Quentin J. Fisher

Mudrock‐hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition

Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly under‐estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow (<500 m) depths and (iv) cementation can create a framework that prevents compaction and preserves porosity. Cement textures can be used to distinguish two modes of growth; concentric growth, where successive layers of cement are added to the outer surface (radius increases with time), and pervasive growth, where cement crystals grow simultaneously throughout the concretion volume (little or no radius increase with time). Cement textures of siderite concretions are mostly consistent with pervasive growth, but many calcite microsparite concretions show no diagnostic textural features and could grow either concentrically or pervasively. Concretionary cementation, whether concentric or pervasive, occurred such that there was accessible porosity which could be filled by later cements. Pervasive growth in particular is associated with the retention of substantial amounts of porosity which may be filled by chemically and isotopically distinct phases. The resulting chemical gradients across concretions may then reflect variations in the relative proportions of early and later cements more than variations in porewater composition. Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from −30‰ to +15‰, and thus overlaps 13C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).

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.

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.

Anisotropic permeability and bimodal pore-size distributions of fine-grained marine sediments

Abstract We present preliminary results illustrating that overconsolidated fine-grained sediments without a compactional fabric but favourably aligned microfractures can show significant anisotropic permeability. At much reduced effective stress, substantial flow can occur parallel to the fabric such that anisotropy indices — calculated by dividing horizontal permeability by vertical permeability — increase by factors above 10. SEM observations show no intense particle alignments in the materials, but the presence of parallel oriented microfractures; mercury-intrusion porosimetry data indicate that this enhanced flow is due to microfractures opening when favourably oriented. Such an effect is displayed as a distinctly bimodal pore-size distribution when intruded by mercury parallel to the fabric; identical samples with the fabric oriented perpendicular to intrusion direction show only unimodal pore-size distributions. While the results, at present, are only directly applicable to the shallow subsurface, it is proposed that directional bimodal pore-size distribution needs further investigation for deeper, more consolidated sediments in light of the potential ramifications highlighted by the results. For example, in some instances shales may not be as efficient hydrocarbon seals as traditionally thought.

Faulting and fault sealing in production simulation models: Brent Province, northern North Sea

Faults can severely compartmentalize pressures and fluids in producing reservoirs, and it is therefore important to take these effects into account when modelling field production characteristics. The Brent Group fields, northern North Sea, contain a complex arrangement of fault juxtapositions of a well-layered sand-shale reservoir stratigraphy, and fault zones containing a variety of fluid flow-retarding fault rock products. It has been our experience that these fault juxtapositions impact the ‘plumbing’ of the faulted layering system in the reservoirs and the models that are built to mimic them – and are, in fact, a first-order sensitivity on compartmentalization of pressures and fluid flow during production simulation. It is important, therefore, to capture and validate the geological feasibility of fault- horizon geometries, from the seismic interpretation through to the static geocellular model, by model building in conjunction with the interpretation. It is then equally important to preserve this geometrical information during geocellular transfer to the simulation model, where it is critical input data used for calculation of fault zone properties and fault transmissibility multipliers, used to mimic the flow-retarding effects of faults. Application of these multipliers to geometrically weak models tends to produce ambiguous or otherwise potentially misleading simulation results. We have systematically modelled transmissibility multipliers from the upscaled cellular structure and property grids of geometrically robust models – with reference to data on clay content and permeability of fault rocks present within drill core from the particular reservoir under study, or from similar nearby reservoirs within the same stratigraphy. Where these transmissibility multipliers have been incorporated into the production simulation models, the resulting history matches are far better and quicker than had been achieved previously. The results are particularly enhanced where the fault rock data are drawn from rocks that have experienced a similar burial–strain history to the reservoir under study.

Siderite concretions from nonmarine shales (Westphalian A) of the Pennines, England: Controls on their growth and composition

Back-scattered electron microscopy has been used to examine the microstructure of nonmarine-shale-hosted siderite concretions. The concretions are composed of 50-100 micrometer, zoned crystallites, which exhibit no noticeable center-to-edge variation within any individual concretion. This indicates that siderite crystallites nucleated at virtually the same time across the entire concretion and that the concretions did not grow by radial addition of siderite layers around a central nucleus. Further siderite precipitation took place by crystal growth onto the nuclei. The total proportion of siderite in any part of the concretion bears no simple relationship to the porosity of the enclosing shale at the time of precipitation, and growth by passive precipitation in pore space is unlikely. Integration of microprobe data with bulk mineral-chemical and stable-isotope data suggests that the siderite crystallites are composed of an Fe-Mn-rich end member with a delta 13 C value of approximately +10 per mil and a Mg-Ca-rich end member with a delta 13 C value of approximately 0 per mil to -5 per mil. The mineral-chemical and stable-isotope compositions of these concretions resulted from microbially mediated processes operating close (<10 m) to the sediment-water interface, during methanogenesis. Methanogenesis can generate low-delta 13 C as well as high-delta 13 C carbonate cements, hence deep-burial diagenetic reactions, such as decarboxylation of organic matter, need not be invoked to generate solutes for siderite precipitation.

Authigenic mineralization and detrital clay binding by freshwater biofilms: The Brahmani river, India

Epilithic biofilms, growing on submerged boulders, were collected upstream and downstream of sites of industrial discharge into the Brahmani River, Orissa State, India. Transmission electron microscopy (TEM) showed that the outer cell walls of attached bacteria in all samples were often encrusted with fine‐grained (<1 μm) inorganic precipitates. The density of mineralization ranged from a few epicellular grains to complete encrustation by clayey materials. Energy‐dispersive x‐ray spectroscopy (EDS) and selected‐area electron diffraction (SAED) indicated that the most abundant inorganic phase was a complex, poorly ordered, (Fe, Al)‐silicate of variable composition, containing minor amounts of potassium. No trace metals were detected in the authigenic precipitates. Bacterial cells were also found to entrap or adsorb detrital minerals such as kaolin, mica, quartz, iron oxide, and gibbsite onto their outer surfaces. Because epilithic microbial biofilms have a very large and highly reactive surface area, bindi...