Structural Genesis of Hydrocarbon Traps of Iraq

Richard P. George¹*, Tayfoor Rushdi², Kirk W. Schafer¹, and Thomas J. Frantes^1
1ExxonMobil, USA
²Iraq Oil Exploration Company
*[email protected]

Iraq’s hydrocarbons (134 billion barrels of oil and oil-equivalent: OPEC, 2004) occur in many structural habitats. Using major fields, we illustrate structural styles from Iraq’s four main hydrocarbon provinces and give interpretations of their genesis. The Kirkuk embayment in northeast Iraq involves Late Miocene and younger SW-verging, fault-propagation folds (Zagros-driven) fed by slip along Lower Jurassic detachments. The principal Tertiary reservoirs at the Kirkuk field include an Eocene through Lower Miocene carbonate-prone section beneath a lower Middle Miocene angular unconformity along which truncation increases to the northwest. The post-unconformity Middle Miocene section carries the topseal. In southern Iraq, within the southwest flank of the Mesopotamian Foredeep (Zagros Foreland Basin), the major traps (e.g. Rumaila and Zubair fields) are large, N-trending anticlines, each with several crestal culminations and gently-dipping flanks (2° to 4°). The Mesozoic reservoirs are little faulted. Long-lived, episodic evacuation of infra-Cambrian Hormuz Salt beginning as early as Late Jurassic controlled trap-genesis. Basement grain, reactivated during the Hercynian, controlled the N-S trend of the later evacuation synclines. In contrast, Central Iraq’s Mesopotamian traps have NW-trending basement-involved faults, some of which had reverse slip (transpression?) during both the Late Jurassic and the Neogene and others of which had normal slip (transtension?) during the Late Cretaceous. Iraq’s lightly explored Western Desert has Paleozoic-sourced exploration potential at depths much shallower than elsewhere in Iraq, owing to (1) post-Late Jurassic to pre-Albian southward tilting, uplift, and erosion; and (2) Late Cretaceous N-S extension. Iraq’s structural styles reflect variable impact, from one region to the next, of (1) basement grain and faults, (2) Hormuz Salt distribution, (3) Hercynian orogeny, (4) creation of Tethyan passive and transform margins and their destruction resulting from Arabia’s collision with Eurasia.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain