Modern coastal systems of Qatar as analogues for arid climate carbonate reservoirs: improving geological and reservoir modelling

Abstract

Maximizing recovery in oil and gas fields relies on geological models that realistically portray the spatial complexity, composition, and properties of reservoir units. Present day arid climate coastal systems, like the coastline of Qatar provide analogues for depositional and diagenetic processes that control reservoir quality in ancient reservoirs. Many major reservoirs in Qatar and the Middle East formed under conditions that are remarkably similar to those shaping the Qatari coastlines of today. Major controls on coastal sedimentation patterns are: 1) coastline orientation, 2) wind, wave and tidal energy, 3) climate, 4) relative sea level, 5) depositional relief, and 6) sediment sources. Strong NW prevailing winds (Shamal winds) drive shallow marine circulation patterns, creating four very distinct depositional profiles: windward, leeward, oblique, and protected. Windward coastlines are marked by reef development and intertidal sheet and beach sands. The leeward coastal profile is dominated by an eolian sediment supply, as sand dunes are blown into the sea. Along windward and oblique coastlines, hardgrounds (beachrock) stabilize circulation patterns, creating mud-prone back beach areas of micobial mats and mangroves. Protected coastlines are characterized by finer-grained peneroplid sands and low-relief beaches. Grain size, composition, and dimensions of coastal sands vary due to wave energy. Coastal deposits are equally affected by high-frequency oscillations in sea level. Approximately 6000 years ago, sea level was about 2 to 4 metres higher than it is currently and the Qatari coastline was up to 10 km inland. Most coastal deposits and sabkhas are relics of this ancient highstand in sea level. Punctuated sea-level drops to present-day level have led to the formation of seaward-stepping beach spit systems. Sedimentation patterns and their diagenetic overprint were studied in detail at the coastal sabkha of Mesaieed, which represents an oblique coastal system relatively to the predominant wind direction. Detailed field mapping, radiocarbon age dating analyses, and the integration of geotechnical borehole data, as well as data from numerous shallow pits allowed reconstructing the thickness of the Holocene, the dating and spatial reconstruction of the progradational pattern of the beach spits relative to the varying sea level, and the mapping of the amount and distribution of porosity destroying gypsum. The observed spatial complexity and heterogeneity of modern coastal systems are important aspects to be considered for conditioning three-dimensional geological models. Modern depositional systems along the Qatar coastline, like the one studied at the Mesaieed sabkha, are particularly useful as analogs for conditioning subsurface data sets in geologic (static) and reservoir (dynamic) models.