Alexander Foote’s research while affiliated with Badley Ashton and other places

Throughout the UAE and the wider region, several broadly E-W orientated structural lineaments are observed on seismic within the Cretaceous successions and are described as strike-slip faults. However, in the studied field, these features have not been readily observed in well data. Instead, networks of fractures and deformation features are present in core and borehole images. A study was carried out in an attempt to calibrate well and seismic data and to understand the relationship between the seismically-resolved faults and the fractures observed on core. This study focuses on a dataset from the north-east part of the field, which includes BHI images, cores, full 3D CT scans and conventional logs in four penetrations, three of which are horizontal, drilled through the faults; as well as 3D seismic data and relevant derived horizons and fault polygon interpretations. The available data have been investigated in detail, with all structural features in core, circumferential CT scans and BHI images systematically classified using simple and reproducible descriptive schemes. All the structural features have been orientated using directional data from BHI. The understanding of the character and fill of the fractures observed in core has also been incorporated. A further calibration with seismic and integration of results with information from previous studies allowed a full description of the fracture networks, of their densities within and outside the potential fault corridors of the studied field, as well as an assessment of their potential for reactivation and their possible impact on localised formation compaction. On the BHI images, several sub-vertical fractures have been identified, consisting mainly of mixed resistivity and resistive fractures, striking dominantly WNW-ESE. Particular zones along the wells have noticeably higher fracture densities, where features are organised in clusters; they are intercalated with zones where fractures are rarer. The clustering of fractures within fracture corridors are believed to be fault-related, subvertical and tabular fracture clusters that traverse an entire reservoir unit vertically and extend for several hundreds to thousands of feet laterally. These zones are believed to represent fracture corridors, which correlate with the structural lineaments observed on seismic. The fracture corridor network in the study area shows a variable deformation signature at the different scales of observations, but consists mainly of sub-vertical (dominantly >60°) deformation bands (c.50% of the features identified) and partially-cemented fractures (c.25-40%). Some of these features show a small displacement and it is believed this scaled variation in deformation within the corridors accounts for the overall larger, but relatively minor displacement observed on seismic (c.10-40ft vertical throw and possibly up to c.500m cumulative strike-slip observed in seismic).

In the onshore sector of the United Arab Emirates, the Lower Arab D Member (Kimmeridgian) typically encompasses a thick succession of rather homogeneous low-energy mid-ramp carbonate mudstones interbedded with minor storm-induced cm-scale skeletal-rich floatstones. Within these deposits, the pore volume is dominated by locally abundant matrix-hosted micropores, along with variably abundant open to partially cemented fractures, primary intraparticle macropores and rare moulds and vugs. As a result of this variably developed pore system, measured porosity varies from poor to very good, while permeability changes from extremely poor to rarely good. Detailed petrographic observations (thin-sections, SEM) carried out within six cored wells in a sour gas reservoir highlight that the variations in reservoir properties are primarily linked to the micron-scale variations in the micritic fabric. Indeed, anhedral compact micrites with coalescent intercrystalline contacts are associated with very small and poorly connected micropores, while polyhedral to subrounded micrites with facial to subpunctic intercrystalline contacts show locally well-developed micropores and therefore better reservoir potential. δ18O and δ13C isotope measurements do not discriminate both micritic fabrics, indicating a recrystallisation of the matrix within shallow burial conditions. However, bulk XRF measurements, and especially SiO2, Al2O3 and Fe2O3 content indicate that poorly porous anhedral compact micrite host more insoluble material and have been prone to a greater compaction compared to porous polyhedral micrites. Log-derived elastic properties, including Young's Modulus (YME) along with porosity data, have been used in two wells to explore the potential relationship between micritic fabric, porosity, permeability and elastic properties. With the evolution of micritic fabric from anhedral compact to polyhedral / subrounded, Young's Modulus decreases with increasing porosity, indicating a decrease in the overall stiffness of the rock. Based on these two learning wells, specific porosity and YME cut-offs have been identified to discriminate the various micrite fabrics. Those cut-offs have been successfully tested in four other wells used as a blind test for the vertical prediction of the micritic fabrics, in which accurate predictions reached up to 90%. Following these results, porosity and YME cut-offs have been used to produce the first model of the distribution of the various micritic fabrics at the field-scale. These results have a fundamental impact on how sedimentologically homogenous microporous limestones can be described and predicted at the well and field-scales, especially in the context of exploring tight carbonate plays associated with intrashelf basins.

This paper summarises the results of an intense reservoir appraisal programme that was conducted within a sour gas reservoir, onshore UAE, in order to determine the controls on porosity presence, distribution and effectiveness. This multiwell study was conducted over less than 5 years in order to define and predict porosity sweet-spots, and prepare for first gas, which was achieved in 2015. Focus is on the Arab D Member of the Arab Formation, the thickest and volumetrically most significant reservoir interval. It comprises an upward-cleaning succession of lime mudstone, peloidal and skeletal packstone and oolitic grainstone, culminating in a clay-rich, brecciated emergent surface. The entire succession shallows upwards from very low energy, basinal mudstones into cross-bedded oolitic grainstones that were deposited in a shoal complex that protected a gently agitated lagoon. Low skeletal diversity and the abundance of ooids are indicative of slightly elevated salinities; the abundance of anhydrite within the overalying Arab A/B/C and Hith Formation is indicative of basin dessication. The lowermost Arab D Member is dominated by lime mudstones and is highly microporous. A very low matrix permeability is locally enhanced by short, partially cemented, vertical fractures formed perpendicular to stylolites, usually cutting thin skeletal floatstone beds with abundant biomoulds. The main reservoir interval is the overlying upper Arab D Member. Porosity occurs principally as intraparticle micorporosity in ooids but in the uppermost upper Arab D Member, interparticle cement volumes are lower and macropores are preserved. The best reservoir properties occur within this bioturbated oolitic grainstone facies, and in cross-bedded oolitic grainstone near the palaeo-crest of the field, probably because early oil emplacement inhibited cementation. There is good evidence that the base of the upper Arab D Member and the lower Arab D member are in communication. Although there is evidence of thermochemical sulphate reduction in the Arab D Member, it is volumetrically minor and unlikely to have generated the high volumes of H2S that are known to occur in the field.

The Lower Arab D Member (Kimmeridgian) in onshore UAE is typically characterised by a thick succession of homogeneous mudstones with local cm-scale interbedded bivalve-rich floatstones, which are thought to have been deposited in a low-energy mid-ramp setting. This sedimentological unit is located at the base of a sour gas reservoir that includes the oolitic grainstones of the Upper Arab D Member. The pore system in these micritic deposits is dominated by matrix-hosted microporosity, along with open to partially cemented fractures, primary intraparticle macropores and rare biomoulds in the shell beds, hence a poor to very good porosity and extremely poor to rarely excellent permeability. Variations in porosity and permeability values appear to be strongly related to variations in the micritic fabric: both porosity and permeability increase when the micritic fabric evolves from anhedral compact with coalescent intercrystalline contacts (associated with very little and poorly connected micropores) to subrounded with facial to subpunctic intercrystalline contacts (with locally well-developed micropores). Micritic fabrics also clearly impact the elastic properties of the rock. Through analysis of elastic moduli calculated from standard density, and shear/compressional sonic wireline logs, the relationship between micritic fabric, porosity, permeability and geomechanical properties has been explored. With the evolution of micritic fabric from anhedral compact to subrounded, Young's Modulus decreases with increasing porosity and permeability, indicating a decrease in the overall stiffness of the mudstones. The implication of this observation is fundamental for the development of natural fractures within the Arab D, which are used as conduits for the vertical fluid flow. Indeed, stylolites with associated partially cemented tension gashes are commonly observed at the rheological boundaries, providing further secondary macroporosity and permeability anisotropy within the reservoir. In this study, the observed link between micritic fabrics, log-derived porosity and elastic moduli within cored intervals has been used to predict micron-scale micritic fabric distribution in uncored wells from wireline logs only.

Reservoir quality of carbonate rocks is usually controlled by the interplay of both the primary depositional and secondary diagenetic parameters. The assessment of the respective impact of these controls together with the understanding of the field-scale sedimentological organisation and diagenetic trends assist in the reconstruction of reservoir architecture and help production and appraisal programs. This work focuses on three formations recorded in the onshore Abu Dhabi area with the final aim of understanding their field-scale architecture through the study of six wells. Sediments reflect deposition in clay-prone and cleaner inner ramp to distal mid-ramp, where biotic assemblage is either dominated by Lithocodium/Bacinella (i.e. within lower Shuaiba and lowermost Lekhwair), rudists or peloids (i.e. within Kharaib). The sedimentological framework has been established through a detailed sedimentological description of c.2545ft of core and sequence stratigraphy interpretation. The occurrence of diagenetic processes (i.e. dissolution, cementation and fracturing/compaction) and their respective impact on pore system have been assessed through the observations of 804 thin-sections and the structural logging of c. 1936ft of core. The integration of the field-scale sedimentological organisation together with the distribution of the assessed reservoir quality controls and porosity/permeability data results in the establishment of the reservoir architectures of the three formations. In this area, the depositional fabric is characterised to be the primary parameter on the reservoir properties of the cored Thamama deposits with the exception of the lowermost cored Lekhwair and fractured reservoir: the best reservoir quality is found within inner ramp (locally dominated by Lithocodium/Bacinella) to backshoal grainstones and rudist-rich floatstones associated with a grainstone matrix. The reservoir quality decreases with the progressive increase in micrite matrix and clay content. The lowest porosity/permeability values are linked to the clay-rich inner ramp deposits recorded within the Dense Units, forming thick seals between the cleaner carbonate reservoir units. The localised occurrence of late dissolution phases has locally enhanced pore connectivity and preferentially affects the deposits which are initially characterised by good pore connectivity and volume (as observed in the lowermost Lekhwair and upper Kharaib Formations). Finally, cementation only locally decreases the reservoir properties and is broadly preferentially developed within and nearby clay-enriched deposits and at sharp lithological contacts often overprinted by stylolites. The focus of cementation on these surfaces results in one extensive ft-thick baffle within the Kharaib reservoir while a similar baffle is locally breached by rare 20cm-long fractures in Lower Shuaiba reservoir developed at the hinge of the anticline. Finally, the abundant and connected fracture network occurring within the lower Lowermost Lekhwair is likely to play a role on fluid flow in subsurface. The reservoir architecture will be integrated in the rock typing workflow to assist in the prediction of rock type vertical distribution and their lateral extent.