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Organic geochemical studies of some early cretaceous sediments, Abu Gharadig Basin, Western Desert, Egypt
Journal of African Earth Sciences, Vol.
PII:SO899-5382(98)00050-5
All rights
27, No. 1, pp. 115-l 27, 1998 D 1998 Elsevier Scmce Ltd reserved. Pmted in Great Britam 089%5362/98 $19.00 + 0.00
COMMUNICATION
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin, Western Desert, Egypt
Geology Department,
ADEL WAHEED FELESTEEN Faculty of Science, University of Aswan, Aswan,
Egypt
Abstract-A total of 81 shale samples from the Early Cretaceous sediments of seven wells in the Abu Gharadig Basin, were analysed for organic matter quantity, type and maturation. The total organic carbon content of analysed samples ranges between 0.5 and 2.67 wt%, indicating a fair to good organic content. The location of investigated samples on the van Krevelen diagram reflects the presence of kerogens type IV (without hydrocarbon potential), type III (gas-prone) and type ll-lll (oil and gas-prone). The estimated temperature of maximum pyrolytic hydrocarbon generation of analysed shales ranges between 426 and 447’C, indicating immature to thermally mature organic matter. The shale beds of Kharita Member from the northcentral part of the study area, at depths more than 2164 m, represent gas-prone source beds, while those of Alam El-Bueib Member, at depths more than 2545 m, can be considered potentially as active oil and gas-prone source beds. The present study reveals that the Early Cretaceous Alam El-Bueib Member is more enriched in thermally mature type III and ll-lll organic-rich shale beds, and consequently can be regarded as the potential source for the oil and gas accumulations in the carbonate reservoirs of.the Alamein Member and sandstone reservoirs of the Kharita Member in the Abu El-Gharadig Basin. Q 1998 Elsevier Science Limited. Resume-Un total de 81 Bchantillons de shales du C&ace precoce, preleves dans 7 forages dans le Bassin d’Abu Gharadig, a Bte Btudie pour la quantite, le type et la maturation de leur mat&e organique. Les teneurs en carbone organique total varient de 0.5 a 2.67% en poids, ce qui indique des valeurs moyennes a Blevees. La localisation des echantillons dans le diagramme de van Krevelen reflete la presence de kerogene de types IV (sans hydrocarbures potentiels), III (a gaz potentiels) et ll-lll (a gaz et p&role potentiels). La temperature estimee pour la formation d’hydrocarbures par pyrolyse varie de 426 a 447OC. ce qui indique que la mat&e organique est immature a thermiquement mQre. Dans la partie centre-nord de la zone d’etude, les lits de shale du Membre de Kharita, situ& a plus de 2164 m de profondeur, representent des gites favorables pour le gaz, alors que ceux du Membre d’Alam El-Bueib, a plus de 2545 m de profondeur, peuvent constituer un source potentielle de p&role et de gaz. Cette etude r&Ye que le Membre c&ace precoce d’Alam El-Bueib est le plus riche en shales thermiquement mfirs de types III et ll-lll et, par consequent, peut Qtre considere comme source potentielle d’accumulations de petrole et de gaz dans les reservoirs carbonates du Membre d’Alamein et les reservoirs greseux du Membre de Kharita du Bassin d’Abu El-Gharadig. c 1998 Elsevier Science Limited. (Received 3 March 1997: revised version received 3 October 1997)
INTRODUCTION
The Abu Gharadig Basin (Fig. 1) represents one of the most important sedimentary basins in the northern Western Desert of Egypt in terms of its hydrocarbon potential. Such evaluation of any
sedimentary basin requires the determination of its potential source rocks. The geology and hydrocarbon potential of Early Cretaceous sediments in the northern
Journal of African Earth Sciences 115
A. W. FELESTEEN
_---____-_
----
___----_.,
__ .a-
QATTAAA
DEPRESSION ,_\,&___~
I \
-’
-._.
---L,’
_?\_.’
cl’--& -\ \
\ ‘L. ._,---.__-_I
N t
KATTANIYA-1 .,’
SW
MUBARAK-1
WD
57-1
p
Figure 7. Key map showing the location of studied wells and lines of stratigraphical
Western Desert have been dealt with by many authors (Parker, 1982.; Abu El-Naga, 1984; Labib, 1984; Schlumberger, 1984; Robertson Research International and associates, 1985; Kholeif era/., 1986; Abdel Aal and Moustafa, 1988; Barakat ef a/., 1988; Nakhla et al., 1992; etc.). The present work is concerned with organic geochemical studies on the potential source members of the Early Cretaceous Burg El-Arab Formation (Kharita Member and Alam El-Bueib Member), in the Abu Gharadig Basin, to shed more light on their hydrocarbon content.
STRATIGRAPHICAL
SETTING
The Early Cretaceous sequence of the Abu Gharadig Basin represents a transgressive series which was deposited after a period of uplift and erosion that took place during Late Jurassic time and continued until the Late Neocomian. This sequence is represented by the Burg El-Arab Formation which is mainly composed of shales and sandstones (continental to shallow marine deposits). It is subdivided into three members: Alam El-Bueib, Alamein and Kharita. The Alam El-Bueib Member forms the lower part of the
116 Journal of African Earth Sciences
100
3,O km
correlation in the Abu Gharadig Basin.
Burg El-Arab Formation, and attains a maximum thickness of 1044 m. It is composed of alternating beds of shale and sandstone, with coal seams, and is considered to have been deposited in a deltaic to shallow marine environment. A Late Neocomian to Aptian age is assigned to this member. A maximum transgressional phase occurred during the Aptian, with the deposition of the Alamein Member in a restricted marine to lagoonal environment. This member is mainly composed of a white to light-brown, hard, dense, microcrystalline dolomite, which posses laterally a tan white limestone with few shale intercalations (Figs 2 and 3). It reaches a maximum thickness of 151 m, and represents the main carbonate reservoir for petroleum in the study area. The Albian saw widespread deposition of the thick marginal-marine elastics of the Kharita Member. This upper member of Burg El-Arab Formation, extends over almost the entire study area. It is mainly composed of thick sequence of sandstones with shale intercalations (Fig. 41, and attains a maximum thickness of 908 m. The Kharita Member rests unconformably on either the Alamein Member or the Alam El-Bueib Member, and is overlain conformably by the
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
I308 -
SHEIBA
lT22 -
1882 -
SH
18-2
SHEIBA
18-1
2120 -
2588 -
2588 -
2720 -
2880 -
3108 -
$305 .
Figure 2. Stratigraphical
correlation of Early Cretaceous rocks in the Abu Gharadig Basin (see Fig. 1: line A-AI.
WD
38-l
KATTANIYA--1
MUEARAK-1
Figure 3. Stratigraphical
correlation of Early Cretaceous rocks in the Abu Gharadig Basin (see Fig. 1: line B-BJ.
Journal of African Earth Sciences 117
A. W. FELESTEEM
Laboratories (Denver) and Robertson Research International (Wales). The kerogen type was determined by plotting the hydrogen index (HI) versus the oxygen index (01) on a van Krevelen diagram. The stage of maturity was estimated from the temperature of maximum pyrolytic hydrocarbon generation (T,,,).
AGE
APTlAh
m
Shale
B
Limestone
1.‘.‘.....1 Sandstone
m
Dolomitic
m B
E
Coal
Sandy shale Dolomite
limestone
Figure 4. Generalised stratigraphical column of Early Cretaceous sediments in the Abu Gharadig Basin.
Lower Cenomanian Bahariya Formation. The sandstones of the Kharita Member are distinguished by good porosity/permeability characteristics, and host important hydrocarbon accumulations.
MATERIAL AND METHODS The present study involves the investigation of 81 shale samples from the Burg El-Arab Formation (34 samples from Kharita Member and 47 samples from Alam El-Bueib Member), collected at various depths in seven wells drilled in the Abu Gharadig Basin (Fig. 1). The total organic carbon (TOC) content of the samples was determined using a LECO carbon analyser. The kerogen type and maturity were determined using Rock Eval Pyrolysis (Espitalie et a/., 1977). The samples were analysed at Exploration Logging International (Cairo), Brown and Ruth
118 Journal of African Earth Sciences
SOURCE ROCK EVALUATION Organic content TOC determinations of shale samples from the Kharita and Alam El-Bueib Members are listed in Tables 1 and 2, respectively. They are mainly above the critical lower limit (0.5 wt%) which is generally accepted as the minimum value of organic carbon for a potential source rock (Welte, 1965; Tissot and Welte, 1978; Hedberg et a/., 1979). Peters (1986) rated a content of < 0.5 wt% TOC as poor, 0.5-l .O fair, 1 .O-2.0 good and >2.0 very good. The TOC values of analysed samples from the Kharita Member (Table 1) range between 0.51 wt% and 1.42 wt%, indicating a fair to good organic content. The most organic rich shale beds of the Kharita Member occurred at depths ranging from 2057 to 2420 m. The average TOC content of this member (Table 3) is less than 0.1 wt%, ranging between 0.59 and 0.84 wt%, indicating a fair organic content. The highest values (0.84 and 0.82 wt%) were determined in the central part of the study area (SW Mubarak-1 well and Sheiba 18-2 well, respectively), while the lowest value (0.59 wt%) was recorded in the southeastern part (WD 381 well) (Fig. 5). Hence, the Kharita beds with a favourable organic content are concenterated in the central part of the study area. The TOC values of analysed samples from the Alam El-Bueib Member (Table 2) range between 0.5 wt% and 2.67 wt%, indicating a fair to very good organic content. The most organic rich shale beds are concentrated at depths ranging from 2484 to 2707 m (except the sample at 2195 m). The average TOC content of this member ranges between 0.55 and 1.48 wt% (Table 31, reflecting a fair to good organic content. The lowest value was recorded also in the southeastern part of the study area (WD 38-l), increasing northwards where it attains 1.48 wt% in the northeastern part (Kattaniya-1 well) and 1.24 wt% in the northwestern part (Sheiba 42-l well) (Fig. 6). Consequently the most organic rich beds of Alam El-Bueib Member are concenterated in the northern part of the study area.
Organic geochemical
studies of some Early Cretaceous sediments, Abu Gharadig Basin
Table 1. Total organic carbon and Rock-Eva1 pyrolysis results for samples from the Kharita Member
Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Well Name WD 38-1 WD 38-1 WD 38-1 WTI 38-1 WD 38-1 WD 38-1 WD 38-l WD 57-l WD 57-l WD 57-l WD 57-1 WD 57-l WD 57-l Sheiba 18-2 Sheiba 18-2 Sheiba 18-2 Sheiba 18-2 Sheiba 42-l SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-1 SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-l Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-l Sheiba 18- 1 Sheiba 18-1
Depth (m)
TOC (wt.%)
1433 1448 1463 1509 1554 1600 1615 1875 1890 1905 1920 1951 1966 2057 2067 2076 2085 2140 2164 2182 2201 2219 2237 2256 2277 2338 2356 2420 2609 2646 2673 2710 2719 2737
0.71 0.53 0.61 0.52 0.65 0.62 0.53 0.72 0.70 0.78 0.70 0.62 0.70 1.15 1.12 0.51 0.52 0.66 0.69 0.90 0.72 1.10 0.86 0.65 0.76 0.76 0.52 1.42 0.52 0.57 0.59 0.67 0.52 0.55
S1
S2
S3
TIllFix (“C)
HI
01
0.07 0.07 0.07 0.07 0.07 0.13 0.07 0.09 0.15 0.13 0.16 0.10 0.20 0.18 0.22 0.13 0.14 0.44 0.17 0.12 0.13 0.19 0.12 0.14 0.13 0.44 0.11 3.60 0.08 0.10 0.07 0.08 0.08 0.11
0.43 0.31 0.38 0.29 0.46 0.29 0.31 0.37 0.49 0.65 0.55 0.36 0.70 1.61 2.01 0.68 0.70 1.12 0.61 0.81 0.61 1.24 0.89 0.74 0.80 0.88 0.42 3.54 0.58 0.61 0.56 0.87 0.57 0.83
0.34 0.49 0.60 0.77 0.54 0.82 0.69 0.87 0.99 0.83 0.88 0.83 1.20 0.31 0.63 0.31 0.39 1.40 1.18 1.16 0.98 1.31 1.09 0.89 1.10 1.16 0.95 2.63 0.42 0.67 0.43 0.52 0.36 0.80
432 435 433 427 432 434 437 434 433 430 431 430 432 434 430 435 432 430 440 441 437 439 438 440 442 432 433 416 438 441 441 441 440 441
61 58 62 56 71 47 58 51 70 83 79 58 100 140 179 133 135 170 88 90 85 113 103 114 105 116 81 249 112 107 95 130 110 151
48 92 98 148 83 132 130 121 141 106 126 134 171 27 56 61 75 212 171 129 136 119 127 137 145 153 183 185 81 118 73 78 69 145
S,: mg HC/g rock; S,: HClg rock; S,: mg CO,/g rock. HI: S,/TOG,
Organic quality The kerogen type controls the quality and amount of hydrocarbons generated from a potential source rock. The characterisation of kerogen types within the Kharita and Alam ElBueib Members, based on pyrolysis data (Tables 1 and 21, are illustrated by Fig. 7. The location of Kharita Member samples on the van Krevelen diagram (Fig. 7) indicates kerogens of type III (gas-prone) and type IV (without hydrocarbon potential). The samples
mg HG/g; TOG 01: S,/TOC,
mg CO,/g TOC.
with type IV kerogen are concentrate’d at depths ranging from 1433 to 1951 m, while those with type III kerogen are localised at depths between 1966 and 2737 m. The facies pattern of the Kharita Member shows that the Albian was a period of predominantly elastic deposition, reflecting marginal-marine conditions. A shaly sandstone facies predominates to the south of the study area, with localised type IV non-source rock, passing northwards into a sandy shale facies with type III gas-prone source rock (Fig.
Journal of African Earth Sciences 179
A.
Table 2. Total organic El-Bueib Member
Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Well Name WD 38-1 WD 38-l WD 57-l WD 57-1 WD 57-1 WD 57-1 WD 57-l WD 57-l WD 57-l WD 57-l WD 57-l WD 57-1 WD 57-1 WD 57-l WD 57-1 WD 57-1 WD 57-1 WD 57-l WD 57-l kattaniya- 1 kattaniya- 1 kattaniya- 1 Sheiba 18-2 Sheiba 42-l Sheiba 42-1 Sheiba 42-l Sheiba 42- 1 Sheiba 42-l Sheiba 42-1 Sheiba 42- 1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18- 1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1
carbon and Rock-Eva1 pyrolysis
Depth (m)
TOC (wt.%)
2088 2134 2179 2195 2225 2240 2256 2271 2286 3204 2317 2332 2347 2362 2438 2454 2469 2484 2499 2545 2554 2563 2573 2633 2643 2652 2661 2688 2697 2707 2847 2865 2893 2920 2957 3048 3094 3130 3167 3 185 3203 3222 3249 3277 3328 3347 3374
0.51 0.60 0.81 2.67 0.65 0.84 0.70 0.56 0.65 0.56 0.52 0.58 0.86 0.58 0.56 0.54 0.62 1.96 0.51 1.31 1.37 1.76 0.50 0.92 2.38 1.04 0.73 1.13 1.37 1.09 0.73 0.93 0.76 0.78 0.65 0.50 0.60 0.53 0.55 0.58 0.66 0.59 0.58 0.53 0.96 0.53
See Table 1 for explanation of symbols.
120 Journal of African Earth Sciences
W. FELESTEEN
S1
0.10 0.07 0.10 0.17 0.09 0.10 0.10 0.05 0.10 0.06 0.06 0.04 0.11 0.07 0.06 0.10 0.10 0.18 0.19 0.15 0.15 0.18 0.14 0.10 0.28 0.14 0.08 0.21 0.15 0.18 0.13 0.18 0.12 0.26 0.13 0.12 0.14 0.13 0.10 0.09 0.12 0.12 0.11 0.15 0.34 0.22
results for samples from the Alam
S2
S3
0.47 0.71 0.29 1.40 0.15 0.40 0.24 0.14 0.27 0.15 0.15 0.08 0.24 0.25 0.24 0.23 0.24 2.81 0.26 1.25 2.06 2.72 0.80 0.98 4.96 1.54 1.27 1.82 1.72 1.45 0.92 1.02 0.77 0.61 0.36 0.48 0.43 0.29 0.53 0.52 0.68 0.68 0.91 0.56 1.41 0.69
0.35 0.60 1.59 1.69 0.99 1.09 0.73 0.80 1.22 0.90 1.03 0.61 0.74 0.67 0.53 0.79 0.58 5.31 0.74 0.98 0.54 0.39 0.28 1.58 0.99 0.94 0.55 0.56 1.11 1.71 0.40 0.45 0.25 0.21 0.21 0.38 0.15 0.28 0.25 0.19 0.25 0.26 0.27 0.25 0.51 0.30
Tlllax (“C) 446 440 430 443 431 430 428 430 428 428 426 426 432 428 429 427 428 428 433 446 436 439 430 438 435 435 439 437 436 434 444 447 446 443 438 441 438 442 436 437 437 436 441 439 439 437
HI
01
92 118 36 52 23 48 34 25 42 27 29 14 28 43 43 43 39 143 51 95 150 155 160 107 208 148 174 161 126 133 126 110 101 78 55 96 72 55 96 90 103 115 157. 106 147 130
69 100 196 63 152 130 104 143 188 161 198 105 86 116 95 146 94 271 145 75 39 22 56 172 42 90 75 50 81 157 55 48 33 27 32 76 25 53 45 33 38 44 47 47 53 57 17
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
Table 3. Average values of total organic carbon in analysed samples
Kharita Member 0.66 0.60 0.82 0.84 n.d. 0.59 0.70
Well Sheiba 42- 1 Sheiba 18-l Sheiba 18-2 SW Mubarak- 1 Kattaniya- 1 WD 38-l WD 57-l n.d.:
Alam El Bueib Member 1.24 0.65 0.59 n.d. 1.48 0.55 0.83
not determined.
I
29'
‘3’ .-. -------____________ .._=--.__ --__
QATTARA
DEPRESSION
.
Figure 5. lsocarbon map of the Kharita Member, Abu Gharadig Basin.
81, reflecting the terrestrial origin of its organic matter. The Alam El-Bueib samples fall within the kerogen types IV, III and 11-111 (oil and gas-prone) fields of the van Krevelen diagram (Fig. 7). The shale beds with type IV kerogen occur at depths ranging from 2179 to 2499 m, while those with type III kerogen are generally localised at depths from 2088 to 2134 and from 2697 to 3374 m. The shale beds with type 11-111kerogen are concentrated at depths ranging between 2545 and 2688 m. The facies pattern of Alam El-Bueib Member shows a similar picture to that of the Kharita Member. A shaly sandstone facies, with
type IV non-source rock predominates to the south, passing northwards to an area of type IIIII oil and gas-prone source rock, and then to an other area of a sandy shale facies with type-Ill source rock (Fig. 91, reflecting a mixed organic matter. Thermal maturity The level of thermal maturity of saimples was estimated from the temperature of maximum pyrolytic hydrocarbon generation (T,,,). Values less than 435OC indicate immature organic matter, while a T,,, value of about 460°C represents the bottom of the oil window and
Journal of African Earth Sciences 12 1
A. W. FELESTEEN
QATTARA
DEPRESSION ‘a’____T
,__-
f
lo_
20
30(
km
29
Figure 6. lsocarbon map of the Alam El-Bueib Member, Abu Gharadig Basin.
the beginning of the condensate and wet gas zone (Espitalie et a/., 1977). TmaXfor each of the Kharita and Alam El-Bueib members not shows a regular increase in depth as expected. Instead, T,,, data are clearly clustered according to locality (Figs 10 and 1 I) This phenomena can be attributed to the different values of geothermal gradient shown by the investigated wells. The increasing of geothermal gradient in some parts is due to the presence of normal faults and igneous intrusions feeding the sedimentary section by a heat flow. The Abu Gharadig Basin was opened during the Early Cretaceous by right-lateral diagonal-slip movement on right-stepped, en echlon faults of the east-west to east-northeast orientated normal fault set (Abdel Aal and Moustafa, 1988). The analysed samples from the Kharita Member, at depths from 1433 to 2140 m, have Tmaxvalues from 427 to 437OC, and are generally immature to marginally-mature. Those at depths between 2164 and 2737 m, having Tmaxvalues from 437 to 442OC, are marginally-mature to mature. However, the samples at depths from 2338 to 2420 m are immature to marginallymature, with Tmaxvalues ranging between 416 to 433OC. Based on Tmax data, an area of thermally mature source rock of the Kharita Member was delineated in the north central part of the study area, surrounded by an other area
122
Journal
of African
Earth
Sciences
of immature to marginally-mature source rock (Fig. 12). The T,,, values of samples from the Alam ElBueib Member, at depths from 2179 to 2499 m (WD 57-l well), generally range between 426 and 433OC, indicating thermally immature organic matter. The samples at depths from 2545 to 2707 m generally have Tmax values between 434 and 439OC, indicating marginallymature to mature organic matter, while those at depths from 2088 to 2134 and from 2847 to 3374 m have Tma, values from 436 to 447OC, indicating thermally mature organic matter. An area of mature source rock of the Alam El-Bueib Member was recorded also in the northern part of the study region, surrounded by a marginallymature to mature source rock, passing southwards to an other area of immature source rock (Fig. 13).
POTENTIAL SOURCE BEDS The obtained results show that the shale beds of the Kharita Member, at depths from 1433 to 1951 m, have a poor to fair organic content, and are generally characterised by immature type IV kerogen. Hence, these beds are considered to be without hydrocarbon potential. The beds at depths from 1966 to 2140 m have a fair to good organic content, with immature
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
900 G co
800
cn g
700
0
Atom
0
Kharita
1.2.3 F 0”
600
0 I\
500
El
Bueib
Member
Member
Sample
Number
cz v 4x
400
c c Q, ul 0 ;
300 28 0
200
1
100
0 0
50
Oxygen
150
100
Index
(S3/TOC,
200
250
mg CO2 /g ~0~1
Figure 7. Characterisation of kerogen types in samples from the Kharita and Alam El-Bueib Members plotted on a van Krevelen diagram. I, II, Ill, IV: the various kerogen types as discussed in the text.
to marginally-mature type III kerogen. Consequently, they are interpreted as potential gas-prone source beds. The beds at depths of more than 2164 m are generally distinguished by a fair to good content of organic matter, as well as marginallymature to mature type III kerogen. Hence, they can be regarded as active gas-prone source beds in the study area. This result is in agreement with Barakat et al. (1988) who stated that argillaceous siltstone and claystone intercalations within the Kharita
Formation in the northern Western Desert are rich in organic matter of land plant origin, which favors the generation of gas and condensate if the maturation level is reached. By the combination of source rock type, distribution and maturity, a gas-generating area of the Kharita Member is delineated in the northcentral part of the study region (Fig. 12). Robertson Research International and associates (1985) reported that the Alam ElBueib Member contains some fair to good quality
Journal of African Earth Sciences 123
A. W. FELESTEEN
I
2 8’
29’
_-- -__----
---
30”;‘+ y4cj7-_,,
___----__
*.
‘._----.__ QATTARA
DEPRESSION
i_-‘* _7._/*‘-.
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of type - III rock
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‘\“__ ._----.__-)
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-
Q ‘\ ‘--.4?-. I QC-_ &‘.
---_.
.-_
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/
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F
1020
3: km
Area of type - IV
non - source rock
Figure 8. Lithofacies and source rock distribution of the Kharita Member, Abu Gharadig Basin.
----__---____
.,$44- - . __ ,_ Q/ s. -. ._ “1
___----._ ‘._----._.
--__ QATTARA
DEPRESSION
A____,
,__-
_r._;-‘-
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I-,’
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7
;_ -_\ *.___-_..
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Area oi’lype source rock
y-__S’
- 111
30’
_ .-. ,-
-__.P.-.
8’
of type source rock Area
II-III
\1-:
:\.::...:
-. LT. . . . ___.-.I
I [WI
Shaly
Sandstone
Sandy
Shale
..
. .
.. .
=._L_._?. . . . IV _._.-.t
..
.
. . . . .
. -.-
I
Area ol type - IV non - source rock
0
20
10 km
Figure 9. Lithofacies and source rock distribution of the Alam El-Bueib Member, Abu Gharadig Basin.
124
Journal of African
Earth
Sciences
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
E E
2000-
5 2
2200 -
Shoiba 10-e
2400 SW Muborok-l
,f
,Sh.ibe 420
W-10
1 440
430
3500 I 425
450
430
435
Tmax t-C 1 Figure 10. A plot of Tm_ versus depth for the Kharita Member, Abu Gharadig Basin.
!‘a’
Figure 17. A plot of T,., versus depth for the A/am El-Bueib Member, Abu Gharadig Basin.
ILl’r--+ s,, 4;;‘.
29’
,*---------_
_ ________
UATTARA
I\*’ \
DEPRESSION
*-_--
-‘.
-*..q
-\ ._=--.__
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::--:
7._,’
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460
449
440
Tmax 1% J
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\.
-+..,
._~($A - _--.:
ofmature soucerock (es - generating area)
f
30
/------“-
0
// Area of immature to marginal mature source rock
0
10
20
30 km
Figure 12. Organic maturity map of the Kharita Member, Abu Gharedig Basin.
Journal of African
Earth
Sciences 125
A. W. FELESTEEN
‘a’
3d’:-$. ‘:Q 4;;‘.
2i’
,-- -------____________
_:._*.-‘-
,
,__-
\ ,
‘.
._-__ “-_ I m-c
‘\
‘_ 9tii. -_ _--1:
Area of mature sourcerock (gas - generatingares)
3 i_ -_\
DEPRESSION *-\_ F.____-
.. =+;=
--__ QATTAAA
-\
,-.*q ..__---.__
\I
‘CL_* ,
31
,p\*0
“_ I’ f
.
f
--
I
.G
3 ,111
Area of marginal - mature \ Ii7 to mature sourcerock (oil - gas generating area)
-_
El?_,
-
Ares of immature eon - sourcerock
0
10
20
30 km
<
Figure 13. Organic maturity map of the Alam El-Bueib Member, Abu Gharadig Basin.
oil-prone source rocks with occasional gas-prone humic organic matter in most parts of the Northern Western Desert of Egypt. In the present study, the shale beds of the Alam El-Bueib Member, at depths from 2179 to 2499 m, have a fair organic content with immature type-IV kerogen, and are considered without hydrocarbon potential. The shale beds at depths from 2545 to 2688 m have a good organic content and marginally-mature to mature type II-III kerogens, while those at depths between 2697 and 3374 m have a fair to good organic content and are characterised by mature typeIII kerogens. Hence, the shale beds of Alam ElBueib Member, at depths from 2545 to 2688 m, can be seen as active oil and gas-prone source rocks, while those at depths between 2697 and 3374 m are gas-prone source beds in the Abu Gharadig Basin. Based on the source rock evaluation, gas and oil-gas generating areas of the Alam El-Bueib Member are delineated in the northern part of the study region (Fig. 13). It can be seen clearly that the hydrocarbon generating potential of the Alam El-Bueib Member is more than that of the Kharita Member. Hence, the Alam El-Bueib Member can be considered potentially as the hydrocarbon generating member for the petroleum accumulations of the Early Cretaceous Alamein and Kharita members in the Abu Gharadig Basin.
126
Journal of African Earth Sciences
ACKNOWLEDGEMENTS The author would like to express his gratitude to the Egyptian General Petroleum Corporation (EGPC) and the Gulf of Suez Company (GUPCO) for permitting and supplying the material upon which the present paper has been based. Editorial Handling - G. W. McNeill.
REFERENCES Abdel
Aal,
A. and Moustafa,
A.
R. 1988.
Structural
framework of the Abu Gharadig Basin, Western Desert, Egypt. Proceedings of EGPC Ninth Petroleum Exploration and Production Conference,
2, ~~23-50.
EGPC, Cairo.
Abu El-Naga, M. 1984. Paleozoic and Mesozoic depocenters and hydrocarbon generating areas, Northern Western Desert. Proceedings of EGPC Seventh Exploration Seminar, pp269-287. EGPC, Cairo. Barakat, M. G., Darwish, M. and Ghanem, M. F. 1988. Evolution, sedimentary environments and hydrocarbon potentials of the Kharita Formation (Albian) in the North Western Desert, Egypt. Proceedings of EGPC Ninth Petroleum Exploration and Production Conference, 2, pp5 l73. EGPC, Cairo. Espitalie, J., Madec, M., Tissot, 8. P., Mennig, J. J. and Leplate, P. 1977. Source rock characterization method for petroleum exploration. Proceedings of the Ninth Annual Offshore Technology Conference, OTC 2935,3, pp439-448. Hedberg, H. D., Moody, J. D. and Hedberg, FL M. 1979. Petroleum prospects of deep offshore. Bulletin American Association Petroleum Geologists 63. 286-300. Kholeif, W., Work, J. G. and Sanad, S. 1986. Meleiha - Its history and its significance. Proceedings of EGPC Eighth Exploration Conference, II, ~~17-34. EGPC, Cairo.
Organic gaochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
Labib, M. 1984. Contribution to the geology of Upper Cretaceous with special emphasis on Turonian-Senonian sedimentation patterens and hydrocarbon potentials in the Abu Gharadig area, North Western Desert, Egypt. Ph. D. dissertation, Cairo University, Egypt. Nakhla, F. M., Dardir, A. A., Saleh, S. A. and Gad, N. L. 1992. Distribution and geochemical characters of subsurface coal and kerogen-bearing Mesozoic formations and their potentialities to generate oil and gas in North Western Desert (NWD), Egypt. Proceedings of EGPC Eleventh Petroleum Exploration and Production Conference, 2, ~~184-198. EGPC, Cairo. Parker, J. R. 1982. Hydrocarbon habitat of the Western Desert, Egypt. Proceedings of EGPC Sixth Exploration Seminar, 1, ~~106-129. EGPC, Cairo. Peters, K. E. 1986. Guidelines for evaluating petroleum
source rocks using programmed pyrolysis. Bulletin American Association Petroleum Geologists 70, 315329. Robertson Research International limited & Scott, Pickford and associates limited 1985. Further evaluation of the Abu Sennan-Alam El-Shawish area, Western Uesert, The Arab Republic of Egypt, with emphasis on stratigraphic trapping potential. Unpublished report General Petroleum Company (GPC), Cairo. Schlumberger 1984. Geology of Egypt. Well Evaluation Conference, Egypt. ~~1-64. Cairo. Tissot, 8. P. and Welte, D. H. 1978. Petroleum Formation and Occurrence. A Mew Approach to Oil and Gas Exploration. 538~. Springer-Verlag, Berlin. Welte, D. H. 1965. Relation between petroleum and source rock. Bulletin American Associa tion Petroleum Geologists 49, 2246-2268.
Journal of African Earth Sciencas 12 7
PII:SO899-5382(98)00050-5
All rights
27, No. 1, pp. 115-l 27, 1998 D 1998 Elsevier Scmce Ltd reserved. Pmted in Great Britam 089%5362/98 $19.00 + 0.00
COMMUNICATION
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin, Western Desert, Egypt
Geology Department,
ADEL WAHEED FELESTEEN Faculty of Science, University of Aswan, Aswan,
Egypt
Abstract-A total of 81 shale samples from the Early Cretaceous sediments of seven wells in the Abu Gharadig Basin, were analysed for organic matter quantity, type and maturation. The total organic carbon content of analysed samples ranges between 0.5 and 2.67 wt%, indicating a fair to good organic content. The location of investigated samples on the van Krevelen diagram reflects the presence of kerogens type IV (without hydrocarbon potential), type III (gas-prone) and type ll-lll (oil and gas-prone). The estimated temperature of maximum pyrolytic hydrocarbon generation of analysed shales ranges between 426 and 447’C, indicating immature to thermally mature organic matter. The shale beds of Kharita Member from the northcentral part of the study area, at depths more than 2164 m, represent gas-prone source beds, while those of Alam El-Bueib Member, at depths more than 2545 m, can be considered potentially as active oil and gas-prone source beds. The present study reveals that the Early Cretaceous Alam El-Bueib Member is more enriched in thermally mature type III and ll-lll organic-rich shale beds, and consequently can be regarded as the potential source for the oil and gas accumulations in the carbonate reservoirs of.the Alamein Member and sandstone reservoirs of the Kharita Member in the Abu El-Gharadig Basin. Q 1998 Elsevier Science Limited. Resume-Un total de 81 Bchantillons de shales du C&ace precoce, preleves dans 7 forages dans le Bassin d’Abu Gharadig, a Bte Btudie pour la quantite, le type et la maturation de leur mat&e organique. Les teneurs en carbone organique total varient de 0.5 a 2.67% en poids, ce qui indique des valeurs moyennes a Blevees. La localisation des echantillons dans le diagramme de van Krevelen reflete la presence de kerogene de types IV (sans hydrocarbures potentiels), III (a gaz potentiels) et ll-lll (a gaz et p&role potentiels). La temperature estimee pour la formation d’hydrocarbures par pyrolyse varie de 426 a 447OC. ce qui indique que la mat&e organique est immature a thermiquement mQre. Dans la partie centre-nord de la zone d’etude, les lits de shale du Membre de Kharita, situ& a plus de 2164 m de profondeur, representent des gites favorables pour le gaz, alors que ceux du Membre d’Alam El-Bueib, a plus de 2545 m de profondeur, peuvent constituer un source potentielle de p&role et de gaz. Cette etude r&Ye que le Membre c&ace precoce d’Alam El-Bueib est le plus riche en shales thermiquement mfirs de types III et ll-lll et, par consequent, peut Qtre considere comme source potentielle d’accumulations de petrole et de gaz dans les reservoirs carbonates du Membre d’Alamein et les reservoirs greseux du Membre de Kharita du Bassin d’Abu El-Gharadig. c 1998 Elsevier Science Limited. (Received 3 March 1997: revised version received 3 October 1997)
INTRODUCTION
The Abu Gharadig Basin (Fig. 1) represents one of the most important sedimentary basins in the northern Western Desert of Egypt in terms of its hydrocarbon potential. Such evaluation of any
sedimentary basin requires the determination of its potential source rocks. The geology and hydrocarbon potential of Early Cretaceous sediments in the northern
Journal of African Earth Sciences 115
A. W. FELESTEEN
_---____-_
----
___----_.,
__ .a-
QATTAAA
DEPRESSION ,_\,&___~
I \
-’
-._.
---L,’
_?\_.’
cl’--& -\ \
\ ‘L. ._,---.__-_I
N t
KATTANIYA-1 .,’
SW
MUBARAK-1
WD
57-1
p
Figure 7. Key map showing the location of studied wells and lines of stratigraphical
Western Desert have been dealt with by many authors (Parker, 1982.; Abu El-Naga, 1984; Labib, 1984; Schlumberger, 1984; Robertson Research International and associates, 1985; Kholeif era/., 1986; Abdel Aal and Moustafa, 1988; Barakat ef a/., 1988; Nakhla et al., 1992; etc.). The present work is concerned with organic geochemical studies on the potential source members of the Early Cretaceous Burg El-Arab Formation (Kharita Member and Alam El-Bueib Member), in the Abu Gharadig Basin, to shed more light on their hydrocarbon content.
STRATIGRAPHICAL
SETTING
The Early Cretaceous sequence of the Abu Gharadig Basin represents a transgressive series which was deposited after a period of uplift and erosion that took place during Late Jurassic time and continued until the Late Neocomian. This sequence is represented by the Burg El-Arab Formation which is mainly composed of shales and sandstones (continental to shallow marine deposits). It is subdivided into three members: Alam El-Bueib, Alamein and Kharita. The Alam El-Bueib Member forms the lower part of the
116 Journal of African Earth Sciences
100
3,O km
correlation in the Abu Gharadig Basin.
Burg El-Arab Formation, and attains a maximum thickness of 1044 m. It is composed of alternating beds of shale and sandstone, with coal seams, and is considered to have been deposited in a deltaic to shallow marine environment. A Late Neocomian to Aptian age is assigned to this member. A maximum transgressional phase occurred during the Aptian, with the deposition of the Alamein Member in a restricted marine to lagoonal environment. This member is mainly composed of a white to light-brown, hard, dense, microcrystalline dolomite, which posses laterally a tan white limestone with few shale intercalations (Figs 2 and 3). It reaches a maximum thickness of 151 m, and represents the main carbonate reservoir for petroleum in the study area. The Albian saw widespread deposition of the thick marginal-marine elastics of the Kharita Member. This upper member of Burg El-Arab Formation, extends over almost the entire study area. It is mainly composed of thick sequence of sandstones with shale intercalations (Fig. 41, and attains a maximum thickness of 908 m. The Kharita Member rests unconformably on either the Alamein Member or the Alam El-Bueib Member, and is overlain conformably by the
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
I308 -
SHEIBA
lT22 -
1882 -
SH
18-2
SHEIBA
18-1
2120 -
2588 -
2588 -
2720 -
2880 -
3108 -
$305 .
Figure 2. Stratigraphical
correlation of Early Cretaceous rocks in the Abu Gharadig Basin (see Fig. 1: line A-AI.
WD
38-l
KATTANIYA--1
MUEARAK-1
Figure 3. Stratigraphical
correlation of Early Cretaceous rocks in the Abu Gharadig Basin (see Fig. 1: line B-BJ.
Journal of African Earth Sciences 117
A. W. FELESTEEM
Laboratories (Denver) and Robertson Research International (Wales). The kerogen type was determined by plotting the hydrogen index (HI) versus the oxygen index (01) on a van Krevelen diagram. The stage of maturity was estimated from the temperature of maximum pyrolytic hydrocarbon generation (T,,,).
AGE
APTlAh
m
Shale
B
Limestone
1.‘.‘.....1 Sandstone
m
Dolomitic
m B
E
Coal
Sandy shale Dolomite
limestone
Figure 4. Generalised stratigraphical column of Early Cretaceous sediments in the Abu Gharadig Basin.
Lower Cenomanian Bahariya Formation. The sandstones of the Kharita Member are distinguished by good porosity/permeability characteristics, and host important hydrocarbon accumulations.
MATERIAL AND METHODS The present study involves the investigation of 81 shale samples from the Burg El-Arab Formation (34 samples from Kharita Member and 47 samples from Alam El-Bueib Member), collected at various depths in seven wells drilled in the Abu Gharadig Basin (Fig. 1). The total organic carbon (TOC) content of the samples was determined using a LECO carbon analyser. The kerogen type and maturity were determined using Rock Eval Pyrolysis (Espitalie et a/., 1977). The samples were analysed at Exploration Logging International (Cairo), Brown and Ruth
118 Journal of African Earth Sciences
SOURCE ROCK EVALUATION Organic content TOC determinations of shale samples from the Kharita and Alam El-Bueib Members are listed in Tables 1 and 2, respectively. They are mainly above the critical lower limit (0.5 wt%) which is generally accepted as the minimum value of organic carbon for a potential source rock (Welte, 1965; Tissot and Welte, 1978; Hedberg et a/., 1979). Peters (1986) rated a content of < 0.5 wt% TOC as poor, 0.5-l .O fair, 1 .O-2.0 good and >2.0 very good. The TOC values of analysed samples from the Kharita Member (Table 1) range between 0.51 wt% and 1.42 wt%, indicating a fair to good organic content. The most organic rich shale beds of the Kharita Member occurred at depths ranging from 2057 to 2420 m. The average TOC content of this member (Table 3) is less than 0.1 wt%, ranging between 0.59 and 0.84 wt%, indicating a fair organic content. The highest values (0.84 and 0.82 wt%) were determined in the central part of the study area (SW Mubarak-1 well and Sheiba 18-2 well, respectively), while the lowest value (0.59 wt%) was recorded in the southeastern part (WD 381 well) (Fig. 5). Hence, the Kharita beds with a favourable organic content are concenterated in the central part of the study area. The TOC values of analysed samples from the Alam El-Bueib Member (Table 2) range between 0.5 wt% and 2.67 wt%, indicating a fair to very good organic content. The most organic rich shale beds are concentrated at depths ranging from 2484 to 2707 m (except the sample at 2195 m). The average TOC content of this member ranges between 0.55 and 1.48 wt% (Table 31, reflecting a fair to good organic content. The lowest value was recorded also in the southeastern part of the study area (WD 38-l), increasing northwards where it attains 1.48 wt% in the northeastern part (Kattaniya-1 well) and 1.24 wt% in the northwestern part (Sheiba 42-l well) (Fig. 6). Consequently the most organic rich beds of Alam El-Bueib Member are concenterated in the northern part of the study area.
Organic geochemical
studies of some Early Cretaceous sediments, Abu Gharadig Basin
Table 1. Total organic carbon and Rock-Eva1 pyrolysis results for samples from the Kharita Member
Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Well Name WD 38-1 WD 38-1 WD 38-1 WTI 38-1 WD 38-1 WD 38-1 WD 38-l WD 57-l WD 57-l WD 57-l WD 57-1 WD 57-l WD 57-l Sheiba 18-2 Sheiba 18-2 Sheiba 18-2 Sheiba 18-2 Sheiba 42-l SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-1 SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-l SW Mubarak-l Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-l Sheiba 18- 1 Sheiba 18-1
Depth (m)
TOC (wt.%)
1433 1448 1463 1509 1554 1600 1615 1875 1890 1905 1920 1951 1966 2057 2067 2076 2085 2140 2164 2182 2201 2219 2237 2256 2277 2338 2356 2420 2609 2646 2673 2710 2719 2737
0.71 0.53 0.61 0.52 0.65 0.62 0.53 0.72 0.70 0.78 0.70 0.62 0.70 1.15 1.12 0.51 0.52 0.66 0.69 0.90 0.72 1.10 0.86 0.65 0.76 0.76 0.52 1.42 0.52 0.57 0.59 0.67 0.52 0.55
S1
S2
S3
TIllFix (“C)
HI
01
0.07 0.07 0.07 0.07 0.07 0.13 0.07 0.09 0.15 0.13 0.16 0.10 0.20 0.18 0.22 0.13 0.14 0.44 0.17 0.12 0.13 0.19 0.12 0.14 0.13 0.44 0.11 3.60 0.08 0.10 0.07 0.08 0.08 0.11
0.43 0.31 0.38 0.29 0.46 0.29 0.31 0.37 0.49 0.65 0.55 0.36 0.70 1.61 2.01 0.68 0.70 1.12 0.61 0.81 0.61 1.24 0.89 0.74 0.80 0.88 0.42 3.54 0.58 0.61 0.56 0.87 0.57 0.83
0.34 0.49 0.60 0.77 0.54 0.82 0.69 0.87 0.99 0.83 0.88 0.83 1.20 0.31 0.63 0.31 0.39 1.40 1.18 1.16 0.98 1.31 1.09 0.89 1.10 1.16 0.95 2.63 0.42 0.67 0.43 0.52 0.36 0.80
432 435 433 427 432 434 437 434 433 430 431 430 432 434 430 435 432 430 440 441 437 439 438 440 442 432 433 416 438 441 441 441 440 441
61 58 62 56 71 47 58 51 70 83 79 58 100 140 179 133 135 170 88 90 85 113 103 114 105 116 81 249 112 107 95 130 110 151
48 92 98 148 83 132 130 121 141 106 126 134 171 27 56 61 75 212 171 129 136 119 127 137 145 153 183 185 81 118 73 78 69 145
S,: mg HC/g rock; S,: HClg rock; S,: mg CO,/g rock. HI: S,/TOG,
Organic quality The kerogen type controls the quality and amount of hydrocarbons generated from a potential source rock. The characterisation of kerogen types within the Kharita and Alam ElBueib Members, based on pyrolysis data (Tables 1 and 21, are illustrated by Fig. 7. The location of Kharita Member samples on the van Krevelen diagram (Fig. 7) indicates kerogens of type III (gas-prone) and type IV (without hydrocarbon potential). The samples
mg HG/g; TOG 01: S,/TOC,
mg CO,/g TOC.
with type IV kerogen are concentrate’d at depths ranging from 1433 to 1951 m, while those with type III kerogen are localised at depths between 1966 and 2737 m. The facies pattern of the Kharita Member shows that the Albian was a period of predominantly elastic deposition, reflecting marginal-marine conditions. A shaly sandstone facies predominates to the south of the study area, with localised type IV non-source rock, passing northwards into a sandy shale facies with type III gas-prone source rock (Fig.
Journal of African Earth Sciences 179
A.
Table 2. Total organic El-Bueib Member
Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Well Name WD 38-1 WD 38-l WD 57-l WD 57-1 WD 57-1 WD 57-1 WD 57-l WD 57-l WD 57-l WD 57-l WD 57-l WD 57-1 WD 57-1 WD 57-l WD 57-1 WD 57-1 WD 57-1 WD 57-l WD 57-l kattaniya- 1 kattaniya- 1 kattaniya- 1 Sheiba 18-2 Sheiba 42-l Sheiba 42-1 Sheiba 42-l Sheiba 42- 1 Sheiba 42-l Sheiba 42-1 Sheiba 42- 1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18- 1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1 Sheiba 18-1
carbon and Rock-Eva1 pyrolysis
Depth (m)
TOC (wt.%)
2088 2134 2179 2195 2225 2240 2256 2271 2286 3204 2317 2332 2347 2362 2438 2454 2469 2484 2499 2545 2554 2563 2573 2633 2643 2652 2661 2688 2697 2707 2847 2865 2893 2920 2957 3048 3094 3130 3167 3 185 3203 3222 3249 3277 3328 3347 3374
0.51 0.60 0.81 2.67 0.65 0.84 0.70 0.56 0.65 0.56 0.52 0.58 0.86 0.58 0.56 0.54 0.62 1.96 0.51 1.31 1.37 1.76 0.50 0.92 2.38 1.04 0.73 1.13 1.37 1.09 0.73 0.93 0.76 0.78 0.65 0.50 0.60 0.53 0.55 0.58 0.66 0.59 0.58 0.53 0.96 0.53
See Table 1 for explanation of symbols.
120 Journal of African Earth Sciences
W. FELESTEEN
S1
0.10 0.07 0.10 0.17 0.09 0.10 0.10 0.05 0.10 0.06 0.06 0.04 0.11 0.07 0.06 0.10 0.10 0.18 0.19 0.15 0.15 0.18 0.14 0.10 0.28 0.14 0.08 0.21 0.15 0.18 0.13 0.18 0.12 0.26 0.13 0.12 0.14 0.13 0.10 0.09 0.12 0.12 0.11 0.15 0.34 0.22
results for samples from the Alam
S2
S3
0.47 0.71 0.29 1.40 0.15 0.40 0.24 0.14 0.27 0.15 0.15 0.08 0.24 0.25 0.24 0.23 0.24 2.81 0.26 1.25 2.06 2.72 0.80 0.98 4.96 1.54 1.27 1.82 1.72 1.45 0.92 1.02 0.77 0.61 0.36 0.48 0.43 0.29 0.53 0.52 0.68 0.68 0.91 0.56 1.41 0.69
0.35 0.60 1.59 1.69 0.99 1.09 0.73 0.80 1.22 0.90 1.03 0.61 0.74 0.67 0.53 0.79 0.58 5.31 0.74 0.98 0.54 0.39 0.28 1.58 0.99 0.94 0.55 0.56 1.11 1.71 0.40 0.45 0.25 0.21 0.21 0.38 0.15 0.28 0.25 0.19 0.25 0.26 0.27 0.25 0.51 0.30
Tlllax (“C) 446 440 430 443 431 430 428 430 428 428 426 426 432 428 429 427 428 428 433 446 436 439 430 438 435 435 439 437 436 434 444 447 446 443 438 441 438 442 436 437 437 436 441 439 439 437
HI
01
92 118 36 52 23 48 34 25 42 27 29 14 28 43 43 43 39 143 51 95 150 155 160 107 208 148 174 161 126 133 126 110 101 78 55 96 72 55 96 90 103 115 157. 106 147 130
69 100 196 63 152 130 104 143 188 161 198 105 86 116 95 146 94 271 145 75 39 22 56 172 42 90 75 50 81 157 55 48 33 27 32 76 25 53 45 33 38 44 47 47 53 57 17
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
Table 3. Average values of total organic carbon in analysed samples
Kharita Member 0.66 0.60 0.82 0.84 n.d. 0.59 0.70
Well Sheiba 42- 1 Sheiba 18-l Sheiba 18-2 SW Mubarak- 1 Kattaniya- 1 WD 38-l WD 57-l n.d.:
Alam El Bueib Member 1.24 0.65 0.59 n.d. 1.48 0.55 0.83
not determined.
I
29'
‘3’ .-. -------____________ .._=--.__ --__
QATTARA
DEPRESSION
.
Figure 5. lsocarbon map of the Kharita Member, Abu Gharadig Basin.
81, reflecting the terrestrial origin of its organic matter. The Alam El-Bueib samples fall within the kerogen types IV, III and 11-111 (oil and gas-prone) fields of the van Krevelen diagram (Fig. 7). The shale beds with type IV kerogen occur at depths ranging from 2179 to 2499 m, while those with type III kerogen are generally localised at depths from 2088 to 2134 and from 2697 to 3374 m. The shale beds with type 11-111kerogen are concentrated at depths ranging between 2545 and 2688 m. The facies pattern of Alam El-Bueib Member shows a similar picture to that of the Kharita Member. A shaly sandstone facies, with
type IV non-source rock predominates to the south, passing northwards to an area of type IIIII oil and gas-prone source rock, and then to an other area of a sandy shale facies with type-Ill source rock (Fig. 91, reflecting a mixed organic matter. Thermal maturity The level of thermal maturity of saimples was estimated from the temperature of maximum pyrolytic hydrocarbon generation (T,,,). Values less than 435OC indicate immature organic matter, while a T,,, value of about 460°C represents the bottom of the oil window and
Journal of African Earth Sciences 12 1
A. W. FELESTEEN
QATTARA
DEPRESSION ‘a’____T
,__-
f
lo_
20
30(
km
29
Figure 6. lsocarbon map of the Alam El-Bueib Member, Abu Gharadig Basin.
the beginning of the condensate and wet gas zone (Espitalie et a/., 1977). TmaXfor each of the Kharita and Alam El-Bueib members not shows a regular increase in depth as expected. Instead, T,,, data are clearly clustered according to locality (Figs 10 and 1 I) This phenomena can be attributed to the different values of geothermal gradient shown by the investigated wells. The increasing of geothermal gradient in some parts is due to the presence of normal faults and igneous intrusions feeding the sedimentary section by a heat flow. The Abu Gharadig Basin was opened during the Early Cretaceous by right-lateral diagonal-slip movement on right-stepped, en echlon faults of the east-west to east-northeast orientated normal fault set (Abdel Aal and Moustafa, 1988). The analysed samples from the Kharita Member, at depths from 1433 to 2140 m, have Tmaxvalues from 427 to 437OC, and are generally immature to marginally-mature. Those at depths between 2164 and 2737 m, having Tmaxvalues from 437 to 442OC, are marginally-mature to mature. However, the samples at depths from 2338 to 2420 m are immature to marginallymature, with Tmaxvalues ranging between 416 to 433OC. Based on Tmax data, an area of thermally mature source rock of the Kharita Member was delineated in the north central part of the study area, surrounded by an other area
122
Journal
of African
Earth
Sciences
of immature to marginally-mature source rock (Fig. 12). The T,,, values of samples from the Alam ElBueib Member, at depths from 2179 to 2499 m (WD 57-l well), generally range between 426 and 433OC, indicating thermally immature organic matter. The samples at depths from 2545 to 2707 m generally have Tmax values between 434 and 439OC, indicating marginallymature to mature organic matter, while those at depths from 2088 to 2134 and from 2847 to 3374 m have Tma, values from 436 to 447OC, indicating thermally mature organic matter. An area of mature source rock of the Alam El-Bueib Member was recorded also in the northern part of the study region, surrounded by a marginallymature to mature source rock, passing southwards to an other area of immature source rock (Fig. 13).
POTENTIAL SOURCE BEDS The obtained results show that the shale beds of the Kharita Member, at depths from 1433 to 1951 m, have a poor to fair organic content, and are generally characterised by immature type IV kerogen. Hence, these beds are considered to be without hydrocarbon potential. The beds at depths from 1966 to 2140 m have a fair to good organic content, with immature
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
900 G co
800
cn g
700
0
Atom
0
Kharita
1.2.3 F 0”
600
0 I\
500
El
Bueib
Member
Member
Sample
Number
cz v 4x
400
c c Q, ul 0 ;
300 28 0
200
1
100
0 0
50
Oxygen
150
100
Index
(S3/TOC,
200
250
mg CO2 /g ~0~1
Figure 7. Characterisation of kerogen types in samples from the Kharita and Alam El-Bueib Members plotted on a van Krevelen diagram. I, II, Ill, IV: the various kerogen types as discussed in the text.
to marginally-mature type III kerogen. Consequently, they are interpreted as potential gas-prone source beds. The beds at depths of more than 2164 m are generally distinguished by a fair to good content of organic matter, as well as marginallymature to mature type III kerogen. Hence, they can be regarded as active gas-prone source beds in the study area. This result is in agreement with Barakat et al. (1988) who stated that argillaceous siltstone and claystone intercalations within the Kharita
Formation in the northern Western Desert are rich in organic matter of land plant origin, which favors the generation of gas and condensate if the maturation level is reached. By the combination of source rock type, distribution and maturity, a gas-generating area of the Kharita Member is delineated in the northcentral part of the study region (Fig. 12). Robertson Research International and associates (1985) reported that the Alam ElBueib Member contains some fair to good quality
Journal of African Earth Sciences 123
A. W. FELESTEEN
I
2 8’
29’
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---
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Figure 8. Lithofacies and source rock distribution of the Kharita Member, Abu Gharadig Basin.
----__---____
.,$44- - . __ ,_ Q/ s. -. ._ “1
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Figure 9. Lithofacies and source rock distribution of the Alam El-Bueib Member, Abu Gharadig Basin.
124
Journal of African
Earth
Sciences
Organic geochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
E E
2000-
5 2
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Shoiba 10-e
2400 SW Muborok-l
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,Sh.ibe 420
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Tmax t-C 1 Figure 10. A plot of Tm_ versus depth for the Kharita Member, Abu Gharadig Basin.
!‘a’
Figure 17. A plot of T,., versus depth for the A/am El-Bueib Member, Abu Gharadig Basin.
ILl’r--+ s,, 4;;‘.
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ofmature soucerock (es - generating area)
f
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0
10
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Figure 12. Organic maturity map of the Kharita Member, Abu Gharedig Basin.
Journal of African
Earth
Sciences 125
A. W. FELESTEEN
‘a’
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,-- -------____________
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<
Figure 13. Organic maturity map of the Alam El-Bueib Member, Abu Gharadig Basin.
oil-prone source rocks with occasional gas-prone humic organic matter in most parts of the Northern Western Desert of Egypt. In the present study, the shale beds of the Alam El-Bueib Member, at depths from 2179 to 2499 m, have a fair organic content with immature type-IV kerogen, and are considered without hydrocarbon potential. The shale beds at depths from 2545 to 2688 m have a good organic content and marginally-mature to mature type II-III kerogens, while those at depths between 2697 and 3374 m have a fair to good organic content and are characterised by mature typeIII kerogens. Hence, the shale beds of Alam ElBueib Member, at depths from 2545 to 2688 m, can be seen as active oil and gas-prone source rocks, while those at depths between 2697 and 3374 m are gas-prone source beds in the Abu Gharadig Basin. Based on the source rock evaluation, gas and oil-gas generating areas of the Alam El-Bueib Member are delineated in the northern part of the study region (Fig. 13). It can be seen clearly that the hydrocarbon generating potential of the Alam El-Bueib Member is more than that of the Kharita Member. Hence, the Alam El-Bueib Member can be considered potentially as the hydrocarbon generating member for the petroleum accumulations of the Early Cretaceous Alamein and Kharita members in the Abu Gharadig Basin.
126
Journal of African Earth Sciences
ACKNOWLEDGEMENTS The author would like to express his gratitude to the Egyptian General Petroleum Corporation (EGPC) and the Gulf of Suez Company (GUPCO) for permitting and supplying the material upon which the present paper has been based. Editorial Handling - G. W. McNeill.
REFERENCES Abdel
Aal,
A. and Moustafa,
A.
R. 1988.
Structural
framework of the Abu Gharadig Basin, Western Desert, Egypt. Proceedings of EGPC Ninth Petroleum Exploration and Production Conference,
2, ~~23-50.
EGPC, Cairo.
Abu El-Naga, M. 1984. Paleozoic and Mesozoic depocenters and hydrocarbon generating areas, Northern Western Desert. Proceedings of EGPC Seventh Exploration Seminar, pp269-287. EGPC, Cairo. Barakat, M. G., Darwish, M. and Ghanem, M. F. 1988. Evolution, sedimentary environments and hydrocarbon potentials of the Kharita Formation (Albian) in the North Western Desert, Egypt. Proceedings of EGPC Ninth Petroleum Exploration and Production Conference, 2, pp5 l73. EGPC, Cairo. Espitalie, J., Madec, M., Tissot, 8. P., Mennig, J. J. and Leplate, P. 1977. Source rock characterization method for petroleum exploration. Proceedings of the Ninth Annual Offshore Technology Conference, OTC 2935,3, pp439-448. Hedberg, H. D., Moody, J. D. and Hedberg, FL M. 1979. Petroleum prospects of deep offshore. Bulletin American Association Petroleum Geologists 63. 286-300. Kholeif, W., Work, J. G. and Sanad, S. 1986. Meleiha - Its history and its significance. Proceedings of EGPC Eighth Exploration Conference, II, ~~17-34. EGPC, Cairo.
Organic gaochemical studies of some Early Cretaceous sediments, Abu Gharadig Basin
Labib, M. 1984. Contribution to the geology of Upper Cretaceous with special emphasis on Turonian-Senonian sedimentation patterens and hydrocarbon potentials in the Abu Gharadig area, North Western Desert, Egypt. Ph. D. dissertation, Cairo University, Egypt. Nakhla, F. M., Dardir, A. A., Saleh, S. A. and Gad, N. L. 1992. Distribution and geochemical characters of subsurface coal and kerogen-bearing Mesozoic formations and their potentialities to generate oil and gas in North Western Desert (NWD), Egypt. Proceedings of EGPC Eleventh Petroleum Exploration and Production Conference, 2, ~~184-198. EGPC, Cairo. Parker, J. R. 1982. Hydrocarbon habitat of the Western Desert, Egypt. Proceedings of EGPC Sixth Exploration Seminar, 1, ~~106-129. EGPC, Cairo. Peters, K. E. 1986. Guidelines for evaluating petroleum
source rocks using programmed pyrolysis. Bulletin American Association Petroleum Geologists 70, 315329. Robertson Research International limited & Scott, Pickford and associates limited 1985. Further evaluation of the Abu Sennan-Alam El-Shawish area, Western Uesert, The Arab Republic of Egypt, with emphasis on stratigraphic trapping potential. Unpublished report General Petroleum Company (GPC), Cairo. Schlumberger 1984. Geology of Egypt. Well Evaluation Conference, Egypt. ~~1-64. Cairo. Tissot, 8. P. and Welte, D. H. 1978. Petroleum Formation and Occurrence. A Mew Approach to Oil and Gas Exploration. 538~. Springer-Verlag, Berlin. Welte, D. H. 1965. Relation between petroleum and source rock. Bulletin American Associa tion Petroleum Geologists 49, 2246-2268.
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