Petrographic characterization and comparison of Abu Dhabi coastal microbial associated sediments and the upper Jurassic Arab C member

  • Wang Jiayi

    Student thesis: Master's Thesis

    Abstract

    The Arab Formation is one of the most prolific formations in the UAE. The formation has been interpreted to have been deposited in depositional environments similar to those of the modern coast of Abu Dhabi. Microbial mat horizons have previously been identified from the Jurassic age Arab-ABC interval. However, how the presence of the microbial mats has influenced the sediments of this peritidal system has remained unclear. Previous studies of the microbial mat system have focused on the mediation of carbonate precipitation. Geologic studies of sedimentary characteristics and pore evolution of microbial mat system are scarce. The study used a range of methodologies, including core logging, petrographic description and mineralogical and electron microscopy analysis. Pore types were characterized and pore size measured, sediment compositions and their morphology, distribution pattern and interrelationships were documented in detail for both the Arab Formation core from the Arzanah Field and for facies from modern sabkha sediments. These attributes were then analyzed to identify the timing of diagenesis and build a paragenesis diagram to show the porosity evolution. (Semi-lithified) carbonate grainstone, buried microbial mat zone, gypsum zone, anhydrite zone and halite crust form a typical vertical sabkha sequence, from the base hard ground to the surface. The sabkha gypsum has preferential replacement of high-magnesium calcite and aragonite shown by the remaining of low-magnesium calcite benthic foraminifera. All of the 8 lithofacies observed in the sabkha microbial mat and associated sediments can be linked to 7 of the 9 lithofacies in the Arab-C based on the texture, presence of microbial mat and anhydrite. The crenulate fine laminations and associated sediments that provide evidence of the microbial mat system are well-preserved in the Arab-C core. The comparison of their paragenetic sequence indicates that in the Arab Formation core, the alteration from gypsum to anhydrite, dolomitisation and cement riming of allochems all occurred at deeper burial than the gypsum layer, carbonate precipitation of microbial mat of the sabkha sediments. The Abu Dhabi sabkha microbial mat sediment system can be used as an analogue to part of the Arab-C. This systematic study of the sedimentary characteristics of microbial mats helps us to better-understand the peritidal system of Abu Dhabi. It also provides an analog to ancient microbial mat bearing reservoirs and can aid us in building more-constrained depositional models. These findings can help us identify microbial mat and associated systems in the ancient formations. The direct illustration of the diagenetic processes that occurred in the syn-depositional and early burial sediments in Abu Dhabi sabkha can help determine the characteristics and timing of the diagenetic processes that occur at deeper burial. The better identification of microbial mat and associated sediments, understanding of porosity evolution of the Arab-C can help us with further engineering work such as reservoir modeling for later production.
    Date of Award2016
    Original languageAmerican English
    SupervisorStephen Lokier (Supervisor)

    Keywords

    • Applied sciences
    • Abu
    • Arab
    • Coastal
    • Dhabi
    • Jurassic
    • Microbial
    • Petrographic
    • Sediments
    • Petroleum engineering
    • 0765:Petroleum engineering

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