A modern analogue for Middle East carbonate reservoirs: The lagoon of Al Dakhirah in Qatar

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: (1) a high-energy, sand-dominated barrier and back-barrier complex, (2) a low-energy, mud-dominated embayment.