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PD3 (Prediction of Deposition, Deformation and Diagenesis in Carbonates) is a Joint Industry Project that combines expertise from leading carbonate geologists from Universities of Manchester, Bergen, Bristol and Liverpool. Phase I of PD3 is now complete and Phase II will launch in 2019. A summary of the project is provided below.

For more details please contact cathy.hollis@manchester.ac.uk

PD3 (Prediction of Deposition, Deformation and Diagenesis in Carbonates) is a Joint Industry Project that combines expertise from leading carbonate geologists from Universities of Manchester, Bergen, Bristol and Liverpool. Phase I of PD3 is now complete and Phase II will launch in 2019. A summary of the project is provided below and results of Phase I are documented at PD3 Phase I Summary of results

For more details please contact cathy.hollis@manchester.ac.uk

These pages provide a workflow for the systematic interpretation and evaluation of carbonate platforms. This is provided in the form of a decision matrix that can be used to:

1. establish the presence, or not, of a carbonate platform
2. appraise reservoir presence and the development potential of a discovered carbonate platform
3. recommend cost-effective and scientifically robust methods to improve recovery from producing carbonate fields

The decision matrix should be used as a guideline by which carbonate platform presence, structure, stratal architecture and diagenetic overprint can be assessed. It is not intended that a full range of reservoir properties will be provided, but guidance will be given on the most appropriate reservoir analogues.

A summary of the workflow is provided here workflow.pdf

The rationale behind the workflow for this phase of reservoir evaluation is to establish

a) the likelihood of carbonate strata development in the basin at a given point in the stratigraphy

b) assess whether a structure is likely to be a carbonate platform based on its shape, size, structural setting and seismic properties

c) if there is a high probability of a carbonate platform presence, present a range of appropriate reservoir analogues and

d) advise, based on a known set of structural, stratigraphic and sedimentological parameters, likely reservoir risks and opportunities for whatever carbonate strata may be present (eg. presence or absence of karst, presence or absence of dolostone).

The proposed workflow for each phase of work is outlined below, whilst details of the importance of determining individual parameters is described in the Appendices.

The structural and stratigraphic framework of the basin is often a critical control on whether a carbonate platform becomes established, and where it grows. The starting point for seismic interpretation should therefore be an analysis of the literature on the region to determine plate- and basin-scale tectonic evolution. Guidance as to the importance of plate and basin-scale tectonics and stratigraphic age on the probability of carbonate platform development is given in Appendix I. In addition, examine the World Stress Map to assist in the interpretation of the effect of in situ stresses on the sealing capacity of the mapped faults.

The identification of carbonate platforms on seismic data is inherantly challenging, because of poor quality imaging and the high potential for misinterpretion of high relief features, such as basement highs and volcanos, as carbonate platforms. The following workflow is modified from Burgess et al., 2013 (AAPG Bulletin) in order to improve confidence in carbonate platfom identification

In order to improve confidence in seismic interpretation results, screening of potential outcrop and subsurface analogues is recommended using the Carbonate Reservoir Database

During appraisal, the absence of data from which facies architecture and diagenetic modification can be mapped means multiple scenarios can be envisaged to both explain platform evolution through time, and also the resultant reservoir properties. Stratigraphic forward models and reactive transport models therefore offer an opportunity by which the sensitivity of the platform to particular environmental parameters – and its resultant response – can be analysed.

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Once the Carbonate Reservoir Database has been reviewed, and analogues iterated with the seismic interpretation, the play should be de-risked with respect to reservoir presence, seal quality and the distribution of potential flow-controlling sedimentological, structural and diagenetic units.

The rationale behind the workflow for this phase of reservoir evaluation is to refine estimates of in-place volumes and determine the reservoir recovery factor based on a range of potential recovery mechanisms. The workflow largely follows that defined for basin scale/frontier exploration, but assumes-

a) Prior analysis of basin formation mechanism, tectonostratigraphy and palaeo-plate reconstruction

b) A richer dataset, for example reprocessed seismic data, well data and/or more evolved analogue studies

c) A larger, more multi-disciplinary team, including petrophysicists, reservoir engineers and well & production engineers

Petrophysical analysis involves the integration of core, log and seismic data to determine the volume and distribution of porosity, saturation, net reservoir and permeability. During appraisal, it is assumed there are only 1-2 wells available with data, and possibly no core data.

Once core description, wireline log analysis and seismic interpretation has been conducted, the spatial distribution of facies and diagenetic overprint can be assessed and their impact on petrophysical properties interpreted.

During appraisal, the absence of data from which facies architecture and diagenetic modification can be mapped means multiple scenarios can be envisaged to both explain platform evolution through time, and also the resultant reservoir properties. Stratigraphic forward models and reactive transport models therefore offer an opportunity by which the sensitivity of the platform to particular environmental parameters – and its resultant response – can be analysed.

Reservoir models are critical to concept testing and selection for field development, including optimisation of well placement and analysis of full-field economics. In order that the most economically and technically feasible development option is selected, it is imperative that reservoir models are

a) constructed using geologically robust rules sets, providing confidence in interwell permeability prediction

b) have appropriately upscaled petrophysical properties, so that flow controlling layers are not obscured by averaging

c) have assigned dynamic data (e.g. relative permeability) that reflects, rather than obscures, reservoir heterogeneity

d) able to incorporate past and present reservoir performance within the geological interpretation, so that flow-controlling layers (e.g. baffles, barriers and high permeability streaks) are represented in the model, and

e) fundamentally linked to a fracture model

A range of geostatistical methods can be used for modelling carbonate reservoir architecture. None are used consistently by the industry, and the modelling workflow is often driven by corporate workflows (perhaps based on clastic reservoirs), reservoir architectural elements, data type, quality and volume, the timeframe available for model construction and prior experience of the reservoir modeller.

Once faults have been picked, surfaces mapped and the model grid constructed, most carbonate reservoirs should be modelled by a workflow that includes facies modelling, diagenetic modelling, petrophysical modelling and fracture modelling. However, during appraisal, it is unlikely that there will be sufficient data for this workflow to be developed in full. At this point it is more important to undertake a robust risk and uncertainty analysis and thereby assess the impact of these uncertainties by running multiple scenarios. One approach for ensuring that the most appropriate range of scenarios is modelled, taking account of the combined uncertainty of different parameters, is through experimental design (e.g. Hollis et al., 2011).

Many mature fields have a poor or disparate historical dataset, particularly with respect to core. Even where there has been diligent data collection, the varying vintages of core and log material, as well as a succession of different interpreters, alongside changing scientific paradigms might mean that there is a confusing, inconsistent opinion as to the origin and dimensions of geological parameters. Often there are perceptions as to why a field performs as it does which are not founded on systematic data collection and analysis. Consequently, prior to a full field re-evaluation of development strategy, a period of intense data collection and analysis is strongly recommended.

The aim of this phase of work is to ensure that the structural evolution of the carbonate platform is understood as completely as possible. This includes the style and timing of faulting, the relationships between faulting, burial and fracture distribution, and the timing of hydrocarbon charge

Normally during appraisal, reservoir architecture is established in the context of basin-scale sequence stratigraphy. This will have been driven largely by seismic data, with limited well calibration. With the addition of well data during appraisal and development drilling, this sedimentological interpretation can be revisited using the well and core interpretation workflows

Many reservoirs do not have a detailed conceptual model to explain the character and distribution of the diagenetic overprint and how it has influenced reservoir properties, even though most carbonate reservoirs have undergone sufficient diagenesis to significantly alter reservoir properties. Although sedimentary facies (associations) often form a template for diagenetic modification, porosity and permeability are typically also strongly influenced by dolomitization, cementation, post-depositional dissolution and/or fracturing. However, in the absence of clear workflows to link depositional rock properties to reservoir properties, and without guidelines for construction of diagenetic overprint within geocellular models, this phase of the workflow is often overlooked.

The aim of this phase of work is to group data into clusters of genetically associated rock types (i.e. mappable units which have evolved via the same depositional and diagenetic pathways) with consistent petrophysical properties. The process relies heavily on prior determination of a robust diagenetic framework and should build on data collected during Stages 1 and 2.

Reservoir models are critical to concept testing and selection for field development, including optimisation of well placement and analysis of full-field economics.

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

Microporosity is often used in the literature as a descriptive term, but it’s definition varies from < 1 micron, < 10 microns or < 30 microns. In all cases, microporosity may be primary or secondary in origin. Here it is defined as all matrix pores that are <1 micron diameter,

This database contains information from a spectrum of outcrop and subsurface studies, detailing their basin type, stratigraphic age, depositional facies, diagenesis, structural and petrophysical properties

This section provides links to stratigraphic and seismic forward models, fluid flow and reactive transport models and 3D geocellular models

This page provides a workflow for forward modelling, which is applicable to stratigraphic, seismic, geomechanical or fluid flow modelling, including reactive transport modelling.

For examples of stratigraphic and seismic forward models, go to Stratigraphic and seismic forward models

For examples of reactive transport models, got to Reactive transport models