Assessment of petrophysical parameters of clastics using well logs: The Upper Miocene in El-Wastani gas field, onshore Nile Delta, Egypt

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 4, August 2013 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(4): 488–494.

RESEARCH PAPER

Assessment of petrophysical parameters of clastics using well logs: The Upper Miocene in El-Wastani gas field, onshore Nile Delta, Egypt Ezz S El-Din1, Maher A Mesbah2, Mohamed A Kassab3, Ibtehal F Mohamed4, Burns A Cheadle5, Mostafa A Teama2,* 1. Petroleum Eng. Dept., Faculty of Engineering, Sirte University, Sirte, Libya; 2. Geology Dept., Faculty of Pet. & Min. Eng., Suez University, Suez, Egypt; 3. Dept., of Exploration, Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt; 4. Geology Dept., Faculty of Science, Suez Canal University, Ismailia, Egypt; 5. Department of Earth Sciences, Faculty of Science, University of Western Ontario, London, Ontario, Canada N6A 5B7

Abstract: Based on logs of ten wells in the El-Wastani gas field, onshore Nile Delta, Egypt, petrophysical evaluation for the Upper-Miocene Qawasim Formation and Abu Madi Formation in the El-Wastani gas field was accomplished. The lithology of these formations was analyzed using cross plots of logging parameters, and petrophysical parameters including shale volume, porosity, water saturation and hydrocarbon pore volume were assessed. The Neutron/Density, Neutron/Gamma-ray and Litho-saturation cross plots of the wells show that the main lithology of the Lower Abu Madi unit is sandstone with shale intercalations, while the lithology of the Qawasim Formation is mainly composed of shale with sandstone intercalations. The cut-offs of shale volume, porosity and water saturation for productive hydrocarbon pay zones are 50%, 10% and 70%, respectively, which were got based on cross-plots and Gamma-ray log data. Contour maps for parameters such as net pay thickness, average porosity, shale volume and water saturation were prepared and it is found out that the Abu Madi Formation and Qawasim Formation in the study area have promising reservoir characteristics; A prospective region for gas accumulation trends northwest-southeast in the study area, especially favorable in the central part. Key words: well log; petrophysical parameter; Upper Miocene; El-Wastani gas field; Nile Delta; Egypt

1

Introduction

The Nile Delta is a giant gas province that has attracted attention due to its approximately 42 TCF of proven reserves and approximately 50 TCF of undiscovered potential. Following the first commercial discovery in 1966 when the Abu Madi-1 well proved the production potential of the late Messinian Abu Madi sandstones[1], exploration on the onshore Delta has focused on Oligocene/Early Miocene through Pleistocene clastic reservoirs. This study focuses on the El-Wastani gas field, one of the most important gas fields in the onshore Nile delta (Figure 1), where the main pays are the Messinian (uppermost Miocene) Qawasim Formation (Q) and the Late Messinian Abu Madi Formation[2−5]. This study involves lithology analysis of Qawasim and Abu Madi Formations based on logging parameter crossplot by using logging data of 10 gas wells, and evaluation of petrologic parameters such as shale content, porosity, water saturation and gas-bearing pore volume.

Fig. 1

2

Location of the study area

Data and methodology The hydrocarbon potential of the Late Miocene (Lower Abu

Received date: 19 Mar. 2013; Revised date: 17 May 2013. * Corresponding author. E-mail: [email protected] Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Ezz S El-Din et al. / Petroleum Exploration and Development, 2013, 40(4): 488–494

Madi and Qawasim) clastic reservoir rocks in the study area was evaluated in ten wells (EW-4, EW-5, EW-6, EW-7, EW-8, EW-9, EW-10, EW-12, EW-13 and EW-15). The basis for reservoir oil and gas potential evaluation is the petrophysical analysis of drilled targets in all the wells, including the vertical distribution of petrophysical parameters, lithology interpretation from logging crossplot and contour diagram of various parameters. Open-hole 1og data for the studied units in all the wells were collected and digitized, including deep and shallow laterologs (RLA5, RLA3 and RXOZ), neutron porosity (APLC), bulk density (RHOZ), acoustic log (DTCO) and gamma ray (HSGR). The borehole environmental calibration and interpretation was carried out using Schlumberger software Interactive Petrophysics (IP, v3.5), including identification of drilling fluid and wellbore properties according to logging curve subtitles, and removal of the effect flush on shale section and rough borehole[6]. Comparison of original log and corrected log shows the differences between them are negligible. The lithology and components of Abu Madi and Qawasim Formation in all wells were investigated by using logging crossplot (including neutron porosity-density crossplot and neutron porosity- HSGR crossplot), the results from different crossplots are slightly different. The increase in shale content would make the crossplot points offset from the baseline of pure sand to the right or lower right[7]. The shale content (VClay) quantity is defined as the volume of wet shale (clay) per unit volume of reservoir rock. The shale is described as “wet” because water is bound to clay surface (i.e.: grain-rimming, pore-lining, and pore-filling clay) as the hydration water layer. Shaly sand reservoirs require petrophysical methods to figure out the effect of bound water because increased VClay decreases the effective reservoir storage capacity while conductive clay reduce the formation resistivity. In shale content correction, the effect of ineffective bound water should be taken into account in correcting porosity and water saturation from logging[8], which will have significant effect on reserves estimation. Shale content may be worked out using a variety of petrophysical indexes, such as Gamma-ray, neutron porosity, resistivity and neutron porosity/density as a double clay indicator. The arithmetic averages of shale content from various methods (single or double indicators) are taken as the shale content close to the actual value. In this study the corrected porosity was estimated using a combination of the density and neutron logs (Neutron Density Porosity Model)[9]. The effective water saturation (Sw) was computed using the Dual-Water Model for shaly formations[10].

3 3.1

Fig. 2 Lithological identification with neutron/density crossplots of Lower Abu Madi Unit (LAM) "light blue" and Qawasim Formation (Q) "pink" for well EW-4 (as an example) in El-Wastani gas field, onshore Nile Delta, Egypt

Madi Unit appears to be sandstone with shale intercalations: the lithology of the Lower Abu Madi Unit is composed mainly of sandstone with occasional shale intercalations in wells EW-4, EW-5, EW-6, EW-9, EW-12, EW-13 and EW-15, and becomes more shaly in wells EW-7, EW-8 and EW-10. In contrast, the lithology of Qawasim Formation is composed mainly of shale with few sandstone intercalations in all wells except EW-8, EW-7 and EW-13 where the Qawasim Formation becomes more sandy. 3.2

Neutron (APLC) vs. Gamma-ray (HSGR) cross-plot

In the neutron/Gamma-ray cross-plots, medium Gammaray (40–55 API) and medium neutron porosity indicate shaly sandstone, while high Gamma-ray (greater than 55 API) and high neutron porosity suggest rich shale[11], and all wells show neutron porosity and Gamma ray go up with the increase of shale content. By comparison, the Neutron/Gamma-ray cross plots (Figure 3) and the Neutron/Density cross plots are consistent, showing the main lithology of the Lower Abu Madi Unit is sandstone with occasional shale intercalations in wells EW-4, EW-5, EW-6, EW-7, EW-9, EW-12, EW-13 and EW-15, while the shale content increases in the lower part of the Unit in wells EW-8 and EW-10. On the other hand, the Qawasim Formation exhibits high shale content in all

Lithological interpretation with cross-plots Neutron (APLC) vs. Density (RHOZ) cross-plot

The crossplots of Neutron (APLC) vs. Density (RHOZ) (Figure 2) show that the main lithology in the Lower Abu

Fig. 3 Gamma-ray/neutron porosity cross-plots of Lower Abu Madi Unit (LAM) "Light blue" and Qawasim Formation (Q) "pink" for well EW-8 (as an example) in El-Wastani gas field, onshore Nile Delta, Egypt

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wells except well EW-8 and limited intervals of EW-12, EW-9 and EW-13, which is in agreement with the results from neutron porosity-density crossplot. 3.3

Lithology- saturation cross-plot

Lithology-saturation crossplot shows the irregular variation of lithology, water saturation and oil and gas content vertically, Gamma ray, density, acoustic log, neutron porosity, resistivity and photoelectric absorption coefficient were adopted to evaluate various reservoir intervals. Analysis of lithology-saturation crossplot of various wells (Figure 4 is the lithology-saturation crossplot for well EW-13) led us to the following findings: lower part, top and bottom of Abu Madi Formation are all shale with sandstone intercalation, most middle section is sandstone, all wells (except well EW-7, EW-8, and EW-9 have more shale, and EW-10 have a large number of shale) have this distinct feature, which can be seen clearly on Gamma-ray log. Qawasim Formation is made up of shale and small amount of sandstone, this Formation in well EW-8 has more sand. It can be seen from well-tie lithologic profile (Figure 5) that the thickness of low Abu Madi Formation in Well EW-9 and EW-10, 106 m and 105 m respectively, is the minimum; the lower part of Abu Madi Formation is the thickest in Well EW-15, about 195 m.

4

Cut-off determination

4.1

Cut-off of shale content

Shale content cut-off is used to tell sand from shale and allows the identification of total sand intervals, Which can be determined according to shale content-effective porosity rela-

tionship of a number of wells and Gamma ray log. Figure 6 shows the shale content-porosity crossplot and Gamma ray log used for determination of shale content cut-off. The plot shows the volume of shale cut-off (Vshc) value for reservoir and non-reservoir rock determined is 0.5, meaning that rocks with more than 50 percent of shale are regarded as non- reservoir rock, while rocks with equal to or less than 50 percent of shale are regarded as reservoirs. 4.2

Porosity cut-off

The porosity cut-off is used to discriminate between porous & permeable and tight sand intervals in the gross sand interval, equivalent to the porosity corresponding to the minimum permeability allows oil and gas flow. Multi-well cross-plot of conventional core porosity-permeability data with gamma-ray log plot (Figure 7) was used to define the porosity cut-off. Figure 7 shows the porosity-permeability crossplot and gamma ray log used for determination of porosity cut-off, it can be seen from this figure porosity of 10% (corresponding to per－ meability of 1×10 3 μm2) can be taken as the cut-off points for reservoir and non-reservoir. 4.3

Water saturation cut-off

Water saturation cut-off is used to discriminate between net pay and non-pay (water wet) intervals in the porous interval, which can be determined according to effective water saturation-effective porosity crossplot and gamma ray log. Figure 8 shows an example. Intervals that have water saturation greater than 70 percent are assumed to be wet or non-productive intervals, while intervals with water saturation of less than 70% are pay zones.

GR—Gamma ray; φN—Neutron porosity; ρ—lithologic density; Δt—interval transit time; Pe—photoelectric absorption coefficient; Rt—formation resistivity (RLA5 suite); RXO—flushed zone resistivity; Rw—formation water resistivity; Rmf—drilling filtrate resistivity; SXO—flushed zone water saturation; Sw—water saturation; φt—total porosity; φe—effective porosity; Vc—shale content; Vs—silt content

Fig. 4

Litho-saturation crossplot for well EW-13 (as an example) in El-Wastani gas field, onshore Nile Delta, Egypt (scale 1:3200)

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Fig. 5

Well-tie profile of El-Wastani gas field

Fig. 6 Shale content versus porosity crossplot and gamma ray log for cut-off determination

Fig. 8 Porosity-water saturation crossplot for cut-off determination

reservoirs extracted from well logging data were averaged and shown in Table 1. Contour maps for these parameters were prepared using Golden Software Surfer v.8 to reflect the general distribution throughout the Lower Abu Madi and Qawasim reservoirs. 5.1

Fig. 7 Porosity-permeability crossplot and gamma ray log for cut-off determination

In conclusion, most of the productive hydrocarbon pay zones are marked by decrease in water saturation and increase in effective porosity, as well as low clay contents.

5

Formation evaluation The reservoir parameters of Lower Abu Madi and Qawasim

Contour diagram of net pay thickness

The gross thickness of sandstone in the Lower Abu Madi reservoir increases from the southeast to the northwest of the study area, from 29 m at well EW-10, to about 147m at well EW-4. The net pay was calculated according to the cut-offs (effective porosity ≥ 10%, shale volume ≤ 50% and water saturation ≤ 70%). The net pay thickness map of Lower Abu Madi reservoir (Figure 9a) shows that the net pay thickness ranges between 3m at well EW-10 in the eastern part of the study area to 33m at EW-6 and EW-9 in the central and southeastern parts of the study area respectively. This defines a narrow northwesterly trend that bisects the study area.

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Table 1 Well name

EW-4

Top depth/m

Average Effective Total Net/gross Average Bottom water satuthickness/ thickratio porosity/% depth/m ration/% ness/m m

Lower Abu Madi Unit

2 756.34

2930.19

174

23.91

0.22

13

50

14

1.92

0.92

Qawasim Formation 2 930.19 2 993.00

63

1.10

0.04

17

60

36

0.21

0.08

2 740.16 2 887.50

147

23.45

0.19

13

51

38

3.36

1.86

Qawasim Formation 2 887.50 3 120.00

219

16.43

0.09

13

53

40

2.34

1.05

2 736.99 2 920.83

184

32.92

0.29

15

38

36

6.89

4.15

Qawasim Formation 2 920.83 2 985.00

65

12.34

0.21

18

49

20

2.43

1.16

2 767.90 2 910.54

142

21.20

0.16

14

50

42

3.09

1.58

Qawasim Formation 2 910.54 3 018.00

108

14.60

0.21

17

44

26

2.79

1.39

2 734.00 2 905.50

172

14.70

0.09

16

60

39

2.53

1.05

Qawasim Formation 2 905.50 3 149.00

244

68.90

0.35

15

51

34

10.76

5.47

2 741.90 2 848.20

106

33.06

0.26

12

46

30

4.03

2.16

Qawasim Formation 2 848.20 2 931.00

78

4.26

0.05

12

42

36

0.48

0.27

2 714.50 2 818.80

105

3.00

0.04

15

62

37

0.47

0.18

Qawasim Formation 2 818.80 2 907.00

88

0.60

0.00

12

64

39

0.07

0.03

2 754.00 2 916.00

162

16.15

0.12

17

55

36

2.75

1.36

QawasimFormation 2 916.00 3 095.00

179

7.01

0.04

15

63

44

1.11

0.58

2 758.10 2 912.30

154

23.40

0.14

16

59

38

4.33

2.05

Qawasim Formation 2 912.30 3 008.00

96

13.20

0.18

15

50

32

2.26

1.14

2 744.00 2 939.00

195

24.75

0.14

14

52

42

3.35

1.73

Qawasim Formation 2 939.00 2 999.00

59

0.25

0.02

11

50

41

0.03

0.01

Lower Abu Madi Unit

EW-6

Lower Abu Madi Unit

EW-7

Lower Abu Madi Unit

EW-8

Lower Abu Madi Unit

EW-9

EW-10

EW-12

EW-13

EW-15

Average shale content/%

Strata

Lower Abu Madi Unit

EW-5

Reservoir parameters of Lower Abu Madi and Qawasim Formation

Lower Abu Madi Unit Lower Abu Madi Unit Lower Abu Madi Unit Lower Abu Madi Unit

φeh/m φeSoh/m

Note: So—average oil saturation, %; h—effective thickness, m

Consequently, the best sites for drilling new productive wells should be along this trend. The net pay thickness map of Qawasim reservoir (Figure 9b) shows that the minimum net pay thickness is only 0.25m at wells EW-15, and the maximum thickness is 69m in the middle well EW-8. 5.2

Contour map of average porosity

The effective porosity map of the Late Miocene reservoir (Figure 10a) reveals that the porosity at well EW-12 in central southern part of the study area reaches the highest value of 17%, and lowest of 12% at well EW-9. The effective porosity map of Qawasim reservoir (Figure 10b) indicates that the porosity reaches its highest of 18% at wells EW-6, and the minimum of 11% at wells EW-15 in the northwestern study area. The middle, southern and central southern parts of the study area are higher in average porosity. 5.3

Contour of shale content

The contour of shale content of the Lower Abu Madi res-

ervoir (Figure 11a) shows that it reaches the maximum value of 42% at wells EW-7 and EW-15. The minimum shale content is 14% at well EW-4 in the northwest part. The shale content contour of Qawasim reservoir (Figure 11b) demonstrates that the shale content increases to over 40% at wells EW-12 in the southwest of the study area. It decreases to 20% at well EW-6 in the central part of the study area. The shale volume decreases along the direction where both effective porosity and net pay thickness increase. 5.4

Water saturation contour

The water saturation contour of the Lower Abu Madi reservoir (Figure 12a) indicates maximum values of over 60% at wells EW-10 and EW-8 in the east and central eastern parts of the study area. The minimum water saturation is 38% at well EW-6 at the central part of study area. The water saturation map of Qawasim reservoir (Figure 12b) shows that it increases eastward and westward to reach 64% at well EW-10 and 63% at well EW-12, respectively. The area with low water

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Fig. 9 Net pay thickness distribution contour of (a) Lower Abu Madi Unit and (b) Qawasim Formation

Fig. 11 Average shale content contour of (a) Lower Abu Madi Unit and (b) Qawasim Formation

Fig. 10 Average porosity contour map of (a) Lower Abu Madi Unit and (b) Qawasim Formation

Fig. 12 Average water saturation contour map of (a) Lower Abu Madi Unit and (b) Qawasim Formation

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favorable for oil and gas accumulation.

Acknowledgements Mostafa Teama would like to thank the Egyptian Missions (Ministry for Higher Education, Research and Technology) for the scholarship that enabled the joint research between Suez Canal University, Egypt, and the University of Western Ontario, Canada. The authors would like to thank those who offered help during sample preparation and data interpretation. Also, we would like to thank the Egyptian General Petroleum Corporation (EGPC) and El-Wastani Company, Egypt, for supplying the well log data for this study. The review comments of Lichen Song greatly helped improve the organization of this paper.

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Geological information and the results obtained from well log analysis in El-Wastani gas field have been used to study and evaluate the petrophysical characteristics and hydrocarbon prospect of the Late Miocene. We conclude from crossplot analysis that the lithology of the Lower Abu Madi reservoir is dominated by thick and porous sandstone with few shale intercalations. Conversely, the cross-plots show that the main lithology of Qawasim reservoir is shale with few sandstone intercalations. The cut-offs of effective pays are: shale content 50%, porosity 10%, and water saturation 70%. Contour maps of effective pay, average porosity, shale content and water saturation show the clastics in lower part of Abu Madi Formation has good physical properties, the NW-SE premium reservoir area, especially the central part in the study area, is

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