The Gachsaran Formation across Onshore Abu Dhabi and possibly across U.A.E poses high potential of generating Shallow Biogenic Gas (mainly methane) and as such has taken the attention to further investigate, understand and evaluate its capability for promising Gas Resources. The paper provides a detailed G&G analysis that has potentially allowed an appropriate characterization of this unique formation that has first time uncovered interesting data responses in differentiating the sweet spot. For the first time in the history of U.A.E., new data was acquired targeting specifically the Miocene, Gachsaran Formation. This includes; 2D Seismic and party 3D Seismic interpretations, thousands of feet continuous core, conventional and advanced subsurface and surface loggings, Formation Pressure, Fluid sampling, Geochemical and Geomechanical labs measurements, stimulations and Frac tests data. The Gachsaran Formation is very challenging due to complex, thinly bedded and intercalated lithological varitions, and tightness provides difficulties in identifying the promising areas of Gas bearing layers. A comprehensive analysis was performed, in the light of regional understanding, by integrating the results of all available data in the form of correlation, cluster analysis, cross plotting and well based rock physics to differentiate the effect of Gas existence within the formation. The potential zones were further tested and results were integrated to confirm the analysis. The Gachsaran Formation has been subdivided into Lower, Middle and Upper Gachsaran Members. The Lower Member is predominantly evaporitic, becoming more argillaceous carbonate and shale –bearing in the the Middle Member with comparatively less anhydrites. The Upper Member contains mainly anhydrites with interbedded shales and carbonates. The potential sequences which represent high Total Organic Carbon and Gas Shows are found within the Middle Gachsaran. Consequently, the Middle Gachsaran Member was analyzed based on the robust data acquisition performed. Several relationships among GR, TOC, Gas shows, Lithology, RHOB, NPHI, Sonic, AI, Vp/Vs, Gradient Impedance and XRD Clay mineralogy have been attempted to check possible identification of Gas existence effect on the data. This has led to identify the sweet spots caused by the existence of any dominant Gas within the Study Area. The potential zones were confirmed by well testing. Furthermore, data variables were distributed within a 3D Grid and based on the analysis performed the area of sweet spots were identified. In the next phase of the study, the results will be integrated with the upcoming Geophysical Seismic Inversion studies to further optimize the possibility of identifying the sweet spot across the Study Area. The robust data acquisition targeting Gachsaran was performed first time in the history of U.A.E. The results are encouraging in establishing the relationship to identify the dominant existence of Gas effects within the area. The estimation of realistic Gas In-Place and its confirmation of commercial discovery will open a new era of Shallow Gas resources within U.A.E.

The Hanifa and Tuwaiq Mountain formations (Upper Jurassic Sila Group) are underexplored units in Abu Dhabi. Although there are several discoveries from the Hanifa Formation, both onshore and offshore, and encouraging indications in the Tuwaiq Mountain Formation, the conventional potential of these formations has not been fully realised. Uncertainties exist between onshore-offshore and west-east correlations, with conflicting stratigraphic terminologies, and the distribution of the Hanifa and Tuwaiq Mountain formations across Abu Dhabi, particularly details of facies distributions and transitions and their impact on the distribution of depositional environments through time. To reduce uncertainties and unlock resources a fully integrated study of the lower Sila Group across Abu Dhabi Emirate has been undertaken. Approximately 150 wells were correlated and integrated with the interpretation of six regional seismic transects (3 W-E and 3 N-S). More detailed interpretation over areas with 3-D seismic coverage helped to improve stratigraphic understanding. Cored wells were used to refine sedimentological interpretationsand extrapolated into uncored wells. Resolution of stratigraphic uncertainties mean that, for the first time, there are consistent lithostratigraphic and sequence stratigraphic schemes across Abu Dhabi Emirate that provide a reliable framework for basin modelling and play mapping. Furthermore, integration of all the sub-studies has led to the development of new depositional models and more detailed GDE maps. These are important inputs to basin modelling and the development of revised play maps that clearly highlight the prospective areas in Abu Dhabi. Focus on these prospective areas has led to the recognition of a significant number of structural leads and three new stratigraphic play concepts. These concepts include: a truncation trap geometry to the east,a possible collapse/mass movement trap on the slope, anda possible combined stratigraphic/structural trap where four-way dip closures overlap both the shoal and the slope, with the facies change to muddier slope deposits providing potential down-dip seal.

Understanding reservoir architecture is key to comprehend the distribution of reservoir quality when evaluating a field's prospectivity. Renewed interest in the tight, gas-rich Middle Miocene anhydrite intervals (Anh-1, Anh-2, Anh-3, Anh-4 and Anh-6) by ADNOC has given new impetus to improving its reservoir characterisation. In this context, this study provides valuable new insights in geological knowledge at the field scale within a formation with limited existing studies. From a sedimentological point of view, the anhydrite layers of the Miocene Formation, Anh-1, Anh-2, Anh-3, Anh-4 and Anh-6 (which comprise three stacked sequences: Bur1, Bur2 and Bur3; Hardenbol et al., 1998), have comparable depositional organisation throughout the study area. Bur1 and Bur2 are characterised by an upward transition from intertidal-dominated deposits to low-energy inner ramp-dominated sedimentation displaying reasonably consistent thickness across the area. Bur3 deposits imply an initial upward deepening from an argillaceous intertidal-dominated to an argillaceous subtidal-dominated setting, followed by an upward shallowing into intertidal and supratidal sabkha-dominated environments. This Bur3 cycle thickens towards the south-east due to a possible deepening, resulting in the subtle increase in thickness of the subtidal and intertidal deposits occurring around the maximum-flooding surface. The interbedded relationship between the thin limestone and anhydrite layers within the intertidal and proximal inner ramp deposits impart strong permeability anisotropy, with the anhydrite acting as significant baffles to vertical fluid flow. A qualitative reservoir quality analysis, combining core sedimentology data from 10 wells, 331 CCA data points, 58 thin-sections and 10 SEM samples has identified that reservoir layers Anh-4 and Anh-6 contain the best porosity and permeability values, with the carbonate facies of the argillaceous-prone intertidal and distal inner ramp deposits hosting the best reservoir potential. Within these facies, the pore systems within the carbonate facies are impacted by varying degrees of dolomitisation and dissolution which enhance the pore system, and cementation (anhydrite and calcite), which degrade the pore system. The combination of these diagenetic phases results in the wide spread of porosity and permeability data observed. The integration of both the sedimentological features and diagenetic overprint of the Middle Miocene anhydrite intervals shows the fundamental role played by the depositional environment in its reservoir architecture. This study has revealed the carbonate-dominated depositional environment groups within the anhydrite stratigraphic layers likely host both the best storage capacity and flow potential. Within these carbonate-dominated layers, the thicker, homogenous carbonate deposits would be more conducive to vertical and lateral flow than thinner interbedded carbonates and anhydrites, which may present as baffles or barriers to vertical flow and create significant permeability anisotropy.

The COVID-19 pandemic limited global travel and access to core facilities. However, by adopting an innovative remote core description workflow, potential delays to an important reservoir characterisation study were avoided and mitigated. Over c.1700ft of middle Miocene core from an Onshore well in Abu Dhabi was described using high-resolution core photos, CT scans and CCA data. Detailed (1:20ft scale) descriptions of heterogeneous, mixed lithology sediments from a gas reservoir were produced. The aim when developing the workflow was not to try and replicate the process of in-person core description, but to create a workflow that could be executed remotely, whilst maintaining technical standards. Ideally, we wanted to find a solution that also had the potential to improve the overall quality of core description, by integrating more data from the onset. The workflow used a matrix to generate a confidence score for the description of each cored interval. Factors such as core condition were considered, which highly influences the extractable core information. The confidence score was used to make decisions, such as whether an in-person review of the core was necessary, especially where core condition was below a reasonable threshold. This helped prioritise cored intervals for review, ensuring time in the core store was focused, and allowed accuracy and reliability of the remote description to be assessed. The 4-phase workflow is summarised as: Image extraction of white light (WL), ultraviolet (UV) and computed tomography (CT) core images. Digital chart creation, core-to-log shifts and sample selection: Wireline data, CCA data and core images loaded Core images used to determine core-to-log shifts Thin section, SEM and XRD samples selected Remote core description: Conducted using all core imagery, CCA and wireline data Thin section, SEM and XRD data were used to refine the description when they became available A confidence score was given to each cored interval QC and finalization: Using the results from phase 3, a selection of cored intervals for in-person review was made. Intervals included those with a poor match between remote description and petrographic data, or areas with a low confidence score. Following the review, charts were finalised and quality-checked for data export Using this workflow, ensured work on an important study could continue during the pandemic. Such an approach has continued value for future studies as it increases efficiency and accounts for more data to be considered in core description prior to viewing the core in-person; it has been used on recent studies with great success. Another benefit to this approach is that less time in the core store is required, reducing potential HSE risks and helping to manage core store availability in busy facilities.