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Carbonate Reservoir Distribution in a Complex, Mixed Depositional System: A New Stratigraphic Model for the Relationships Between the Paleocene - Eocene Sinjar, Khurmala, and Kolosh Formations in the Kurdistan Region of Iraq

Abstract

The coeval Paleocene - early Eocene Sinjar, Khurmala, and Kolosh Formations of the Kurdistan Region of Iraq (KRI) exemplify a complex, mixed carbonate-clastic foreland basin depositional system. This study presents a new model for the depositional relationships between these formations that designates the Khurmala formation as resedimented carbonate deposits in a slope-basin environment. This finding contrast with previous studies that designate Khurmala formation as in-situ carbonate deposits in a restricted platform interior. Our revised interpretation has several implications for hydrocarbon prospects that target this interval in northern KRI. The KRI is a prolific hydrocarbon region within the Zagros fold and thrust belt consisting of stacked carbonate reservoirs that range in age from Triassic to Tertiary. Paleogene and early Neogene carbonate reservoirs were deposited in a subsiding foreland basin during a waning period of intra-oceanic subduction that pre-dated the collision of the Arabian and Eurasian plates during the Zagros orogeny. This tectonic event, along with eustatic-driven fluctuations in relative sea level, clearly influenced the distribution of carbonate facies and stacking patterns. We present new observations and interpretations from recent field work adjacent to the Sarta PSC, and subsequent integration of data from 40+ regional wells and seismic data from Northern KRI. These observations reveal that the Paleocene - early Eocene interval can be grouped into four stratigraphic successions that occur within distinct, mapped fairways and have predictable trends in reservoir gross thickness and continuity. Our findings indicate that the stratigraphic succession containing Khurmala formation consists of relatively thin (10’s m-scale) and laterally discontinuous carbonate sediment gravity flows that overlay 100’s of meters of finely bedded clay-silt-sand of the Kolosh formation. These sediment gravity flow likely originated from the carbonate platform deposits of the laterally adjacent Sinjar formation. Interpretations based on regional, multi-scale datasets improved our understanding of both the Sinjar and Khurmala reservoirs and altered the course of on-going appraisal and exploration activities in the region.