Palaeoenvironments and sequence development of the Upper Palaeogene-Lower Neogene Succession in Kirkuk, Bai Hassan and Khabaz oil Fields, Northern Iraq
Authors: Yaseen S. Al-Jwaini  , Ali D. Gayara  
Published: April 2023

Microfacies analysis of the Upper Palaeogene-Lower Neogene succession which include Jaddala, Palani, Sheikh Allas, Shurau, Tarjil, Baba, Bajwan, Ibrahim, Azkand, Anah and Jeribe formations within Kirkuk area led to the recognition of many microfacies. They were grouped into nine facies associations ranging from supratidal to basin setting. These facies associations were deposited within a rimmed shelf with a barrier reef during the Palaeogene and a steepened ramp setting with fringing patch reef during the Neogene. The digenetic imprints on the recognized microfacies are prominent and dominated by cementation, neomorphism, dolomitization, precipitation of authigenic minerals, dissolution (leaching), compaction, mechanical degredation, micritization and geopitalstractures.

The Upper Palaeogene-Lower Neogene succession is represented by a 2nd order cycle, It includes five 3rd order cycles within the shallowing upward upper part (The highstand systems tract) of this cycle. Cycles A and B were deposited during the Oligocene and present in the wells of Kirkuk oil field and bounded below by a transgressive surface and above by Type1 sequence boundary, Cycles C and D were deposited during the Aquitanian Early Miocene and found in Khabaz Field only and bounded below and above by a Type1 sequence boundary. These cycles were formed where the tectonic component was the major controlling factor on their development, This have caused successive episodes of relative sea level rises and stillstands, followed by sea level fall. Another 3rd order cycle within the Jeribe Formation was identified at Bai Hassan and Khabaz fields, this cycle is bounded below and above by a Type1 sequence boundary and also represent a brief episode of relative sea level rise followed by a major fall eroding most of the formation.

The development of the Upper Palaeogene-Lower Neogene succession went through three main stages. The first stage was represented by the deposition of the basinal Jaddala and Palani formations (Eocene-Early Oligocene) all over the study area. The second stage was characterized by the basin trend being to the south toward the Khabaz Field and the reef buildup was located near Kirkuk Field and an interfingering took place between the Sheikh Allas and Palani formations deposited as a 3rd order cycle (A) therefore the fore reef and reef of Sheikh Allas and the back reef Shurau were deposited as a second 3rd order cycle (B) in Kirkuk Field only whereas the deposition of the basinal facies of the Palani Formation continued in both Bai Hassan and Khabaz areas. The third stage took place during the Aquitanian Early Miocene where a positive area developed as the Baba Dome and the Bai Hassan Field were uplifted with Avanah and Khurmala domes in Kirkuk Field; therefore the Bajwan and Baba formations (Late Oligocene) became subject to erosion, and the Khabaz Field represent the area of reef buildup, and the depositional system was changed from the shelf to ramp setting. During the Langhian new transgression took place where the Bai Hassan and Khabaz areas represent a tidal flat and the Jeribe Formation was deposited, whereas the Kirkuk Field was a positive area.