. The model presented incorporates these reactions into the deformation processes and accounts for the concentration of phengite and iron-poor chlorites in to one microstructural domain and paragonite, quartz and Fe-rich chlorites into the other domain. Two transformations appear to be fundamental in the slate studied: 1. (1) the transformation of illite to phengite 2. (2) the transformation of Fe-rich chlorite to Fe-poor chlorites, and both can be related to particular responses of grains to the reorientation processes at domain interfaces.

Faulting, fault sealing and fluid flow in hydrocarbon reservoirs: an introduction

Abstract A predictive knowledge of fault zone structure and transmissibility can have an enormous impact on the economic viability of exploration targets and generate considerable benefits during reservoir management. Understanding the effects of faults and fractures on fluid flow behaviour and distribution within hydrocarbon provinces has therefore become a priority. To model fluid flow in hydrocarbon reservoirs, it is essential to gain a detailed insight into the evolution, structure and properties of faults and fractures. Generation of realistic flow models also requires calibration with data on the fluid distributions and flow rates from hydrocarbon fields. Most hydrocarbon geologists at one time or another have asked the question ‘What is the behaviour of this fault?’. This question, as emphasized by the contributions to this volume, should more fundamentally be phrased; ‘What is the geometry of this fault zone, what are the nature and petrophysical properties of any fault rocks developed and how are they distributed in the subsurface?’. An additional important question is ‘What impact could the fault zone have on fluid flow through time?’. The properties and evolution of fault zones can be evaluated using the combined results of structural core and down-hole logging, microstructural and physical property characterization, together with analysis of faults from seismic and outcrop studies and well test data. Successful fault analysis depends upon the amalgamation of these data and incorporation into robust numerical flow models.

Fault seal analysis: successful methodologies, application and future directions

Fault seal prediction in hydrocarbon reservoirs requires an understanding of fault seal mechanisms, fault rock petrophysical properties, the spatial distribution of seals, and seal stability. The properties and evolution of seals within fault zones can be evaluated using the combined results of structural core logging, microstructural and physical property characterisation, together with information on fault populations from seismic and outcrop studies and well test data. The important structural elements of fault zones which require characterisation are: u — the microstructural/petrophysical properties of the different fault rocks present; — the population of faults and fractures which define damage zones around large faults; — the spatial distribution, orientation and clustering of the deformation in individual fault zones; — the history of fault activity, diagenesis and migration; — the distribution and volume of fault rocks with different properties. Fault rocks in siliclastic sequences range from quartz-rich cataclasites, developed from pure sandstones, to phyllosilicate smears developed from shales. Fault rocks developed along sand/sand fault juxtapositions can have transmissibility reduction factors of >10 6 . The exact value depends upon the conditions of faulting and the amount of self-sealing experienced by the fault rock. An important class of intermediate fault rocks are those generated from impure sandstones, or from sandstones with concentrations of fine phyllosilicate laminations. The localisation of cement precipitation within the damage zone may occur, which will remove the applicability of simple seal prediction based only on the host-rock lithology and fault displacement. The density of structures present in damage zones around faults is related to the cumulative displacement across the zone. The detailed internal structure of a fault zone is dependent on the conditions of deformation, the lithological architecture present and the position in the fault array. Successful seal analyses depends upon the amalgamation of data from the micro-scale to the macro-scale. This review demonstrates that improvements in fault seal risk evaluation are possible. The future directions for improving fault seal risk evaluation are also discussed. The most critical of these are; characterisation of the internal structure of fault zones, generation of a database for fault rock petrophysical properties and incorporation of the impact of realistic fault zone geometries into reservoir modelling programs.

Fluid-flow properties of faults in sandstone: The importance of temperature history

Sandstone rheology and deformation style are often controlled by the extent of quartz cementation, which is a function of temperature history. Coupling findings from deformation experiments with a model for quartz cementation provide valuable insights into the controls on fault permeability. Subsiding sedimentary basins often have a transitional depth zone, here referred to as the ductile-to-brittle transition, above which faults do not affect fluid flow or form barriers and below which faults will tend to form conduits. The depth of this transition is partly dependent upon geothermal gradient. In basins with a high geothermal gradient, fault-related conduits can form at shallow depths in high-porosity sandstone. If geothermal gradients are low, and fluid pressures are hydrostatic, fault-related conduits are only formed when the sandstones have subsided much deeper, where their porosity (and hence fluid content) is low. Mineralization of faults is more likely to occur in areas with high geothermal gradients because the rocks still have a high fluid content when fault-related fluid-flow conduits form. The interrelationship between rock rheology and stress conditions is sometimes a more important control on fault permeability than whether the fault is active or inactive.

Footwall geometry and the rheology of thrust sheets

Abstract The inter-relationships between the exact footwall geometry and the rheology of thrust sheets are investigated. Deviations in the thrust fault surface from an ideal plane will induce a local heterogeneous deformation. The resulting deformation processes depend upon the rate of thrust sheet displacement, the geometry of the feature causing heterogeneous flow, the deformation conditions and the lithologies involved. Two classes of features are particularly important in causing heterogeneous deformation in thrust sheets. The first features are small perturbations on bedding planes which may be inherited sedimentary structures or produced during layer-parallel shortening; the second class of features are ramps, where the thrust sheet climbs up the stratigraphic section. Displacement over these features causes repeated, cyclic straining in the hanging-wall during movement. The strain rates associated with deformation at perturbations, ramps of different geometries and different displacement rates are estimated and used to discuss the influence of footwall geometry on the structural evolution of a thrust sheet. Particular attention is given to the range of fault rocks and deformation microstructures preserved after movement over a footwall with a complex geometry. Perturbations are suggested to be important in the localization of ramps, either because they create ‘sticking points’ near the fault tip during propagation or because they induce eventual failure in the hanging-wall after the movement over a number of these features raises the accumulated damage to a critical level. Analysis of the influence of the exact geometry of ramps on deformation processes during displacement leads to two important conclusions. Firstly, the exact geometry of ramps (i.e. the maximum dip angle and the straining distance from a flat to this maximum angle) may be used to estimate a maximum displacement rate of the thrust sheet. Secondly, the listric geometry of ramps may be an equilibrium shape adjusted to the displacement rate and the rheology of the hanging-wall. Adjustments towards the final geometry may involve the generation of shortcuts on either hanging- or footwall which reduce the imposed deformation rate in the hanging-wall during displacement.

Deformation mechanisms accommodating faulting of quartzite under upper crustal conditions

Abstract Analysis of the deformation microstructures associated with a high-level fault in quartzite (Skiag Bridge, Assynt, NW Scotland) reveals a complex variation in the deformation mechanisms active during faulting. The different mechanisms have been identified using an integrated study involving optical, cathodoluminescence and electron (both SEM and TEM) microscopy. The specific mechanisms identified include: intragranular cleavage fracture (types 1, 2 and 3), brittle intergranular fracture (types 1 and 2), low-temperature ductile fracture, diffusion mass transfer and low-temperature crystal plasticity. Fracturing dominates the deformation (faulting), initially via intragranular extension cleavage fractures due to stress concentrations at grain contacts (although many of these may be healed by quasi-simultaneous diffusive mass transfer processes). These eventually link and are then exploited as shear fractures, leading to the development of microbreccia-cataclasite zones which define a three-dimensional fracture array. Quasi-simultaneous diffusive mass transfer processes may heal these through-going fractures. Continued fault zone deformation involves the development of a damage (‘wake’) zone along the displacement zone borders where low-temperature plasticity and subsequent low-temperature ductile fracture processes aid the expansion of the fault zone. This study emphasizes that the evolution of the Skiag Bridge fault zone has involved three main categories of deformation mechanisms: fracture, crystal plasticity and diffusion mass transfer. The interrelationship between these categories, and the transition between individual fracture mechanisms, are significant aspects of this evolution. The examples presented demonstrate the complex interrelationships which exist between a group of deformation mechanisms and emphasize the potential importance of low-temperature plasticity and low-temperature ductile fracture processes during faulting under upper crustal conditions.