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.

Several challenges are associated with the characterization of organic rich unconventional plays, most significantly with the identification of sweet spots for optimum placement of horizontal wells, estimation of producible hydrocarbons and subsequent stimulation design. This paper presents the petrophysics and geomechanics integration approach from the X Formation and the important factors for the identification of sweet spots. The case study concentrates on the X Formation that consists of a succession of argillaceous limestone, mostly fine grained packstones and wackestones together with subordinate calcareous shales in the lower part. The complex carbonate lithology and fabric combined with low porosity and the requirement to evaluate total organic carbon presents a challenge to conventional logs and evaluation of them. Amid all the rock properties, the low permeability and productivity dictate the requirement to stimulate the wells effectively. Detailed integration of advanced and conventional log data, core data, mud logs and geomechanical analysis plays a critical role in the evaluation and development of these organic rich unconventional reservoirs. Extensive data gathering was done with wireline logging suite, which covered Resistivitiy/Density/Neutron/Spectral GR- Acoustic logs – Resistivity & Acoustic Images – Dielectric- NMR - Advanced Elemental Spectroscopy technologies and microfrac tests to characterize the hydrocarbon potential, sweet spots and in-situ stress contrast within the organic rich X Formation. The azimuthal and transverse acoustic anisotropies were obtained from X-dipole data to fully characterize the elastic properties of the formation. The static elastic properties were obtained using empirical core correlations as triaxial core tests were not available at the time of the study. The stress profile was calibrated against straddle packer microfrac tests to identify intervals with stress contrast for proper hydraulic fracturing interval selection. The integration of conventional and advanced logs enabled the accurate evaluation of total organic carbon (TOC), petrophysical volumes, and sweet spot selection. The advanced elemental spectroscopy data provided the mineralogy, amount of carbon presence in the rock, and consequently the associated organic carbon within the X Formation. The NMR reservoir characterization provided lithology independent total porosity. The difference between the NMR and density porosities provides additional information about organic matter. NMR data was utilized in this case study to identify and differentiate the organic matter and hydrocarbon presence within the X Formation. Acoustic and image logs provided the geomechanical properties that enable selection of the best intervals for microfrac stress measurement and proper fracture containment modeling. Geomechanical workflow allowed identification of intervals with a good stress contrast in X formation. The core data and stress measurements are recommended for the accurate calibration of the stress profiles and hydraulic fracture propagation modeling. The extensive data integration work presented in this single-well study within X Fomation, is a key factor for any organic rich unconventional reservoir characterization that integrated geology, petrophysics, mineralogy, and geomechanics for sweet spot identification within tight oil carbonate reservoirs.