Rock Type and Pore Throat Radius of Zubair Formation in the W Oil Field: Analysis Utilizing Core and Log Data

Abstract

The Zubair Formation is a prolific oil reservoir in several southeastern Iraq oil fields. The formation comprises thick sandstones with interbedded shales and siltstones of Lower Cretaceous age (Hauterivian to lower Aptian). The environment of the Zubair Formation consists of fluviodeltaic, deltaic, and marine sandstones. In southern Iraq, the area is subdivided into informal members (oldest to youngest). The Lower Shale, lower sand, middle shale, main pay upper sand, and the upper shale. At the W oilfield, the main pay member is an oil producer. The thickness is of the main reservoir about 100 meters thick. A recent study focused on the Main Pay upper sand, which is divided into three informal units. The upper unit H and lower unit L reservoir units are connected by shale intervals (K unit) that extend throughout the field area laterally. There exist four depositional lithofacies of Zubair Formation fluvial, channel, mouth bar, and prodelta depositional lithofacies. Four Facies were defined according to gamma-ray sandstone, shaly sand, sandy shale, and shale Facies. The Zubair formation has four types of rocks (RRT1, RRT2, RRT3, and RRT4) due to heterogeneity. The RRT1 is characterized by 10%-25% porosities and 10-1000 mD permeability. The RRT2 has a similar porosity range to RRT1, 15%–25%, but an order ofmagnitude lower permeability range from 10-100 mD. RRT3 is considered to have high porosity and low permeability is thought to have great storage but poor flow potential. RRT 3 is characterized by 6%-15% porosities and <10 mD permeability. Meanwhile, RRT4 is considered to have low porosity and low permeability. Various rock types showed in the H unit due to heterogeneity, which composed this unit. The K unit unveiled RRT4 due to shale Facies. The rocks types common of unit L are RRT1 and RRT2 where the L unit mostly ranged from sandstone to shaly sand Facies.