Microbial dolomites from carbonate-evaporite sediments of the coastal sabkha of Abu Dhabi and their exploration implications

Detailed petrographic studies of Recent dolomites from the coastal sabkha of Abu Dhabi using SEM and Energy Dispersive X-ray (EDX) analyses indicate that most of the dolomite started to form in " micro-niches" (i.e. small isolated pore spaces) within the carbonate sediments. These micro-niches tend to retain connate waters, becoming anoxic and supporting specific microbial activity which ultimately results in the formation of dolomite. This novel concept may help to explain the origin of many dolomite types, since recently deposited carbonate sediments have high porosities (up to 60%) and the pore spaces may serve as nucleation sites or " stations" for dolomite formation. The combination of neighbouring micro-niches in a porous carbonate may result in dolomitization of an entire stratum.Six dolomite types were reported from the studied sediments in the Abu Dhabi sabkha. These were pustular and cluster dolomite; dolomites associated with clay minerals and with evaporites; and dolomites associated with microbial mats and foraminiferal tests. © 2010 The Authors. Journal compilation © 2010 Scientific Press Ltd

Stratigraphy and petroleum systems of the Paleozoic (pre-Khuff) succession, Qatar

A thickness of more than 1500m of Palaeozoic (pre-Khuff) siliciclastic strata were encountered in three deep wells (Matbakh A, and Dukhan A and B) drilled in the State of Qatar. These sedimentary rocks have no formal entries in the Qatari Geologic Lexicon and were not included in sequence stratigraphic schemes of the Arabian Plate region. The rocks are investigated here using cores, well logs and other materials. The pre-Khuff succession has been divided from the base up into the Qasim, Qusaiba, Sharawra, Tawil and Unayzah Formations based on age, stratigraphic status, lithology, sedimentary structures and correlation with the Saudi stratigraphic nomenclature. The succession is divided into a series of thirdorder sequences based on the identification of the principal maximum flooding surfaces. Deposition is interpreted to have occurred in a wide range of environments ranging from fluvial to shallow-marine but with the absence of the glacial facies which are prevalent in the equivalent Saudi section. Core and log analyses indicate that the Qusaiba Formation represents an important source rock which became mature for hydrocarbon generation during the Late Permian; it generated oil until the Late Jurassic and then began to expel gas and condensates, continuing until the present day. Both the Qasim and Sharawra Formations may include potential reservoir rocks which merit further study. The new nomenclature reported here will assist with the understanding of the Palaeozoic palaeogeography across the Arabian Peninsula and will help to delineate reservoir rocks in the Qatari Palaeozoic succession

Assessment of water bodies of united arab emirates coastal sabkhas as potential sites for natural salinity gradient solar ponds

Increasing population and growing demands from both agriculture and industry in the United Arab Emirates (UAE) necessitate the need to provide inexpensive fresh water. The only feasible source of such water is desalination. The coastal area of the United Arab Emirates especially in Abu Dhabi is formed of a series of hypersaline (47%°) lagoons surrounded by salt-encrusted flat sabkhas. Such water bodies may be used as salinity gradient solar ponds (SGSPs) to generate clean electricity that can be used in desalination plants. The geologic and climatic conditions of this region are compared with El Paso, Texas, USA where there is a well-established SGSP technology. It is concluded that the water and air temperature, solar radiation, and geologic setting support the possibility of using this technology in the UAE and especially in Abu Dhabi. Cost analyses also suggest that the generation of electricity and its use in desalination plants are economically feasible. © 2008 ASCE

Comprehensive pore size characterization of Midra shale

Although the Middle East's carbonate reservoirs were the first and most studied rocks in the world, Middle Eastern shale reservoirs are still poorly understood compared with other unconventional formations in the US. We investigate the pore-throat size and pore-body size distributions of Midra shale in Qatar to quantify the pore-scale features that control transport properties at the core scale. We measured the capillary pressure by injecting mercury into samples in drainage. The capillary pressure variation with wetting phase saturation exhibits a plateau-like trend, which differs from the linear trend observed in most shales in the US. The capillary pressure measurements quantify the pore-throat size distribution. We also measure nitrogen adsorption and analyze the measurements to determine the pore-body size distribution. This study shows that the pore-throat size has a narrow distribution, and its average is close to 22 nm. In addition, the pore-body size has a wide distribution, and its average is 18 nm. Thus, the transport properties dependent on the pore-throat size require modifications to account for the pore proximity to represent subsurface conditions. The transport properties, such as density, relevant to the pore volume in the matrix can be estimated with reasonable accuracy from the gas composition in wider conduits. The presented results have applications for the development of unconventional gas, which is the cleanest fossil fuel.The authors would like to acknowledge the support of the Qatar National Research Fund (a member of the Qatar Foundation ) through Grant # NPRP12S-0305–190235 . The findings achieved herein are solely the responsibility of the authors

Characterizing fracture toughness using machine learning

The existing models for fracture toughness characterization based on nanoindentations that account for the fracture length are limited to simple (ideal) geometries that are absent in shales. The present study proposes two conceptual models to estimate the fracture length created by nanoindentations in shales. It also presents a workflow to apply the conceptual models and uses machine learning, enabling a systematic and automated analysis. The conceptual models assume that the induced fracture is in the first mode to determine the fracture toughness. In this study, fracture toughness is also determined by the energy method that relates the load-displacement hysteresis to the fracture toughness without restricting the fracture mode. The present study sheds light on the complexities of characterizing fracture toughness using nanoindentations and has applications in the petroleum industry. The conceptual models are appealing for formation characterization using small pieces, such as drill cuttings, when large samples (~2.5 cm) required for conventional tests are unavailable. The conceptual models have applications in estimating fracture toughness when the induced fracture patterns become more complex.The authors would like to acknowledge the support of the Qatar National Research Fund (a member of the Qatar Foundation ) through Grant # NPRP11S-0109–180241 . The findings achieved herein are solely the responsibility of the authors. The authors are also very grateful for the constructive comments of the reviewers

Life in the Sabkha: Raman Spectroscopy of Halotrophic Extremophiles of Relevance to Planetary Exploration

NoThe Raman spectroscopic biosignatures of halotrophic cyanobacterial extremophiles from sabkha evaporitic saltpans are reported for the first time and ideas about the possible survival strategies in operation have been forthcoming. The biochemicals produced by the cyanobacteria which colonise the interfaces between large plates of clear selenitic gypsum, halite, and dolomitized calcium carbonates in the centre of the salt pans are identifiably different from those which are produced by benthic cyanobacterial mats colonising the surface of the salt pan edges in the intertidal zone. The prediction that similar geological formations would have been present on early Mars and which could now be underlying the highly peroxidised regolith on the surface of the planet has been confirmed by recent satellite observations from Mars orbit and by localised traverses by robotic surface rovers. The successful adoption of miniaturised Raman spectroscopic instrumentation as part of a scientific package for detection of extant life or biomolecular traces of extinct life on proposed future Mars missions will depend critically on interpretation of data from terrestrial Mars analogues such as sabkhas, of which the current study is an example

A NEW DEPOSITIONAL MODEL AND SEQUENCE STRATIGRAPHIC INTERPRETATION FOR THE UPPER JURASSIC ARAB "D" RESERVOIR IN QATAR

Deposition of the Arab Formation on the Arabian Plate followed a eustatic sea-level high during the Oxfordian that deposited the open-marine shelfal carbonates of the Hanifa and Jubaila Formations. Oolite/peloidal shoals and local coral-algal stromatoporoid banks were subsequently deposited on the platform margin. These acted as barriers and led to the differentiation of intrashelf basins from open-marine (Tethyan) waters to the east. During the subsequent Kimmeridgian lowstand, gypsum wedges were laid down in the intrashelf basins. Slight changes in water depth, which exposed or flooded these barriers, are believed to be responsible for the cyclic nature of the Arab Formation sediments. Arab Formation cycles show a 4?h order frequency but have thicknesses more typical of 3rd order Vail-type sequences. This is probably explained by the 4th order flooding events merely topping-up pre-existing accommodation space of tens of metres water depth in the intrashelf basin. Diagenesis associated with movement of hypersaline brines may have been responsible for the development of widespread dissolution porosity and dolomitization. The laminated, organic-rich, bituminous lime mudstones of the Hanifa/Jubaila Formations are the probable source of oil in the Arab Formation in Qatar. The main reservoir types are oolitic-peloidal grainstones and dolomitized limestones.Wiley Online Librar