Depositional Environment and Stratigraphic Sequence of the Oligocene Succession in Fallujah and Melh-Tharthar Oilfields, Central Iraq

The current study dealt with the facies and stratigraphic sequence of the Oligocene succession in the Fallujah and Melh-Tharthar oilfields, which are located in the most part the northern of the Mesopotamian Zone at central Iraq. The studied Oligocene succession consists of Bajawan, Baba, Sheikh Alas and Plani formations in the Fallujah oil field, while in Melh-Tharthar oil field, consists of Anah, Azkand, Sheikh Alas and Palani formations. The facies analysis of the studied Oligocene sequence was described according to the carbonate fabric and the particle types in the Fallujah oil field that showed eleven micro-facies for seven facies associations (back-reef, shallow open, shoal, fore-reef, reef, slope and deep shelf), while in the Melh-Tharthar oil field, the Oligocene succession was characterized by thirteen micro-facies for six facies associations (back-reef, reef, fore-reef, shoal, slope and deep shelf). Four depositional cycles were diagnosed through facies analysis of the studied sequence, the first sedimentary cycle of Palani Formation represents a transgression stage (TST) of the deep-sea facies and ends with the slope facies during the highstand system tract stage (HST). The second depositional cycle of Sheikh Alas Formation represents by progressive stage (TST), which is characterized by deepening upward for fore-reef association facies in Fallujah oil field and reef/fore-reef facies associations in Melh-Tharthar oil field. The third depositional cycle represents Baba Formation which was deposited during three stages of the HST in a succession of fore reef/reef and shoal facies associations. While in Melh-Tharthar oil field represent by Azkand Formation which characterized by the similar facies associations of Fallujah succession, with exception of presence the fourth HST stage that represented by fore-reefs and coral reef facies associations. The high stand stage continued during the deposition of the Bajawan Formation, as a fourth depositional cycle, which showed the deposition in shallow open marine and back-reef associated facies, whereas in Melh-Tharthar oil field such cycle is characterized by alternative of reef and Backreef facies association which deposited during HST stage. The fourth cycle ended with a regional regression stage to form SB1 which marking the end of the Oligocene.