Formation evaluation and reservoir quality assessment of the Albian clastic sequence in the SWM Oilfield, Southwest the Mesopotamian Basin, Iraq

Abstract

This work intends to delineate the petrophysical properties and the reservoir quality of the Albian sequence in the Mesopotamian Basin which has been deposited as a siliciclastic sequence. The well log data (including the caliper, gamma-ray, density, photoelectric factor 'PEF', neutron, sonic, and resistivity logs), derived from eight wells in the SWM (Southwest Mesopotamian) Oilfield, was studied and interpreted using the appropriate software (IP3.6). These borehole data indicate the necessity to divide the sequence into five reservoir rock units (U1, U2, M, L1, and L2 units). This enabled depicting the porosity, shale volume, water saturation, and net-pay thickness in the vertical and horizontal directions. It is indicated that the M and L1 units are the most promising units with good to very good porosity (∅ ≥ 17.2%), low water saturation (Sw ≤ 28.0%), and acceptable shale volume (Vsh ≤ 8.4%). The net-pay thickness varies between 32.8 and 77.4 m in the different wells of the SWM Oilfield. The well log data were supported by more detailed conventional core data including porosity (∅He) and permeability (k), from which the reservoir quality parameters were estimated precisely. In the present study, reservoir quality assessment based on conventional core data is considered for the first time for evaluating and discriminating the Albian–Aptian sequence in the Southwest Mesopotamian field into reservoir zones and hydraulic flow units. The estimated reservoir parameters include the Flow Zone Indicator (FZI), the Reservoir Quality Index (RQI), and the effective pore radius (R35). Based on the available core data and the estimated reservoir quality parameters, the studied sequence was divided into three reservoir rock types (RRT1–3). The best petrophysical properties have been assigned for the medium-grained sandstone of the RRT1 that dominates in the L1 and the M units (av. ∅He = 23.0%, av. k = 1581 mD, av. R35 = 23.7 μm, av. FZI = 7.368 μm, and av. RQI = 2.304 μm). On the other side, the fine-grained sandstone of the RRT3 is characterized by the lowest petrophysical properties and reservoir quality parameters (av. ∅He = 17.1%, av. k = 5.49 mD, av. R35 = 0.92 μm, av. FZI = 0.603 μm, and av. RQI = 0.136 μm). The integration between the well log and core data of the studied Albian sequence stated that the M and L1 units are the most promising reservoir units. They are predominated by medium-grained and fine to medium-grained sandstones of the RRT1 and the RRT2, respectively. This well log-core data integration is applicable to the different oilfields in the different fields in the Mesopotamian Basin and other basins in south Iraq and elsewhere.