Innovative Coastal Research in Qatar: Improving Geological and Reservoir Modeling

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:coastline orientation,wind, wave and tidal energy,climate,relative sea level,depositional relief, andsediment 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 coast profile is dominated by an eolian sediment supply, as sand dunes are blown into the sea. Along windward and oblique coastlines, shoreface hardgrounds stabilize circulation patterns, creating mud-prone areas of stromatolites 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 6,000 years ago, sea level was about 2 to 4 meters higher than it is currently and the Qatari coastline was up to 10km inland. Most coastal deposits and sabkhas are relicts 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. Introduction The peninsula of Qatar is located approximately 25 degrees north of the equator and measures roughly 190km in north-south and 90km in east-west direction. Strong, seasonal northwesterly winds locally called Shamal winds, drive marine circulation patterns. Other factors that control sedimentation patterns in coastal areas include: relative sea level, climate, depositional relief, and sediment sources (Jameson et al., 2009; Jameson et al., 2010). Together, these factors combine to produce four distinct coastal environments: 1) windward coastline: northern coastline (Al-Ruwayis area), oblique coastline: northeastern to eastern coastline (Al-Thakhira and Mesaieed areas), leeward coastline: southeastern coastline (Khor Al-Adaid area), and protected coastline: western coastline (Bir Zekreet and Al Zareq areas). These coastal areas together with the inland sabkhas of Dukhan (east of the Dukhan anticline) and Sawda Nathil are the focus of our research (Fig. 1).