Affections on the Sandstone of Zubair Formation in Luhais Oil Field Southern Iraq

Cretaceous is very important period in the Middle East because most petroleum reservoirs return to this period, Zubair reservoir is the most important reservoir of the lower Cretaceous depositional cycle in Iraq. The importance of studying digenesis processes in the oil-bearing reservoir increased because they can affect porosity and permeability; thereby can affect hydrocarbon flow and accumulations. 73 samples (45 sandstone samples and 28 shale and thin bed carbonates) from six wells of Zubair Formation (upper shale and upper sand members) in Luhais oil field were selected and prepared for petrography inspection to detect mineral compositions of sandstone and diagenesis processes and its affections on the rocks. Eight samples were selected for X-ray diffraction analyses to detect clay mineralogy. Petrographic inspection shows that many postdepositional processes affected the sandstone of Zubair Formation at different stages because of continuous changes in physical and chemical conditions in the sedimentary environment one of which leads to increase porosity such as dissolution and some processes lead to decrease porosity such as compaction, alteration and cementation. As well as there are diagenesis processes played contrasted role that it leads to increase porosity in some places and decrease in others, such as bioturbation which leads to increase vertical porosity and permeability if fine grained bed overlaid by coarse grained beds; at bioturbation start the coarse grains of overlying bed fill the underlying borrow; where as it leads to decrease porosity when it happened in coarse grained bed overlaid by fine grained sediments. Although most of diagenesis processes are destructive, but Zubair Formation is considered one of the main oil reservoirs; because it has good textural maturity (most sand grains have good roundness, sorting and little mud content). So, it can be concluded that the main factors control reservoir quality are rock texture and intensity of diagenesis process not the quantity of diagenesis processes.