Marine and Petroleum Geology 71 (2016) 55e75

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Marine and Petroleum Geology

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Research paper Structural regime and its impact on the mechanism and migration pathways of hydrocarbon seepage in the southern : An approach for finding new unexplored fault blocks

* Shawky Sakran a, Muhammad Nabih b, Ahmed Henaish b, , Abdelmohsen Ziko b a Cairo University, Department of Geology, Faculty of Science, Giza 12613, Egypt b Zagazig University, Department of Geology, Faculty of Science, Zagazig 44519, Egypt article info abstract

Article history: A Natural active oil seepage occurs at the intersection of the NW-oriented rift coastal fault and a NE- Received 5 August 2015 oriented cross fault which bound the southwest dipping Little Zeit tilted fault block at the south- Received in revised form western side of the Gulf of Suez, Egypt. Detailed surface geological mapping followed by subsurface 28 November 2015 mapping using aeromagnetic, seismic and borehole data of Ras El Ush oilfield (the nearest oil field to the Accepted 2 December 2015 seepage) provide a reliable hydrocarbon migration pathway model of the area. Available online 10 December 2015 The proposed model suggests that hydrocarbons migrated upward at the intersection of a NE-oriented and the NW-oriented rift coastal faults where they found their way to the surface. The Nubia Keywords: fi Gebel El Zeit occurs south of Ras El Ush oil eld in a trap door structure and probably entrapped some of the migrating Oil seepage hydrocarbons while a probable oil-water-contact at 1000 m which resulted into the migration of hy- Hydrocarbon migration drocarbon through the damage zone of the northeast fault. Fault connectivity The original oil in place of the predicted reservoir is estimated to be more than 47.5 MMBO which Gulf of Suez rift encourages the design makers for more investigation of this reservoir to increase its certainty and putting it in the plan of the future investments. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction an elongated graben about 300 km long and 30 km wide between the Sinai Peninsula and the Eastern Desert of Egypt (Fig. 1). Various Natural hydrocarbon seepage was the incentive for exploration recognized source rocks deposited in distinct, well-defined envi- drilling by the pioneers of the petroleum industry as long ago as the ronments exist within the Gulf of Suez basin, and several models 1860s in Pennsylvania and Azerbaijan (Williams and Lawrence, for oil generation and oil/source correlations have been proposed 2002). Seepage has also given some of the first indications of the (Rohrback, 1982; Shahin, 1988; Mostafa, 1993; Robison, 1995; presence of petroleum in most of the world's petroleum producing Barakat et al., 1997). regions, with at least half the reserves proved by 1952 discovered Gebel El Zeit is located on the southernmost western shore of by drilling on or near seeps (Judd and Hovland, 2007). Hydrocarbon the Gulf of Suez (Fig. 1). At the southern end of Gebel El-Zeit, a seepage can, in selected geological settings, also delineate subsur- natural active seepage zone of oil occurs on the land surface in an face petroleum accumulations and provide information on hydro- area underlain by asphalt-saturated post- fractured lime- carbon charge type or oil quality. Therefore, the study of natural stone and coral reefs. Near the seep there are two ancient hand-dug hydrocarbon seepage has proven to be a valuable aspect in petro- wells. According to Harrell and Lewan (2002), pottery littering the leum prospectivity assessment and exploration. ground around these wells dates from the late Imperial Roman Since the beginning of the last century, the Gulf of Suez has been Period (1st e 6th centuries A.D.) and the Islamic Period (11th e 16th a highly prospective hydrocarbon location and the focus of much oil centuries A.D.), but also present are a few pottery shreds from the exploration. The Gulf of Suez is an intercontinental rift consisting of Middle Kingdom or second Intermediate Period (16th e 20th centuries B.C.). The presence of liquid petroleum, through which gas bubbles continually rise, indicates that the seepage is still active

* Corresponding author. (Fig. 2). E-mail address: [email protected] (A. Henaish). This paper provides an integrated geological and geophysical http://dx.doi.org/10.1016/j.marpetgeo.2015.12.003 0264-8172/© 2015 Elsevier Ltd. All rights reserved. 56 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 1. Simplified geological map of Gebel El Zeit area and the location of oil seepage. study to afford a reasonable origin of the onshore active oil seepage succession of the Gulf of Suez province is generally characterized by of Gebel El Zeit area. 3 main depositional sequences relative to the Miocene rifting events, as: pre-rift sequence (including the basement rocks and a sedimentary succession up to the ); syn-rift 2. Stratigraphic setting sequences (Early- successions) and post-rift sequence (Late Miocene to Recent successions). The first and sec- The Gulf of Suez stratigraphy has been discussed by many ond sequences include important hydrocarbon source and workers. According to Said (1962, 1990), the stratigraphic S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 57

reservoir rocks while the third depositional sequence is important because of its evaporitic seal. The major sedimentary successions accumulated under different structural settings on the Precambrian Basement complex with distinct inter- and intraformational un- conformities and hiatuses of different magnitudes. In the Southern Gulf of Suez, the Precambrian basement rocks are overlain by a variably eroded section of to Cenozoic sedimentary rocks. Fig. 3 is a generalized stratigraphic section for the Southern Gulf of Suez. The measured stratigraphic section in the study area (Fig. 4), which is about 1000 m thick, revealed that the exposed rocks vary in age from Precambrian to Recent (Base- ment rocks, Naqus, Raha, Wata, Matulla, Abu Gerfan, Gharamaul, Gemsa formations and Post-Miocene sediments). The oldest pre-rift rock unit at Gebel El Zeit is represented by the Precambrian granitic rocks which are intruded by acidic and basic dykes. The Precambrian rocks are unconformably overlain by the

Fig. 2. Oil seepage at Gebel El Zeit area. Photo was taken on the 9th of February 2013.

Fig. 3. Simplified stratigraphic section of the southern Gulf of Suez, modified after Schlumberger (1984, 1995) and EGPC (1996). 58 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 4. Composite stratigraphic section of Gebel El Zeit area. See Fig. 3 for legend.

Cambrian-Ordovician Araba Formation (Hassan, 1967) which at- brownish yellow . The Naqus Formation is likely corre- tains a thickness of 35 m, and is mainly composed of yellowish, red lated with the subsurface occurrences of Nubia “C” (Schlumberger, to orange, medium to coarse-grained sandstones and conglom- 1984). erate. The Araba Formation is dominant at the base of the Nubia The Naqus Formation is unconformably overlain by the Aptian- Sandstone across the entire region and lithologically corresponds to Albian Malha Formation which reaches 35 m thick and is composed the Nubia “D” stratigraphic interval recognized in the nearby oil of yellow to brown, orange to red, and pale earthy sandstones of wells (Schlumberger, 1984). It is overlain by the Upper Ordovician different grain sizes with subordinate sandy siltstone and claystone Naqus Formation (Said, 1971). It attains about 320 m thickness and interbeds rich in kaolinite. The Malha Formation is likely correlated is composed of medium to coarse-grained pale earthy yellow to with the subsurface occurrences of Nubia “A” (Schlumberger, 1984). S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 59

It is overlain by the Raha Formation (Ghorab, 1961), which is about 35 m thick and is made up of alterations of sand- stones and of greenish gray to dark green color, but occa- sionally brown with ledges of fossiliferous limestone and dissected by numerous gypsum veins. The Raha Formation is conformably overlain by the Wata Formation (Ghorab, 1961). The Wata Formation attains 11 m thickness and consists of fossiliferous white to yellowish white limestone and varicolored fissile sandy . It is conformably overlain by the - Matulla Formation (Ghorab, 1961). The Matulla Formation attains 80 m thickness and is composed of cross-bedded, varicolored, fossiliferous sandstones, earthy to dark green gypsifirous shales and hard, fractured, un- fossiliferous, yellow to yellowish brown dolomitic limestone. The syn-rift succession is represented by the Late Abu Gerfan Formation (Ghorab and Marzouk, 1967) which uncon- formably overlies the Matulla Formation. It attains 60 m thickness and is formed of yellowish brown to brown, hard, consolidated, massive, moderately indurated, grain-supported polymectic con- glomerates with a few argillaceous limestone interbeds. It is exclusively composed of bioclastic carbonate grains with a few chert pebbles which are tightly embedded in a sand-rich lime-mud matrix. The conglomerates of Abu Gerfan Formation were previ- ously named as Flint Conglomerate (Perry, 1983) and Nukhul For- mation (Evans, 1988). The Abu Gerfan Formation is unconformably overlain by the Late Burdigalian Gharamul Formation (Ghorab and Marzouk, 1967). It is composed of greenish, laminated marls with thin shale beds intercalations and yellowish brown dolomitic sandstone beds with gypsum intercalations. The Gharamul Formation reaches 100 m and is unconformably overlain by the Late Burdigalian- Early Lan- ghian Gemsa Formation. The Gemsa Formation reaches 250 m in thickness and consists of thick massive gypsum beds with thin intercalations of marls, shales, limestones and sandstones. Locally, the sequence is capped by dolomitic algal stromatolitic limestone patches. The Post-rift succession is represented by Plio-Pleistocene shale, sandstone and few limestone beds. Quaternary gravel terraces and wadi alluvium deposits cover the topographic low land.

3. Structural setting

Mapping of Gebel El Zeit is completed by detailed study of satellite images of different scales that cover the study area, field- work to study and measure the stratigraphic sections in several parts of the area to establish the characteristics of the rock units and their marker horizons, and verification of all structures that were interpreted from satellite images. During this phase of the field-work, all of the structures observed in the field were identi- fied, measured and located in their correct position on the map. Fig. 5. Attitude statistics of surface structural elements at Gebel El Zeit area. (a) Strike Strike and dip measurement of bedding were taken in order to summary plot showing fault frequency distribution, (b) Dip angle versus fault orien- tation diagram, and (c) Summary plot of fault lengths. record the change in attitude of the strata in the study area. Structural data were gathered during the detailed field mapping of the study area (Fig. 5). A total of thirty three mapped faults and Ras Gemsa to the south. Structurally, Gebel El Zeit is divided belong to six prominent sets which are oriented, in a descending into three fault blocks; the Northern Gebel El Zeit Fault Block, Sarg e e e e order of frequency; NW SE, NNE SSW, NE SW, ENE WSW, El Zeit Fault Block, and Little Zeit Fault Block (Fig. 1). The regional WNWeESE and NNWeSSE. The mean orientations of these fault structural architecture of the study area is controlled by three main sets are; N 45 W, N 14 E, N 49 E, N 71 E, N 80 WandN22 W fault systems namely; Gebel El Zeit Boundary fault (GZBF), Little respectively. The measured dip data from eleven fault surfaces Zeit Boundary Fault (LZBF) and Ras El Ush Boundary Fault (RUBF) indicated a steep angle that usually exceeded 55 . (Figs. 1 and 6). The Precambrian rocks of Gebel El Zeit face the Gulf of Suez 4. Surface geological setting shoreline along a NW-trending fault, which is called “Gebel El Zeit Boundary Fault, GZBF”. The GZBF is represented by an eroded fault- e Gebel El Zeit forms a low elongated range trending NW SE scarp separating the basement rocks from the offshore East Zeit parallel to the Gulf of Suez rift, lies between Ras Dib to the north 60 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 6. Regional cross-sections (XeX0 and YeY0) through the southern Gulf of Suez Basin. Locations are given in Fig. 1 (compiled from Gebel El Zeit Petroleum Company (PETROZEIT), internal report, 2003; present study). basin and extends from the northernmost part of Gebel El Zeit (at 4.2. Sarg El Zeit fault block Ras Dib area) to the northern part of Sarg El Zeit where the NW- trending Little Zeit Boundary Fault (LZBF) lies to the west of the The Sarg El Zeit fault block is a topographic saddle (max. GZBF and extends to the southern end of the Little Zeit region elevation is about 150 m) which lies south to Northern Gebel El Zeit (Fig. 1). fault block and west of Ras El Ush. The exposed rocks in Sarg El Zeit The offshore area of Gebel El Zeit which extends from the fault block are mainly represented by the syn-rift Gemsa Formation southern end of the Gebel El Zeit basement rocks to east of Ranim and the post-rift Pliocene sediments. The Gemsa Formation has a Island is called Ras El Ush trend which is delimited from its eastern moderate dip ranges from 15 to 25 towards the SW, except near border by the NW-trending Ras El Ush Boundary Fault (RUBF). the fault planes, it shows a high dip values ranges from 45 to 60 (Fig. 8). The Sarg El Zeit fault block forms a large graben that is delimited from its northern side by the NNE striking Sz1 fault which separates it from the Gebel El Zeit rift block and West Zeit Range, while the NW-striking LZBF separates it from the Little Zeit fault block (Fig. 8). 4.1. Northern Gebel El Zeit fault block The eastern side of Sarg El Zeit fault block is bordered by four normal faults named Sz2, Sz3, Sz4 and Sz5, all of which juxtapose The Northern Gebel El Zeit fault block is an outcrop of basement the Quaternary sediments in their hanging-wall against the Gemsa rocks with a maximum elevation of 430 m. It is bordered to the Formation in their footwall. The first fault strikes NNE while the west by a 350 m high, West Zeit Range which is made up of other three strikes NW (Fig. 8). Paleozoic to Cenozoic sedimentary sequence. These two ridges are trending NW and are separated by the longitudinal depression of the Wadi Kabrit (Fig. 7). 4.3. Little Zeit fault block The exposed pre-rift rocks in the northern Gebel El Zeit fault block and West Zeit Range have a moderately steep dip ranges from The Little Zeit fault block forms an isolated small basement 30 to 45 towards the SW; whereas the syn-rift succession has a outcrop south to Sarg El Zeit block (max. elevation is about 250 m). moderate dip ranges from 15 to 25 towards the SW (Fig. 8). The exposed rocks in the Little Zeit fault block are represented by The body of northern Gebel El Zeit fault block and West Zeit the pre-rift Precambrian rocks, and the Naqus Formations, and the Range are highly affected by normal faults of different orientations; syn-rift succession including the Abu Gerfan, Gharamul, and Gemsa four NE-striking faults (Gz2, Gz3, Gz5 and Gz9), four ENE-striking formations. The Naqus Formation has a high dip ranging from 40 faults (Gz6, Gz7, Gz8, and Gz11), two NNW-striking faults (Gz1, to 50 towards the SW; whereas the syn-rift succession has a gentle and Gz4) and a NW (Gz10) striking fault (Figs. 7 and 8). to moderate dip ranges from 15 to 30 towards the SW (Fig. 9). S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 61

Fig. 7. Geological map of Northern Gebel El Zeit fault block.

The Little Zeit is the southernmost fault block in Gebel El Zeit (Fig. 9). area. It shows more structural complexity and high deformation The Gemsa Formation is affected by a large-scale asymmetric than the other surrounding fault blocks. The Little Zeit fault block is non-plunging synclinal fold whereas; the western limb dips at 30 highly affected by normal faulting. The mapped normal faults are NNE while the eastern limb dips at 19 WNW. The trend of the fold represented by six NW-striking faults (Lz5, Lz6, Lz7, Lz8, Lz10 and axis is NW, follows the strike of the Lz5 fault, which means that this Lz13), four NNE-striking faults (Lz3, Lz4, Lz9, and Lz11), two NE- fold is formed by drag on the Lz5 fault plane (Fig. 9). striking faults (Lz1 and Lz2) and a NNW-striking fault (Lz12) 62 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 8. Geological map of Sarg El Zeit fault block.

5. Subsurface geological setting for the region.

The subsurface geology of the Gebel El Zeit region is investigated 5.1. Potential data by the mapping of Ras El Ush oilfield and its surroundings (north and South Ras El Ush areas). The Ras El Ush oilfield represents the Gravity and magnetic surveys are relatively inexpensive, non- nearest oilfield to the hydrocarbon seepage in the area. Ras El Ush invasive and nondestructive and can quickly cover large areas of oilfield is located within Gebel El Zeit area (Fig. 10). It covers an area ground. They have been used most extensively in the search for oil both onshore and offshore along Ras El Ush trend. All of the onshore and gas. The subsurface structures at the Gebel El-Zeit area were and offshore structures have been investigated in detail and inte- mapped by Ziko et al. (2014) using the Bouguer gravity anomaly grated using a combination of field mapping of all available sub- map of Gebel El Zeit area (scale 1: 100,000) compiled by the surface data including magnetic maps, 2-D seismic lines, well logs, General Petroleum Company (1967). The downward continuation and dipmeter data in order to provide a coherent structural model and first horizontal gradient techniques were applied to the S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 63

Fig. 9. Geological map of Little Zeit fault block.

Bouguer anomaly map in order to identify and delineate the Company acquired a high resolution aeromagnetic data overing possible subsurface structures of the area in order to assist locating Gebel EI Zeit Concession (Fig. 11). The flight lines spacing were new hydrocarbon prospects. Their study revealed that Gebel El Zeit 200 m by 400 m directed NE and NW respectively. The purpose of area is divided into major fault blocks that are separated by NW and this survey was to obtain detailed information on prospective NE-trending faults. Also, they stated that the NNW to NW di- basement as related to rift trend structures and rift orthogonal rections represents the main trend of the study area. faults and also to identify prospective blocks. The second vertical In 1997, a group of exploration companies including Marathon derivative map of the 1997 survey reveals several anomalies of 64 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 10. Simplified base map for the 2-D seismic lines and well locations. elongated shape that can be interpreted as basement blocks limestones of the Eocene Thebes Formation, which are uncon- bounded by clysmic-trending faults and separated by rift orthog- formably overlain by the Early Miocene Nukhul Formation. onal faults. This aided to the evaluation of the preliminary struc- The Nukhul Formation is composed of sand, shale, and lime- tural setting of the area. Fig.11 shows the location of the interpreted stone beds and is overlain by the Early Miocene Rudies Formation faults on the second vertical derivative map. It is obvious that Ras El which is composed of alternating beds of shale and argillaceous Ush area is divided into several tilted fault blocks that are separated limestone. The Middle Miocene is represented by the Kareem and by NW and NE-trending faults. Belayim formations which overly the Nukhul Formation. The Kar- eem Formation is composed of shale, anhydrite, and sand at the top, and the Belayim Formation is composed of alternations of shale, 5.2. Stratigraphy of Ras El Ush oilfield anhydrite, and thin limestone beds at the upper part. A thick sec- tion of salt and anhydrite of the South Gharib Formation overlies The stratigraphic setting of Ras El Ush oilfield, which represents the Belayim Formation and is overlain by the anhydrite and shale the offshore part of the study area, is summarized from a key well beds of the Zeit Formation and in turn the Pliocene-Recent which is compiled from Ras El Ush-3, -5, -6, -7, -8, and South Ras El sediments. Ush-1 wells (Fig. 12). The sratigraphic section ranges in age from Albian to Recent. It is composed mainly of the sandstones of the Albian Nubia A which is 5.3. Seismic data overlain by the sand and shale beds of the Cenomanian Raha For- mation and in turn by the sand and shale beds of the Turonian Wata The area is covered with nine 2d seismic lines which cover an Formation. The Wata Formation is overlain by the sand and shale area of 130 km2. The quality of the 2-D survey ranges from poor to beds of the Coniacian-Santonian Matulla Formation. The limestone good. The data at the Miocene tops ranges from fair to good, while and chalky limestone beds of the - Sudr on top Matulla and Nubia Sandstone are of poor to moderate Formation overlie the Matulla Formation and are unconformably quality. The seismic interpretation was accomplished by identifi- overlain by the shales of the Esna Formation and the cation, picking and correlation of reflectors and faults locations S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 65

Fig. 11. Second vertical derivative aeromagnetic map of the study area (internal report, Gebel El Zeit Petroleum Company, 2003). It shows that Ras El Ush Block is affected by NW and NE-trending faults. detection using hard copies of the 2d seismic lines and tying of the 5.4. Structural cross-sections and maps 2-D seismic lines. Integration of dipmeter data for some wells with the formation Four structural cross-sections of 1:1 scale are constructed to tops and interpreted seismic data, allowed construction of struc- document the structural setting of Ras El Ush area. The dipmeter tural cross sections and maps and for the oilfield. Dip amount and data indicate that dip is generally towards the SW, ranging between dip direction derived from dipmeter logs are posted on the struc- 35 and 40 in the pre-rift formations and about 15e25 in the tural depth maps. Due to closeness of seismic data volume to sur- syn-rift formations. It also indicate that the clysmic faults have dip face exposures, integration between surface and subsurface data angles ranging between 45 and 58, while the rift orthogonal was done. faults have dip angles ranging between 75 and 80. 66 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 12. Simplified key well of Ras El Ush area. S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 67 68 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 14. Top basement structural depth map.

A northeast-southwest cross-section (AeA0) passing through A northeast-southwest cross-section (BeB0) passing through REU-1, and REU-5 wells is shown in Fig. 13a. It includes three REU-2, REU-7, and REU-3 wells is shown in Fig. 13b. It shows four clysmic faults: LZBF, GZBF and RUBF. These three faults affect the clysmic faults: LZBF, RF-3, RF-4, and RUBF. These four faults affect pre- and syn-rift successions and are downthrown towards the the pre- and syn-rift successions and are downthrown towards the northeast. The LZBF has an estimated throw of 1175 m at the level of northeast. The LZBF has an estimated throw of 1610 m at the level of the Precambrian basement rocks, while the GZBF has an estimated the Precambrian basement rocks. The RF-3 has an estimated throw throw of 1400 m at the level of the Nubia Sandstone. of 770 m, while the RF-4 has an estimated throw of 40 m at the level

Fig. 13. (a) A NWeSE cross-section (AeA0) passing through REU-1, and REU-5 wells. (b) A NEeSW cross-section (BeB0) passing through REU-2, REU-7, and REU-3 wells. (c) A NEeSW cross-section (CeC0) passing through S.REU-1 and S.REU-1st wells. (d) A NWeSE cross-section (DeD0) passing through E.Zeit-1 well. See Fig. 10 for location and Fig. 12 for colors. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 69 of the Nubia Sandstone. downthrown towards the southeast. A northeast-southwest cross-section (CeC0) passing through After creating the integrated structural model of the Ras El Ush S.REU-1 and S.REU-1st wells is shown in Fig. 13c. It demonstrates area, three structural depth maps were constructed for top Pre- three clysmic faults: LZBF, RF-3, and RUBF. These three faults affect cambrian basement rocks, top Nubia Sandstone, and Top Rudies the pre- and syn-rift successions and downthrown towards the Formation. The constructed structural depth maps revealed that northeast. The LZBF has an estimated throw of 1750 m at the level Ras El Ush area is a northwest trending rift block which is divided to of the Precambrian basement rocks. The RF-3 was confirmed from six tilted fault blocks namely FB-1, FB-2, FB-3, FB-4, FB-5 and FB-6 S.REU-1 well and has an estimated throw of 1140 m at the level of (Figs. 14e16). the Nubia Sandstone. The regional structural style of the area and the produced A northwest-southeast cross-section (DeD0) passing through structure depth maps revealed that the throw along the GZBF de- E.Zeit-1 is shown in Fig. 13d. It demonstrates one cross fault (Lz4 creases towards the SE where the maximum throw reaches 5000 m fault) which affects the pre- and syn-rift successions and (see cross-section XeX0, Fig. 6) and drops off to be 1400 m at the

Fig. 15. Top Nubia Sandstone structural depth map. 70 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 16. Top Rudies Formation structural depth map. northern extension of Ras El Ush oilfield (Fig. 13a). The maximum The FB-1 tilted fault block lies between the GZBF and the LZBF. throw along the RUBF reaches 3250 m (See cross-section YeY0, The geometry and overlapping of the two faults resulted in the Fig. 6). It decreases towards the northwest direction and reaches its formation of a breached relay ramp by the Lz1cross fault that de- tip at ERDMA-2 well, also the throw decrease towards East Ranim. limits the FB-1 tilted fault block from its northwestern edge. The constructed cross-sections of Ras El Ush area show that the The FB-2 tilted fault block represents the between maximum throw along the LZBF reaches 1750 m and the amount of the two main boundary faults GZBF and RUBF. The geometry and throw decreases towards the northwest direction. throw variation of the GZBF and the RUBF suggest the probability of The FB-1 and FB-2 tilted fault blocks represent the northern presence of a northeast dipping relay ramp which is represented by extension of Ras El Ush oilfield and each covers an area of the FB-2 tilted fault block. approximately 1.8 and 4 km2, respectively. They are delimited to The FB-3 tilted fault block lies in the SE to the FB-1 tilted fault the southeast by the NE-striking RF-1 and RF-5 cross faults, block. It covers an area of nearly 2 km2 and delimited to the respectively, that separate them from Ras El Ush oilfield. southeast by the Lz4 cross fault which intersects the LZBF at the S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 71

Fig. 17. Proposed setting for oil migration. (a) Migration pathway map for the migrated oil on top Nubia Sandstone. (b) Cross-sections CeC0 showing the proposed oil-water-contact. (c) Allan diagram passing parallel to the Lz4 fault on both hanging-wall and footwall. 72 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 southwestern corner of this fault block. responses. The FB-4 and FB-5 tilted fault blocks represent the Ras El Ush In an effort to assess the migration pathway of basinal fluids oilfield itself and each covers an area of about 1.65 and 0.58 km2, expulsion to the surface, seep location and surface structures have respectively. The Nubia Sandstone of the FB-5 tilted fault block been correlated to the subsurface structures of the Ras El Ush area. represents the main productive reservoir at Ras El Ush oilfield. The following observations were put into consideration in order to The FB-6 tilted fault block represent the southern extension of produce an appropriate migration pathway model: Ras El Ush oilfield and cover an area of about 0.85 km2.Itis delimited by the RF-2 cross fault fault from Ras El Ush oilfield and Barakat et al. (2005) evaluated molecular geochemical proper- by the Lz4 cross fault from its southeastern border (Figs. 14e16). ties of crude oil sample from Ras El Ush-3 Well and the surface petroleum seepage at Gebel El Zeit. The characterizations of 6. Petroleum habitat in the study area individual aliphatic, aromatic, and biomarker compounds were based on gas chromatography (GC) and gas chromatography/ Ras El Ush oilfield is located along Ras El Ush trend, which be- mass spectrometry (GC/MS) analyses. They concluded that the longs to the East Zeit basin (Fig. 1). Petroleum source rocks occur in compositional characteristics of the oil seepage are similar to Upper , Eocene, and Miocene deposits. Five potentially those of the crude oil sample from Ras El Ush-3 Well, indicating rich source rock intervals within the East Zeit trough can be iden- a common source. This conclusion appears to support that the tified (Rohrback, 1982; Barakat, 1982; Shaheen and Shehab, 1984; oil seepage originated from deeper reservoirs and migrated Salah, 1992; EGPC, 1996; and Alsharhan and Salah, 1994; longer distances towards the surface. Mohamed et al., 2013). These are: Upper Cretaceous carbonates Monitoring of the oil seepage level indicated minor seasonal (Eocene Thebes Formation, Lower Miocene Rudeis shales, Middle variation (about 10 cm) due to surface temperature difference Miocene Kareem shales, and also Middle Miocene Hammam Faraun through summer and winter. The nearly stable level of the oil Member of the Belayim Formation. In terms of volumes of well- seepage implies the existence of non-producing reservoir preserved source rocks, the syn-rift sequence is more important somewhere near from the oil seepage. than the pre-rift sequence (Alsharhan and Salah, 1994). Oil shows are found in the whole E.Zeit-1 well (202 m depth) The southern Gulf of Suez is known for its multi-reservoir that represents the closest well to the oil seepage (about 200 m character where each field produces from several reservoirs. The distance), which, also, is found penetrating the Lz4 fault. This reservoirs can be classified into pre-rift reservoirs and syn-rift means that the sediments penetrated by the E.Zeit-1 well are reservoirs (Meshref et al., 1988; Khalil and Meshref, 1988; Salah, highly fractured due to faulting, which provides a pathway to 1989; Tewfik et al., 1992). The main reservoir targets within the the oil to escape through basement rocks and Rudies and Kar- southern Gulf of Suez are the fractured basement rocks, Paleozoic eem/Belayim formations. and Lower Cretaceous sediments of the Nubia sandstone, the sandstone present in the Upper Cretaceous Matulla Formation, and In the light of the prior remarks and the detailed study of the the Lower Miocene carbonates of reefal origin of the Nukhul For- constructed structural depth maps and cross-sections, it is possible mation and the sandstones of the Lower Miocene Rudeis for oil to migrate from an undiscovered Nubia Sandstone reservoir Formation. of the FB-3 fault block. The oil entrapment is possible at the pro- The vertical seal of the pre-Miocene reservoirs (Nubia and spected reservoir; where the shales of the overlying Matulla For- Matulla sandstone) can be provided by overlying shales, chalks, and mation represent the vertical seal and the Lz4 fault represent the tight Limestones of Sudr and Esna formations. The lateral seal is lateral seal (seal fault) where the shales of the Rudies Formation are provided by middle to late Miocene . The Miocene clastic juxtaposed in the hanging-wall against the Nubia Sandstone. section, such as shales of the Rudeis and Kareem formations, can act Migration pathway maps, Allan diagram and the proposed as sealing agents especially in areas where some shaly facies have model in the present study are constructed to figure out the oil developed The Sudr and Esna formations provide lateral seal, migration pathway through the area (Figs. 17 and 18). The resultant depending on the amount of stratigraphic offset across the model assumes the transmissibility of the Lz4 fault where oil can structure-bounding fault. move along the strike of this fault due to fractures resulted from The crests of tilted fault blocks are the most common traps and faulting. At a probable oil-water-contact of 1000 m depth, the oil they definitely contain the largest known oil accumulations (EGPC, will move through the Lz4 fault. There is no ability for oil to escape 1996). The closure is provided by the bounding normal fault and the across the Lz4 fault because of the surrounding Upper Cretaceous dip of the tilted strata. The crest of the block; its structurally highest seal rocks and the Miocene Evaporites. Once the oil reaches the part, faces the deepest part of the adjacent half-graben along the intersection point of the Lz4 and LZBF faults, the migrating oil can zone of maximum throws of the bounding fault. This is an ideal move along the LZBF through the fractured basement rocks which position for updip migration of the hydrocarbons along the tilted control the hydraulic properties of rocks by providing permeable sand layers. Such crests can be multilayered traps with a progres- conduits for fluids. After that, oil can move up to the surface sive offset of axis with depth. Migration from the source intervals at through unconsolidated and fractured sediments of the Rudies and the East Zeit trough to the prospects is possible along the Ras El Ush Kareem/Belayim formations, and then through the fractured structural trend. limestone and porous coral reefs of the Recent sediments.

7. Oil seepage system and migration pathway

A petroleum seepage system is defined as the interrelationships among total sediment fill, tectonics (migration pathway), hydro- 8. Estimated oil-in-place carbon generation (source and maturation), regional fluid flow (pressure regime and hydrodynamics), and near-surface processes The original oil-in-place (OIP) for the predicted Nubia Sandstone (zone of maximum disturbance), (Abrams, 1992, 2005). The rates reservoir has been calculated using the volumetric method and volumes of hydrocarbon seepages to the surface modify the (Levorsen, 1967; Dake, 1998). The oil-in-place is expressed math- near-surface geochemical, geophysical, geological, and biological ematically as: S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 73

Fig. 18. Schematic model for oil migration pathway.

The average values of porosity and oil saturation are obtained from Ras El Ush-3 well logs (Fig. 19). The average porosity (F )is OIP ¼ Approximated Rock volume*F *ð1 S Þ (1) av av w estimated from neutron porosity log to be 21%. Water saturation where: (Sw) is estimated to be 8% and oil saturation (So) is estimated to be 92% using Eqs. (3) and (4),(Schlumberger, 2009), respectively:

Fav: Average porosity. Sw: Water saturation. 1=n Sw ¼ðRo=RtÞ (3), (Fig. 19) The total acre ft of the reservoir, or rock volume is calculated Assuming no gas saturation: using the more suitable and accurate approach (i.e., the Trapezoidal Rule). In such an approach the irregularities of the reservoir in both shape and thickness can be considered and consequently to obtain S ¼ 1 S (4) a fair value of the product rock volume. Using Eq. (2) (Levorsen, o w 1967), the approximated rock volume is calculated to be about where: 648,570 acre ft.

Ro: Formation resistivity in case of 100% water saturation. Approximated Rock Volume ¼ hðA1 þ A2Þ=2 (2) Rt: True formation true resistivity. n: Saturation exponent ¼ 2. where: From the prior, the maximum storage capacity is estimated to be h: Formation thickness. approximately 136,200 acre ft. The oil pore volume is estimated to A1: Area of upper surface. be about 6129 acre ft. More or less 47.5 MMBO of original oil-in- A2: Area of lower surface. place is estimated for the predicted Nubia Sandstone reservoir. 74 S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75

Fig. 19. Sample from Ras El Ush-3 well composite log. S. Sakran et al. / Marine and Petroleum Geology 71 (2016) 55e75 75

9. Conclusions in some Egyptian Mummies. Geoarchaeology 20 (3), 211e228. Barakat, H., 1982. Geochemistry criteria for source rock, Gulf of Suez. In: Pro- ceedings of the 6th Exploration and Production Conference, Cairo, 1982. The The structure analysis of surface geological data with subsurface Egyptian General Petrol Corp, pp. 224e252. geological information derived from seismic data are potentially Dake, L.P., 1998. Fundamentals of Reservoir Engineering, seventeenth ed. Elsevier, more meaningful for the purpose of the present research devoted Amsterdam, p. 498. EGPC, 1996. Gulf of Suez Oil Fields-a Comprehensive Overview. The Egyptian to identifying the recent oil seepage found at Gebel El Zeit area, General Petroleum Corporation, p. 736. southern of the Gulf of Suez, Egypt. The available seismic data at Ras Evans, A.L., 1988. Neogene tectonic and stratigraphic events in the Gulf of Suez rift El Ush oilfield -the nearest oilfield to the oil seepage-are poor, due area, Egypt. Tectonophysics 153, 235e247. Gebel El Zeit Petroleum Company “PETROZEIT”, 2003. Gebel El Zeit Concession Final to the occurrence of thick salt; which makes structural modeling of Relinquishment Evaluation Report, p. 52. Unpubl. Internal Rep. the field a challenging task. To conquer this challenge, a combina- General Petroleum Company “GPC”, 1967. Bouguer Gravity Map of Egypt: Scale 1: tion of detailed structural field mapping and geophysical data led to 100,000. Ghorab, M.A., 1961. Abnormal stratigraphic feature in Ras Gharib oil field. In: Pro- a better understanding to the geometry of geological structures ceedings of the 3rd Arab Petroleum Congress, Alexandria, p. 10. used to construct the field structural model. Ghorab, M.A., Marzouk, I.M., 1967. A Summary Report on the Rock Stratigraphic The subsurface structural mapping is completed using the Classification of the Miocene Non-marine and Coastal Facies in the Gulf of Suez integration of magnetic maps, 2-D seismic lines, and available and Red Sea Coast. Unpublished internal report. The Egyptian General Petro- leum Corporation, no. 601, p. 42. composite and dipmeter logs. The result is presented in the form of Harrel, J.A., Lewan, M.D., 2002. Sources of mummy bitumen in ancient Egypt and four cross-sections and three structural depth maps on the top of Palastine. Archaeometry 44, 285e293. basement rocks, top of Nubia Sandstone, and the top of Rudies Hassan, A.A., 1967. A new occurrence in Abu Durba-Sinai, Egypt. In: Proceedings of the 6th Arab Petroleum Congress, Baghdad, Paper 39, p. 8. Formation. Judd, A.G., Hovland, M., 2007. Seabed Fluid Flow: the Impact on Geology, Biology The resulting model demonstrates the presence of unexplored and the Marine Environment. Cambridge University Press, New York, p. 475. Nubia Sandstone reservoir where the oil migrates due to the Khalil, B., Meshrif, W.M., 1988. Hydrocarbon occurrences and structural style of the southern Suez Rift Basin, Egypt. In: Proceedings of the 9th Exploration and transmissibility of the NE-trending Lz4 fault and then upward Production Conference, Cairo, 1988. The Egyptian General Petroleum Corpora- through its intersection with the NW-trending LZBF to reach its tion, pp. 86e109. surface place through the highly fractured basement rocks and Levorsen, A.I., 1967. Geology of Petroleum, second ed. W.H.Freeman and Co., San Francisco, p. 724. unconsolidated sediments of the Rudies, Kareem/Belayim forma- Meshref, W.M., Abu Karamat, M.S., Gindi, M., 1988. Exploration concepts for oil in tions and Recent sediments. the Gulf of Suez. In: Proceedings of the 9th Exploration and Production Con- The interpretation mentioned above is confirmed by the results ference, Cairo, 1988. The Egyptian General Petroleum Corporation, pp. 1e24. Mohamed, N.S., Shahin, A.N., Elkammar, A.M., 2013. Hydrocarbon generating basins of the geochemical studies of Barakat et al. (2005), and the moni- and migration pathways in the Gulf of Suez, Egypt. Life Sci. 10, 229e235. toring of the oil seepage level, which points to pressure stability, Mostafa, A.R., 1993. Organic geochemistry of source rocks and related crude oils in confirm the attendance of the Nubia reservoir to be the source of the Gulf of Suez, Egypt. Berl. Geowiss. Abh. A 147e163. Gebel El Zeit surface oil seepage. Perry, S.K., 1983. The Geology of Gebel El-zeit Region, Gulf of Suez, Egypt. Unpub- lished MSc thesis. South Carolina University, p. 128. The integration between geological and geophysical methods in Robison, V.D., 1995. Source rock characterization of the late cretaceous brown mapping reduces the uncertainty of the resultant structural model. limestone of Egypt. In: Katz, B. (Ed.), Petroleum Source Rocks. Springer, Hei- e In summary, the estimated 47.5 MMBO of original oil-in-place for delberg, pp. 265 281. Rohrback, B.G., 1982. Crude oil geochemistry of the Gulf of Suez. In: Proceedings of the predicted reservoir indicates that the area is promising and the 6th Exploration and Production Conference, Cairo, 1982. The Egyptian needs more studies to improve the geological model for further General Petroleum Corporation, pp. 212e224. exploration. From the prior observations and obtained results, it is Said, R., 1962. The Geology of Egypt. Elsevier, Amsterdam, p. 377. Said, R., 1971. The explanatory notes to accompany the geological map of Egypt. recommended to test the Nubia Sandstone hydrocarbon potenti- Geol. Surv. Egypt 56. ality at the southeast extension of Ras El Ush oilfield structure. Said, R., 1990. The Geology of Egypt. Balkema-Rotterdam, p. 734. Salah, M.G., 1989. Geology and Hydrocarbon Potential of the Southern Sector of the Gulf of Suez, Egypt. Unpublished MSc thesis. Cairo University, p. 140. Acknowledgments Salah, M.G., 1992. Geochemical evaluation of the southern Gulf of Suez, Egypt. In: Proceedings of the 11th Exploration and Production Conference, Cairo, 1992. We are grateful to the authorities of The Egyptian General Pe- The Egyptian General Petroleum Corporation, pp. 383e395. Schlumberger, 1984. Well Evaluation Conference, pp. 1e64. troleum Corporation and Gebel El Zeit Petroleum Company (Pet- Schlumberger, 1995. Well Evaluation Conference, pp. 56e71. rozeit) for permission to publish this paper. We appreciate the Schlumberger, 2009. Log Interpretation Charts, 2009 Edition. Schlumberger Ltd., critical review of the manuscript by Professor Adel Ramadan p. 308 Moustafa and Professor Nabih Abdelhady Elsayed. Also, we are Shahin, A.N., 1988. Oil window in the Gulf of Suez basin, Egypt. Am. Assoc. Petro- leum Geol. Bull. 72, 1024e1025. thankful to Suez Oil Company (SUCO) for the facilities provided Shahin, A.N., Shehab, M., 1984. Petroleum generation, migration and occurrence in during the fieldtrip. the Gulf of Suez offshore, South Sinai. In: Proceedings of the 7th Exploration and Production Conference, Cairo, 1984. The Egyptian General Petroleum Cor- poration, pp. 126e152. References Tewfik, N., Harwood, C., Deighton, I., 1992. The miocene, rudeis and kareem for- mations of the Gulf of Suez. Aspects of sedimentology and geohistory. In: Abrams, M.A., 1992. Geophysical and geochemical evidence for subsurface hydro- Proceedings of the 11th Exploration and Production Conference, Cairo, 1992. carbon leakage in the Bering Sea, Alaska. Mar. Petrol Geol. 9, 208e221. The Egyptian General Petroleum Corporation, pp. 84e113. Abrams, M.A., 2005. Significance of hydrocarbon seepage relative to petroleum Williams, A., Lawrence, G., 2002. The role of satellite seep detection in exploring the generation and entrapment. Mar. Petrol Geol. 22, 457e477. South Atlantic's ultradeep water, in surface exploration case histories: appli- Alshrahan, A.S., Salah, M.G., 1994. Geology and hydrocarbon habitat in a rift setting: cations of geochemistry, magnetics, and remote sensing. In: Schumacher, D., southern Gulf of Suez, Egypt. B Can. Petrol Geol. 42 (3), 312e331. LeSchack, L.A. (Eds.), AAPG Studies in Geology No. 48 and SEG Geophysical Barakat, A.O., Mostafa, A., El-Gayar, M., Rullkotter,€ J., 1997. Source dependent References Series No. 11, pp. 327e344. biomarker properties of five crude oils from the Gulf of Suez, Egypt. Org. Ziko, A., Sakran, S., Nabih, M., Henaish, A., 2014. Subsurface structural mapping of Geochem. 26, 441e450. Gebel El Zeit area, southwestern Gulf of Suez rift using Boguer gravity anomaly Barakat, A.O., Mostafa, A., Qian, Y., Kim, M., Kennicutt, M.C., 2005. Organic maps. Bull. Fac. Sci. Zagazig Univ. 36, 21. geochemistry indicates Gebel El Zeit, Gulf of Suez, is a source of bitumen used