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

Abstract

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 micro quartz 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.