Source Rock Evaluation in July Field Gulf of Suez Egypt by Gamal Ragab Gaafar

Second International Conference on the Role of Applied Geology in Environmental Development, Dec 2009 P. 105 - 126

105

EVALUATION OF SOURCE ROCKS AND HYDROCARBON POTENTIAL OF JULY OIL FIELD, GULF OF SUEZ, EGYPT.

Khaled. A. Khaled*, Gamal Attia*, Gamal. R. Gaafar** and Sameh. M. Ibrahim***

*Geology Department, Faculty of Science, Helwan University, Helwan, Egypt. **Senior Petrophysicist, Petronas Carigali, Kuala Lumpur, Malaysia.

***Geophysicist, PGS Data Processing ME, Cairo, Egypt.

.تقييم صخور المصدر و الجهد الهيدروآربوني بحقل زيت يوليو، خليج السويس، مصر

الخالصة

يتناول هذا البحث تقييم صخور المصدربحقل زيت يوليو بمنطقة خليج السويس وذلك من خالل الخصائص الجيوآيميائية وتاريخ سين عن طريق رسم نماذج ثنائية اآلبعاد لبعض اآلبار بمنطقة الدفن وايضا التطور الحراري لتتابع صخور الميوسين وما قبل الميو

تكاوين بالعيم (و قد تم تقييم الجهد الهيدروآربوني عن طريق تحليل بيانات تسجيالت الكهربية المتاحة لصخور الميوسين . الدراسةآذلك تم تمثيل البيانات . لعضويلتعيين ترآيز محتوى الكربون ا) تكوين طيبة(و صخور ما قبل الميوسين ) روديس , آريم ,

.الجيوآيميائية بعدد من الخرائط التي توضح التوزيع اآلفقي للتدرج الحراري و ترآيز محتوى الكربون العضوي بمنطقة الدراسةعض باإلضافة لذلك تم تقييم الجهد الهيدروآربوني لمنطقة حقل يوليو والجزء األوسط بخليج السويس من خالل نتائج الدراسة وب

وقد تبين من خالل هذة البيانات أن العديد من الوحدات الليثوستراتجرافية بمنطقة . البيانات الجيوآيميائية لدراسات سابقة منشورةيوليو فى خليج السويس تحتوي علي الحد األدنى من محتوي الكربون العضوي، وخاصة صخور الطفلة والحجر الجيري الطفلي

لتى أوضحت الدراسة أنها تتباين من صخور مصدر فقيرة الي غنية بينما صخور ما قبل الميوسين التى الموجودة بعصر الميوسين وا . تعتبر صخور مصدر جيدة الي جيدة جدا

ABSTRACT

July Oil Field is one of the most prolific fields on the Gulf of Suez; it is located in the central part of the Gulf of Suez. The present study aims to evaluate the source rocks based on the organic carbon richness, maturation and thermal burial history. Also, two-dimensional model of burial history and thermal evolution for Miocene and Pre-Miocene rocks in the study area is constructed to illustrate the effect of time and temperature on the oil generation and maturation level of organic matter.

Evaluation of the hydrocarbon generation potential is achieved by the wireline log data analysis of Miocene rocks (Belayim, Kareem and Rudeis formations) and Pre-Miocene rock (Thebes Formation) for determination of the organic matter concentration (measured as Total Organic Carbon Content TOC ,%). Moreover, a number of iso-parametric maps are constructed to show the horizontal distribution of the Geothermal Gradient and the Total Organic Carbon Content (TOC, wt. %) in the studied area. In addition, evaluation of hydrocarbon generation potential in July area within the offshore central part of Gulf of Suez is discussed using the obtained results which are supported by some geochemical data collected from some previous published works. The data reveals that all of the lithostratigraphic units in the area have enough TOC values and the Miocene shale and argillaceous limestone are a variably poor to good source rock, while the Pre-Miocene rocks (Thebes, Esna Shale, Sudr, and Matulla formations) are considered good to very good source rocks.

INTRODUCTION

The geology and stratigraphy of the Gulf of Suez have been attracted the attention of a large number of

authors, out of them in addition to mentioned before, Ghorab, (1961); Said and El-Hiny, (1967); Abd El-Gawad, (1970); Bartov, et al.,(1980); Evans, (1990); Zahran and Meshref, (1988); Hassouba, et al. (1994), and Zein El- Din et al., (1995 and 1997).

The petroleum geology and hydrocarbon potentiality of the Gulf of Suez have been discussed by several authors, out of them, Ball, (1916); Bobbitt and Gallagher, (1978); Abd El-Azim, (1970); Rohrback, (1981); Barakat, (1982); Shahin and Shehab, (1984) ; Robertson Research International, (1986); Chowdhary and Taha, (1987); Shaheen, (1988); Shahin et al., (1994); Al-Sharhan and Salah (1995); Al-Sharhan (2003)and Afify et al. (2005).

July oilfield is located in the central part of the Gulf of Suez, Egypt. It is delineated by latitudes 28° 13\ and 28° 18\ to the north, and longitudes 33° 11\ and 33° 17\ to the east (Fig. 1). The study area is located 18 km from Ras-Gharib Town and approximately 20 km from El Morgan oil field. The field is the fifth largest oil field in Egypt.

Khaled. A. Khaled, et al

106

Lithostratigraphy The generalized stratigraphy of the study area and the Gulf of Suez for which three depositional phases are

generally assumed (Fig. 2). Said, (1990) mentioned that, the stratigraphic sequence in the Gulf of Suez province is characterized by three depositional phases related to the Miocene rifting events. These are: Pre-rift phase (Early Paleozoic to Eocene); Syn-rift phase (Early-Middle Miocene) and Post-rift phase (Late Miocene and Pliocene).

The first phase comprises the deposition of formations ranging in age from a postulated Devonian to Eocene. These formations, which include the Nubia sands, are important as reservoir rocks and to a lesser extent as source rocks. The second phase is represented by the Lower Miocene and is characterized by its overall excellent qualities as source, reservoir and seal rocks. The third phase, of the Upper Middle Miocene to Upper Miocene and Pliocene age in essence and is characterized by its evaporite seal. It closes the depositional history of the Gulf of Suez graben area (Darwish and El Araby, (1993)).

Evaluation of the Geological Setting of the Gulf of Suez The structural evolution of the Gulf of Suez area was the subject of numerous investigations, such as those of

Said (1962), Garfunkel and Bartov (1977), Meshref, (1990), and Patton et al. (1994). The evolution of the Gulf basin is characterized by tectonic extensional episodes producing tension block faulting (horst and graben) and block subsidence. However, Kingston et al. (1983) , Rashed, (1990) and Saoudy, (1990) suggested five distinctive evolutionary stages:

1- Deposition of Paleozoic terrestrial clastics over Precambrian crystalline basement with minor tectonic. The Hercynian epiogeny folded and uplifted the Paleozoic deposits. The hiatus caused by these movements is evident in the thinning or absence of sedimentation in many parts of the Gulf of Suez, where Cenomanian strata rest unconformably on Carboniferous strata.

2- Local subsidence and minor transgression occurred during the Permain-Triassic to Jurassic. This is led to deposition of fluviomarine red shales and sandstones.

3- Rifting of the continental crust, under tension, in the Early Cretaceous led to formation of a system of grabens via block faulting. Depressions were later filled with nonmarine sandstone and shale.

4- This stage extended from Middle Cretaceous to Miocene, normal faulting continued and the graben system gradually subsided to form a deep basin. Early and Middle Alpine movements occurring in this stage had significant effects on the structure of Mesozoic and Paleogene strata and gave rise to a series of folds in the areas of tectonic compression. Marine water invaded the basin and deposited a range of different sedimentary facies, ranging with location in the basin. Marine sandstone and shallow marine limestone, including reefal limestone, were deposited on structural highs, whereas shale and globigerinal marl accumulated in the low areas. The last strata of this stage were thick salt deposits.

5- This stage was the final stage of rift evolution. The interior fracture system widened during the Pliocene-Holocene, the basin fill was uplifted at the rift margins because of continued block faulting, and nonmarine wedge-top strata (mainly sandstone) penetrated the basin. Purpose and Scope

This paper attempts to evaluate the source rocks in July Field at the Central Part of the Gulf of Suez, based on the organic carbon richness [wireline log data for determination of the organic matter concentration (measured as Total Organic Carbon Content TOC, %)], maturation and thermal burial history. Moreover, a number of iso-parametric maps are constructed to show the horizontal distribution of the Geothermal Gradient and the Total Organic Carbon Content (TOC, wt. %) in the studied area. In addition, evaluation of hydrocarbon generation potential in July area within the offshore central part of Gulf of Suez is discussed using the obtained results which are supported by some geochemical data collected from some previous published works. Methodology Quantity of Organic Material

The amount of the organic matter (OM) required for a sedimentary rock to be considered as a petroleum source rock was discussed by a number of researchers. Among them are Cook (1974), Waples (1979), Cornford (1984), Tissot and Welte (1984), Espitalie' et al. (1985), Peters (1986) and Omokawa et al. (1992). In general, the quantity of oil generated from a given volume of a source rock is proportional to its TOC (Cook, 1974 and Waples, 1979).

The amount of organic material present in the sedimentary rocks is almost always measured as Total Organic Carbon (TOC) content, which is the first and most important screening technique used to indicate which rocks are of no interest to us (TOC < 0.5%), which ones might be of slight interest (TOC between 0.5% and 1%), and which are definitely worthy further consideration (TOC > 1%), (Waples, 1985, 1980). According to Peters, 1986 the source rocks are classified as follows (Table 1):

EVALUATION OF SOURCE ROCKS

107

Quality TOC (wt. , %) ranges

Poor 0 - 0.5

Fair 0.5 - 1.0

Good 1.0 - 2.0

Very Good > 2.0

Table 1 Source Rock Types (after Peters, 1986)

Schmoker (1979) drew the attention to the determination of the organic matter within shale sequence using the combination of compensated formation density log (FDC) with the gamma-ray log (GR). The use of this method is preferred because the density log is more common and available than core samples. Moreover, the continuously recorded density log eliminates the statistical uncertainties of the limited sampling of the formation. Also, determination of TOC by log analysis proved to be less expensive than the classical analysis of core samples. In the present work, the total organic carbon is calculated by using Schmoker and Hester (1983) equation and Meyer and Nederlof (1984) equation, after applying the borehole corrections to the density log readings:

Qo = (ρb - ρ) / (ρb – ρo) (Meyer and Nederlof, 1984) (1) where:

Qo is the organic content by volume (vol., %) ρb is the bulk density of a compacted shale sequence with no organic matter (ρb = 2.7 gm/cc). ρ is the density of the shale sequence within the studied units. ρo is the organic matter density.

TOC = Qo (100 * ρo) / (R * ρ) (Schmoker and Hester, 1983) (2) where:

R is the ratio between the weight percent of organic matter and organic carbon and depends on certain parameters as depth and temperature, Schmoker used R= 1.3. Maturity of Organic Material

Many parameters are used in petroleum exploration for evaluating the thermal evolution of source rocks and their OM during the different stages of maturation. The atomic H/C versus O/C diagram was developed by Van Krevelen (1961) is considered the most reliable method for following the chemical processes that occur during coal maturation. These atomic ratios are replaced by HI and OI in case of using the Rock-Eval Pyrolysis. The concentration and molecular distribution hydrocarbons contained in a rock depend on both the type of the parent organic matter and its degree of thermal alteration (Stoneley, 1995). In this study, the thermal maturation analysis has been carried out through Vitrinite Reflectance (Ro, %), Geothermal effect and Burial history curves are used. i) Vitrinite Reflectance (Ro, %)

Thermal evolution of source rocks changes many physical and chemical properties of the organic matter, so the changes in these properties are used as indicators for maturity. The most common parameter used, as a standard against which all other parameters are calibrated, is the vitrinite reflectance. For most kerogens the onset of oil-generation is taken to be near 0.6% Ro. Peak generation and migration is about 0.9% Ro and the end of liquid-hydrocarbon generation is thought to be about 1.35% Ro, as shown in the following (Table 2):

Ro % Stages of Maturation Types of Hydrocarbons

0.4 0.5 Immature stage Condensate from resinite 0.6 Early mature stage 0.65 0.7


108

Table 2 Stages of maturation and types of hydrocarbon products based on Vitrinite Reflectance values (after Waples, 1985)

In this study, the calculation of vitrinite reflectance from temperature histories utilizing PETROMODE program was carried out for two Wells (J58-82 and J37-93) using the EASY%Ro algorithm of [Sweeney and Burnham 1990] (Equation 3) which is based on reaction kinetic results that allow the calculation of vitrinite reflectance values between 0.3 and 4.5 % VRr. This is the most popular methods of modeling, VRr. It uses a set of twenty reactions with a common pre-exponential factor A= 1.0 x 1013 s-1, weighted factors and activation energies (Table 3). The EASY%Ro breaks the thermal history of a source bed into segments of constant heating rates.

The resulting composition is used to calculate vitrinite reflectance via correlations between elemental compositions and reflections. In a simplified model (Easy-Ro model), only the fraction of unconverted vitrinite is used to calculate the vitrinite reflection values. Then it is possible to overlay the distributions of initial potentials and activation energies of all four reactions and to obtain one resulting distribution for the unconverted vitrinite potential x. The reflection value is finally derived by an exponential mapping of the TR to the interval (0.2%, 4.66%), which yields to the following equation.

Vitrinite reflectance estimation based on (Sweeney and Burnham 1990):

Ro%= 0.20 (4.66/0.20)TR (3) Reaction number

(i) Stoichionmeteric

factor (Fi) Activation Energy

Ei (kJ mol-1)

1 0.03 142 2 0.03 151 3 0.04 159 4 0.04 167 5 0.05 176 6 0.05 184 7 0.06 192 8 0.04 201 9 0.04 209 10 0.07 218 11 0.06 226 12 0.06 234 13 0.06 243 14 0.05 251 15 0.05 259 16 0.04 268 17 0.03 276 18 0.02 285 19 0.02 293 20 0.01 301

Table 3 Weighted Factors and activation energies used in EASY%Ro.

0.8 Peak generation Oil generation 1.0 1.35 Late generation

2.0 Wet gas

> 2.0 Dry gas


109

The thermal modeling procedures include the reconstruction of the present-time temperature regime and the

temperature history evaluation. Bottom-hole temperatures (BHTs) were used to calculate the present-day temperature (Figure 3). Deficiencies in the database must be recognized. Normally, more than one interpretation fits the observable data. The calculated parameters are compared with measured data so that the thermal model can be calibrated (Figure 3). If necessary, the conceptual model is adjusted or modified to lead to a better match between simulation results and calibration data.

ii) Geothermal Effect

It is unanimously accepted that the temperature and time are important agents influencing the process of oil generation and the subsequent cracking of oil to methane. The effect of increasing temperature is essentially exponential, as compared to that of increasing time. So that, the calculation of geothermal gradient (GG) is important in determining the maturation of organic matter. The geothermal gradient is calculated by using (Helander, 1978) equation as follows:

GG= [(bottom hole temperature – surface temperature)/ total depth] * 100 (4) Before calculating the geothermal gradient, bottom hole temperature must be corrected. In the present study, the bottom hole temperature was corrected by using Shell (1978) chart. Application

In this paper, the author has used the previous published data due to lack of rock samples and geochemical analysis data in July Field. This geochemical data on offshore Central Gulf of Suez, table 4 is collected from some previous published works which are Shahin (1988), Shahin et al. (2000) and Abd El-Baki (2000).

The first look at Table 4 declares that all the lithostratigraphic units have enough organic carbon contents, which exceed the cut off value (0.5 wt. %) to consider them as potential source rocks. The most significant TOC values are recorded in the shale rocks of the Rudeis Formation (up to 7.3 wt. %) and the limestones of the Brown Limestone (up to 4.5 wt. %). Also, the other formations have good TOC values which are always bigger than 1 wt. %. This indicates that the offshore central part of the Gulf of Suez has a sedimentary sequence containing good to excellent source-rock beds. These beds could be the source rock of the hydrocarbon accumulations in the discovered oil fields in the area. In addition, these source-rock beds are able to generate addition hydrocarbons in the future.

Figure 4 shows the relationship between the genetic generation potential (S1+S2) and the organic matter richness (TOC) of the different lithostratigraphic units in the offshore central area of the Gulf of Suez. The figure declares that Raha, Abu Qada, Wata, Matulla and Belayim (limestones) formations are not potential source rocks. Meanwhile, Brown Limestone, Sudr, Esna Shale, Thebes, Nukhul, Rudeis, Kareem and Belayim (Shales) could act as potential source rocks.

The total genetic potential (S1+S2, mg HC/g rock) of the potential lithostratigraphic units differs from one to another. Using the classification of Peters (1986), the obtained data (Table 4) shows that Brown Limestone (17.2) and Sudr Chalk (18.4) show very good genetic potential (>10 mg HC/g rock), Esna Shale (8.2) and Belayim Shale (7.0) have a good genetic potential (5-10 mg HC/g rock). Meanwhile, Thebes (2.4), Nukhul (2.7), Rudeis (2-3.3), and Kareem (2.5-3) show fair genetic potential (2-5 mg HC/g rock). Also, the production index [S1/ (S1+S2)] of these lithostratigraphic units indicate the oil window zone as the values lie between 0.08 and 0.4 (Table 4).

Quality of organic matter

The H/C and O/C atomic ratios can be expressed also using the Rock-Eval Pyrolysis results (S2, S3) and the calculated indices HI and OI plotted on the modified Van Krevelen diagram. Figure 5 shows the plot of HI versus OI for the different lithostratigraphic units of the offshore central Gulf of Suez. This figure indicates that the Brown Limestone contains mainly Type I Kerogen which is mixed with a less amount of Type II, while Sudr and Thebes formations are characterized by Type II Kerogen. Meanwhile, Esna Shale and Belayim formations have mainly Kerogen of Type II mixed with a less amount of Type III. Abu Qada limestones, Nukhul, Rudeis, Kareem and Wata (limestones) formations contain mainly Type III Kerogen with less amount of Type II. On the other hand, Matulla, Abu Qada (shales) and Wata (shales) formations are characterized by Type III Kerogen (Table 5).


110

R o

, %

0.45

0.51

0.45

0.5

0.47

0.46

0.51

0.62

0.54

0.57

0.61

0.6

Tmax

(o C

)

423

429

430

427

431

434

441

427

438

429

442

438

434

437

438

437

436

431

OI (m

g CO

2/g

TOC)

123

150

123

170

140

181

140 53

108 59

45

106 70

68

193

122

172

126

HI (m

g HC

/g

TOC)

469

341

266

255

210

289

279

407

384

549

589

168

151

104

222

155

299

197

S1 / (

S1+S

2)

0.24

0.10

0.08

0.09

0.10

0.11

0.38

0.63

0.20

0.10

0.09

0.11

0.08

0.13

0.12

0.11

0.09

0.09

S1+S

2

7.00

2.64

3.1

2.53

1.98

3.35

2.66

2.41

8.23

18.4

17.2 1.9

1.19

1.5

1.59

1.45

3.02

2.08

S2

5.30

2.37

2.84

2.31

1.78

3.00

2.40

0.90

6.60

16.50

15.70

1.70

1.10

1.30

1.40

1.29

2.75

1.90

S1

1.7

0.27

0.25

0.22

0.20

0.35

0.26

1.51

1.63

1.90

1.50

0.20

0.09

0.20

0.19

0.16

0.27

0.18

OM

0.9 -

2.5

0.9

0.9 -

2.8

0.9 -

2.7

0.9 -

9.1

1.0 -

3.0

1.0 -

3.8

1.4 -

4.1

1.8 -

2.5

1.7 -

4.6

1.1 -

5.6

0.9 -

3.3

0.9 -

1.2

0.8 -

2.4

0.7 -

0.9

0.6 -

2.5

0.7 -

1.2

0.8 -

1.3

Orga

nic R

ichne

ss

(wt.,

%)

TOC

1.1 -

2.0

0.70

0.7 -

2.2

0.7 -

2.1

0.7 -

7.3

0.8 -

2.3

0.8 -

2.95

1.1 -

3.1

1.4 -

2.0

1.3 -

3.6

0.9 -

4.5

0.7 -

2.6

0.7 -

0.9

0.6 -

1.9

0.5 -

0.7

0.5 -

2.0

0.5 -

0.9

0.6 -

1.0

Rock

Typ

e

Shale

Limes

tone

Shale

Limes

tone

Shale

Limes

tone

Limes

tone

Limes

tone

Shale

Limes

tone

Limes

tone

Shale

Limes

tone

Shale

Limes

tone

Shale

Limes

tone

Limes

tone

Dept

h, (m

)

1478

-211

8

2774

1645

-291

1

1661

-312

4

1200

-349

0

2499

-347

5

3215

-327

6

1359

-336

8

2444

-339

8

2286

-347

5

2502

-341

4

1521

-358

1

1530

1658

-361

2

1704

-364

2

1722

-373

4

3795

1740

Age

Serra

valia

n

Lang

hian-

Serra

valia

n

Burd

igalia

n-La

nghia

n

Aquit

anian

-Bu

rdiga

lian

Eoce

ne

Paleo

cene

Late

Seno

nian

Late

Seno

nian

Early

Se

nonia

n

Turo

nian

Turo

nian

Ceno

mania

n

Form

atio

n

Belay

im

Kare

em

Rude

is

Nukh

ul

Theb

es

Esna

Sha

le

Sudr

Cha

lk

Brow

n Lim

eston

e

Matul

la

Wata

Abu Q

ada

Raha

Table 4 the average values of some geochemical characteristics of litholostratigraphic units in the Offshore Central Gulf of Suez Region.* Depth intervals are not in successive increasing order since the data are collected from different wells in the area.


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Kerogen Types Litho-stratigraphic Units I II II & III III & II III Belayim X Kareem X

Rudeis (Limestone) X Rudeis (Shale) X

Nukhul X Thebes X

Esna Shale X Sudr X

Brown Limestone X X Matulla X

Wata (Limestone) X Wata (Shale) X

Abu Qada (Limestone) X Abu Qada (Shale) X

Raha X

Table 5 Quality of organic matter (Kerogen Types) of lithostratigraphic units, Offshore Central Gulf of Suez.

Total Organic Carbon (TOC) The Total Organic Carbon Content (TOC) was calculated from formation density log using equations (1 and 2) in

our studied Wells, Table (6). The TOC wt, % for Belayim Formation ranges from 0.7 wt, % at J4-4 Well to 2.2 wt, % at J25-28A Well, while TOC wt, % for Kareem Formation is about 0.72 wt, % at J37-93 Well and 2.32 wt, % at J10-70 Well. Regarding Rudeis Formation, the TOC wt, % ranges from 2.5 wt, % at J4-4 Well to 4.3 wt, % at J25-28A Well and the TOC wt, % for Thebes Formation equals 0.75 wt, % at J37-93 Well and 3.5 wt, % at J4-4 Well.

Contour maps were constructed for the calculated TOC of the studied formations (Figs. 6, 7, 8 and 9). These maps illustrate the average TOC wt, % increases towards the south eastern part of the studied area.

Well Belayim FM TOC (wt, %)

Kareem FM TOC (wt, %)

Rudeis FM TOC (wt, %)

Thebes FM TOC (wt, %)

J4-4 0.7 0.8 2.5 3.5 J10-70 1.75 2.32 3.2 2.03 J15-98 0.68 1.2 2.6 1.4

J25-28A 2.2 1.08 4.3 1.0 J37-93 0.98 0.72 3.89 0.75 J58-82 1.04 1.55 3.5 1.8

SG310-5A 1.5 1.8 2.97 2.0 SG310-5C 1.48 1.72 2.98 2.3 GS302-3 1.62 2.2 4.0 3.1

Table 6 Estimated average Total Organic Carbon Content (TOC) wt, % from wireline log data.

Maturity of Organic Material In this study, the thermal maturation analysis has been carried out through Vitrinite Reflectance (Ro, %),

Geothermal effect and Burial history curves are used. Vitrinite Reflectance (Ro, %)

The calculation of vitrinite reflectance from temperature histories utilizing PETROMODE program was carried out for two Wells (J58-82 and J37-93) using the EASY%Ro algorithm of [Sweeney and Burnham 1990] (Equation 3). The calculated Vitrinite Reflectance (Ro %) values are range from (0.46%) at J58-82 Well to (0.61%) at J37-93 Well for Raha Formation, range from (0.45%)at J58-82 Well to (0.61%) at J37-93 Well for Abu Qada Formation, range from (0.43%)at J58-82 Well to (0.60%) at J37-93 Well for Wata Formation, range from (0.42%)at J58-82 Well to (0.58%) at J37-93 Well for Matulla Formation, range from (0.40%)at J58-82 Well to (0.56%) at J37-93 Well for Brown Limestone, range from


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(0.39%)at J58-82 Well to (0.54%) at J37-93 Well for Sudr Formation. range from (0.38%)at J58-82 Well to (0.53) at J37-93 Well for Esna Formation, range from (0.38%)at J58-82 Well to (0.52%) at J37-93 Well for Thebes Formation, range from (0.37%)at J58-82 Well to (0.50%) at J37-93 Well for Nukhul Formation, range from (0.36%)at J58-82 Well to (0.48%) at J37-93 Well for Rudeis Formation and range from (0.35%)at J58-82 Well to (0.43%) at J37-93 Well for Kareem Formation. The calculated Vitrinite Reflectance (Ro %) values for the studied formations reveals that the early stage of generation (0.6% Ro) in Raha Formation at J37-93 Well.

In the present work, the lithostratigraphic units encountered in the area show in general low values of Vitrinite Reflectances. Table 4 indicates Vitrinite Reflectance less than 0.5% for the Syn-rift rocks (Belayim, Kareem, Rudeis, Nukhul formations) and the Pre-rift Thebes Formation. Meanwhile, most of Pre-rift formations exert relatively higher values (Ro, 0.51-0.62 %). As much as the Vitrinite Reflectances are a function of both the depth and the geothermal gradient, the roof of the catagenesis stage of maturation (oil generation zone and expulsion, Ro > 0.5 %) is suggested to be at depth value of about 2800 m (8500 ft) as indicated by the depth intervals of the different lithostratigraphic units in the area (Table 4, Fig. 10).

Due to the structural configuration of the Gulf of Suez basin, which is the result of the effect of faulting giving rise to a successive fault-blocks (grabens and horsts), it is expected to find the source rocks buried in the catagenesis stage (oil-generation) zone within the graben blocks. In addition, the roof of oil window (Ro = 0.85 %) is delineated at depth value about 4750 m (14300 ft) at the East July Trough (Shahin et al., 2000).

Figure 11 declares the situation of the different lithostratigraphic units in the area using the plot of HI versus the Tmax values. It is obvious that Belayim, Kareem, Thebes, Sudr and Raha formations are located within the immature zone, while other formations are recorded in the mature (oil-generation) zone. These results are proved by relationship between production Index and Tmax (Fig. 12). However, the Brown Limestone and Nukhul formations shows the highest Tmax values indicating their high maturity level within the main stage of oil generation (oil window) at depth intervals deeper than 3000 m (9840 ft).

It is worth to mention that, there is a slight differences concerning maturity level of some lithostratigraphic units in the area. For example, Sudr Formation in figure 10 is located within the mature zone depending on the average Vitrinite Reflectance value (Ro = 0.51 %), while in figure 11 Sudr Formation lies within the immature stage as its Tmax equals 429 oC (less than 430 oC). This is could be attributed to the data used which is collected from different Wells and depths. However, using of different maturity parameters overcomes this problem. Figure 12 which is based on the Pyrolysis results (Tmax and Production Index) proves presence of Sudr Formation in the mature zone. Geothermal Effect

The geothermal gradients within July Field are calculated for the studied Wells, (Table 7). The calculated values range from 1.57 oF/100 ft (2.87 oC/100m) at J15-98 Well to 2.05 o F/100 ft (3.74 oC/100m) at J25-28A Well. A Geothermal Gradient map in July Oil Field (Fig. 13) shows that, the geothermal gradient decreases gradually toward J15-98 Well toward NE, where the sediment thickness increases. Whereas, a relatively high geothermal gradient values occurred in the vicinity of recent rifting and vulcanicity. However, the average value of the calculated geothermal gradient in July area equals about 1.81 oF/100 ft (3.31 oC/100m) (Table 7, Fig. 13).

Well Geothermal Gradient F/100 ft Geothermal Gradient C/100 m

J4-4 1.73 3.16

J10-70 1.66 3.03

J15-98 1.57 2.87

J25-28A 2.05 3.74

J37-93 1.69 3.09

J58-82 1.89 3.45

SG310-5A 1.99 3.63

SG310-5C 1.99 3.63

GS302-3 1.73 3.16 Table 7 Geothermal Gradient of the studied wells in July Oil Field.


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Burial History of July Field The burial history of July Field is carried out utilizing PETROMODE program, represented by time-depth history

plots for (J58-82 and J37-93 wells) (Figures 14 and 15) that show the burial of different horizons traced through time, from deposition to present day. These plots indicate the main features of deposition in July Field.

The thermal burial histories of the drilled sections (Figure 14) suggested two thermal anomalies associated with two regional tectonic events. The first is the Lower Miocene Middle Rudies event, at 16.6 MYBP. The second is post Miocene Messinian events at 5.3 MYBP. These two events are likely to be associated with higher than normal heat flow (Shahin et al., 1994).

Figures 14 and 15 reveals that the drilled sections in J58-82 and J37-93 Wells are thermally immature as demonstrated by burial history model. The calculated Vitrinite Reflectance (Ro %) values for the studied formations reveals that the early stage of generation (0.6% Ro) in Raha Formation at J37-93 Well. This indicates that oil in the reservoir rocks on July Field is migrated from another basin. In the offshore central province, where July Field is located, two major troughs are traced and delineated by Patton et al. (1989) and Shahin et al. (1994 and 2000). These two troughs are the South Belayim to the northeast from July Field and the East July trough to the southeast from the field (Fig. 16). Alsharhan (2003) denoted these two troughs as one trough which in South Belayim trending NW-SE to the east of July Field in the Central province of the Gulf of Suez.

According to Alsharhan (2003), in the Southern Province the Pre-rift source rocks lie within the oil generation window. The oil generation threshold is believed to have been established at 10 Ma for the Pre-rift source rocks and around 4 Ma for the Syn-rift source rocks (mainly Miocene). The depth to the level of onset of oil generation ranges in the area from about 2290 to greater than 3660 m and decreases southward. In the Northern Province (Darag trough) oil generation threshold is believed to have been established at 9 Ma for the Pre-rift source rocks and around 3 Ma for the Syn-rift source rocks (mainly Miocene). The depth to the level of onset of oil generation ranges in the area from about 2280 to greater than 3650 m and decreases southward.

Shahin et al. (1994 and 2000), indicated that in south Belayim trough the Pre-rift source rocks (Thebes, Esna Shale and Lower Sudr formations) reached top oil window and expelled their hydrocarbons since 8 to 4 Ma. Meanwhile, the oldest Pre-rift source rocks (Matulla-Wata, Abu Qada-Raha formations) started to expel their hydrocarbons at time between 9 and 2.3 Ma. On the other hand in East July trough, the hydrocarbons generated from all source formations were expelled between 13 to 8 Ma.

Figure (1) Location of the available wells and cross sections at the July oilfield, Gulf of Suez.


114

Figure (2) Generalized lithostratigraphic column of the Gulf of Suez (Schlumberger, 1984).


115

Figure 3 Plot of paleotemperature calibrated with measured corrected static bottom hole temperature and the measured vitrinite reflectance data in a reference wells (J58-82 and J37-93) against depth. The calculation of vitrinite reflectance from temperature histories was carried out using the EASY%Ro algorithm of [Sweeney and Burnham 1990] which allows the calculation of vitrinite reflectance values between 0.3 and 4.5% VRr. The cross-plot of observed and computed reflectance shows a good fit.


116

Figure 4 Evaluation of source rock potential using genetic generation potential and organic matter abundance of the lithostratigraphic units in the Offshore Central Gulf of Suez (Relationship after Bissada, 1982)

Figure 5 Plot of HI versus OI for the lithostratigraphic units in the Offshore Central Gulf of Suez (developed by Van Krevelen, 1961).


117

Figure 6 Average Organic Carbon Content of Belayim Formation map of July Oil Field (C.I:0.10 wt, %)

Figure 7 Average Organic Carbon Content of Kareem Formation map of July Oil Field (C.I:0.10 wt, %)


118

Figure 8 Average Organic Carbon Content of Rudeis Formation map of July Oil Field (C.I:0.10 wt, %)

Figure 9 Average Organic Carbon Content of Thebes Formation map of July Oil Field (C.I:0.10 wt, %)


119

Figure 10 Vitrinite Reflectances versus Depth of the lithostratigraphic units Offshore Central Gulf of Suez showing maturity level of rocks.

Figure 11 A plot of HI versus Tmax for lithostratigraphic units in Offshore Central part of the Gulf of Suez (after Delvaux et al., 1990).


120

Figure 12 Maturation evaluation of lithostratigraphic units based on Tmax and Production Index, Offshore Central part of the Gulf of Suez (Relationship after Bissada, 1982).

Figure 13 Geothermal Gradient map of July Oil Field (C.I:0.05 oF/100 ft)


121

Figure 14 Quantitative simulated geohistory, burial history and recalibrated temperature development history as a function of time and space at (J58-82 and J37-93 wells), July Field using the paleotemperature determined by Easy% Ro approach.


122

Figure 15 Simulated burial history at (J58-82 and J37-93 wells), July Field, with maturity overlay using the Easy%Ro algorithm [Sweeney and Burnham 1990] against geologic time scale (Ma).


123

*: Prospective Area : Oil Field : Generating Trough : Migration pathway I: South Belayim Trough II: East Ramadan Trough III: East July Trough Figure 16 Top Pre-Miocene Depth Structure contour map showing the outlines of generating troughs and the suggested migration pathways of hydrocarbon. (After Patton et al., 1989, Shahin et al., 1994 and 2000)


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CONCLUSIONS The data reveals that all of the lithostratigraphic units in the area have enough TOC values and the

Miocene shale and argillaceous limestone are a variably poor to good source rock, while the Pre-Miocene rocks (Thebes, Esna Shale, Sudr, and Matulla formations) are considered good to very good source rocks.

The geothermal gradients within July Field range from 1.57 oF/100 ft (2.87 oC/100m) at J15-98 Well to 2.05 oF/100 ft (3.74 oC/100 m) at J25-28A Well. A Geothermal Gradient map shows that, the geothermal gradient decreases gradually toward J15-98 Well toward NE, where the sediment thickness increases. Whereas, a relatively high geothermal gradient values occurred in the vicinity of recent rifting and vulcanicity.

The thermal burial histories reveal that the drilled sections are thermally immature as demonstrated by burial history model. The calculated Vitrinite Reflectance (Ro %) values for the studied formations reveals that the early stage of generation (0.6% Ro) in Raha Formation at J37-93 Well. This indicates that oil in the reservoir rocks on July Field is migrated from another basin. In the offshore central province, where July Field is located, two major troughs are traced and delineated. These two troughs are the South Belayim to the northeast from July Field and the East July trough to the southeast from the field.

The oil generation threshold is believed to have been established at 10 Ma for the Pre-rift source rocks and around 4 Ma for the Syn-rift source rocks (mainly Miocene). Also it is indicated that in south Belayim trough the Pre-rift source rocks (Thebes, Esna Shale and Lower Sudr formations) reached top oil window and expelled their hydrocarbons since 8 to 4 Ma. Meanwhile, the oldest Pre-rift source rocks (Matulla-Wata, Abu Qada-Raha formations) started to expel their hydrocarbons at time between 9 and 2.3 Ma. On the other hand in East July trough, the hydrocarbons generated from all source formations were expelled between 13 to 8 Ma.

RECOMMENDATIONS • Apply log methodology to other areas of the Gulf of Suez, where source quality is unknown. • Carry out more geochemical studies on July Field to trace the source of oil in the reservoirs.

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Documents

Source Rock Evaluation in July Field Gulf of Suez Egypt by Gamal Ragab Gaafar