The South Diyur exploration block of nearly 38,000 sq km is located in the Farafra Oasis region in the Western Desert of Egypt. It is a frontier exploration area, the nearest well being Ammonite-1, a dry hole drilled by Conoco in 1979 immediately outside the southwestern corner of the block. The South Diyur Block is located on the probable northeast extension of the Kufra Basin in southeast Libya. Although prolific reserves of oil and gas occur in Paleozoic basins in North Africa and throughout the Middle East, to date, the targets for petroleum exploration in the northern Western Desert have been in Jurassic and Cretaceous rocks.
The regional structural surface features in the South Diyur Block are the NE-trending Bahariya and Farafra anticlines interpreted as a deeply eroded and inverted Late Cretaceous structure on the southern extension of the Syrian Arc system. The oldest exposed rocks are a Cretaceous sequence of sublittoral sediments (the Campanian Wadi Hennis Formation) in the core of the anticline.
The interpretation of the subsurface is based on 1,175 line-km of reprocessed 1970s-vintage 2-D seismic. Four sequence boundaries have been identified from the seismic data. SB-1 correlates with the Jurassic/Cretaceous boundary in Ammonite-1. SB-2 is regionally correlated with the Late Devonian to Early Carboniferous Hercynian unconformity that overlies deeply eroded and truncated Paleozoic sequences and possibly marks the regionally extensive Late Paleozoic basin inversion. SB-3 near the base of the interpreted Silurian sequence coincides with the ‘hot shale’ petroleum source rock that is present throughout North Africa and the Middle East. SB-4 is interpreted as a major unconformity at the top of an Upper Proterozoic sedimentary section that was misinterpreted as the Precambrian acoustic basement in Ammonite-1.
Five seismic sequences relate to the seismic boundaries. SS-1, from the surface to SB-1 is characterized by subparallel seismic stratification and is composed mainly of sandstone with shale interbeds in Ammonite-1. SS-2, bounded by SB-1 and SB-2, is distinguished by parallel to subparallel seismic stratification. In Ammonite-1, the sequence of interbedded sandstone and shale is fresh-water bearing and lacking in top seals, thus reducing its prospectivity. The underlying SS-3 (SB-2 to SB-3) directly underlies the Hercynian unconformity and is characterized by semi-transparent seismic facies that may correspond to a thick Silurian shale sequence. SS-4 (SB-3 to SB-4) of probable Cambrian–Ordovician age has parallel seismic stratification. Deep channels are interpreted as evidence of a Late Ordovician–Early Silurian glacial phase that is present throughout North Africa and the Middle East. SS-5 (below SB-4) is marked by partial subparallel seismic stratification and block faulting. It probably belongs to the Late Proterozoic (Pan-African) phase of block faulting and pull-apart basins. Similar seismic geometries and facies occur in the Kufra Basin in southeast Libya and in many parts of the Arabian Plate, including the prolific petroleum systems of Oman.
Exploration plays in the South Diyur Block are a combination of Paleozoic structural and stratigraphic traps associated with prospective fairways, and possible stratigraphic traps in the Late Ordovician–Early Silurian glacial channels. In addition, the interpreted Late Proterozoic sequences (SS-5) warrant further evaluation.
In order to identify future exploration plays and drill targets, additional 2-D seismic (4,490 line-km), aeromagnetic and airborne gravity surveys will be integrated with the present seismic data and drilling results from Ammonite-1. This will allow a proper assessment of the magnetic basement, basin configuration and prospective fairways.
The South Diyur Exploration Block covers an area of 37,678 sq km in the Western Desert of Egypt (Figure 1a). Conoco (Continental Sahara Oil Company) explored the South Diyur and adjacent South Siwa and Dakhla blocks in the 1970s. The exploration program culminated in 1979 with the drilling of the Ammonite-1 and Foram-1 exploration wells, both of which intersected Tertiary, Mesozoic and Paleozoic strata and bottomed in commercial basement without identifying oil or gas shows. After nearly three decades without exploration activity, a consortium lead by India’s Gujarat State Petroleum Corporation Ltd (GSPC) is currently exploring the South Diyur Block.
The South Diyur Block is centered on the Farafra Oasis at latitude 27.06°N and longitude 27.97°E, approximately 500 km southwest of Cairo and 300 km west of the River Nile at Asyut, and midway between the Dakhla and Bahariya oases. Figure 1b is a satellite image of the South Diyur Block overlain with the major geographic and cultural features. The Farafra depression is the second largest in Western Egypt after the Qattara Depression 300 km to the north-northwest. The Farafra Oasis is linked by road to Cairo via the railhead at Bawiti in the Bahariya Oasis to the northeast, and with Qena on the Nile through the Dakhla oasis and the railhead of El Kharga in the Great Oasis to the southeast.
The Farafra Oasis is located within an irregularly shaped steep-sided triangular depression with its apex to the north. The northwestern and northeastern scarps have elevations of 275 m above sea level (a.s.l). The floor of the depression (about 100 m a.s.l near Qasr El Farafra) rises gradually southward to the Dakhla escarpment. The El Quss Abu Said plateau on the northeastern edge of the depression, separated from the bounding escarpment by Wadi El Obeiyed, has a maximum elevation of 343 m a.s.l. Within the Farafra depression is the so-called White Desert located 45 km north of Qasr Farafra formed by the wind erosion of chalky deposits of the Upper Cretaceous Khoman Formation. The hot springs of Bi’r Sitta occur on the southeastern flank of El Quss Abu Said near Qasr El Farafra.
Our study presents the interpretation of about 1,175 line-km of 1970-vintage reprocessed seismic data acquired by Conoco in the South Diyur Block. It is part of the minimum work commitments for the GSPC-led consortium, which includes the acquisition of aeromagnetic and gravity surveys over the entire block, approximately 4,490 line-km of new seismic data, and the drilling of four exploration wells. The interpretation of the data from 14 NW-trending and 6 NE-trending tie lines, reprocessed in Cairo on behalf of GSPC, identified four regionally mappable geoseismic reflectors which can be tied to Ammonite-1 that had a measured depth (MD) of 2,395 m (7,857 ft). The paper discusses the importance of these geoseismic reflections in understanding the geologic evolution, petroleum system and exploration potential of the South Diyur Block. The results are critical to the interpretation of recently acquired seismic data and in the planning of the proposed exploration wells. The combination of geoseismic data and the recently acquired aeromagnetic and gravity data should enable the identification of basinal areas having petroleum potential within the South Diyur exploration block.
One of the prime targets of the present study is to predict a suitable stratigraphic column from surface to basement together with expected thicknesses as an aid to exploration, and to forecast the commercial total drillable depth in the basinal area for a projected combination stratigraphic well.
In Egypt, Proterozoic metamorphic and igneous rocks of the African Craton are exposed in the Red Sea Hills bordering the Gulf of Suez, in the southwestern corner of Egypt at Gebel Uweinat on the border with Libya and Sudan. In southern Libya they occur on the border with Chad in the Tibesti-Ghiarabub Mountains and in the Mourizidie area south of Gebel Bin Ghanimah near the Libyan border with Chad and Niger.
According to Dahi and Shahin (1992) the crystalline Precambrian basement that underlies the Western Desert sedimentary section has been penetrated in the Abu Roash-1, Almaz-1, Ammonite-1 (now considered unlikely), Baharaiya-1, BRE6-1, Dawabis-1, Diyur-1, Fayoum-1, Foram-1, Nashfa-1, Qarun-1 and 2, Umm Baraka-1 and Wadi Gheiba-1 wells.
In the Mourizidie area of Libya, greenschist-facies metasediments of probable Proterozoic age were intruded by an Upper Proterozoic post-tectonic granodiorite batholith (Hunting Geology and Geophysics Ltd, 1974a). The original sediments were a geosynclinal assemblage of greywacke and siltstone with minor limestone and chert. Also in southern Libya about 25 km northwest of Gebel Uweinat, minor exposures of clastic sedimentary rocks have been assigned a possible Late Proterozoic age (Hunting Geology and Geophysics Ltd, 1974b). They consist of ripple-marked sandstones, marbles, graywackes, cherts and sedimentary breccias. A similar and equivalent sequence may occur in the subsurface of the South Diyur Block.
In a recent publication, Craig (2012) indicated that the Neoproterozoic Eon extending from 1,000 Ma to the base of the Cambrian at 542 Ma, is poorly recognized as a prospective petroleum habitat, despite the existence of potential, proven or producing plays in many parts of the world. These include Oman, Mauritania, Siberia, China, India, Pakistan, Australia and North and South America (Bhat et al., 2009; Craig et al., 2009; Ghori et al., 2009). The most prospective Upper Proterozoic plays in northern and western Africa appear to be in the Taoudenni Basin in Mali, Mauritania and southern Algeria, and in the Kufra Basin in southeastern Libya (Figure 2a). These basins have broadly similar stratigraphy and tectonic evolution, despite being more than 2,500 km apart.
Paleozoic and Mesozoic Sedimentation
Paleozoic sedimentary rocks of the Western Desert were deposited on the northern rim of the African Craton in a series of transgressions and regressions directed mainly from the Paleo-Tethys in the north. The region was part of the unstable Tethyan shelf of what became North Africa and the Mediterranean region. Various orogenic episodes created depocenters throughout the Late Proterozoic (Neoproterozoic), Paleozoic and Mesozoic times in which were deposited thick tectono-stratigraphic sequences of fluviatile to shallow-marine sediments.
During Late Proterozoic to Late Carboniferous times, most of the northern part of the African-Arabian Plate, including the South Diyur area, was a site of thick clastic deposition. As a result, the subsurface geologic sequences in South Diyur are expected to show both similarity and continuity with similar sequences found in North Africa, particularly Libya, the Northern Province of the Western Desert of Egypt, together with Saudi Arabia and Jordan (Mahmoud et al., 1992; Mahmoud et al., 2000).
Late Ordovician-Early Silurian polar glaciations
According to Konert et al. (2001), the African-Arabian Plate moved into high southern latitudes during the Early Paleozoic with the northern part of the Plate at about 55°S in the Late Ordovician. A major polar glacial phase was followed by a deglaciation phase in the earliest Silurian due to the anticlockwise rotation of the African-Arabian Plate and its consequent northward movement to lower latitudes (McClure, 1978; Vaslet, 1987; Vaslet, 1990; Konert et al., 2001).
Le Heron et al. (2004) reported that many features interpreted as Late Ordovician glacial paleovalleys have been reported from Mauritania, southern Algeria, Saudi Arabia and Jordan (Beuf et al., 1969, 1971; Bennacef et al., 1971; Vaslet, 1987, 1990; Powell et al., 1994; Ghienne and Deynoux, 1998; and Deynoux et al., 1994). Analogous structures have also been reported from the subsurface of Algeria, Libya and Saudi Arabia (Aoudeh and Al-Hajri, 1995; Smart, 2000; Hirst et al., 2002); accordingly, the same should be expected in the Western Desert of Egypt, including the South Diyur Block.
Publications by Boote et al. (1998) (Figure 2a), Guiraud (1998), MacGregor and Moody (1998) and Moustafa et al. (2003) (Figure 3b) provide details on the structural history and petroleum systems of North Africa. A synthesis by Badalini et al. (2002) located more than 20 Paleozoic basins in North Africa, many of which are world-class petroleum provinces. The structural evolution of these basins has controlled to a large degree the petroleum geology of North Africa. According to Badalini et al. (2002), large-scale plate tectonic controls provided the main driving force for regional subsidence and inversion, and consequently for basin formation and evolution, sediment supply and distribution, and trap formation and style.
These major orogenic events deformed the African paleocontinent during its migration from high southern latitudes in the Early Paleozoic to its present-day position. The tectonic evolution of these sedimentary basins has influenced reservoir distribution and quality, the timing and state of source-rock maturity oil migration and preservation, and the distribution and types of traps that are present. They include the major Hercynian and Alpine orogenies and the Mesozoic extensional phase, as well as a series of tectonic events that reactivated north-, northeast-, northwest- and east–northeast-striking structures inherited from the Pan-African Orogeny.
The Western Desert of Egypt consists of a series of small rift basins. Although some are of Permian age, Guiraud (1998) considered that most were initiated in the Late Jurassic–Early Cretaceous, contemporaneous with the creation of the Mediterranean basins and with the southernmost Sirte rifts in Libya. Extensional tectonic activity was terminated in the Late Cretaceous by the Syrian Arc inversion phase (MacGregor and Moody, 1998). According to Sestini (1995), the dominant structural style of the Western Desert consists of a deep series of low-relief horst and grabens separated by master faults of large throw, and broad Late Tertiary folds at shallower depth.
In the Western Desert of Egypt, petroleum exploration has been concentrated in its northern part and, hence, the geology is better known there than elsewhere. According to Sestini (1995), several carbonate-clastic alternations that, together with the enclosed secondary transgressive-regressive cycles characterize the stratigraphic succession of northern Egypt, constitute one of the main elements of the Mesozoic–Early Tertiary petroleum systems of the Western Desert. The occurrence of oil is closely linked with the tectono-stratigraphic history of the area that has created multiple reservoir and seal combinations. Adequate potential source rocks are stratigraphically and areally widespread in the Western Desert (Abu El Naga, 1984; Elzarka, 1983). Most fields are related to structures formed in the Late Cretaceous–Eocene and are located in, or at the edge of, early depocenters that later became petroleum generative areas (Abu El Naga, 1984). The Paleozoic to Mesozoic depocenters contain thick sequences of fluviatile and shallow-marine sedimentary rocks.
In broad terms, in the northern Western Desert the north-trending Paleozoic Faghur Basin underlies the border region between Egypt and Libya with its axis in Egypt (Dahi and Shahin, 1992). Inversion tectonics converted the Faghur Basin into a platform that acted as a high on the western margin of the Shushan Basin (see Figure 2b) during the Mesozoic (Mahmoud and El-Barkooky, 1998). As a result, Paleozoic sediments are progressively onlapped by Mesozoic sediments westwards toward the main Faghur high.
Farther south, the geology of the Farafra-Dakhla-Kharga region has been studied by Hermina (1990). The Dakhla Basin is separated by the northeasterly plunging Kharga uplift from the Assuit-Upper Nile Basin to the east (Figure 3a). Basement in the northern part of the Dakhla Basin is at least 4,000 m below sea level (b.s.l.); that in the Assiut Basin is about 3,000 m b.s.l. (Hermina, 1990). The Kharga uplift forms the southeastern margin of the Dakhla Basin and was active during Jurassic and Cretaceous times. The Dakhla Basin was filled with thick continental and marine strata of Paleozoic to Early Eocene age in the northwest and by thinner Jurassic (or Early Cretaceous) to Early Eocene in the south and east. The Ammonite-1 well in the central part of the Basin reached a previously interpreted basement at 2,000 m b.s.l. and intersected at least 860 m of Carboniferous and older strata below the Jurassic.
Hercynian Orogeny and Basin Inversion
The Hercynian Orogeny is a major Late Paleozoic tectonic and basin inversion event that affected the whole of North Africa and the Arabian Plate during the Late Devonian to Early Carboniferous (Konert et al., 2001). Several publications have focused on the Hercynian unconformity in the Western Desert. Moustafa et al. (2003) interpreted it as occurring at the boundary between the Upper Carboniferous–Lower Permian sequence and the Middle Cambrian sequence, with the probable removal of a thick Paleozoic section due to erosion caused by regional uplift and basin inversion. Evidence of the removal of several kilometers of sediments from uplifted and inverted basinal areas are reported from most of North Africa and the Arabian Plate. Changes in basin geometry, regional uplift, basement-cored uplifts, and the evidence of folding and inversion tectonics, suggest that the northern part of the African Plate and the Arabian Plate underwent multiple phases of compression during this orogenic period (Konert et al., 2001). As a result, the major Hercynian unconformity is recorded throughout the region and is interpreted in the South Diyur Block as occurring at Seismic-sequence Boundary-2.
SURFACE GEOLOGY OF THE SOUTH DIYUR BLOCK
Reconnaisance fieldwork in the area of the Farafra Oasis by GSPC took place in June 2011. Figure 3b is the geologic map of the South Diyur Block after Hermina (1990) updated from the fieldwork (the GPS track taken is shown in yellow; black dots are the geologic field observation points). Exposures of Upper Cretaceous, Paleocene and Eocene rocks occur in the South Diyur Block. The exposed Paleocene and Eocene rocks are composed of chalk, limestone, sandstone, shale, and silt-clay facies.
The Upper Mesozoic and Lower Tertiary strata of the Farafra area are subdivided according to Figure 4a (Hermina, 1990) and comprise a Campanian to Middle Eocene transgressive-regressive open-marine sequence. From oldest to youngest they are the Wadi Hennis, El Hefhuf (Duwi), Dakhla, Khoman, Tarawan, Esna, Farafra and Naqb formations. In addition, the post-Miocene Minqar El Talh Formation crops out at the southeastern end of the El Quss Abu Said plateau. Figure 4b is a correlation of Upper Cretaceous to Lower Tertiary formations in the Farafra Oasis. The Campanian transgression deposited the Wadi Hennis and El Hefhuf formations in the Farafra Depression. Figure 4c is a stratigraphic section of the Wadi Hennis and El Hefhuf formations in Wadi Hennis.
Figure 5 is a composite of field photographs of the Wadi Hennis, El Hefhuf, Khoman, Dakhla, Tarawan, Esna and Farafra formations. Eustatic sea-level changes accompanied by slow, long-term tectonics through Late Cretaceous to Eocene times were responsible for the main sequences and sequence boundaries.
In the Farafra area, tectonic uplift at the Campanian–Maastrichtian boundary related to the formation of the Bahariya Arch to the northeast caused a break in sedimentation so that the Khoman Formation is unconformable on the El Hufhuf Formation. However, in the Dakhla-West Dakhla area, sedimentation was continuous into the overlying Dakhla Formation. In the Farafra Depression, chalky limestone of the Khoman Formation is time-equivalent to the middle part of the shale-marl Dakhla Formation. Hermina (1990) surmised that the intertonguing of the Dakhla and Khoman formations was probably due to contemporaneous tectonic events. Here, a Late Cretaceous–Early Paleocene unconformity varies in magnitude, but is more pronounced in the Ain El Maqfi area where the pseudomenardii Zone directly overlies the Late Cretaceous Globotruncana gansseri Zone.
As a result of the major Late Cretaceous–Early Paleocene regression, shallow-marine shelf environments developed on regional highs to the south of the South Diyur Block. On these platforms were deposited reef-like limestones and intercalations of near-shore sediments whereas in middle to outer shelf environments off the platforms more shaley facies were deposited; these are the upper Dakhla Formation overlain by the Tarawan and Esna formations. Overlying the Paleocene units are predominantly shallow-water shelf carbonate rocks of the Thebes Group (Farafra and Naqb formations) that crops out on the El Quss Abu Said outlier and the northern scarp of the Farafra Depression. They represent a marine regression that began with the deposition of the late Esna and Garra formations. Post-Miocene deposits consist of the terrestrial Minqar El Talh Formation. Table 1 is a lithostratigraphic compilation of the surface units in the South Diyur Block.
Playa deposits occur in the Farafra Depression and wind-blown sand covers large areas of the South Diyur Block. The Great Sand Sea, a component of the Sahara, formed of northwest-trending sand dunes, mainly of the seif type, borders the Farafra region to the west. It is the world’s third largest dune field spanning 650 km from Siwa in the north to the Gilf Kebir plateau to the south and eastward to the Farafra Depression.
Structurally, the Farafra area has relatively intense tectonics as it represents the southern extension of the Syrian Arc system, more fully developed in the Bahariya area to the northeast. According to El-Ramly (1964), NE-trending anticlines alternate with synclines. Two sets of faults strike northeast and north-northwest and two sets of joints (contemporaneous with the folding) strike N80°E and N80°W. The clastic sequence of sublittoral sediments forms the core of the northeast-trending Farafra-Bahariya anticline.
SUBSURFACE GEOLOGY OF THE SOUTH DIYUR BLOCK
Direct evidence of the subsurface in the vicinity of the South Diyur Block has been provided by four exploratory wells (see Figure 1 for location). They are Ammonite-1, Foram-1, El Desouqy-1 and Bahariya-1.
Ammonite-1 was drilled by Continental Sahara Oil Company (Conoco) in 1979 immediately outside the southwestern corner of the South Diyur Block (latitude 26°24′27.254″N; longitude 26°51′31.6274″E) at an elevation of 315 m above sea level. It is the nearest well to the South Diyur Block. It was spudded on March 1, 1979 to test a large NE-trending anticlinal feature (seismic high) and was completed in suspected acoustic basement on April 20 at a measured depth (MD) of 2,395 m (7,857 ft) below rig kelly-bush elevation (RKB). No evidence of hydrocarbons was found except for a few lost-circulation zones in some Cretaceous strata. Much of the Jurassic–Cretaceous sections consisted of porous sands; however, all Mesozoic sequences are considered fresh-water bearing. The lack of adequate seals due to the considerable amounts of sandstone in the succession precluded retention of any hydrocarbons that might have been generated. The well was logged, plugged and abandoned.
Foram-1 was spudded on June 1, 1979 by Conoco in the northwestern corner of the South Siwa Block and completed in October 1979. The exploratory well, located at latitude 27°39′11.1899″N and longitude 25°06′05.1536″E near the border with Libya, targeted a large seismic closure. No hydrocarbon shows were intersected and basement was reached at 3,923.4 m (12,868 ft) below RKB. Most of the sedimentary sequence was porous sandstone. Although both the Devonian and Silurian are watered-out reservoirs in this well, the high gamma-ray ‘hot shale’ within the Devonian section may be a good source rock. No drill stem tests were conducted. The well was logged, plugged and abandoned.
El Desouqy-1 was spudded on August 21, 1972 by the Egyptian General Petroleum Company (GPC) to the north of the South Siwa Block and was completed in November 1972. The wildcat, located at latitude 28°22′25.45″N and longitude 25°56′34.39″E, targeted a seismic closure. It intersected no hydrocarbon shows and reached MD at 4,141 m below RKB. Most of the sequence consisted of porous sandstone. The well was logged, plugged and abandoned.
Bahariya-1 was drilled by Sahara Petroleum Company in 1957. It reached Precambrian basement at 1,718 m below sea level. The location of the well was determined mainly from surface geologic mapping and gravity surveys (Moustafa et al., 2003). The well was spudded in the exposed Middle Cretaceous Bahariya Formation and intersected rocks ranging in age from Early Cretaceous to Precambrian.
Devon Energy (Egypt) Corporation carried out a detailed paleontological analysis of the Bahariya-1 well that revealed a thick Paleozoic section (Moustafa et al., 2003). The two units identified are the 588-m-thick Late Carboniferous–Early Permian Safi Formation consisting of massive sandstones interbedded with minor shale units, and the Middle Cambrian Shifah Formation that is a 436-m-thick predominantly shale unit with some sandstone interbeds. The major unconformity between the two formations is interpreted by Moustafa et al. (2003) as resulting from the Late Paleozoic Hercynian Orogeny.
Figure 6 is a composite lithologic log of the Ammonite-1, Foram-1 and El Desouqy-1 wells. In addition, stratigraphic information is available from the Bahariya-1 well drilled in 1957 about 200 km northeast of the Farafra Oasis (for location see Figure 1). Figure 7 is a simplified stratigraphic column showing the Mesozoic and Paleozoic formations of the Western Desert that are expected to be intersected by drilling in the South Diyur Block.
The paleontological and palynological analyses that were carried out on rock samples from the three offset wells identified biozones and faunal assemblages. Based on the electric logs and the paleontological and palynological analyses, the following eight horizons were recognized (Table 2a):
(Surface); Base Cenomanian (?); Base Cretaceous Unconformity (?); Paleozoic-Mesozoic Unconformity; Middle–Early Carboniferous; Near Top Devonian; Top Lower Devonian; Base Silurian; and Basement.
Ammonite-1 has the smallest penetrated section of the three wells and bottomed in the Carboniferous by regional correlation; both the Foram-1 and El Desouqy-1 wells penetrated thicker Paleozoic sections and bottomed in basement and Cambrian–Ordovician, respectively.
Ammonite-1 Synthetic Seismogram
The surface geology of the South Diyur Block correlates with the relevant Tertiary and Upper Cretaceous sequences intersected in Ammonite-1. However, the Paleozoic sequences in Ammonite-1 have no surface expression in South Diyur. It is therefore necessary to base the expected stratigraphy on seismic interpretation. The seismically interpreted Paleozoic sequences are analogous to those found is the subsurface in the prolific petroleum-producing regions of North Africa and the Middle East. Old tentative depth-to-basement maps indicated an average sedimentary thickness of 3,000 to 10,000 ft in the central part of the South Diyur exploration block. This important feature has been validated by the recently acquired magnetic and gravity data that indicated an estimated depth to basement of approximately 15,000 to 17,000 feet in basinal areas in the South Diyur Block.
To tie the key Ammonite-1 well drilling results to the seismic data, a synthetic seismogram was constructed (Figure 8). It incorporated time/depth curves with the existing 2-D seismic data to formulate geoseismic horizons. At the same time, a reinterpretation of the lithology was made, particularly with regard to the proportions of sandstone interbeds, as sandstone was overestimated in the original lithologic interpretation. A relatively good match occurs between four geoseismic horizons (SB-1 to SB-4) and the synthetic seismogram and includes a well-defined unconformity that was recognized from lithologic and seismic data, particularly in the southwestern part of the block. The four geoseismic horizons are interpreted as sequence boundaries, as follows: SB-1 of Jurassic–Cretaceous age; SB-2 marks a regional Late Devonian–Carboniferous unconformity; SB-3 occurs near to the base of the Silurian Shale sequence; and SB-4 is near what is described in Ammonite-1 as basement, but which is probably an Upper Proterozoic sequence overlying a crystalline Precambrian basement. It is most probable that Ammonite-1 did not reach basement as the tentative depth-to-basement magnetic map indicates at least 1,000–2,000 ft of additional sedimentary section below reached total depth.
Preliminary correlation with similar geoseismic sequence boundaries in Libya and the Middle East, indicates that SB-2 is the major Hercynian unconformity at the top of the interpreted Silurian geoseismic sequence that spans the period from Late Devonian to Early Carboniferous. In Ammonite-1, the age of the unconformity was confirmed by the dating of rock samples spanning the interval. The detection of the Hercynian unconformity is crucial to the exploration of the Paleozoic rocks in the South Diyur Block because of the major differences in petroleum prospectivity between Mesozoic and Paleozoic sequences.
The four sequence boundaries are recognized wherever good-quality seismic data are available and correlate with the typical Paleozoic stratigraphic sequences of the Western Desert. In particular, the Hercynian unconformity and the basal Silurian boundary have important connections with the potential petroleum source rocks of the black Silurian ‘hot shale’ facies (see Figure 7).
Stratigraphic Framework of the Paleozoic Rocks
Abu El Naga (1984) wrote one of earliest reviews of the hydrocarbon habitat of the northern part of the Western Desert of Egypt based on published and unpublished reports by the oil companies Gupco, Wepco, GPC and Conoco. In his paper he used the names of the Paleozoic formations established in Libya. Gueinn and Rasul (1986) published a paper on the biostratigraphy of the Paleozoic of the Western Desert based on a commercial report by Paleoservices Ltd in 1986. Paleoservices had analyzed about 1,200 rock samples taken from 11 exploration wells and identified 15 Paleozoic biozones. Gueinn and Rasul (1986) named the zones (from oldest to youngest) as WD1 to WD15 and correlated them with biozones recognized in Algeria, Libya, Morocco, Spain and Turkey.
Dahi and Shahin (1992) described and named seven Paleozoic formations in a comprehensive monograph covering the oil and gas fields in the Western Desert of Egypt (see Figure 7). At the time, 30 wells had penetrated the Paleozoic formations. The Paleozoic formations comprise the Siwa and Faghur groups that had been established by Paleoservices in 1986. The Cambrian to Devonian Siwa Group consists of the Shifah, Kohla and Basur formations, and the Zaitoun (Zeitoun), Desouqy, Dhiffah and Safi formations make up the Middle Devonian–Permian Faghur Group.
Cambrian–Silurian Siwa Group (
Shifa, Kohla and Basur formations)
In the Western Desert, the Cambrian–Ordovician rocks are represented by the Shifa Formation.
Shifah Formation (EGPC, 1992)
Type location: Siwa-1 well (latitude 29°07′18″N; longitude 23°25′50″E) in Western Desert at depth of 3,058–3,374 m KB.
Lithology and paleoenvironment: Interbedded mudstones and siltstones, locally rich in pyrite and glauconite, intercalated with sandstones and conglomerates, locally rich in kaolinite. Predominantly continental.
Thickness: 315 m in Siwa-1 well; maximum thickness of 2,820 ft in Bahariya-1X well.
Age: Middle Cambrian (Solvan) to Middle Ordovician (Llanvirnian).
Boundaries: Conformably overlain by Kohla Formation.
Hydrocarbon potential: Potential sandstone reservoir.
Kohla Fomation (EGPC, 1992; Tawadros, 2001)
Type location: Zeitoun-1 well (latitude 29°14′43″N; longitude 25°43′32″E) at depth of 2,616–3,242 m KB.
Lithology and paleoenvironment: Mudstones and siltstones with minor sandstone and shale. Fluviatile, tidal flats and shoreface facies and transgressive shallow-marine.
Thickness: 626 m in Zeitoun-1; varies between 60 and 626 m in the Ghazabat Basin.
Age: Silurian (Llandoverian–Early Ludlovian) based on unspecified acritarchs. In Foram-1, an assemblage of leiospheres and other palynomorphs gives an age of possible Early Llandoverian. It is probably equivalent to the Tannezouft Formation in Libya.
Boundaries: Conformable on Shifah Formation; unconformably overlain by the Basur Formation.
Hydrocarbon potential: High gamma-ray values (‘hot shale’) indicate source rock potential. Potential gas-prone in Siwa basin.
Basur Formation (EGPC, 1992; Tawadros, 2001)
Type location: El Basur-1 well (latitude 29°54′23″N; longitude 25°50′08″E) in Western Desert at a drilled depth of 2,549–3,165 m.
Lithology and paleoenvironment: Interbedded siltstones and conglomerates. Regressive alluvial fans and braided streams.
Thickness: 616 m in El Basur-1.
Age: Silurian–?Early Devonian (Llandoverian–Ludlovian–?Geddinnian) based on palynology; includes palynological zones WD5 and WD6 of Gueinn and Rasul (1986).
Boundaries: Unconformable on Kohla Formation; overlain with sharp contact by Devonian Zeitoun Formation.
Hydrocarbon potential: Potential source-rock and reservoir facies.
Middle Devonian–Permian Faghur Group
(Zeitoun, Desouqy, Dhiffah and Safi formations)
Devonian strata are mostly variably indurated fine- to coarse-grained sandstone. The maximum thickness is 3,400 ft in well NWD 302-1. The presence of foraminifers, ostracods, conodonts, acritarchs and sparse megafossils, such as brachiopods, bryzoans and echinoderms, indicate marine depositional conditions. The upper boundary with the Carboniferous is paleontologically determined where there are no lithostratigraphic breaks. The Devonian is represented by the Zeitoun Formation.
Carboniferous lithologies are predominantly variably cemented, fine- to coarse-grained sandstones and interbedded shales and partly dolomitic limestone. The Carboniferous formations are the Desouqy and Dhiffah. The Safi Formation is of Permian age or possibly Early Carboniferous.
Type location: Zeitoun-1 well (latitude 29°54′23″N; longitude 25°50′08″E) in Western Desert at a drilled depth of 1,670–1,958 m.
Lithology and paleoenOironment: Predominantly sandstones and conglomerates consisting of upward-coarsening sequences with limestone near the base. Fluvial continental and shallow marine.
Thickness: 288 m in Zeitoun-1 and 759 m in Faghur FRX-1.
Age: Early–Middle Devonian based on acritarchs; in Foram-1 (2,547–2,550 m) palynomorphs indicate a Late Emsian–Early Givetian age.
Boundaries: Unconformable on Basur Formation and unconformably overlain by Desouqy Formation.
Hydrocarbon potential: In Foram-1, high-value gamma-ray dark shales are potential petroleum source rocks; possibly equivalent to Frasnian ‘hot shales’ in Algeria. Oil-prone in Siwa Basin.
Desouqy Formation (EGPC, 1992; Tawadros, 2001)
Type location: Desouqy-1 well (latitude 28°22′23″N; longitude 25°56′36″E) in Western Desert at drilled depth of 1,933–2,295 m KB.
Lithology and paleoenvironment: Fining-upward sequence of kaolinitic fluvial sandstones grading up into marginal marine, deltaic and prodeltaic siltstone and mudstone.
Thickness: 100–300 m (362 m in El Desouqy-1).
Age: Early Carboniferous (Tournasian–Visean).
Boundaries: Unconformable on Zeitoun Formation and conformably overlain by Diffah Formation.
Hydrocarbon potential: Potential source-rock and reservoir facies.
Type location: Dhiffah plateau on northern Libyan-Egyptian border; subsurface West Faghur-1 well (latitude 28°22′23″N; longitude 25°56′36″E) in Western Desert at depth of 7,365–8,665 ft KB.
Lithology and paleontology: Oolitic and bioclastic limestone interbedded with mudstone and shale; subsurface, mainly mudstone and siltstone grading up into sandstone with limestone and dolomite predominant northward.
Age: Carboniferous (Early Visean–Late Namurian).
Boundaries: Conformably overlies Desouqy Formation; conformably overlain by Safi Formation.
Hydrocarbon potential: Oil- and gas-prone in Siwa Basin.
Type location: Siwa-1 well (latitude 29°07′18″N; longitude 23°25′50″E) in Western Desert at drilled depth of 976–812 m.
Lithology and paleoenvironment: Coarsening-upward argillite and sandstone sequence overlain by carbonate unit. Prograding deltaic and near-shore sequence followed by shallow-marine deposition suggesting a marine transgression possibly related to deltaic subsidence.
Thickness: 155 m.
Age: Permian; but presence of Polytaxis could indicate an Early Carboniferous age.
Boundaries: Conformable on Dhiffah Formation; unconformably overlain by Bahrein Formation.
Hydrocarbon potential: Potential source-rock and reservoir facies.
Mesozoic units are not exposed in the South Diyur Block. However, several Jurassic and Cretaceous formations are predicted to occur subsurface (Figure 7). These Mesozoic formations form the southern rim of the Dakhla Basin.
Jurassic units in this area have yet to be drilled, evaluated and named. From evidence in the northern part of the Western Desert they may be clastic deposits, equivalent to the Middle Jurassic Bahrein Formation (see below) that are prospective hydrocarbon reservoirs.
Type location: Betty-1 well (latitude 29°40′N; longitude 27°45′E) in Western Desert.
Lithology and paleoenvironment: Fine- to coarse-grained red quartzose sandstone with thin pebble interbeds, siltstone and shale. Predominantly continental to marginal (coastal) marine paleoenvironments; locally, anhydrite indicates supratidal to lagoonal environment.
Thickness: Maximum thickness of 550 m in Betty-1.
Age: Early to Middle Jurassic (Bathonian–Bajocian in Zeitoun-1; Callovian in Bahrein-1). Terrestrially derived pteridophytic (fern)-dominated pollen and spores and sporadic marine dinoflagellates and microforams.
Boundaries: Unconformable on Paleozoic units; conformably overlain by Khatatba Formation (Said, 1990).
Hydrocarbon potential: Prospective reservoirs in Salam-3X well.
The Cretaceous strata that may be expected to occur in the subsurface consist of the predominantly clastic Six Hills, Sabaya and Taref formations that belonged to what was formerly known as the Nubian Sandstone, and the intervening shallow-marine Abu Ballas and Maghrabi formations. In addition, gradations of the Sabaya Formation (the Kharita Formation) and of the Maghrabi Formation (the Bahariya Formation/El Heiz Member) may also occur. The succession begins with clastic rocks of the Six Hills Formation that were deposited before the Aptian transgression and ends with the clastic Turonian Taref Formation.
Type location: Six Hills (latitude 24°21′N; longitude 29°15′E) about 100 km south of Mut in the Dakhla Oasis.
Lithology and paleoenvironment: Continental fluvial sandstone, paleosol; minor nearshore marine sandstone. Subsiding basin before the Aptian transgression advanced from the north.
Thickness: Up to 500 m in type area; elsewhere 600–700 m.
Age: Late Jurassic to Early Cretaceous; subsurface evidence (pollen and foraminifera) suggest a Late Jurassic age.
Boundaries: Transitionally overlain by the fluviatile Abu Ballas Formation.
Hydrocarbon potential: Potential reservoir facies.
Type location: Escarpment south of Abu Ballas (latitude 24°23′N; longitude 27°35′E).
Lithology and paleoenvironment: Shale, siltstone and sandstone; shallow marine transgression, probably of high salinity. Periods of continental deposition occurred during the marine transgression.
Thickness: 60 m.
Age: Early Cretaceous (Aptian). Lamellibranchs, brachiopods, gastropods and plant remains present but index fossils absent; Aptian age based on palynology.
Boundaries: Transitional with the underlying Six Hills Formation; unconformably overlain by the Sabaya Formation due to the epeirogenic Kharga uplift.
Hydrocarbon potential: Good reservoir potential.
Type location: Qulu El Sabaya hills on the Kharga-Dakhla road (latitude 25°21′N; longitude 29°43′E).
Lithology and paleoenvironment: Regressional fluviatile deposits. Basal 30 m of kaolinitic sandstone (paleosol) overlain by fluviatile sandstone. In the deeper parts of the Dakhla basin there may have been some marine incursions.
Thickness: 170 m in type area; maximum thickness of 200 m.
Age: Mid-Cretaceous (probably Albian to Early Cenomanian). A correlative unit in Ammonite-1 yielded a microflora of Late Albian–Early Cenomanian age.
Boundaries: Unconformably overlies the Abu Ballas Formation; transgressively overlapped by marine deposits of the Maghrabi Formation.
Hydrocarbon potential: One of the best aquifers of the Dakhla basin; potential hydrocarbon reservoir.
Kharita Formation (Said, 1990) mainly occurs in the northern part of the Western Desert but it might be present in the South Diyur Block. The Formation has also been named as the Kharita Member of the Burg El Arab Formation (Said, 1990). It is time equivalent with the Sabaya Formation.
Type location: Kharita-1 well (latitude 30°33′38″N; longitude 28°35′32″E) in Western Desert at drill depth 2,501–2,890 m.
Lithology and paleoenvironment: Fine- to coarse-grained sandstone with subordinate shale and carbonate interbeds; characterized by amorphous silica. Near-shore, high-energy, marine shelf environment; deeper-water deposition in northern areas.
Thickness: Maximum 1,100 m in Mersa Matruh-1 probably due to accumulation in a local graben.
Age: Mid-Cretaceous (Aptian–Albian–Cenomanian).
Boundaries: Conformable on Dahab (Shale) Member or unconformable on older units from Early Cretaceous to basement; underlies Bahariya Formation with marked lithologic change.
Hydrocarbon potential: Together with the overlying Dahab (Shale) Member, it shows the first development of depocenters in northern Western Desert. Oil and gas in Abu Gharadig, Alamein and Badr El Din fields.
Type location: The Abu Tartur plateau lying between the Kharga and Dakhla oases at about latitude 25°30′N; longitude 29°45′E.
Lithology and paleoenvironment: Interbedded claystone, siltstone and sandstone overlap the paleorelief on top of the Sabaya Formation. Basal fossiliferous (mainly angiosperms) flaser-bedded sandstone are overlain by less-fossiliferous shale, siltstone and sandstone. Beds of glauconitic sandstone indicate shallow-marine to tidal-flat sedimentation.
Thickness: About 60 m but thins over the Kharga uplift in the Baris area south of the Kharga Oasis.
Age: Plant remains suggest a probable Late Cretaceous (Cenomanian) age.
Boundaries: Overlaps the Sabaya Formation; unconformably overlain by the Taref Formation.
Hydrocarbon potential: Good reservoir potential.
Bahariya Formation (Said, 1990) occurs mainly in the northern part of the Western Desert but might be present in the South Diyur Block. It represents the southern extent of the Cenomanian transgression.
Type location: Outcrops in the Bahariya Oasis.
Lithology and paleoenvironment: Cross-bedded, coarse-grained, non-fossiliferous sandstone in the lower part are overlain by fine-grained, well-bedded ferruginous fossiliferous (vertebrates and marine fossils) clastics and an upper part that consists of fossilerous dolomite, sandy dolomite and calcareous grit. Sedimentation varied upward from fluviatile through estuarine to lagoonal.
Thickness: Exposed section is at least 170 m thick; average thickness in wells is 50–500 m; maximum thickness is 1,143 m in Kattaniya-1.
Age: Late Cretaceous (Late Cenomanian). Fossils include oysters and reptilian (lepidosaur) and dinosaur (sauropod and theropod) remains (Weishampel et al., 2004).
Boundaries: Base unexposed in outcrop and lower limit is questionable in Bahariya-1 well; conformably overlain by the Abu Roash Formation.
Hydrocarbon potential: Oil and gas in the Abu Gharadig, Razzak, Salam, WD-19 and Alamein fields; source rocks present.
El Heiz Member (Said, 1990). This unit may be the upper member of the Bahariya Formation and equivalent to the ‘G’ (lowest) member of the Abu Roash Formation; formerly given formational status by Akkad and Issawi (1963).
Type location: Outcrops in the Bahariya Oasis.
Lithology and paleoenvironment: Interbedded fossiliferous carbonates (dolomite, sandy dolomite and calcareous grits) and clastics.
Thickness: About 20 m.
Age: Late Cretaceous (Late Cenomanian).
Boundaries: Upper member of the Bahariya Formation.
Hydrocarbon potential: Good reservoir potential.
Type location: Gebel Taref in the north Kharga depression (about latitude 25°32′N; longitude 30°29′E).
Lithology and paleoenvironment: Typical ‘Nubian’ sandstone. Thick tabular-planar cross-bedded sandstone probably formed on a prograding alluvial plain during the Santonian regression. Thin interbeds of clay and shale. Minor intertidal influences.
Thickness: At least 100 m at type locality.
Age: Late Cretaceous (Turonian–Santonian)
Boundaries: Probably coeval with the shallow-marine Abu Roash Formation of the northern part of the Western Desert.
Hydrocarbon potential: Good reservoir potential.
The currently available seismic data for the South Diyur Block consists of 1,175 line-km of 2-D seismic data acquired by Continental Sahara Oil Company, probably in 1978. Figure 9 shows the seismic coverage. Seismic data quality is fair in the western part of the block but inadequate in the east where only Line-104 and its extension L-4 are present. Extensive sand cover in the north and southeast of the block has caused a deterioration of seismic quality. The Ammonite-1 well was drilled in 1979 on a pronounced seismic high immediately outside the southwestern corner of the South Diyur Block in an area of fair- to good-quality seismic data. This quality of seismic data enabled the drilling results from Ammonite-1 to be utilized in seismic interpretation and the delineation of the geoseismic sequences.
The seismic data has been reprocessed by WesternGeco, Cairo on behalf of Gujarat State Petroleum Corporation Ltd (GSPC). Of the 20 reprocessed lines, 14 trend north-northwesterly and the others are NE-trending tie lines. Data was interpreted using a Petrel Workstation with optional utilization of SMT’s Kingdom software. The quality of the reprocessed 2-D seismic data ranges from fair to good for seismic interpretation and mapping in the western and southwestern parts of the block. However, because of low frequency from datum to 2,100 msec (average two-way-time, TWT), below 2,000 msec the data quality rapidly deteriorated and became difficult to interpret due to poor reflectivity from the interpreted acoustic basement. In the north-central and eastern areas of the block, the poor quality of seismic data is probably due to the thick limestone cover and the presence of extensive sand dunes.
Interpretation was also made difficult by the lack of tie lines except in the zone extending northeast from the mid-western to mid-northern boundaries of the block (Figure 9). In order to rectify this lack of information, in-fill 2-D seismic lines will need to be acquired following the integration of the present seismic data with the recently acquired aeromagnetic-gravity surveys.
As the best quality north-northwest trending seismic lines occur in the western part of the block, L-1, L-107/L-7, L-116/L-6 and L-102/L-2, together with tie lines L-126 and L-122B (Figure s 10 through 15), this area was selected for seismic interpretation together with the determination and definition of traceable seismic horizons. Currently, four seismic boundaries (SB-1 to SB-4) have been recognized wherever the quality of data has allowed. They have been tied into the Ammonite-1 lithostratigraphy and designated as the following sequence boundaries (Figure s 8 and 10):
Sequence Boundary-1 (SB-1) is a good geoseismic reflector within the Jurassic–Cretaceous section that coincides with the ‘Desert Roses’ unit intersected in Ammonite-1.
Sequence Boundary-2 (SB-2) is Hercynian (Late Devonian–Early Carboniferous) unconformity. This important sequence boundary overlies deeply eroded Paleozoic sequences and possibly marks the Late Paleozoic basin inversion that occurred in North Africa and the Middle East.
Sequence Boundary-3 (SB-3) interpreted near to the base of the Silurian. It marks the seismic-boundary near or at the base of the prolific Silurian black ‘hot shale’ petroleum source rock that is present throughout Algeria, Libya and the Middle East.
Sequence Boundary-4 (SB-4) interpreted as a major unconformity near the top of the Upper Proterozoic sedimentary and metasedimentary section that overlies crystalline basement. This geoseismic horizon was misinterpreted as Precambrian acoustic basement in Ammonite-1. It overlies an interpreted Late Proterozoic phase of basin formation due to block faulting.
The four sequence boundaries bound five corresponding isochron mappable seismic sequences SS-1 to SS-5.
Seismic Sequence-1 (SS-1): from surface to SB-1 and characterized by subparallel seismic stratification. In Ammonite-1, this section is fresh-water bearing and consists of interbedded sandstone and shale (sandstone is dominant).
Seismic Sequence-2 (SS-2): from SB-1 to SB-2 and characterized by parallel to subparallel seismic stratification. As with SS-1, in Ammonite-1, this section is fresh-water bearing and consists of interbedded sandstone and shale in approximately equal proportions.
Seismic Sequence-3 (SS-3): from SB-2 to SB-3 and directly underlies the regional early Hercynian unconformity. It is truncated by the unconformity in the central area of the block but preserved in the southern area. The seismic sequence is characterized by semi-transparent seismic facies and most probably belongs to the thick Silurian shale sequence.
Seismic Sequence-4 (SS-4): from SB-3 to SB-4. It directly underlies the Silurian sequence and is characterized by truncation at the unconformity surface in the central area of the block and preservation in the southern area. The SS-4 seismic sequence consists of parallel seismic stratification and deep channeling. It is interpreted as of Latest Ordovician–Earliest Silurian age and marks a period of Late Ordovician polar glaciation that is evident in North Africa and the Middle East, including Saudi Arabia.
Seismic Sequence-5 (SS-5): occurs below SB-4 and is marked by partial subparallel seismic stratification and block faulting. It probably belongs to the Upper Proterozoic block faulting together with pull-apart basins formed during a major phase of Pan-African deformation. Similar seismic geometries and facies in the Kufra Basin in southeast Libya (Forum-Geopex Ltd, unpublished report to Libyan National Oil Company, 1993-94) and in large parts of the Arabian Plate including the Upper Proterozoic basins and prolific petroleum system in Oman.
The NW-trending seismic line L-116/L-16 (Figure 12) shows the four seismic boundaries SB-1 to SB-4 and their encompassed seismic sequences SS-1 to SS-5. The marked change in the seismic isochrons that group SB-2 (purple) with SB-4 (yellow) and SB-1 (light blue) with SB-2 (purple) could mark a Late Paleozoic basin-inversion phase that matches the Paleozoic mega-regional correlations in both North Africa and the Middle East.
The geoseismic boundaries recognized in L-116 can be correlated with seismic lines elsewhere within the South Diyur Block. It is apparent that the southern region of the block demonstrates a more complete geoseismic sequence to the northwest of Common Depth Point (CDP)-940. In addition, a reduction in the thickness of the isochron groups below SB-2 is associated with truncation to the southeast of CDP-940 and is considered to be direct evidence of a regional unconformity at the SB-2 geoseismic boundary.
As noted above, there is a marked variation in the quality of the seismic data between the western and eastern parts of the South Diyur Block, and only a limited number of tie lines. As a result, mapping of detected and correlated seismic sequence boundaries was possible only in areas of fair to good seismic quality.
Also, as noted above, Figures 10 to 13 are the interpreted seismic lines in the South Diyur Block in sequential order from west to east, and Figures 14 and 15 are tie lines. The seismic boundaries and their encompassed seismic sequences are indicated, correlated and tied, then mapped using all available seismic lines in the block. In addition, the key Ammonite-1 exploration well is projected and tied to the L-1 seismic line on Figure 10.
Ammonite-1 was drilled on a pronounced seismic high (Figure 10). A thin seismic sequence (SS-3), which is possibly the Silurian shale source-rock facies is preserved below the regional unconformity, SS-2, most of the upper section of SS-3 having been eroded. No glacial channels (apparent elsewhere in SS-4) were intersected in the well.
REGIONAL PETROLEUM POTENTIAL
The South Diyur Block is located at the boundary between the Northern and Southern Provinces in the Western Desert of Egypt. The Northern Province from latitude 28°N to the Mediterranean shoreline is operated under the administration of the Egyptian General Petroleum Corporation (EGPC). The Southern Province extends from the latitude 28°N south to the Egyptian-Sudanese border and is administered by Ganob El-Wadi Holding Company (GANOPE).
In the Northern Province, the petroleum habitat and petroleum systems are well understood and defined with several petroleum discoveries having been made, mainly in the Cretaceous and Jurassic sequences. Proven petroleum systems relate to traps in fault blocks that were formed during Jurassic extension normal faulting; the faulting continued through the Cretaceous to Late Cretaceous regional basin inversion (Moustafa et al., 2003).
In contrast, the petroleum habitat of the Southern Province is not well understood or properly assessed as a result of the very minimum exploration activities in the fluviatile-dominated sedimentary rocks of the uplifted and deeply eroded Cretaceous and Jurassic sequences. These Cretaceous-Jurassic sequences are important groundwater aquifers in the oases of this part of the Western Desert.
To date, the only presence of oil in the Southern Province of the Western Desert was found by Repsol in Komombo-1 drilled in 1997 in Upper Egypt (Dolson et al., 2001). The Komombo oil discovery is located in the vicinity of the Nile Valley and interpreted to be in a Lower Cretaceous rift system that may be similar to those in the Sudan. In addition, the large-scale regional Paleozoic petroleum habitat and occurrences in both the Arabian Plate and North Africa suggest the possibility of similar petroleum systems existing in both provinces of the Western Desert of Egypt due to the known occurrence of deeply buried Paleozoic sequences in the Northern Province (Mahmoud et al., 2000).
Western Desert (North)
Abu El Naga (1984) reviewed the hydrocarbon habitat of the northern part of the Western Desert based on published papers and unpublished reports of oil companies such as Gulf of Suez Petroleum Company (Gupco), Western Desert Operating Petroleum Company (Wepco), General petroleum Company (GPC) and Conoco.
An increasing proportion of the Egyptian oil and gas production comes from the Western Desert. In the late 1990s, oil production in the Western Desert accounted for about 16% of Egypt’s total oil production. The gas fields produced 400 million cubic meters per day (MMcmpd), an amount that accounted for 30% of the country’s total production. The Western Desert still has a significant hydrocarbon potential as indicated by recent oil and gas discoveries in EGPC northern province. Exploration is ongoing in the Faghur and Abu Gharadig basins where, in September 2011 the Apache Corporation reported two new commercial discoveries of oil. One was in the Alam El Buieb Formation and the other in the lower part of the Bahariya Formation, both of Early to Late Cretaceous age. A production test in the Alam El Buieb Formation in Tayim South 1-X well flowed at a rate of 8,196 barrels of oil per day. The Lower Bahariya Formation in the AG-90 development well flowed 5,200 barrels of oil and 5 million cubic feet (MMcf) of natural gas per day during a completion test. In 2010, Apache reported oil discoveries in the Jurassic Safa Formation. The Apache Corporation has drilled 13 exploratory wells on a 40-km long east-west fault trend, of which 11 have been successful discoveries. As of September 2011, the Abu Gharadig Field produced daily totals of 24,000 barrels of oil and 60 MMcuf of gas.
Western Desert (South)/Kufra Basin
The Kufra Basin of Libya extends northeastward into the western part of the Western Desert of Egypt. The South Diyur Block is located 700 km north-northeast of the Libyan Kufra Basin (Figure 2a). To date, no oil or significant oil shows have been found in the Kufra Basin south-southwest of the South Diyur Block (Hallet, 2002). Although there are suitable reservoirs and structures within the basin, there appears to be an absence of suitable source rocks. Both field work and the limited well data have indicated that in general the facies of the Tanezzuft Formation (the rich ‘black shale’ of Early Llandovery–Middle Rhuddanian age) that is the principal petroleum source rock in North Africa, was not favorable to the development of a ‘hot shale’ facies; sedimentological data support this conclusion.
However, the investigations have not ruled out the possibility of ‘hot shales’ being present in local depocenters, in the same way that ‘hot shales’ are preserved in local depressions in the Murzuq Basin of Libya and also on the Arabian Plate. The question then arises as to the possibility of the maturity of the shales if they are present. Lüning et al. (2003), having reviewed the burial history of the basin, concluded that the Tanezzuft shales probably reached maturity in the center of the Kufra Basin. Using the analogy of Oman along with Mali and Algeria in northwest Africa, they also suggested that potential source rocks could be present in Neoproterozoic rocks preserved within basement graben structures. This is unproven, and the probability is that such source rocks are not present in the Kufra Basin. However, this does not preclude the possibility that potential source rocks might be present in basinal deeps within the South Diyur Block.
A remnant Neoproterozoic to Lower Cambrian basin containing sedimentary rocks at least 1,500 m thick has been interpreted from seismic data in the Kufra Basin of southeastern Libya. The ancient sediments have a markedly unconformable relationship below the relatively uniform and flat-lying Paleozoic succession (Craig et al., 2008). The basin has been interpreted as an erosional remnant of a once much more extensive Upper Proterozoic to Lower Cambrian succession, possibly preserved in a graben or a pull-apart basin. The succession has not been drilled but appears to consist of three distinct stratigraphic units that are truncated by two well-defined unconformities in its upper part. These unconformities have been interpreted as representing the two main phases of Pan-African compression that are present in the Taoudenni Basin in Mali and elsewhere (Craig, 2012).
SOUTH DIYUR BLOCK PETROLEUM POTENTIAL
As yet, no major petroleum reserves have been found in the southern region of the Western Desert (Dolson et al., 2001). Significant exploration only began in the 1990s and is being continued by several oil companies. To date, little exploration drilling has been focused on the petroleum potential of the Paleozoic or Upper Proterozoic sequences. This may be due to an incorrect assumption that most of the Paleozoic petroleum source sequences, particularly the Silurian black ‘hot shale’, were not deposited in Egypt (Dahi and Shahin, 1992). The key exploratory wells (see Figure 1) that intersected Paleozoic rocks in the vicinity of the South Diyur Block are Ammonite-1 (drilled by Conoco in 1979), Foram-1 (Conoco in 1979), and El-Desouqy-1 (General Petroleum Company in 1972). The drilling results from the three wells are discussed in this paper. In addition, Moustafa et al. (2003) discussed the drilling results of Bahariya-1, which is another key exploration well that was drilled by Sahara Petroleum Company in 1957 in the Bahariya Oasis (see Figure 1 for well location).
The reinterpretation of the lithologic data from the three wells showed that the porous Mesozoic (Jurassic–Cretaceous) units are fresh-water bearing; accordingly, their structural-trap potential should be downgraded. In addition, the lack of effective seals within the Mesozoic sequences confirms the possibility of recent fresh-water flushing. In contrast, the abundance of relatively thick shale units within the Devonian–Carboniferous and Silurian sections could act as effective top seals for any potential underlying reservoir facies.
Upper Proterozoic Petroleum Habitat
Al-Husseini (2000) presented a model, which showed that the prolific Najd syn-rift petroleum system of the Arabian Plate (Huqf Supergroup of Oman) is more widespread than was generally suspected. Two examples of the system are the frontier undrilled salt basins in the western Rub’ al-Khali of Saudi Arabia (Dyer and Al-Husseini, 1991; Faqira and Al-Hauwaj, 1998), and the Phillips-operated Ghudun Basin in southwestern Oman (Blood, 2000). The Al-Husseini (2000) extensional model also implied that grabens between the great anticlines of eastern Arabia may have thick syn-rift sequences, including salt.
Craig (2012) stated that the main Neoproterozoic hydrocarbon play in the Taoudenni Basin in Mali is the Atar Group stromatolitic limestone reservoirs. Hydrocarbons were sourced laterally and vertically from organic-rich shales deposited in restricted sub-basins between the stromatolitic carbonate complexes and are sealed by the interbedded shales. However, the subsurface portion of the Taoudenni Basin remains extremely under-explored.
At the time of writing, the Upper Proterozoic sequence in the South Diyur Block was inferred on the basis of an interpretation of the available 2-D seismic data. The sequence underlies the defined Paleozoic geoseismic sequences and the seismic data indicated block faulting together with sedimentary and possible metasedimentary basin- and graben-fill deposits. Stratified and parallel seismic facies are recognized within the basin- and graben-fill geometries; however, chaotic to non-seismic facies characteristics suggest possible up-thrown and deeply eroded basement blocks related to the major bounding faults.
The top boundary of the Upper Proterozoic sequence is defined as SB-4, which is interpreted as a major unconformity surface that is easily recognizable on seismic data. It indicates a possible four-way dip closure of considerable size in the central-northern area of the South Diyur Block (see Figure 12). It is thought that the presence of prolific petroleum source rocks similar to the sequences in Oman is unlikely due to considerable distance of the South Diyur Block from the Paleo-Tethys during that time. Perhaps the Upper Proterozoic sequence in South Diyur would show similarities with similar sections found in Jordan and eastern Saudi Arabia.
Paleozoic Petroleum Habitat
In addition to the considerable petroleum reserves discovered in the Upper Proterozoic sequences in Oman, the discovery of the prolific gas reserves in the Permian Khuff reservoirs in the Arabian Gulf and Zagros regions in the early 1970s marked another major phase of Permian Khuff and Upper Proterozoic exploration in the Arabian Plate (Konert et al., 2001).
The interpretation of geoseismic data from the South Diyur Block has revealed a large-scale channel-fill system at SB-3 (see Figure 10) that is considered to be a series of Late Ordovician–Early Silurian glacial channels. The paleolatitude of North Africa at the time precludes the possibility of the channels being the result of glaciation that took place during the latest Ordovician–earliest Silurian time. In North Africa and on the Arabian Plate, the Upper Ordovician–Lower Silurian indurated preglacial deposits, together with the less consolidated Upper Ordovician deposits, are considered an excellent reservoir facies that directly underlies the petroleum source rocks of the Lower Silurian black ‘hot shale’.
Konert et al. (2001) suggest that in addition to the sedimentary facies patterns, the prospectivity of the Paleozoic sequences is largely determined by the pre- and post-Hercynian burial and thermal histories. These dramatically impact on reservoir quality and the availability of hydrocarbons. Therefore, a non-traditional approach is unavoidable to constrain thermal histories due to the complex burial/uplift history. Although porosity might be largely destroyed during the deep burial of the section, it was locally preserved due either to the presence of an early diagenetic phase, or to early emplacement of hydrocarbons. Moreover, Konert et al. (2001) added that secondary porosity was selectively created in thin carrier beds by leaching during fluid flow.
McGillivray and Husseini (1992) indicated that the drilling by Saudi Aramco in the late 1980s and 1990s resulted in the discovery of several large fields with light oil and sweet gas in the previously unexplored central province of Saudi Arabia, which marked another advanced phase of Paleozoic pre-Permian exploration in Central Arabia. In comparison to the producing fields of eastern Arabia that have a Mesozoic source and reservoirs, the newly discovered fields of Central Arabia consist of Paleozoic reservoirs with a Silurian source rock (McGillivray and Husseini, 1992).
Konert et al. (2001) stated that hydrocarbons were mainly derived from the prolific Silurian ‘hot shale’ that extends over most of the basin. Tectono-stratigraphic relationships indicate that the Arabian platform southwest of the Zagros Suture was generally stable until the Hercynian Orogeny that began in the latest Devonian and climaxed in the Early Carboniferous. The Orogeny is manifested by regional upwarps (Syria, Central Arabia and Oman) and sags (Palmyra and Rub’ al-Khali), and narrow N-trending basement-cored uplifts (e.g. Ghawar Field). In the Early Permian, rifting along the eastern margin led to the opening of the Neo-Tethys Ocean.
Abu-Ali and Littke (2005) concluded that the only proven source rock for the Paleozoic hydrocarbons in Eastern and Central Saudi Arabia is the Early Silurian basal ‘hot shale’ of the Qusaiba Member of the Qalibah Formation (Abu-Ali et al., 1991, 1999, 2001; Mahmoud et al., 1992; McGillivray and Husseini, 1992; Cole at al., 1994; Jones and Stump, 1999). The basal Qusaiba contains type-II organic matter with a ‘hot shale’ thickness ranging from 10 to 250 ft (3–70 m) (Mahmoud et al., 1992; Wender et al., 1988; Abu-Ali et al., 1999, 2001). Carbon isotopes strongly correlate hydrocarbons found in the Devonian, Carboniferous and Permian reservoirs to the Qusaiba source rock extracts. Biomarker data also clearly distinguishes the Silurian shales as the main source rock for the Paleozoic oils and condensates (Abu-Ali et al., 1991; Moldowan et al., 1994; Cole et al., 1994). The Silurian source rock is also well established in North Africa (Yahi et al., 2001; Lüning et al., 2000) and reflects a regional transgression that followed the deglaciation of Gondwana.
Boote et al. (1998) concluded that large volumes of hydrocarbons have been generated, trapped and sometimes dispersed on the northern part of the African Plate since the Late Paleozoic (Figure 17). The petroleum systems were charged by the Lower Silurian Tanezzuft shale, the chronostratigraphic equivalent of the Qusaiba ‘hot shale’ source rock on the Arabian Plate (Mahmoud et al., 1992) and Upper Devonian source rocks. They developed around discrete sedimentary depocenters, first in the Paleozoic and later during the Mesozoic and Early Tertiary. Most of the hydrocarbons generated during the Paleozoic were dispersed by the later Hercynian uplift and deep erosion.
Similar to the situation on the Arabian Plate, Boote et al. (1998) reported that in this structurally simple region, highly continuous reservoirs and seals are intimately associated with excellent source rocks. The productivity of active petroleum systems vary according to migration style and effectiveness of migration focusing, degree of impedance and stage of maturity. The petroleum systems range from those of enormous accumulations (Hassi Massaoud and Hasi R’Mel fields) that are characterized by broad regional culminations with very efficient migration focusing and high-impedance entrapment style, to small scattered pools now largely dispersed by hydrodynamic flushing (Hamra Basin systems).
According to Boote et al. (1998), each of these systems is ephemeral in various stages of an evolutionary cycle from initial genesis, to maturity, destruction and final extinction. The Triassic–Early Jurassic evaporite-sealed systems of the northern ‘Triassic Basin’ in North Africa, are all in the mature phase of evolution. Petroleum systems are either in early (oil-dominant) maturity or possibly now in late (gas-dominated) maturity because of late trap development; these are similar to situation on the Arabian Plate (Wender et al., 1988; McGillivray and Husseini, 1992; Konert et al., 2001; Abu-Ali and Littke, 2005).
In summary, Boote et al. (1998) defined three categories of Paleozoic petroleum systems together with one potential Upper Proterozoic to Lower Cambrian petroleum system (Figure 17) in the North African basins:
Mesozoic to Early Tertiary charged systems with Triassic–Jurassic shale and evaporite seals in the Mesozoic sag or ‘Triassic’ Basin of the northern Sahara Platform. These are reported to include about 75% of the total discovered reserves with approximately 55% in the supergiant oil fields, such as Hassi Massaoud and Hasi R’Mel.
On basis of the currently available information and data from the South Diyur region, this class of petroleum system is not expected to be present in the South Diyur Block.
Mesozoic to Early Tertiary charged systems with intra-Paleozoic shale seals in basins to the south and east of the Triassic Basin. These include about 20% of the total discovered reserves, mostly in the prolific Illizi Basin in eastern Algeria (see Figure 2a).
Due to the fresh-water hydrodynamic flushing of the Mesozoic sequences in the South Diyur region, this class of petroleum system is not expected to be present in the South Diyur Block.
Largely inactive Paleozoic charged systems with intra-Paleozoic seals in basins in southwest Algeria and Morocco contain about 5% of the discovered reserves. In this category of petroleum systems, the assessment of the deformation that resulted from the Hercynian Orogeny and the related basin inversion are extremely important.
Based on the preliminary interpretation of the existing seismic data along with Ammonite-1 drilling results and dating, this class of petroleum system is expected to be present in the South Diyur Block.
The Upper Proterozoic to Lower Cambrian potential petroleum system, which has been proved to be valid in both the Taoudenni Basin in Mali and the Salt Basin in Oman, has good seismic expression in the Kufra Basin in southeast Libya but has not been tested.
Detailed interpretation of Seismic Sequence-5 (SS-5) indicated the presence of block faulting together with possible graben to pull-apart basins similar to those described in the Kufra Basin (Craig, 2012). The Upper Proterozoic to Lower Cambrian succession is expected to be present in the South Diyur Block and the presence of an associated petroleum system is inferred. This interpretation will need to be tested by stratigraphic-exploratory drilling.
Potential Source Rocks
Lower Llandovery–Middle Rhuddanian
According to authors such as Mahmoud et al. (1992), McGillivray and Husseini (1992), Lüning et al. (2003) and Fello et al. (2006), the Early Silurian in North Africa and Arabian Plate was characterized by the widespread deposition of organic-rich shales in euxinic paleo-depressions due to the rapid glacio-eustatic rise in sea level that resulted from the deglaciation of the Late Ordovician–Early Silurian ice cap. The shales are an important hydrocarbon source rock throughout the entire region and can be easily detected in well logs by gamma-ray spectroscopy due to the strong uranium-related natural radiation (Mahmoud et al., 1992).
The Silurian clastics grade from very fine- to fine-grained detrital sediments of an outer to inner shelf environment, to fine- to medium-grained deposits of a marginal marine environment. The progradational nature of the Silurian sequence and its restriction to the Lower Silurian in Saudi Arabia are both consistent with diachronism as demonstrated in northwest Africa, which occupied a similar strike position to Arabia along the broad shelf of Gondwana.
The distribution of the Silurian organic-rich shales in the Saharan outcrops is poorly known because surface weathering has commonly destroyed the organic matter and the black color of the shales, making it difficult to identify the previously organic-rich unit in the field. Graptolite biostratigraphic data suggest that the anoxic event was centered in Early Silurian–Middle Rhuddanian times, with more-oxygenated conditions and the deposition of organically leaner shales having commenced sometime during the Late Rhuddanian.
Silurian shales are also exposed in the Kufra Basin in southeast Libya (Lüning et al., 2003). The basal shale interval was exposed in a locality in Gebel Eghei on the western margin of the basin. Natural uranium radiation values of particular horizons in this basal interval are up to double (ca. 10 ppm) the normal lean shale baseline values (ca. 5 ppm), as measured in the organically lean middle and upper parts of the Silurian Tanezzuft Formation in the same area. This elevated uranium content is interpreted as a relic of the radioactive ‘hot shale’, and therefore can be used to identify the unit at exposure even though the original organic matter is now oxidized. In the same strata circular structures (20 mm diameter) with a fine radial structure were seen that Seilacher (2001) interpreted as leached-out pyrite disc shadows, characteristic of originally organic-rich shales.
The North African-Arabian Plate mega-regional correlation of the Paleozoic chrono- and tectono-stratigraphic sequences (Figure 16) indicated that the possible deposition sites of the Lower Llandovery–Middle Rhuddanian ‘hot shale’ petroleum source rocks are within shelfal euxinic paleo-depressions that could exist in the South Diyur Block. To further assess the exploration potential on the exploration acreage, an aeromagnetic and airborne gravity survey of the entire block has been acquired; in addition, an extensive 2-D seismic survey of 4,490 line-km is currently in progress.
Interpretation of the aeromagnetic and airborne gravity data has confirmed a high level of correlation between the reduced-to-pole (RTP) (Figure 18) and the residual Bouguer gravity values (Figure 19) in the sub-basins near the southern and the eastern sides of the block. Other recognized features indicated further confidence in the interpreted basins and highs in the South Diyur Block. The southern basinal area of the block is interpreted as having a confined geometry and, accordingly, may be capable of accommodating a thick Upper Proterozoic sedimentary section together with Paleozoic and Mesozoic sequences.
In Libya, a second petroleum system in the Ghadamis Basin occurs within the Middle and Upper Devonian succession (Hallet, 2002). Several shales have source-rock potential in this interval including the Emghayet Shale (Eifelian) with a total organic content (TOC) of about 1.0% and radioactive calcareous shales of Frasnian age in the Cues Limestone that have TOC values of up to 10%. Oil derived from the Middle and Upper Devonian source rocks is contained in sandstone reservoirs of the Devonian Awaynat Wanin Formation, but has also leaked into younger formations. The geochemistry of the Middle and Upper Devonian source rocks was discussed by Hallet (2002).
Hallet (2002) surmised that in Libya the Devonian source rock is less extensive than the Silurian Tanezzuft Shale and is less mature; TOC values average 2.0% to 4.0% and kerogen is types I and II. Maturity ranges from late mature in the basin center to immature on the margin. The source rock is high in paraffin and low in aromatics. Peak petroleum generation was during the Eocene.
In Saudi Arabia, Abu-Ali and Littke (2005) has shown there are isotopic correlations between Paleozoic hydrocarbons and other potential source rocks, but of lesser importance than the rich Lower Silurian petroleum source rocks. These include, but are not restricted to, the Ordovician Ra’an and Hanadir shales, Upper Devonian shales, Permian Unayzah shales, and Permian basal Khuff shales (Abu-Ali et al., 1991; Cole et al., 1994). They are of limited thickness and lateral extent and have not been thoroughly sampled and analyzed. Deeper Neoproterozoic units have not been penetrated in Saudi Arabia, but may be source rocks as in Oman (Grantham et al., 1987). Proterozoic–Cambrian-age basins have been identified from seismic data in the western Rub’ al-Khali Basin.
Following the evidence from North Africa and the Arabian Plate, the presence of potential Ordovician and Upper Devonian petroleum source rocks should not be excluded in the South Diyur Block. In Foram-1 and El Desouqy-1 exploration wells (see Figure 6), the Devonian sequence gave high gammaray responses, which may indicate the presence of potential petroleum source-rock facies; however, validation of the data is required.
To date, Upper Proterozoic or Paleozoic petroleum reserves have yet to be proved in the Western Desert of Egypt. This could be because of the deficiency in the mega-regional understanding of the prospectivity of the Paleozoic sequences. This prospectivity is largely determined by its facies and by the pre- and post-Hercynian burial and thermal histories, both of which dramatically impact reservoir quality and availability of hydrocarbons (Konert et al., 2001). In addition, most of the wells drilled in the Western Desert are within Mesozoic basinal areas that formed on deeply eroded Hercynian highs. Accordingly, most of the Paleozoic sequence was removed during the Hercynian basin inversion. Previously, this sequence was erroneously interpreted as not having been deposited (Dahi and Shahin, 1992). Moreover, the deposition of the Lower Llandovery–Middle Rhuddanian rich petroleum source rocks within restricted euxinic shelf depressions, rather than as blanket shelf shale sequences, is now recognized.
Reservoirs and Seals
On basis of the available drilling data, clastic reservoir-rock facies are thought to have a widespread distribution in the South Diyur Block. Paleozoic sandstone reservoir facies probably occur in sequences of Cambrian, Cambrian–Ordovician, Devonian, and/or Carboniferous age. Additionally, porous and permeable Upper Proterozoic rocks might act as reservoir units given the appropriate petroleum system requirements and effective petroleum migration pathways.
The Late Ordovician–Early Silurian glacial deposits are probably the best potential reservoir facies, especially where overlain directly by Lower Silurian shales that act as a top seal. Similarly, Devonian and Carboniferous shales provide seals to petroleum accumulations in underlying sandstone reservoirs.
As Hallet (2002) postulated with regard to the Murzuq Basin of Libya, petroleum generated from the potential Lower Silurian black shale source rocks was expelled and migrated directly into the underlying subjacent Upper Ordovician glacial sandstone reservoir. The incised Ordovician glacial valleys filled with periglacial clastics may have provided additional migration pathways. In addition, petroleum might be trapped in a variety of structures, including Paleozoic and possibly ‘non-flushed’ protected Mesozoic structures, some of which might be affected by reverse faulting, and remnant paleo-topographic structures of possible glacial origin draped with Silurian shales. Subsequent diagenesis may also have produced local traps (Hallet, 2002).
Ammonite-1 is the nearest well to the South Diyur Block. The high sand content of the entire Mesozoic section in the well suggests the lack of reasonable seals. It also has a negative impact on structural plays due to the poor retentive capability of laterally juxtaposed strata.
Structural Deformation and Trap Formation
On the basis of recent seismic and aeromagnetic-gravity interpretations in the South Diyur Block, five periods of structural deformation were recognized, including two major phases of deformation that were identified by Moustafa et al. (2003) in the Gebel Radwan–Gebel El Hefhuf–El Harra structural belt in the Bahariya-Farafra region. The deformational phases are expected to have had an impact on structural trends and petroleum traps in the South Diyur Block. Accordingly, further investigations and assessment are needed after the acquisition of the new regional 2-D seismic data and the drilling of the first exploratory and stratigraphic well in the South Diyur Block. The deformational episodes are as follows:
The earliest deformation is of Late Proterozoic age and was the strongest phase. It probably occurred following the deposition of the Late Proterozoic clastics and their carbonate interbeds. The sedimentary succession is equivalent to the productive Huqf Supergroup of Oman, the Jibalah Group of Saudi Arabia, and the Saramuj conglomerate of Jordan (Al-Husseini, 2000, 2011). This deformational phase is related to the Pan-African regional tectonic events and led to the formation of tilted and deeply eroded Upper Proterozoic faults blocks that might contain sedimentary, metasedimentary, and igneous rocks.
The oldest unconformity is a mid-Silurian to Middle Devonian hiatus in Iraq and Syria. It reflects an episode of epeirogenic uplift that is probably related to the mid-Silurian rifting of the Hun Superterrane (Ruban et al., 2007) on the northern margin of Gondwana. This depositional hiatus caused the south-to-north diachronism of the Silurian sequences in both North Africa and the Arabian Plate (Mahmoud et al., 1992). In the South Diyur region it is probable that the upper boundary of the Lower Silurian sequences is a depositional disconformity due to the absence of the Upper Silurian deposits.
The intermediate and extremely important mid-Carboniferous unconformity, which is sometimes correlated to the Hercynian Orogeny (Ruban et al., 2007). The deformation caused major periods of basin inversion, deep erosion and the removal of thick Paleozoic sections from inverted basins, transpressional to reverse movements on normal faults, and the possible dispersion of the early generated petroleum during the Paleozoic.
The unconformity is interpreted as occurring at Sequence Boundary-2 and shows truncation of the Late Paleozoic sequences (see Figure 12). It is probable that the uplifted Hercynian areas and inverted basins were the site of later collapsing and downwarping and the consequential formation of most of the Mesozoic basins in North Africa and the Arabian Plate (Mahmoud et al., 2000).
According to Moustafa et al. (2003) a strong deformational phase took place after the deposition of the Campanian El Hefhuf Formation and before the deposition of the Middle Eocene Naqb Formation in the Bahariya region. Right-lateral strike-slip movement is indicated by sub-horizontal slickensides on some faults in the structural belt, and by the right-stepped en echelon folds that make acute angles with the fault segments. This major Late Cretaceous period of uplift and erosion is expected to contribute to the structural maturity of potential petroleum traps in South Diyur Block.
Moustafa et al. (2003) concluded that the latest phase of deformation was post-Middle Eocene in age as indicated in the Farafra area at the eastern end of the Gebel Tobog Fault in the Bahariya Oasis where the Naqb Formation in the eastern plateau to the north of Naqb El Harra is faulted. The sense of slip on the faults was not easy to determine because of the lack of Eocene exposures within the structural belt. This period of deformation was concurrent with the early opening of the Gulf of Aden followed by the opening of the Red Sea. It is believed that this deformational period has led to the continued uplift and erosion of the Bahariaya-Farafra region, including the South Diyur Block.
The five phases of deformation are expected to occur in the South Diyur Block. It is therefore important to assess the structural integrity of potential structural traps particularly during the Hercynian and later deformational episodes. In addition, the possibility of paleogeomorphic and stratigraphic traps occurring in the South Diyur Block should be considered and permeability barriers may also form traps.
In 2010-2011, the Egyptian Nuclear Materials Authority (NMA) acquired for GSPC about 51,000 line-km of aeromagnetic data and 23,500 line-km airborne gravity data. As is often the case, the involvement of field observations and gravity and magnetic data in the overall interpretation process not only assured a greater uniformity of interpretation cover but was important in locating features that are difficult to recognize seismically, such as the true basement surface, strike-slip faults, regional discontinuities, and dikes.
A close examination of the reduced-to-the-north magnetic pole (RTP) map (Figure 18) and the Bouguer gravity map (Figure 19), magnetic-gravity interpretations indicated two groups of anomalies. The first group consists of high-value anomalies that reflect the frequency range and the depth of the corresponding sources; they may indicate uplifted blocks. In places, these high magnetic and gravity anomalies are seen to ‘nose’ into the regional anomalies. The second group of anomalies shows up as areas of closely spaced gravity and magnetic contours relative to the more gentle gradients associated with deeper sources and may also be related to structurally uplifted blocks.
From an examination of the surface geology (see Figure 3b) and the Bouguer gravity (Figure 19), the Farafra structure is seen to be a doubly-plunging anticline, trending northeast with the steeper plunge on the northeast. Figure 4 shows that the clastic rocks of the Campanian Wadi Hennis Formation are the oldest rocks exposed in the central part of the anticline in the area of Ain El Maqfi and Wadi Hennis.
Three basinal areas (A, B and C) were interpreted from the Bouguer gravity map (Figure 19). These coincide with strong negative magnetic anomalies on Figure 18. As a result, a new 2-D seismic grid has been designed (Figure 20) for a total length of 4,490 line-km. In addition, three 2-D seismic lines are planned in the far east of the concession to check the presence of basin B interpreted from the Bouguer gravity data.
The Paleozoic seismic sequences in the GSPC South Diyur Block are considered to be prospective but are believed to be partially eroded or thinned near the far southwestern corner of the block where the Ammonite-1 exploration well was drilled.
Ammonite-1 drilling results indicated that the Jurassic–Cretaceous sequences are fresh-water bearing. Accordingly, they have a reduced potential for structural traps exploration plays in the block.
Interpreted Paleozoic sequences in Ammonite-1 and the available 2-D seismic data correlate with similar sequences in the productive Paleozoic sequences in North Africa, especially Libya, and in the Middle East.
The recognition of the petroleum potential of the Paleozoic sequences in Egypt has suffered from the lack of mega-regional understanding of the petroleum geology, a limited knowledge of the Late Paleozoic basin inversion and the impact of the Hercynian tectonic event on Paleozoic petroleum prospectivity. Similarly, details of the geologic history of the Paleozoic sequences in Egypt, for example, the Cambrian–Ordovician glaciations, the Early Silurian abrupt rise in sea level following deglaciation and the subsequent deposition of the organic-rich black shale in euxinic basin lows, have been largely ignored or misinterpreted.
Relatively thick Devonian to Carboniferous shales that overlie seismic Sequence Boundary-2 and the Silurian shales preserved below the Hercynian unconformity could act as effective top seals for potential petroleum accumulations. Underlying them are good reservoir facies in the Upper Ordovician glacial channels.
The interpreted 2-D seismic data together with the drilling results from the Ammonite-1 exploration well and the estimated depth to basement, further stressed the importance of acquiring magnetic-gravity data to enable the proper assessment of the basement and basin configuration in the South Diyur Block.
In addition to the Paleozoic petroleum potential of South Diyur Block, that of the Upper Proterozoic Seismic Sequence-5 is considered interesting and warrants further evaluation after acquiring, interpreting and integrating the acquired magnetic-gravity data. In Oman, most of the petroleum reserves are of Late Proterozoic age.
Exploration plays in the South Diyur Block include a combination of Paleozoic structural and stratigraphic traps associated with prospective fairways, and stratigraphic traps in Cambrian–Ordovician glacial channels. Other possible Paleozoic structural traps may occur but they need additional seismic data interpretation for proper delineation and maturation to drillable prospects.
To date, the Mesozoic sequences have been the primary exploration objectives in the Western Desert. Wells in the vicinity of the block, such as Ammonite-1, and the Fagur wells in the northern part of the Western Desert, were drilled on typical Mesozoic–Paleozoic four-way closures on pronounced seismic highs. However, the prospective rock sequences are detrimentally freshwater flushed. In addition, lack of an effective top seal due to the extensive sandstone content further decreased the chances of top-sealed traps occurring in the Jurassic–Cretaceous sections in the wells.
The present modeling and integration of additional aeromagnetic and gravity data have provided crucial understanding to the configuration of the basement in the South Diyur Block and of the prospective Paleozoic fairways and potential generative basinal areas. This next phase of interpretation, together with the acquisition of additional 2-D seismic lines, will lead to the identification of future exploration plays in the South Diyur Block.
We thank Gujarat State Petroleum Corporation Ltd (GSPC), Gandhinagar, India, and in particular Mr Tapan Ray, IAS, Managing Director, for granting permission to publish this paper and with supporting the project. We also thank the two anonymous reviewers for their constructive comments and review of the manuscript. Our thanks and appreciations go to David Grainger for editing and significantly improving the manuscript, to Kathy Breining for proof-reading the manuscript, and to Moujahed Al-Husseini for helpful comments, encouragement and support. The drafting and design work by GeoArabia Graphic Designer Nestor ‘Nino’ Buhay IV, is greatly appreciated.
ABOUT THE AUTHORS
Mohammad Yusuf Farooqui joined Gujarat State Petroleum Corporation (GSPC), Egypt in 1994 and is currently General Manager (Overseas). He received his BSc (Hons.) in Geology in 1985, and an MSc and MPhil from Aligarh Muslim University, India, in 1988 and 1990, respectively. While working for GSPC, Mohammed has been responsible for the exploration and production of oil and gas in India and overseas. He is currently overseeing assets in Australia, Indonesia, Egypt and Yemen. He has had many papers published in international journals.
Khamis Farhoud joined Gujarat State Petroleum Corporation, Egypt in 2011 as Exploration Manager for Egypt and Yemen. Khamis received his BSc in Geophysics from Cairo University in 1997 and an MSc and PhD from Ain Shams University, Egypt, where he specialized in aeromagnetic and gravity techniques. From 1998 to 2002 he was Potential Field Geophysicist in the Airborne Geophysics Department of the Nuclear Materials Authority of Egypt. In 2003 he joined East Zeit Petroleum Company as Senior Seismic Geophysicist. From 2005 to 2010 Khamis was Deputy Exploration Manager and Exploration Project Leader for Edison International, Egypt.
Dia Mahmoud has 46 years of experience in petroleum exploration and development in Egypt, Saudi Arabia, and adjacent countries. His experience includes 8 years with the Gulf of Suez Petroleum Company in Egypt, 18 years with Saudi Aramco Exploration, and 20 years self-employed as the President of GEO Petroleum & Exploration Services Limited (GEOPEX Ltd.) and the Managing Director of Spectrum-GEOPEX Egypt Limited. He is an active member and certified petroleum geologist of the AAPG, SEG, EAGE and EPEX.
Ahmed N. El-Barkooky graduated from Cairo University with a BSc (Hons.) in Geology and an MSc and PhD. He teaches in the Geology Department of Cairo University and supervises MSc and PhD students. Ahmed has more than 30 years of experience in academia and as a petroleum industry consultant. He has been involved in many exploration projects and special studies regarding basin architecture and the tectonostratigraphic controls of petroleum systems. He conducts geological field seminars for both students and professional geoscientists. His broad regional experience of the geology of Egypt, North Africa and Middle East has been obtained through various consultation and research projects.