Much of the research has suggested that there is a clear relationship between porosity and acoustic impedance in carbonate reservoirs, being the main objective of reservoir characterization using seismic amplitudes. However, this relationship can be particularly complicated in the setting with presence of various lithofacies and different elastic responses, which cannot be completely distinguished simply by acoustic impedance. This study entailed the discrimination and occurrence probability prediction of different lithofacies through the integration of simultaneous pre-stack seismic inversion and Bayesian-based classification. Consequently, the final estimation of porosity was obtained through the integration of lithofacies occurrence probability and the lithofacies-variant linear regressions between porosity and acoustic impedance. An adequate understanding of the diverse lithofacies distribution and their different elastic properties responses to reservoir properties is essential for an appropriate reservoir characterization. Firstly, a suitable petroelastic rock physics analysis study was executed to determine the possibility of discriminating different lithofacies and predicting different reservoir properties from elastic properties, and which elastic properties act best. The four lithofacies classifications (oolitic grainstones, pack/wackestones, mudstones and dolomites) in the target reservoir were distinguished based on the cross plot between compressional wave and shear wave velocity ratio (Vp/Vs) and acoustic impedance (Zp). For this reason, a simultaneous pre-stack seismic inversion was executed to obtain the elastic properties previously identified as discriminants (Vp/Vs and Zp), which were accordingly used in a supervised classification approach through the application of Bayesian-based probability density functions computed from lithofacies information at wells. This classification scheme delivered the occurrence probability cubes related to each lithofacies and the most probable lithofacies. At the end of the process, a pseudo porosity cube was computed through the integration of the lithofacies occurrence probability (denoted as Poo, Pp/w, Pmdst and Pdol,) and the lithofacies-variant linear regressions between porosity and acoustic impedance observed at wells. The linear regressions were estimated independently for oolitic grainstones (F(ϕ,Zp)oo), pack/wackestones (F(ϕ,Zp)p/w), mudstones (F(ϕ,Zp)mdst) and dolomites (F(ϕ,Zp)dol). The resultant pseudo porosity was estimated with the following equation: Pseudo Porosity = Poo*F(ϕ,Zp)oo*Zp + Pp/w*F(ϕ,Zp)p/w*Zp + Pmdst*F(ϕ,Zp)mdst*Zp + Pdol*F(ϕ,Zp)dol*Zp. In this work, volumes of Vp/Vs and Zp consistent with variant lithofacies behaviors were obtained through pre-stack seismic inversion and served for launching a litho-seismic classification, which honors the uncertainties related to the classification approach by the probability density functions. Results have demonstrated a promising conformance with geological information and a perfect positive correlation with the well data when blind tested. Given the above, the methodology presented in this study has demonstrated the potential advantages of integrating simultaneous pre-stack seismic inversion, Bayesian-based classification and lithofacies-variant linear regressions in reservoir characterization in areas with complex lithological settings. The obtained results would be an extremely valuable and rewarding guidance and bring novel insights in further exploration and development activities in study area.

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.

The Abu Dhabi onshore Anticlinal Fields are trending NE-SW and N-S, with their eastern flanks being of steeper dip (~3°-5°) than their western flanks (~1°-2°). In previous research, kinematics, and mechanics of onshore Abu Dhabi folded anticlines had not been addressed. The main objective of this paper is to understand the kinematics and mechanics of the folding systems.A new approach to understanding the mechanical evolution of these anticlines is presented here through the use of conceptual modelling and theoretical approaches. A combination of geological and geophysical tools is used for determining the direction of the shear stresses along the flexural slip planes. Although textural evidence, such as karstification and diagenesis, suggests local material redistribution on different scales, compositional trends support negligible volume flux at the scale of the anticlines. The preservation of bed length and bed thickness in most of the onshore Abu Dhabi Anticlines requires the development of bed-parallel shear. Abu Dhabi Anticlinal fields are mechanically evolved by flexural shear and flexural slip folding, which is characterized by differential simple shear along the bedding planes, where deformation is concurrent with the diagenesis. The layered horizons of these folds are mostly of carbonates, where the competency contrast is low and the friction between the layers is high leading to flexural shear. In the locations, where incompetent shale layers alternate with the carbonates, flexural slip is considered -although this is very limited within Abu Dhabi anticlinal structures. In these folds, the bedding planes of less competent layers act as boundaries of shear zones. Therefore, most of the internal deformation occurs within the less competent layers by partial recrystallization and development of fractures. However, the competent layers undergo pressure solution, development of sigmoidal extensional fissures, and stylolites in most cases. Thus, the overall folding system is heterogeneous and the competent layers simply slide over one another. The dominant direction of the over-riding shear is directed toward the fold axial surface and the stresses are released along the bedding plane discontinuities. The bedding plane discontinuities are considered to represent the finite neutral surface of the fold. Folds having parallel geometrical forms have opposite directions of structural shear in transtensional and transpressional regime.In the Abu Dhabi flexural slip shear folds, deformation in the hinge zone is characterized by pure shear, and deformation on fold limbs is characterized by components of both simple and pure shear. Strain models of pure bending, layer-parallel shear, and shear parallel to the hinge plane are shown to be the main mechanism of these folds. Preservation of these will play an important role in revealing a new play concept for the Abu Dhabi fields.

Recently there has been a growing interest in gas exploration, much of this focus has been directed toward thermogenic gas derived from cracking kerogen in the highly mature kitchens. However, a significant proportion of the global gas reserve is not thermogenic but of bacterial origin (Katz, 1995). Biogenic gas is an important exploration target because it occurs in geologically predictable circumstances, in areally widespread area, and in large quantities at shallow depths as free gas (Schneider et al, 2016). The recent exploration wells drilled in the northeast onshore Abu Dhabi showed elevated total gas readings during the drilling of the Gachsaran formation. Consequently, mud-weight was increased to control the gas flow. In addition, the recorded wireline logs indicate the presence of relatively high hydrocarbon saturations in several high porous zones of Gachsaran and Asmari formations. To assess the productivity and commerciality of the Biogenic gas potential in Abu Dhabi, several exploration wells are planned to be drilled before the end of 2019. The positive results of these wells will open the door for a new era of sweet gas exploration activities in Abu Dhabi and its surrounding areas. The primary gas reservoirs are thin carbonate and clastics layers in the Gachsaran Formation at a depth that ranges between 1600-5200 feet below sea level. Organic carbon isotopes, Rock Eval analysis, TOC log data and gas shows analysis indicated that the methane gas found in the Gachsaran Formation is of a biogenic origin and sourced mainly from the organic-rich argillaceous limestone of the Middle Gachsaran. Gachsaran formation is comprised of alternating thin layers of anhydrite, limestone, marl and shale sediments in addition to the presence of salt layers in the lower part. This mixed lithology resulted in the reservoirs property deterioration in particular by shale and anhydrite nodules cementation. The biogenic basin areal extent, significant thickness of the Gachsaran in this basin and the organic richness distribution, conclude possible generation of a huge volume of biogenic gas in northeast onshore Abu Dhabi. However, additional work is required to estimate the volume of gas that is accumulated and that can be produced from the Gachsaran and Asmari formations.

Drilling deep exploration wells to Pre-Khuff formations requires considerable investment and therefore data gathering through logs and cores should be of highest quality to maximize value of information. The abrasive nature combined with high downhole temperatures and bit sticking present a uniquely hostile environment. In this paper, we discuss challenges in data acquisition and propose best practices for logging and coring programs tailored for Pre-Khuff exploration targets. The oldest Pre-Khuff stratigraphic unit penetrated offshore is Silurian Qusaiba equivalent. Apart from Permo-carboniferous Unayzah, the other stratigraphic units encountered in Abu Dhabi Offshore are Permian Basal Khuff Clastics (BKC), Early Carboniferous Berwath, Silurian Tawil /Sharawra Formations. In recent years, a number of deep exploration wells penetrated Pre-Khuff in various offshore fields, often under challenging high temperature and high pressure conditions. Considering extensive data gathering programs, this includes mud logging, LWD, wireline logging, and core extraction followed by detailed core analysis. Log data acquired through wireline is frequently impaired by wash outs and bad hole conditions, hence posing a challenge to assess rock properties reliably. Cores were successfully acquired as whole core and sidewall core and extensive core analysis programs were conducted. The key objectives for data gathering in Pre-Khuff are lithostratigraphy, sedimentology and age dating, petrophysical properties and prove hydrocarbon presence as well as geomechanical properties for hydraulic fracturing of the formation for subsequent well testing operations. Palynological analysis at well site is important for age determination. Correlation of mud logging results with open hole logs and Borehole image logs proved essential to determine hydrocarbon presence. The complex lithology mix is best resolved with elemental spectroscopy logging. LWD log acquisition is preferred over wireline because of a reduced risk of stick and pull and minimum invasion. However, some useful measurements such as Dielectric log are currently only available through wireline and industry should develop this tool for LWD. NMR tools generally cannot cope with the high temperature environment. Hence, service industry is encouraged to provide higher spec NMR tools. Due to low matrix permeability, core analysis for crushed rock or retort methods are preferred to obtain permeability measurements through pulse decay and pressure decay methods. Recommendations are given for optimal data gathering to ensure maximum value of information and best possible data quality. Logging operations should be a combination of Logging-While –Drilling and Wireline operations. Core acquisition will have to exclude BKC (Basal Khuff Clastic) to limit the risk of core jamming. A typical core analysis program to assess petrophysical and geomechanical properties for Pre-Khuff is proposed, leading to a successful risk assessment prior to hydraulic fracturing of the formation. Further lithostratigraphic and age-determination techniques are included in the scope.