This study has been accomplished by testing three different models to determine rocks type, pore throat radius, and flow units for Mishrif Formation in West Qurna oilfield in Southern Iraq based on Mishrif full diameter cores from 20 wells. The three models that were used in this study were Lucia rocks type classification, Winland plot was utilized to determine the pore throat radius depending on the mercury injection test (r35), and (FZI) concepts to identify flow units which enabled us to recognize the differences between Mishrif units in these three categories. The study of pore characteristics is very significant in reservoir evaluation. It controls the storage mechanism and reservoir fluid properties of the permeable units while pore structure is a critical controlling factor for the petrophysical properties and multiphase-flow characteristics in reservoir rocks. Flow zone indicator (FZI) has been used to identify the hydraulic flow units approach (HFUs). Each (HFU) was reproduced by certain FZI and was supposed to have similar geological and petrophysical properties. The samples were from four lithofacies, mA, CRII, mB1, and mB2. Because of the wide range of cored-wells samples (20 wells), this paper is updated the previous studies and indicated some differences in the resulting categories. It was noticed as results of this study that the rocks types of the lower Mishrif were mostly ranged from wackestone to packstone in the upper part of mB2 which reflected mid-ramp facies while the upper part of mB2 referred to shoal facies and for the mB1 unit the rocks types mostly range from packstone to grainstone with some points as wackestone marked as shoal and rudist bioherm facies. Grainstone relatively decreases with the increasing of depth from upper to lower Mishrif while wackestone and packstone indicated increasing in the same direction. The unit mA is marked as mesopores and macropores, while megapores and macropores feature increased in mB1 which has been noticed in the northern part of West Qurna oilfield due to increasing shoal and rudist bioherm facies, the mB2 unit revealed increasing in mesoporous and decreasing in megaporous. The upper Mishrif (mA) has three flow units, while the lower Mishrif (mB1, mB2) has eight flow units four for each reservoir unit.

In this study, the researchers attempted to build models that are more accurate by using lithofacies data along the field. Using a wider field area and extended data from both electrofacies and lithofacies could provide an accurate model of the reservoir description that will be useful for future field developments. The stochastic and deterministic models were presented to build the facies model as the basis for the construction, comparison, and selection of the more accurate model of the Mishrif Formation in the West Qurna oilfield/Iraq. The results obtained from this study indicated that the Sequential Gaussian indicator simulation algorithm offered a more accurate stochastic model between lithofacies data extracted from 20 wells and electrofacies from 75 wells, while the Truncated Gaussian simulation algorithm has a better match than others for the distribution percentage of lithofacies and electrofacies. The stochastic model shows that Mishrif Formation consists of approximately 35–40% as carbonates, 32–35% as shale, and 28–30% as coarse sand, Dolomite, and other materials according to the SISIM and GTSIM, while the deterministic by the kriging algorithm referred to as 62%, 24%, and 14%, respectively. The best variogram that gives the best results indicated that the direction of facies distribution is mainly South-North with an inclination to North-West.

Summary: This study considered the complexity of Mishrif geology and its effect on fluid movement within and across Mishrif reservoir intervals. For this purpose, we analyzed the following items: the multiple interval communication with high permeability contrast, the geological setting of the upper Mishrif (mA) interval, the channel structure in the Lower Mishrif-Part 1 (mB1) interval, the thin layers in the upper part of Lower Mishrif-Part 2 (mB2U) of very high permeability, and the microporous interval of the lower part of Lower Mishrif-Part 2 (mB2L); none of them were well defined before this work. The bottom interval of Mishrif or Rumaila (mC) is predominantly microporous, and the best reservoir is at the top of intermediate quality. Two high-porosity layers are systematically found in the mC unit, which is casually referred to as “rabbit ears.” The mB2L contains grainstones in the far north of the West Qurna/1 oil field (WQ1). In the south of mB2L, some of the toe sets from the clinoforms in a distal depositional setting have developed into rather important vertical pressure baffles and barriers to vertical flow. The mB2U generally consists of grainstones with thin streaks of mudstone high flow layers (HFLs), and the rocks underneath are described generally as grainstone shoals. About 80% of stock tank oil originally in place (STOOIP) in mB2U exists in grainstones. There are no known microporous reservoirs in mB2U. The pressure difference across the boundaries between mB1 and mA can be positive or negative. At the base, mB1 channels are always in pressure communication with the mB2U below. The best flow from the mA comes from HFLs, which are found around faults. Reservoir quality within mA is generally best in the first section of the upper Mishrif (mAa), and the majority of STOOIP in mA exists in microporous rocks, while some 30% of STOOIP is contained in grainstones.