Hydrocarbon Reservoir Characterization Using Well Logs of Nahr Umr Formation in Kifl Oil field, Central Iraq

This study aims to determine the petrophysical characteristics of the three wells in the Kifl Oilfield, central Iraq. The well logs were used to characterize hydrocarbon reservoirs to assess the hydrocarbon prospectivity, designate hydrocarbon and water-bearing zones, and determine the Nahr Umr Formation's petrophysical parameters. The Nahr Umr reservoir mainly consists of sandstone at the bottom and has an upper shale zone containing a small proportion of oil. The geophysical logs data from three oil wells include gamma-ray, resistivity, neutron, density, acoustic, and spontaneous potential logs. A gamma-ray log was employed for lithology differentiation, and a resistivity log was used to determine the response of distinct zones' productivity. The petrophysical parameters of shale volume, total porosity, effective porosity, water saturation, hydrocarbon saturation, and flushed zone saturation were evaluated using computer processing analyses (CPI). These parameters were plotted by interactive petrophysics (IP) software. The effective porosity (PHE) in the sandstone unit ranges between 3.2 and 31.2%, water saturation (SW) ranges between 6.6 and 100%, and hydrocarbon saturation and (Sh) range from 6 to 65%. The Nahr Umr Formation has moderate to low reservoir characteristics in the lower sandstone unit.

This study is concerned with estimating various petrophysical characteristics, a synergistic approach that combines many disciplines to define rock, pore, and fluid systems by evaluating the physical and chemical qualities that explain the occurrence and behavior of rocks and fluids. A comprehensive knowledge of basic reservoir characteristics is necessary to study any well log input data for reservoir modelling and characterization in the discovery and development of hydrocarbon resources [2]. Exploratory wells were drilled through promising geological features. They are helpful in assessing the hydrocarbon potential; however, some fundamental petrophysical criteria must be examined to determine the amount of hydrocarbon accumulation in reservoir rocks (sandstone, limestone, or dolomite). Porosity, formation thickness and extent, hydrocarbon saturation, and permeability. These factors have been calculated using a variety of logs, including electrical, nuclear, and acoustic. Well ilogs iwere iused ito icorrelate izones isuited ifor ihydrocarbon iaccumulation, iidentify iproductive izones, imeasure idepth iand ithickness iof izones, idiscriminate ibetween igas, ioil, iand iwater iin an ireservoir, iand iestimate ihydrocarbon saturation.

Materials and Methods
The Nahr Umr Formation was stuided by wireline well logs. The well logs were digitized using the neuralog programme. For the selected wells (KF-1, KF-3, and KF-4), one reading every 0.25 m depth was chosen. Then, the well logs were subjected to an environmental adjustment. Corrections and interpretation procedures such as hydrocarbon saturation (Sh), porosity (Φ), water saturation (Sw), and bulk volume water (BVW) were performed using the IP software. As illustrated in Figure 3, the petrophysical characteristics were calculated using conventional equations inferred from Asquith and Krygowski [2].

Results and Discussion
Identifying the promising zones (i.e. clean sand with a significant amount of hydrocarbons and/or gas) is the first step in evaluating the petrophysical parameters. The lithology (sandstone and shale) was determined using the gamma-ray log and a sand/shale baseline. The Nahr Umr Formation was divided into two lithological units based on the calculation of shale volume. These two units were the upper shale unit and the lower sandstone unit. The correlation section of the Nahr Umr units was built along three studied wells (Figure 4) using lithological interpretation track, which consists of shale volume calculated from the gammaray log and density and neutron logs are used to calculate total porosity, and then corrected by clay volume and hydrocarbon fluid content to calculate the effective porosity. Water saturation was calculated using the Archie equation. The CPI figures of KF-1, KF-3 and KF-4 wells were derived from well logs using the Interactive Petrophysics software and are shown in Figures 5,6 and 7. These figures depict the steps of the complete interpretation process. The components of effective porosity (PHIE), water-filled porosity in the invaded zone (BVWXO), and water-filled porosity in the non-invaded zone (BVW) were illustrated by the fluid analysis track, which includes: A-iThe iresidual ihydrocarbons iare iexpressed iin ithe iarea ibetween iPHIE iand iBVWXo. B-The izone iof ithe imoveable ihydrocarbons iis ibetween iBVWXo iand iBVW. C-iTotal ihydrocarbons in ithe iarea are between iPHIE iand iBVW.
Track of saturation represents a flushed and uninvaded area of water saturation. Track of porosity is the tracking of total porosity and effective porosity. In addition to the lithology track in the last track, the raw data of porosity and resistivity logs are shown on tracks 1 and 2, respectively [12]. Depending on CPI figures, effective porosity increases towards well KF-4, it ranges between (3-31%) at sandstone unit whereas shale volume is the least unit at this well which ranges between (0-28%). The water saturation decreases towards well KF-4 which ranges between (6-90%) whereas it increases towards well KF-1, which ranges between (17-100%). The average petrophysical parameters for Nahr Umr Formation in the wells under study are presented in tables 1,2, and 3.

Conclusions
The present study deals with the petrophysical characteristics and hydrocarbon potentiality of the Nahr Umr Formation in the Kifl oil field. It is one of the most important subsurface geological formations and one of the productive and promising reservoir oil fields in central Iraq. Nahr Umr Formation was divided into two lithological units based on shale volume. These units are defined as upper shale unit and lower sandstone unit. The CPI figures show that the sandstone unit is a principle oil-bearing unit in the Nahr Umr Formation, which is characterized by high porosity, low shale volume and moderate water saturation, especially at the top of this unit. On the other hand, the shale unit at the upper part of the formation represents cap rock it characterised by very high shale volume and low porosity. The sandstone unit in the Nahr Umr Formation (well KF-4) represents the best oil-production unit in the Kifl oil field. The Nahr Umr Formation in the Kifl oil field has good reservoir properties in the well KF-4, as shown in CPI figures characterized by bad reservoir properties toward well KF-1.