Characteristics of the Oil Water Contact Zone of Nhar Umr and Mishrif Reservoirs in Kumait and Dujaila Oil Fields, Southern Iraq, Using Vp/Vs Ratio and Porosity Logs Data

The detailed data of the Vp/Vs ratio and porosity logs were used to detect the Oil-Water Contact Zone (OWCZ) of Nahr Umr sandstone and Mishrif limestone reservoir formations in Kumiat (Kt) and Dujaila (Du) oil fields, southeastern Iraq. The results of OWC were confirmed using P-wave, Resistivity, and Water Saturation (Sw) logs of Kt-1 and Du-1 wells. It was found that the values of the oilwater contact zone thickness in Nahr Umr sandstone and Mishrif limestone were approximately one meter and eight meters, respectively. These results suggest that the OWCZ is possibly thicker in the carbonate rock than clastic rock formations. The thickness of OWCZ in the clastic rocks changed from one part to another, depending on several factors including mineral composition, grain size, porosity, pore shape, and fluid type.


Introduction
The oil-water contact (OWC) is an important parameter to evaluate the petrophysical properties and calculate the volume of oil in the reservoir. The OWC does not reflect a sharp surface boundary, but it is a transition zone. The resistivity log is usually used to determine OWC and water saturation (Sw) [1]. The OWC identification depends on low resistivity contrast. Akbar and Permadi [2] believe that OWC which is determined using electrical logs is occasionally uncertain. The capillary transition zone in reservoirs depends on the pressure difference across the fluid contact [3]. Some authors believe that the thickness of the OWCZ changes during the production process [4]. Therefore, it is important to monitor the OWC continuously. The thickness of the OWCZ may vary among the different parts, ranging from 1 to 15 meters. OWC is parallel to the tectonic stress, but the actual situation shows that this contact is more complicated. Han and Batzle [5] found that the Vp/Vs ratio is more sensitive to the change in fluid type. Thus, it can be used to identify the hydrocarbon contact. The Vp/Vs ratio is used by many authors in lithology discrimination, degree of consolidation determination, and pore fluid identification [6,7]. The porosity of rocks is usually decreased with the increase of compaction, which is accompanied by an increase of burial depth. However, for sandstone rocks, the mineralogical and chemical factors are more significant than the physical forces [8]. Temperature and time influence porosity variation with the depth, mostly at shallow and moderate depths, whereas chemical agents and composition of the grains are more effective at depths more than three kilometers [9]. This study is concerned with the identification of OWCZ in Nahr Umr and Mishrif formations (Cretaceous age), particularly in Kt-1 and Du-1 wells, southern Iraq.

Location and geology of the oil fields
Kumait and Dujaila oil fields are located in Maysan governorate and northeast of Nasiriyah City, southeastern Iraq, as shown in Figure -1.

Figure1
-Location and base map of the study area [10].

Hydrocarbon-Water Contact analysis of Nahr Umr and Mishrif formations in both oil fields
The resistivity logs are used to determine OWC and Sw. High contrast significant increase in the Sw values indicates the OWC [16]. Nahr Umr (Sand-2 layer) Sandstone Formation is the main reservoir in Kt-1 well. The well logs show that the sand-2 unit of this Formation is approximately 11 m in thickness at a depth between 3804 and 3815 m. The values of P-wave and S-wave, obtained using Castagna's equation, density, Poisson's ratio, Vp/ Vs ratio and Sw logs of Nahr Umr Formation, are shown in Figure-4. The well logs of P-wave, S-wave, water saturation, volumetric shale, Vp/Vs ratio, and effective porosity of the lower part of the reservoir of Mishrif Formation between the depths 2860-2900 are shown in Figure-5.

The usage of Vp/Vs ratio and porosity logs to detect the OWC Zone
The Vp/Vs ratio was reported to be changed at the OWC [6]. The Vp/Vs and porosity logs are used in the present study to detect the OWCZ in the two selected wells. Three boundaries are established and shown in Figure-6. The first is the upper boundary (Green), at the level of 3804-3807 m depth, indicates the starting oil zone (only oil). The second boundary (Red) at the level of 3807-3808 m depth, represents the top of the OWCZ. The third boundary (Blue) at the level of 3808 m depth, indicates the lower boundary (the actual boundary) of the OWCZ. The three boundaries, which were traced by Vp/ Vs ratio log, were also traced by the porosity log, as demonstrated Figure-7. The location of the detected boundaries coincides with the P-wave, resistivity, and Sw logs of Nahr Umr in Kt-1 well (Figure -8). The same above procedure is used for Mishrif Limestone Formation in Du-1 well of Dujaila oil field. The OWCZ was traced using the Vp/ Vs ratio and porosity logs, as illustrated in Figures-(9) and (10), respectively. The P-wave, resistivity, and Sw logs were employed to support the results obtained using Vp/ Vs ratio and porosity logs and confirm the detection of the OWCZ in Mishrif Formation (Figure-11).

Conclusions
It is concluded that the detailed study of the Vp/Vs and porosity logs can be used to detect the actual location of the oil-water contact zone. Studying the logs data of Nahr Umr and Mishrif formations in Kt-1 and Du -1 wells of Kumait and Dujaila oil fields resulted in obtaining some important characteristics concerning the oil-water contact zone. It was found that the thickness of this zone in Nahr Umr sandstone Formation in Kt-1 well is approximately one meter. While the thickness of the oil-water contact zone in Mishrif Formation in Du-1 well reached eight meters. The authors believe that the oil-water contact zone may be thicker in carbonate rocks than clastic rocks. The