A Modern Analogue for Middle East Carbonate Reservoirs: The Lagoon of Al Dakhirah in Qatar
Authors: Caline Bruno , Billeaud Isabelle , Pabian-Goyheneche Cecile , Laurier Dominique , Dubrule Olivier , Klein Blandine , Fraisse Christian 
Published: May 2016

This reference is for an abstract only. A full paper was not submitted for this conference.

Abstract
The modern carbonate-evaporite depositional environments along the Arabian shores of the Gulf are considered as useful shallow-marine and coastal analogues for Middle East reservoirs such as the Khuff, the Arab, the Mishrif or the Shuaiba Fm (Alsharhan and Kendall, 2003; Warren, 2006).
The lagoon of Al-Dakhirah, which is located on the north-eastern coast of the Qatar peninsula, some 60 km north of Doha, provides a modern carbonate-evaporite system suitable for documenting the distribution and geometry of depositional geobodies, which are commonly used in geological reservoir model. The depositional model of this lagoon is unique for illustrating recent coastal accretion by carbonate sediment influenced by longshore transport, first described by Shinn in 1973. The resulting distribution of facies belts is markedly different from the pattern derived from the Abu Dhabi coastline (Purser and Evans, 1973).
New insights into the depositional model and early diagenetic overprint of the Al Dakhirah lagoon have been achieved thanks to extensive field work including collection of onshore and dredged offshore samples and 20 shallow cores. The sedimentological study of the lagoon highlights the variety of depositional processes and products. The impact of early diagenesis resulting in cementation, dissolution and recrystallisation of skeletal grains and evaporite crystals has been carefully studied using 3D medical scanner, petrographic thin sections, quantitative mineralogy and sample examination under scanning electron microscopy. The rapid evolution of the coastal landscape during the last fifty years has been monitored by comparing present-day satellite pictures with an aerial photograph survey of 1958. Integration of these new field data has resulted in a detailed geomorphic and sedimentological map of the lagoon at 1:20 000 scale. In addition to the understanding to the modern deposition model of such a carbonate-evaporite lagoon, a step by step reconstruction of the gradual infill of the lagoon during the last 7 000 years (Holocene transgression) has been attempted using some 12 14C age dating on shells.
This carbonate-evaporite lagoon subject to an arid, subtropical climate is characterized by the juxtaposition of two contrasted depositional environments: a high-energy, sand-dominated barrier and back-barrier complex,a low-energy, mud-dominated embayment.

Sand-dominated barrier and back-barrier complex
Longshore currents have resulted in the development of spectacular carbonate sand beach and spit that steadily moved southwards and contribute to narrow the lagoon inlet. Active and inactive examples of such wave and storm-generated barrier and back-barrier bodies are typically made up of clean, well-sorted, coarse-grained to medium-grained skeletal sand and entire shells (grainstone and rudstone texture) with oblique planar lamination. Beach-rocks resulting from syn-depositional cementation by calcite frequently occur in windward and leeward sides of the beach barriers. Several generations of sand beach and spit have created a series of sheltered back-barrier area characterised by muddy carbonate facies (wackestone and packstone texture) deposited in north-south elongated mud flats affected by large tidal channels.

Mud-dominated embayment
Despite low tidal range (maximum 1.8 m) the inner lagoon and associated embayment are characterised by extensive mud-dominated tidal flats. These flats display facies belts, which are arranged in successive tiers. These belts include upper intertidal microbial mats, bioturbated middle intertidal mud flats and mud-dominated lower intertidal flats largely covered by mangrove. The subtidal area is mainly composed of mixed sand/mud sediments with entire shells. The embayment system is also characterized by a hierarchical network of tidal channels filled with composite sediment nature and grain size (shells, pellets, coarse to fine-grained skeletal sand, mud). The wide supratidal area, which covers the major part of the Holocene lagoon, consists of evaporite-rich deposits of sebkha and nebkha environment (gypsum and palygorskite minerals). A thin blanket of carbonate sands (reworked-dolomite grains from the Eocene bedrock) mixed with modern eolian quartz grains, covers the sebkha environment and steadily progrades on the intertidal flats.
The depositional model derived from the Al Dakhirah lagoon provides key information on the organised pattern of facies belts in high-energy and low-energy environments, respectively as well as on the nature of lateral facies variations. It also highlights the importance of syn-depositional diagenesis including cementation/dissolution of skeletal grains as well as the very first stage of dolomite nucleation within the microbial mats. Early diagenesis plays a major role in the preservation potential of the carbonate geobodies and their reservoir properties. As such, this modern depositional system can be used to constrain the subsurface carbonate reservoirs in the Middle East. In practice, this model has been successfully applied for updating the reservoir model of several fields in the Arabian Gulf producing from the Arab Fm.
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