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The PD3 carbonate reservoir atlas provides a summary of the principle structural, sedimentological, diagenetic and petrophysical features that might be encountered on a carbonate platform.

Carbonate platforms are composed of several different depositional elements, often including build-ups and reefs developed on the platform margins, parallel-layered platform interior elements and slope deposits.

This section describes the characteristics of faults and fractures in carbonate reservoirs

A normal fault is a geological fault in which the hanging-wall (the fault block sitting above the fault plane) is displaced downwards relative to the footwall (the fault block sitting below the fault plane). Normal faults characteristically form during regional extension (e.g. in rift basins), but may also form in any type of stress regime.

Transfer (or accomodation) zones are areas of deformation between two normal faults that overstep in map view. Relay zones are areas of deformation between two normal faults that overstep in map view

A reverse fault is a geological fault in which the hanging-wall (the fault block sitting above the fault plane) is displaced upwards relative to the footwall (the fault block sitting below the fault plane).

A strike-slip fault is a geological fault in which the slip vector is (sub-)parallel to strike of the fault plane, i.e. creating a motion where the fault blocks on either side of the fault plane slide alongside one another.

The fault core is the central part of a fault, where most (>80%) of the displacement is accommodated.

Carbonate depositional environments are dominatly shallow marine, but often also include sediments deposited in peritidal zones, where there is daily or longer emergence, and deep subtidal environments. Less commonly, carbonate deposition can be observed within lakes and around terrestrial hot springs.

Peritidal sediments are deposited within the shoreline area that is subjected to daily or less frequent tidal action, above the subtidal zone (which is permanently below sea level) but which is not permanently emergent.

One of the principle depositional environments on the carbonate platform top and inner ramp is the shallow subtidal environment. Often this area is protected from the open ocean by a constructional barrier or grainstone shoal, and therefore depositional energy levels are reduced. This environment is typically referred to as a lagoon.

Shallow water, subtidal lagoons are usally protected from the open ocean by a constructional barrier or grainstone shoal, and therefore the energy levels are reduced. However, when the barrier has a relatively low topography, then the inner rampor platform top might be subject to higher wave and current energy. This is particularly true on platforms that face the open ocean. In this case, sedimentation is dominated more by wave and current activity, and sediment transportation, than in lower energy, protected lagoons.

Carbonate build-up is a generic term used to describe any organic deposit that forms topography above the sea floor. The composition of carbonate build-ups has changed through time as a function of evolution, and they tend to only occur during periods where organisms with the capacity to build topographic relief occur.

Carbonate platform margin facies associations will form within the high energy margin of a carbonate platform; where facies are constructive then they are responsible for the formation of the shelf break. Platform margin facies are typified by two principal lithofacies associations, both of which are indicative of deposition with a moderate to high energy setting in clear, shallow water:

• Constructive carbonate facies such as mounds, reefs and build-ups
• Grain-dominated, remobilised sediment that might collectively be referred to as a ‘shoal’ but includes sandbars and sand waves.

Carbonate slope facies associations will form immediately offshore of the shelf break on a flat-topped carbonate platform and within mid to outer ramp settings on carbonate ramps. They may also be recognised within intrashelf basins formed on the top of flat-topped (usually epeiric) carbonate platforms. The types of facies that are present will be largely dependant on slope angle.

Carbonate basinal facies associations will form distal to the shelf break of a flat-topped carbonate platform and within outer ramp settings, beneath storm-weather wave base. They may also be recognised within intrashelf basins that form on the top of epeiric platforms.

Carbonate sediments are often interbedded with other rock types, which may provide information on palaeoclimatic conditions and basin evolution. Identification of non carbonate facies can also be critical to reservoir quality and architecture.

Precipitation of evaporite minerals can be intimately related to carbonate sedimentation, particularly where platforms have grown in arid basins. Precipitation of evaporites typically occurs in highly saline seawater that has undergone high levels of evaporation, potentially forming laterally extensive layers and, often, a seal to the reservoir.

Siliciclastic sediments can be intercalated with carbonate sediments on carbonate platforms when the platform is land-attached. Delivery of sediment is usually via fluvial and deltaic systems that become established during a fall in relative sea level and/or hinterland rejuvenation. The palaeoclimate has to be sufficiently humid for these systems to develop. The grain size, sorting and composition of the sediment will be dictated by hinterland topography, climate and drainage.

Carbonate platforms may be associated with igneous rocks, since carbonate platforms often grow in rift basins and because isolated carbonate platforms can form on volcanic pedestals.

Marine diagenesis embraces all processes that take place on the sea floor after sedimentation and before burial. It includes bioturbation, boring and micritisation, cementation, dolomitization and, less commonly, dissolution

Calcite and aragonite cementation takes place in the marine phreatic environment on platform margins from seawater that is supersaturated with respect to calcium carbonate. This occurs where there is a continual flux of seawater, at high flow rates, though primary macropore networks. Cementation is facilitated by the warming of rising ocean water.

In the platform interior, marine diagenesis includes bioturbation, boring and micritisation, cementaiton and dolomitization.

The mixing zone forms along coastlines, where the oceanward flux of groundwater interacts with saline water that migrates landwards. Density stratification results in a lower warm saline water body and a cooler, uppermost freshwater lens; circulation entrains seawater into the freshwater lens resulting in salinization and formation of a mixing zone (Hubert, 1940; Plummer, 1975). The position of the mixing zone fluctuates in response to changes in sea-level and groundwater flux.

Hardgrounds form in submarine settings on carbonate platforms when there is a break in sedimentation, which allows circulation of seawater and precipitation of calcite cements. They might be indicative of a local change in sedimentary conditions or a more platform-wide cessation of deposition.

When carbonate platforms become emergent, above sea level, then deposition is terminated and a series of diagentic processes begin. What type of diagenetic modification occurs, and its extent, is dependent upon a number of parameters - in particular palaeoclimate and length of emergence

Incipient surfaces are stratigraphic marker horizons that are indicative of a break in sedimentation, but which are hard to identify and may have a polygenetic origin (e.g Rameil et al., 2012). Despite their subtle features, they might record long periods of non-deposition on a carbonate platform

Calcretes, or caliche, are a highly complex suite of vertically-zoned features formed by inorganic and biologically-mediated calcite precipitation and physical modification of the sediment, particularly by plant roots. When sufficient time and rainfall allows soil formation, aluminosilicate clays become abundant, forming distinctive, red coloured lateritic soils (so-called terra rossa). Calcretes and palaeosols would not have formed in the Lower Palaeozoic, prior to the evolution of land plants.

Karst is a broad term to describe the large (metre-scale and above) solution modification of emergent carbonate platforms. Karstification largely involves dissolution and can occur either immediatly following deposition, or sometime later, following uplift of the platform.

Cementation by groundwater takes place in both the vadose zone (above the water table) and the phreatic zone (beneath the water table), but is most significant in the phreatic zone, where pores are completely saturated by water.

Dolomitization is the diagenetic process by which limestone is converted to dolostone. A large proportion of the World's oil and gas reserves are in dolomitized or partially dolomitized reservoirs. Dolomitization can signifcantly improve reservoir properties, but this is not always the case and therefore it is important to determine the timing and processes governing dolomitization to assess reservoir quality

Dolomitization of limestone to form dolostone by geothermal convection of seawater can occur locally, usually on the platform margin or in the vicinity of faults.

Dolomitization of limestone by reflux can form dolostone over extensive areas of a platform (10’s to 100’s kilometres). The process usually results in a wholesale replacement of the limestone by a crystalline dolomite, which may be fabric-retentive or fabric destructive, with a complete reorganisation of porosity.

Since dolomite cementation and replacement is kinetically favoured by higher temperatures, then dolomitisation should be more favourable under burial conditions than in the near-surface environment. Typically, dolomitization in the burial realm forms non-stratabound bodies, usually around faults, with complex textures and pore networks.

Dedolomitization occurs when replacive dolomite or dolomite cement is calcitized. It is a poorly understood feature and is relatively uncommon. It is usually thought to occur from meteoric fluids, but there is also evidence for dedolomitization from hydrothermal fluids.

Burial diagenesis embraces all diagenetic events that take place within carbonate rocks once they are buried to a depth where they are no longer influenced by unmodified surface fluids

Pervasive carbonate cementation can take place within the burial realm, filling primary and secondary macropores and fractures.

Sulphates can occur as replacive or pore filling cements within carbonate sediments. Anhydrite is by far the most common sulphate, but celestite and barite might also commonly be observed. The presence of depositional and diagenetic anhydrite substantially increases the chance of hydrocarbon-related reactions

Burial diagenesis might be associated with a wide range of non-carbonate minerals, including quartz, fluorite, sulphides, sulphates and clay minerals (particularly kaolinite)

Compaction is a physical process by which porosity is reduced during burial. In carbonate rocks, there are two types of compaction:

• Mechanical compaction, where pores are closed and grains are pushed closer together. This can result in grain breakage and the formation of grain-grain contacts
• Chemical compaction, where carbonate dissolves at point contacts. Soluble carbonate moves away from the point contact and insoluble material remains to define a solution seam or stylolite

Although the burial realm is typically a zone of net porosity degradation, there is growing evidence that porosity can increase during burial as a result of dissolution.

Solution collapse is a generic descriptor for the product of a number of processes which might create porosity or porosity heterogeneity at a metre scale or larger. It embraces all processes where rock matrix has been removed and the subsequent void has collapsed, usually due to overburden pressure.

Porosity in carbonate reservoir rocks in inherently heterogeneous and complex because it is often largely the product of diagenetic processes and usually forms on multiple scales. Many carbonate reservoirs have bi- or multimodal pore networks.

Primary macroporosity includes all pores that were formed during deposition of the rock, and which have been preserved into the burial realm.

Mouldic porosity (non-touching vug porosity of Lucia, 2006) describes all pores that have been formed by the dissolution of specific grains. Mouldic porosity is fabric selective, and is often prefixed by the grain that has been dissolved (e.g. oomould, pelmould, dolomould, biomould).

Vuggy porosity (touching vug porosity of Lucia, 2007) describes all pores, from micron to centimetre scale, that have been formed by the combined dissolution of grains and matrix.

Intercrystalline porosity includes all pores that occur between crystals, normally within dolostones. They might be macro (> 30 microns), meso- (1 -30 microns) or micro (<1 micron); their pore size being dependent upon the crystallinity of the rock. Subsequent dissolution will create moulds or vugs.

Mesoporosity is rarely used in the literature as a descriptive term, but is used here to describe all primary and secondary matrix pores that are >1 micron diameter but < 30 microns diameter. This means that they cannot be seen as individual pores in thin section, but can be imaged using BSEM and X-Ray CT imaging