Use of Well Logs for Petrophysical Evaluation of Abu Madi Reservoir in Abu Madi-ElQara-Nidoco Area, Northern Nile Delta, Egypt

Copyright: © 2018 Ghoneimi A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
The thick sequence of Neogene-Quaternary clastics in the Nile Delta basin is considered to be prospective for oil and gas. The offshore parts and the northern parts of the onshore area of the Nile Delta are favorable sites for hydrocarbon generation and accumulation [1]. Abu Madi sandstone is the main proven reservoir within the Miocene (Messinian) deltaic sequences of the northern Delta basin [2].
The discoveries in the Nile Delta make it one of the most world known deltas. Abu Madi-El-Qara-Nidoco area is a giant field located in the northeastern part of the Nile Delta. Abu Madi field is the first commercial discovery in the Nile Delta, where IEOC (Italian Egyptian Oil Company) achieved the first gas discovery from the early Pliocene Abu Madi formation in the northeastern part of the onshore delta by drilling the Abu Madi-1 well [3].
Abu Madi Formation includes thick bodies of cross-bedded sandstones that are occasionally pebbly with thin shale interbeds in Abu Madi well-1 type section. The lower part of the formation is the most sand rich (level III), passing up into interbedded sandstones and siltstones with shale streaks (level II), to predominantly shales in the upper part (level I) [4].
The main objective of the present study is to quantitatively evaluate the petrophysical parameters of the hydrocarbon reservoir; including the shale content, effective porosity and water and hydrocarbon saturations. Construction of the lithosaturation crossplots in addition to distribution parameters maps have been accomplished to represent the vertical and lateral distribution of petrophysical parameters of the two reservoirs rock units (Abu Madi levels II and III).

Abstract
The open-hole wire line log data of seven wells are used to study the petrophysical characteristics of the Late Miocene Abu Madi Formation (levels II and III) in Abu Madi-El-Qara-Nidoco area, northern Nile Delta, Egypt. The vertical variation of the petrophysical characteristics is presented as lithosaturation cross-plots. The parameters distribution maps (shale volume, effective porosity, fluid saturation, net pay thickness and hydrocarbon saturation) are used to represent the lateral variation of petrophysical characteristics. Cross plots show that the main reservoir lithology is shaly sandstone with laminated to dispersed shale in level II and dispersed shale and minor laminated shale in level III. The shale volume ranges from 0. 19  the Hinge Zone into the South Nile Delta block and the North Nile Delta basins [3,10,16].

Petrophysical Analysis
The well logs data are analyzed using the Interactive Petrophysics software (version 3.5). These conventional logs are used as a supplementary method for getting some results about lithology, porosity, and fluid type. For logs interpretation, the data are needed to be in good conditions. The borehole conditions and mud types can affect the quality of the logs and make some troubles in log readings or cause stationary readings of the logs, which are not correct and lead to miss-interpretation of the logs. In this study, the logs have varying qualities without any evidence of stationary readings.
The gamma ray, density, neutron and sonic logs are used as lithology and porosity indicators. The neutron-density, neutron-sonic and M-N cross-plots are used to determine lithology [17][18][19][20].
The formation water resistivity (R w ) calculated from SP log is used to create a continuous R w curve (Rw sp ). This is useful for estimating Rw values over a number of zones. The result Rw sp curve is corrected to the well temperature curve. Table 2 contains the petrophysical constants used in the log data analysis.
The calculated R w of Abu Madi Formation in wells AM-18, AM-20, AM-21 and Nidoco-7 are ranging from 0.04 to 0.05 ohm-meter. The value in well AM-16 is ranging from 0.04 to 0.07 ohm-meter. The graphical technique, where the mono porosity crossplot (Pickett crossplot), is constructed between the resistivity of uninvaded zone and the porosity deduced from porosity tools [17][18][19][20], this method had been used to calculate R w of Abu Madi Formation in well Nidoco-10, where SP log is not available in this well. The R w value in this well (Nidoco-10) is ranging from 0.02 to 0.05 ohm-meter.
Shale content (V sh ) is calculated from the combination of density and neutron logs using the double clay indicator and the GR clay indicator using the linear method. The determination of effective porosity (Ø eff ) is executed in both clean and shaly rock units by the combination of the density and neutron logs measurement by using a mathematical equation of Poupan and Gaymard [17][18][19][20].
The Archie's water saturation equations assume that the formation water is the only electrically conductive material in the formation. The middle and southern parts of the Nile Delta because of the northward thickening of the Tertiary section [8]. Near the hinge line, the wells penetrating the Pre-Miocene formations include the series from Upper Jurassic to Oligocene. They consist of shore shelf and lagoonal deposits [3,9,10].
Paleocene and Eocene sediments in the Western Desert and the western side of the Delta were mainly deposited in narrow and elongated basins trending northeast-southwest. This situation most probably extends to the area of the Nile Delta [11]. The Early to Middle Eocene were deposited in a narrow elongated E-W trending basin with a distinct flexure zone in the mid-Delta area [12]. Oligocene sediments are represented by the Tineh Formation of Late Oligocene-Early Miocene age, which is composed of a series of marine to fluvio-marine shales and sandstone interbeds [13].
Neogene-Quaternary rock units in the Nile Delta area were grouped into three sedimentary cycles with different environmental parameters [14]. The Neogene-Quaternary subsurface succession in the northern part of the Nile Delta basin has been subdivided into eight formations arranged from base to top: Sidi Salem, Qawasim, Rosetta, Abu Madi, Kafr El-Sheikh, El-Wastani, Mit Ghamr and Bilqas formations [3,13,15]. The generalized litho-stratigraphic column of Abu Madi area [13] is shown in Figure 2.
The Nile Delta is divided into two geological provinces; the deep offshore Nile Delta province, north of the continental shelf and the onshore Nile delta province. The onshore Nile Delta is subdivided by  presence of shale requires the existence of another conductive material other than water. The dual water model is an elaborated model to explain the effects of clay on resistivity. In the dual water model the water saturation formation behaves as if it contains two types of water. The first one is the water near the clay (clay water or bound water), this water is salt free and its conductivity is independent of the ion concentrations and type of clay. The second type is the water far from the clay surface (far water) [21].
The porosity and water saturation interpretation module is used to interactively calculate porosity (PHI), water saturation (S w ), flushed zone water saturation (S xo ), matrix density (RHO MA ), hydrocarbon density (RHO HY ) and wet and dry clay volumes (VW CL and VD CL ).
The well logs analysis is accomplished on Abu Madi reservoir levels II and III. The shale type is determined by choosing the shale model, which reflects the dominant shale types. The shale types are dispersed and few laminated in level III, but it is dispersed to laminated in level II. The petrophysical parameters evaluation results of levels II and III are summarized and presented in Tables 3 and 4. Also the average petrophysical parameters are presented as histograms in Figures 3 and 4.

Evaluation of Lithological Components
The gamma-ray, density, neutron, and sonic logs are used as indicators of lithology using lithology identification charts (Diaporosity and Tri-porosity cross-plots). Density-neutron, neutronsonic and MN cross-plots are used for lithology identification. Figures  5 and 6 illustrate examples to represent the neutron-density crossplots of Abu Madi levels II and III in the available wells. These cross-plots reflect that the main lithology type in Abu Madi level II is shaly sandstone in wells AM-21 and Nidoco-7, but it is sandy-shale in wells     the lithology shown in density-neutron cross plots of Figures 5 and 6 with some differences due to the difference of estimation methods. The shale type of level II is mainly laminated, partly dispersed and rarely structural; while in level III it is mainly dispersed, partly laminated in the top of the level, and rarely structural. Figure 9 illustrates the porosity (total and effective) and fluid saturation (S w and S xo ), in addition to the movable hydrocarbon. These plots shows the predominance of gas in level II in wells Am-18, AM-21 and Nidoco-7; and in level III in wells AM-20, AM-16, Nidoco7 and Nidoco-10. Figure 10 shows the input porosity tools including the neutron, density and sonic curves and the cut-offs that define net-reservoir and net-pay. Figures 7 and 10 shown the prevalence of net-reservoirs and AM-18 and El-Qara-3 and shale or silty shale in wells AM-16, AM-20 and Nidoco-10, while the lithology type in level III is shaly sandstone in wells Nidoco-7, AM-20, and AM-16, and sandy shale in wells El-Qara-3 and Nidico-10 (where the level is not recorded in wells AM-18 and AM-21).

Litho-saturation Crossplots
The vertical variation of shale content, lithology, porosity, saturation and net pay zones are presented as lithosaturation crossplots. The net reservoir and net pay intervals in the wells have been presented in Table 5 and as histogram in Figure 7. The litho-saturation crossplots of the studied wells are summarized as petrophysical and lithological characteristics (Figure 8), petrophysical and fluids saturation parameters ( Figure 9) and net reservoir and net pay intervals ( Figure 10).

Distribution Maps of Petrophysical Parameters
The isoparametric maps (horizontal distribution maps) represent the lateral configuration of the petrophysical analysis in the study area. These maps include thickness maps (isopach) and the petrophysical parameters maps, including shale content (V sh ), effective porosity (PHIE), water saturation (S w ) and hydrocarbon saturation (S h ) distribution maps of Abu Madi Formation, levels II and III.
The shale content (V sh ) of Abu Madi level II reservoir varies from 10% to 70% (Figure 11). It decreases toward the southwestern and northeastern parts and increase at the middle, southeastern and    northwestern parts. In Abu Madi level III reservoir ( Figure 11) the shale content varies from 10% to 50%. It is decreasing at the eastern direction and increasing slowly toward the west (the southwestern part is blanked, because level III is missing in wells AM-18 and AM-21). This distribution shows that the increase or decrease of shale content of the studied fluvio-marine Abu Madi Formation (Figure 2) may be attributed to that it was deposited in a channel of a NW-SE trend. In such depositional environment, the shale is deposited in the channel central course, while the sand or shaly sand was deposited towards the channel flanks to the southwest and northeast directions.
The effective porosity (Ø eff ) of Abu Madi level II reservoir changes from 2% to 23%. It increases toward northeast and southwest directions and is decreasing toward the middle part (Figure 12a). In Abu Madi level III reservoir the effective porosity changes from 3% to 21%. It increases toward the east and northeastern parts, but decreases at the west as shown in Figure 12b. This distribution indicates that the variation of the effective porosity is mostly due to the variation of shale content which decreases the effectiveness of the porosity. Therefore, the porosity is decreasing in the central course of the proposed channel, where the shale is predominant. Towards the channel flanks the porosity is increasing due to shale decrease.
The water saturation (S w ) of Abu Madi level II reservoir fluctuates between 10% and 100%. It is increasing at the northwestern and eastern parts, but decreasing toward the northeastern and southwestern parts, as shown in Figure 13. While in Abu Madi level III reservoir the water saturation fluctuates between 10% and 60%. It decreases at the middle, northwestern and southeastern parts and increasing toward the northeastern direction, as shown on Figure 13. As shown from this distribution, the water saturation is mainly distributed according to the distribution of shale content. Therefore, water saturation increases   at locations where the shale content increases at the channel central course.
The hydrocarbon saturation (S h ) of Abu Madi level II reservoir varies from 1% to 90%. It is decreasing at eastern and northwestern parts and is increasing toward northeastern and southwestern parts, as shown in Figure 14a. In Abu Madi level III reservoir varies from 4% to 90%. It increases at southeastern, middle and northwestern parts, but it is decreasing in the northeastern direction, as shown in Figure 14b The net pay (net flag) thickness of level II (Figure 17a) varies from 0 to 140 m. It decreases to zero in the middle, northwestern and southeastern parts, where the lithology of level II is sandy shale in the available drilled wells in these parts. Then, it increases in the northeastern and southwestern directions. The net pay (net flag) thickness of Abu Madi level III (Figure 17b) varies from 0 to 120 m. It decreases to zero in the southwestern direction, where level III is missing in wells AM-18 and AM-21. Then, the thickness increases gradually toward the middle, southeastern, northwestern and northeastern directions.

Conclusions
The petrophysical analysis carried out in this study is achieved on the most favorable two levels II and III of Abu Madi reservoir. Different cross-plots are constructed (neutron-density, neutron-sonic and M-N crossplots) to display the reservoir lithology. These cross-plots reflect that the lithology in level II and level III varies in the available wells from shaly sandstone to sandy shale to shale or silty shale. The shale type varies from mainly laminated and partly dispersed in level II to mainly dispersed and partly laminated in level III; in addition, it is rarely structural in both levels.
The shale content of Abu Madi level II increases to 60% in the middle, northwestern and southeastern parts of the study area, while in level III, it increases to 50% in the northwestern part. On the other hand, the effective porosity of level II increases to 23% toward the northeastern and southwestern parts, while in level III it increases to 21% toward the east and northeastern parts. This variation may be attributed to the presence of a NW-SE trending channel favoring the deposition of the fluvio-marine Abu Madi Formation.
The hydrocarbon saturation of level II increases to more than 90% in the northeastern and southwestern parts, while in level III it increases to 90% in the middle, northwestern and southeastern parts. The effective thickness of level II, net reservoir and net pay, increase to 160 m toward the northeastern and southwestern parts, while level III the net reservoir and net pay thickness increase to 160 and 120, respectively, in the northeastern and eastern parts.
The lateral distribution of the petrophysical parameters indicate that the most favorable places for hydrocarbon reservoirs occupy the northeastern and southwestern parts for Abu Madi level II reservoir and in the middle, the middle-west, the middle-east and the southeastern parts for Abu Madi level III reservoir.