AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop

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The Lower Fars Seal, West Qurna 1 Field, Southern Iraq

Abstract

Understanding the thickness and continuity of the Lower Fars Seal is critical for the on-going development of the West Qurna 1 Field in Southern Iraq. The Miocene-age Lower Fars Formation consists of an approximately 200 m interval of interbedded gypsum, marl, and limestone in Southern Iraq. Regionally, the Lower Fars is a hydrocarbon seal in Northern Iraq and Kuwait (Al-Juboury and McCann, 2008; Llerena et al., 2014). In the West Qurna 1 area, the Lower Fars seal separates the underlying Dammam Formation, which is a highly permeable dolomite, from the overlying Dibdibba Formation, which is a shallow brackish water aquifer, locally used for aquiculture. By understanding seal strength and continuity, the petroleum industry and Iraqi government can assess the impact of fluid withdraw on shallow aquifers. The West Qurna 1 Field is a major oil producer in southern Iraq and produces oil from multiple reservoirs. Major reservoirs in the field include Yamama, Zubair, Mauddud, Mishrif, Khasib, and Sadi. The major seal in the Lower Cretaceous is the Ratawi Shale, which overlies the Yamama Formation. The master seal in the Upper Cretaceous is the Tanuma Shale which overlies the Khasib and Mishrif. The primary seal in the Tertiary is the Lower Fars, which overlies the Dammam Formation. The multiple seals in the West Qurna 1 Field create a stacked reservoir system with multiple pay zones. The oil reservoirs below the major seals are filled to spill. Whereas, the thinner seals are breached by faults, and the corresponding oil fill is controlled by the position of these faults on the structure. In general, the reservoir seals are not heavily studied in West Qurna 1. This presentation will discuss the general hydrocarbon system of West Qurna 1 and focus on a particular case study of the Lower Fars seal. The study of the Lower Fars seal was necessitated by concern over the impact of fluid withdraw on shallow aquifers. Therefore, a multi-disciplinary team of geoscientists was formed to evaluate the seal continuity and thickness over the field area.