Fault-controlled dolomitization has been documented in Lower Carboniferous (Viséan) platform carbonates at various localities in the Pennine Basin and North Wales. The largest of these dolomite bodies (approx. 60 km2) occurs on the Derbyshire Platform, on the southern margin of the Pennine Basin. This study tests the hypothesis that dolomitization occurred at this locality during deposition, platform drowning, and the earliest stages of burial, coincident with the transition from a late syn-rift to post-rift regime. It also assesses the importance of syn-rift volcanism on dolomitization. Planar, fabric-retentive dolomite with single-phase (i.e., low temperature) fluid inclusions occurs along NW–SE and E–W oriented faults, and in platform margin facies and in proximity to the Masson Hill Volcanic Complex. Oxygen isotope data are consistent with dolomitization from seawater, but slightly depleted δ13C values reflect mixing with magmatic fluids. Volcanic activity is likely to have produced a thermal drive for fluid circulation on the platform margin, and post-depositional alteration of basalts by CO2-rich fluids could have led to alteration of olivine and release of magnesium to convecting seawater. Consequently, the large volume of dolostone on the southern margin of the Derbyshire Platform is attributed to the increased geothermal gradient and a localized increase in the Mg/Ca ratio of dolomitizing fluids at this locality, compared to elsewhere in the Pennine Basin. The results suggest that syn-rift carbonate platforms in volcanically active areas of rift basins have a greater potential for dolomitization from seawater than non-volcanic platforms in the same basin.

The Derbyshire Platform is a Mississippian aged flat-topped, steep sided platform that forms the westernmost expression of the Derbyshire-East Midlands Platform. On the south-east platform margin, 60 km 2 of Visean limestone has been dolomitised, forming two distinct bodies. One of these bodies forms along a major NW-SE trending basement fault and smaller, associated, N-S trending faults and fractures. This study uses outcrop, petrographic and geochemical analysis to better constrain the timing and mechanism for this fault-controlled dolomitisation. Field relationships demonstrate dolomitisation was multi-phase and initiated after the main phase of matrix pore-occluding calcite cementation on the Derbyshire Platform and terminated prior to the main phase of mineralisation. Fluids are interpreted to have fluxed from adjacent basins, primarily along strike-slip crustal faults that were reactivated during basin inversion at the onset of the Variscan Orogeny. Fluid supply was episodic and progressively confined to fractures as matrix porosity became occluded. The study demonstrates the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through seismic valving along faults that connect the carbonate platform to basin compartments. This ultimately controlled the position of dolomite geobodies along faults and provides a record of fluid flow during the transition from thermal subsidence to post-rift basin inversion. The findings have implications for the exploration of both minerals and hydrocarbon within dolomitised host rocks and can inform studies of fluid transfer and reaction on carbonate platforms within the burial realm.

The Lower Cretaceous Lekhwair Formation is one of the most prolific oil reservoirs in onshore and offshore UAE, yet the available literature on this interval remains limited. Based on a recent study carried out in collaboration with ADNOC Offshore, the present paper provides new insights into the comprehension of the interplay between primary depositional and secondary diagenetic controls on the reservoir performance, which is of crucial importance for the refinement of the static and dynamic models. In offshore Abu Dhabi, the Lower Lekhwair Formation is characterised by an alternation of relatively thick argillaceous (dense zones) and clean limestones (reservoir zones). Reservoir zones consist of basal, low to moderate energy inner ramp deposits, grading upward into thick inner and mid-ramp sediments. Lithocodium/Bacinella is the volumetrically dominant skeletal allochem and can form m-thick, stacked floatstone units. Such Lithocodium/Bacinella-rich floatstones are interpreted to originate from a mid-ramp depositional setting as a result of an increase in the accommodation space. By contrast, the contribution of Lithocodium/Bacinella floatstones is significantly reduced in inner ramp settings where these tend to form cm- to dm-scale, laterally discontinuous interbeds. The combination of sedimentological findings with diagenetic data provided an enhanced understanding of the origin and variations of the reservoir quality across the Lower Lekhwair Formation. In more detail, the best reservoir quality occurs within poorly cemented, Lithocodium/Bacinella-rich floatstones with grain-supported matrices, which favoured the preservation of a macropore-dominated pore system allowing an effective fluid flow. By contrast, the mud-supported textures with only rare and localised occurrence of mm- to cm-scale Lithocodium/Bacinella clumps, present the poorest reservoir quality due to the isolated nature of the macropores and the relatively tight micrite matrix surrounding them. At the large scale, the Lower Lekhwair shows an upward increase in reservoir quality, consistently with the upward increase in abundance and thickness of the Lithocodium/Bacinella-rich floatstones. The integration of depositional features with diagenetic overprint in the Lower Lekhwair Formation shows the fundamental role played by Lithocodium/Bacinella-rich floatstones with grain-supported matrices on the reservoir quality distribution. The impact of the Lithocodium/Bacinella floatstone matrices on the reservoir performance was never investigated before and hence represents an element of innovation and a powerful tool to predict the distribution of the areas hosting the best reservoir properties.

Carbonate reservoirs from the Lower Cretaceous Thamama Group are of major economic interest since they host some of the largest hydrocarbon accumulations in the United Arab Emirates. This study focuses on a Thamama reservoir from the Kharaib Formation and aims at complementing the regional geological understanding through the integration of newly cored and historical wells. The reservoir of interest consists of a thick organic and clay-rich dense zone at its base characterised by mud-supported, discoidal orbitolinid-rich deposits. This interpreted mid-ramp dense succession is overlain by a thick reservoir unit deposited under inner ramp environments, hence describing a large-scale shoaling trend. In detail, the reservoir succession records higher hydrodynamic conditions than the dense unit, as testified by the predominance of grain-supported textures (from packstone to grainstone). Floatstone to rudstone interbeds with grainy matrices are associated with Lithocodium/Bacinella- and rudist-rich accumulations mainly recorded in the lower and upper parts of the reservoir, respectively. A series of depositional cycles of regional significance have been recognised throughout the reservoir succession and are usually bounded by prominent stylolites, correlatable across the field. The reservoir succession is predominantly characterised by micropores, although macropores (especially vugs) also have an important contribution to the pore system. The extent and impact of dissolution is highly variable, but overall, it is responsible for the creation of most macropores (ie. secondary macropores are more abundant than primary macropores). Subsequent to dissolution, the pore system is typically heavily degraded by cementation from non-ferroan calcite cements and, to a lesser extent, by dolomite cements. An emphasis has also been put on recognising the residual hydrocarbon, the abundance of which varies considerably at field scale. To better constrain the vertical and lateral distribution of the reservoir heterogeneities, nineteen layers of interest depicting the main reservoir quality trends have been interpreted. The creation of comprehensive sets of maps – consisting of sedimentological, thickness, diagenetic and hydrocarbon staining maps – for each of these layers allowed a high-resolution understanding of the reservoir architecture. Of interest is the upward increase in reservoir quality reported towards the upper part of the reservoir unit, associated with the development of thick rudist-rich intervals, which favour the development of a macropore-dominated pore system facilitating fluid flow. By contrast, the common presence of stylolites plays a key role in the creation of baffles or barriers throughout the entire reservoir. This integrated approach has allowed a better prediction of flow units at field scale and provided valuable input data for future reservoir modelling.

The interpretation of depositional environments following sedimentological description is one of the most fundamental steps for the understanding of the vertical and lateral lithofacies groups’ distribution. This is directly linked to sequence stratigraphy and reservoir quality heterogeneity. Despite its importance, many interpretative lithofacies association schemes fail to capture, in a clear, systematic and flexible way, the key environmental features affecting the depositional fabrics’ distribution and their associated reservoir quality. This is especially true in carbonate systems, where the reservoir quality evaluation is complicated by the deep interrelation between physical (i.e. hydrodynamism) and biological processes. Our innovative depositional environment scheme (DE) comprises a variety of codes relating to key depositional and environmental characteristics in a way that captures their hierarchical organisation. These codes comprise groups of lithofacies interpreted to be genetically related, which provide metre-scale information commonly correlatable with wireline logs, that constitute ‘building blocks’ for static depositional architecture. The carbonate DE scheme is designed to be generic and flexible and therefore can be applied to any carbonate ramp, shelf or platform setting. The DE nomenclature uses an ‘initials-type’, letter-based abbreviation coding, which provides a straightforward and accessible understanding of the interpreted depositional environments. This comprises a suite of codes organised from a large to a small(er) scale using the formula: ‘1.2’. The first uppercase abbreviation code (1) defines the environmental belt and the energy level (i.e. hydrodynamism) from the continental to the deep offshore realm. Hydrodynamism abbreviations are typically appropriate for inner/proximal settings where a wide range of energy levels may occur.The second lowercase abbreviation code (2) corresponds to qualifiers that are used to highlight subtle variations in environmental conditions favouring the development of specific faunal assemblages (e.g. Lithocodium/Bacinella or rudists) or sedimentological features (e.g. clay content, lamination). These qualifiers allow a direct link to reservoir quality variations at the stage of sedimentological coding. Where specific geobodies can be interpreted within an environmental belt (e.g. an oolitic shoal in a dominantly high-energy inner/proximal setting) a code referring to the geobody can be added as a suffix in uppercase. The hierarchical organisation of this scheme is proven to be of great value for reservoir quality analysis. This is because it allows a simple and direct visualisation of the gross environmental settings on a carbonate platform, alongside its hydrodynamism (i.e. high-energy oolitic grainstones vs low-energy argillaceous-rich mudstones). Furthermore, the qualifiers capturing argillaceous content, allochem composition and/or depositional features are extremely effective to record reservoir quality heterogeneity and enhance sweet-spot prediction.