Depositional Architecture and Stratigraphic Framework of Albian Succession in Balad and Nasiyia Oil Fields

Abstract

The Albian Carbonate-clastic succession in the present study is represented by the Mauddud and Nahr Umr formations which were deposited during the Albian stage within the Wasia Group More than 200 thin sections of cores and cuttings in addition to well logs data for Nahr Umr and Mauddud formations from 4 boreholes within two oil fields (Ba-4, Ba-8, Ns-2, and Ns-4) were used to interpret the different association's facies as well as the facies architectures to describe the sedimentary framework of the basin and development the petrophysical properties. Seven major microfacies were diagnosed in the carbonate succession of the Mauddud Formation, while the Nar Umr Formation includes five lithofacies; their grain types' characteristics and depositional textures enabled to recognition of paleoenvironments. These microfacies were deposited in restricted, shallow open marine, back-reef, slope and deep open marine for the Mauddud succession. While in Nahr Umr, the lithofacies were deposited in bay fill, back-shore, and distributary channels. The studied succession in the Balad oil field represents two 3rd-order and four 4th-order cycles (A1, A2, A3 and A4). These cycles are asymmetrical and start with cycle A1 where the Backshore facies in Lower Mauddud Formation represents the transgressive systems tract and then appear the highstand systems tract bounded below by a transgressive surface which coincides with the unconformable surface (SB1) with the underlying Shuaiba Formation. In Nasyria oil field the studied succession represents two 3rd order cycles and three 3rd order cycles (A1, A2 and A4) with the absence of A3 cycle. These cycles are asymmetrical and start with cycle A1 where the distributary channel in the lower part of Nahr Umr succession represents the transgressive systems tract bounded below by an unconformable surface (SB1) with the underlying Shuaiba Formation. The highstand systems tract of cycle A1 is reflected by shallowing upward for distributary channel, bay fill and back-sore facies of the Nahr Umr Formation. This relatively thick parasequence reflects the time of sea level stillstand where tectonic subsidence is the major controlling factor in producing the necessary accommodation for thick aggregational to progradation clastic succession to deposit.