Facies Analysis and Depositional Stages of The Albian-Aptian Succession in Balad Oil Field, Central Iraq

• Published in: Iraqi geological journal 2021-02, Vol.54 (1B), p.43-56
• Main Author: Amer, Zainab
• Format: Article
• Language: English
• ISSN: 2414-6064; 2663-8754
• DOI: 10.46717/igj.54.1B.4Ms-2021-02-22

Carbonate-clastic succession which includes the Shu'aiba, Nahr Umr and Mauddud formations are representing a part of the Barremian-Aptian Sequence (Wasi'a Group). The present study includes three boreholes (Ba-1, 4 and 8) within the Balad Oil Field. The study area is located in central Iraq. This field represents a subsurface anticline with a northwest to southeast direction axis within the Mesopotamian Zone. Eight types of microfacies were recognized in the succession of the Mauddud and Shu’aiba formations. These microfacies represent shallow open marine, restricted and semi-restricted, reef - back reef, deep open marine and basinal depositional environments. While Nahr Umr Formation includes two successions, the first is the upper unit which is characterized by shale dominated rocks and the second is the lower unit which is characterized by sand-dominated rocks. Four major lithofacies were recognized in these two successions, they represent four depositional environments which are distributary channel, bay fill, delta plain and prodelta. The Albian-Aptian sequence was deposited during three cycles overlying the regional unconformity below the Shu’aiba Formation, the Zubair Formation, and ended with local unconformity with the Ahmadi Formation. The first stage is represented by deposition of Shu’aiba Formation during the sea-level rise after regression stage during the Zubair deposition which deposition in delta association facies. The second stage was showed a regressed of deposit the delta system above the shallow open marine of the Shu’aiba Formation, where the succession became characterized by mud-dominated rock with fissile and organic material. The third depositional stage is represented by the continuation of sea-level rise. This transgression leads to the building of carbonate ramp of the Mauddud Formation above the deltaic system of Nahr Umr Formation with a conformable surface. The Mauddud depositional stage was represented by two cycles of transgression succession, where appeared deepening upward in both cycles. The unconformity between these formations have been determined by observing the glauconite mineral and detected by log response in depth 2880m in well Ba-1.