Diagenetic History and Porosity Evaluation of the Albian-Aptian Succession in Balad Oil Field, Central of Iraq

Abstract

Carbonate-clastic succession which includes the Shu'aiba, Nahr Umr and Mauddud formations are representing a part of Barremian -Aptian Sequence (Wasi'a Group). The present study includes three boreholes; Balad-1, Balad-4 and Balad-8within the Balad Oil Field. The most commo n diagenetic features observed in the studied sections include micritization, cementation, leaching (dissolution) , dolomitization and compaction fabrics in carbonate rocks. While the calstic rocks effected by cementation, compaction, dissolution processes. The Albian-Aptian succession in the study area is affected by many types of diagenetic processes during and post deposition. There are three diageneti c zones in this succession; the first within Shuaiba Formation which characterized by high compacted limestone and dolomitization in all studied wells with low porosity values. The second within Nahr Umr Formation is characterized by different diagenetic patterns in the studied area, where appeared in Ba-1 three late diagenetic subzone within sandstone units. These zones are affected by dissolution process to product an affective porous unit, and separated them by cemented and compacted zones. To the south of study area (Ba-4) these subzones became less porosity values and the early and moderate diagenetic stages (cemented and compacted) were common. To the north the clastic succession was characterized by one affective porous unit and dominantly by cemented and compacted zones. The third upper zone within Mauddud succession is characterized by two high effecti ve porous units with dominance of dissolution process in the upper part of Mauddud Formation in Ba -1. While the lower part of this formation is affected by cementation and dolomitization (early and moderate diagenetic stage) with a breakdown for primary an d secondary porosities. To the south of study area (Ba -4), the effective porous was become weaker because of dominance the dolomitization effect. In addition to developed a new effective porous zone in the middle part of Mauddud Formation. In the northern part of the region (Ba-8), the dissolution and early dolomitized processes were the dominance effects in this unit. Therefore, the effective porous zones were becoming more prevalent and influential. Accordingly, three effective porous have been distinguis hed in the upper part of Maudud Formation and three others in the lower part.