Diagenetic Effects and Porosity Evolution of Zubair Formation in West Qurna Oil Field, Southern Iraq

This present study includes the petrography, lithofacies analysis and depositional development for the Zubair Formation in six boreholes (WQ-1, WQ-13, WQ-15, WQ-60, WQ-148 and WQ-356) within the West Qurna oil field. Zubair Formations was deposited within Mesopotamian Zone during the Barremain stage which belongs to the Early Cretaceous epoch. The Zubair formation in the study area affected by many diagenetic processes through and after the deposition. There are three diagenetic zones in this succession; the lower part of the Zubair Formation is characterized with three effected porous zone, which is separated by high compacted and cemented sandstone. The middle part of this formation is showing high compacted sandstone with appeared the overgrowth quartz and microquartz cementation. While the upper Zubair Formation is affected by compaction shale alternative with high compaction overgrowth quartz. The quartz arenite sandstone affected by the compaction in low amount follows by chemical dissolution led to made the secondary quartz precipitated on the original grains this decrease the primary porosity, the increase of compaction process destroyed the quartz and rock fragment grains this associated with chemical solution which led to precipitate extra grains of quartz as a small grain decreasing the porosity. After this process the changing in chemical and physical properties of depositional basin led to precipitate the calcite cement, and finally as results of organism activity calcite were dissolved and produced the secondary porosity. The effective porosity values distribution in the studied succession, where appeared in the lower unit one moderate permeable zone in all studied wells separated by impermeable zone in WQ-60 to became two zone. Whiles the middle unit characterized by two moderate and two high permeable zones. The first moderate zone is appeared in the lower part of middle unit within WQ-60, 15 and 1, while the second is appeared in the upper part of this unit within WQ-15, 1 and 356. The upper unit is showing one limited moderate permeable zone within WQ-60 and 15.