Chapter 4: The Geological Setting
The present study involves the well test analysis of six wells located in two different basins, the Nile Delta (4 wells) and Western Desert of Egypt (2 wells). The geological setting for these basins is discussed briefly to reveal the lithological nature of the studied reservoirs together with the predominant structural features.
4.1 The Nile Delta
The Nile Delta due to the onshore and offshore gas discoveries in the last few decades had encountered as one of the prolific petroleum provinces in Egypt. It produces mainly gas with few fields producing minor oil associating the gas. In the Nile Delta four wells located in two different fields (A and B) are selected for the present analysis; two gas wells in Field A and two light oil in wells Field B.
Field “A” is discovered in Oct 2005 and Field “B” is discovered in 2006 and both fields started production in November 2007.Field “B” is located within the El Mansoura concession in the southern part of the onshore Nile Delta. The reservoir interval of Field “B” comprises gas- and oil-bearing clastic sediments that stratigraphically fall within the Qawasim Formation. Field B extends over an area of about 17.5 Km² that is enclosed between latitudes 30° 05? & 30° 08? N and longitudes31° 34? & 31° 36? E.
Field “A” is located within the El Mansoura concession in western part of the onshore Nile Delta). It falls close to latitude 31° 18? N and longtitude:31° 15? E. Abu Madhi Formation represents the main gas producing interval of Field A. Figure 4.1 presents a location map to Field “A” and Field “B”.
Figure 4.1: Location Map to Field “A” and “B”
The River Nile owes its origin to a major tectonic event that affected the Mediterranean Sea during the Late Miocene time. The lowering of the sea level is evidenced by the steeply sloping river beds which at that time formed the drainage basin of the surrounding land masses into the lowered sea level (Said, 1981) 47. Moreover, the huge volume of evaporated water provoked a marked climatologically changes with frequent pluvial periods. Torrential streams cut deep gullies along the high mountain ranges of the Eastern Desert and flowed into the Eonile which had developed as a subsequent river (Said, 1981) 47.
Figure 4.2: A block diagram portraying the Eonile canyons 47.
It appears that in the slightly elevated south Delta Block, this ancestral river cut through the Oligocene basalts and clastics, then the underlying Eocene and Cretaceous formations and scoured the Jurassic formations as shown in Figure 4.3 47.
Figure 4.3: A geologic cross section passing through the west and middle Nile Delta (Bilqas – Kafr El Dawar) presenting Eocene and Cretaceous formations 47.
The Eonile, after cascading over the hinge zone into the north Delta embayment spreaded out its sedimentary load in a series of coalescing sedimentary fans. The thickness of the Late Miocene sediments typically ranges between 100 m and 2,000 m. With the beginning of the Pliocene time, a renewed opening of the Mediterranean in the straits of Gibraltar allowed a new inflow of the Atlantic water. The rising sea level eventually reached northern Egypt, drowned the delta of the Eonile, and transgressed into the Nile Valley depression forming a narrow and long gulf extending south of Aswan (Said, 1981) 47.
The Pliocene Sea also covered large parts of the land around the modern Delta, but overlapped the present-day Mediterranean littoral with only small fringes. Thus, it appears that the Nile Delta began to be formed by the Eonile and subsequently by a number of large rivers from Pliocene through Pleistocene to Recent times. The Nile as it is known today appears to be a humble “descendent” of much larger and fast flowing rivers which flowed through the Nile Valley and built up a huge delta (Said, 1981) 47. Detailed sedimentological processes and depositional environment prevailed through the Nile Delta development are discussed according to its geologic time to show modifications in the architecture of the delta.
The lithostratigraphy of the Nile Delta, presented by the Miocene sediments, is subdivided, from bottom to top, by E.G.P.C., (1994) 48 into three formations: Sidi Salem, Qawasim and The Rosetta formations. In addition, the Pliocene-Pleistocene sequences are represented mainly by shale or clay with sandstone interbeds (Figure 4.4). The Miocene sequence is summarized as follows:
4.2.1. The Early Miocene “Moghra Formation”
The Moghra Formation comprises marine to fluvio–marine deposits of deltaic origin that typically associate and alternate with carbonate intervals of marine origin. It is unconformably overlained by Sidi Salem Formation (Younes., 2015) 49.
4.2.2. The Middle Miocene “Sidi Salem Formation”
The lower boundary of this formation to Moghra formation is not recognized at the central parts of the Nile Delta but is well represented at the western and southern parts where it overlies the Moghra Formation. It is made mainly of mudstone interbedded with marl, sandstone, and siltstone. It is developed within the lower neritic slope environment and falls unconformably below Abu Madi Formation (Younes., 2015) 49.
4.2.3 The Middle Miocene “Qawasim Formation”
Qawasim Formation comprises a thick succession of sand and conglomerate series developed through the Tortonian to Messinian age. It typically involves irregular thick sandstones sequence and very thick conglomerate intervals (Schlumberger, 1984) 50. Such a facies demarcates a change in the deposition towards fluvio-deltaic environment. The thick lenticular sand and conglomeratic deposits are usually prone to submarine slumps that could formulate a suitable trapping mechanism to the subsequent Rossetta and Abu Madhi formations.
4.2.5 The Rosetta Formation
The Rosetta Formation is represented by large anhydrite section with interbedded thin claystone as shown in the stratigraphic column of Nile delta region (Figure 2.1). The anhydrite of Rossetta formation typically disappears at the western flank of the Delta (e.g. Abu Qir) but reappears to the north, offshore Alexandria. The Rosetta anhydrite is a good marker to Messinian age but absence of this facies at certain areas of the Delta indicates local shoaling or brine dilution (Younes., 2015) 49.
The Plio- Pleistocene sequences
These sequences are represented by Abu Madi, Kafr El Sheikh, El Wastani, Mit Ghamr, and Bilqas Formations. The nature of these stratigraphic units is presented in the following sections.
4.2.6. The Early Pliocene “Abu Madi formation”
Abu Madhi formation represents the main gas producing horizon in the Nile Delta (Schlumberger, 1984) 50. It is composed mainly of sandstone with siltstone intercalations and shale interbeds. The shale content becomes significantly thick and more frequent in the upper parts of Abu Madi Formation. These sediments are deposited within fluvio-marine to shallow marine deltaic environments, where the braided fluvial distributary channels are usually dominated. The lithofacies of Abu Madi sediments changes spatially by the influences of the northward channels as confined by their topographic and structural highs (El-Heiny and Enani, 1996) 51. The upper section of the Abu Madi Formation is marked by a clay sequence that delineated the lower boundary Kafr El Sheikh Formation. The base of Abu Madi formation is marked by fossiliferous clastics, while the upper sand interval is always associated with clay sediments rich in the content of marine fauna (Ahmed, 2002) 52.
4.2.7. The Early–middle Pliocene Kafr El Sheikh Formation
Kafr El Sheikh Formation consists of soft clay, mainly Kaolinite and montmorillonite, with thin limestone and sandstone interbeds (Schlumberger, 1984) 50. It extends all over the Delta area, but at the Nile Valley accumulated at the outer neritic to upper bathyal zones. Following the major lower Pliocene transgression, the former Eonile canyon is flooded and the Nile Valley is transformed into a long, narrow gulf extending as far as south Aswan 53. Rizzini et al. 54 suggested that Kafr El Sheikh Formation is accumulated as neritic mudstone on the present onshore Delta and in a basinal setting offshore. They suggested that the sand incorporated into this formation is essentially storm sand. This formation is conformably overlained by El Wastani Formation.
4.2.8. El Wastani formation (Late pliocene)
El Wastani Formation is composed of thick quartzose sand with little feldspar, interbedded with thin clays that becomes less thick toward the top, with argillaceous interbeds. This formation represents the transition between the muddy shelf facies of the lower Kafr El Sheikh Formation and the coastal/continental sand of the upper Mit Ghamr Formation. It shows large forests caused by the progradation and could also contain deltaic deposits (Barakat, 1982 and Schlumberger, 1984) 55 50.
4.2.9. Mit Ghamr formation
Mit Ghamr Formation is made of thick sand and gravels section with clay interbeds. The depositional environment falls between shallow marine to fluvial with the basin typically fill-up with coastal sands, clay, coquinas, and peat. This succession conformably underlies the Holocene Bilqas Formation (Nashaat, 1992) 56.
4.2.10. The Holocene Bilqas formation
Bilqas Formation represents the top sedimentary unit within othe Nile Delta province. It involves sand interbedded with clay rich in pelecypods, gastropods and ostracods fragments ( Rizzini et al. 1976)54. The clay contains numerous fragments of plant matter and peats. The deposition most likely took place in lagoons to brackish swamps that could be interrupted by the beach sand.
4.3 Hydrocarbon Habitat
The Nile Delta seems gas prone, basically methane, with a fair content of gas condensate and very few oil shows. The lower Pliocene Abu Madhi sandstone is the main producing formation in both onshore and off shore discoveries. Source rocks are generally classified according to the amount and type of organic matter, degree of maturation, and thermal alteration. The presence of organic rich shales within the Sidi Salem and the Kafr El Sheikh Formations makes them well known source rocks in the Delta but their maturation system remains poorly understood. The reservoirs in the Nile delta are typically sand of 20% average porosity. Abu Madhi sandstones have evidenced to be the most suitable reservoir unit in the Nile Delta and most of the fields produce gas from these sands (Said, 1981) 47.
Traps within the Nile Delta fields are typical asymmetric folds, overthrust faults and in some cases diapers. Other traps of the typical deltas such as channels and scour points and/or growth faults can also be obviously visualized. The maturation of source rocks and generation of gas and condensate is thought to have occurred in relatively recent time, right after the deposition of the Pliocene sequence. Trapping mechanism within Abu Madi reservoir is enhanced by the culmination effect associated the slippage of the succession along the developed growth faults. This slippage could be started and continued due to differential compaction that took place in the underlying shale of Kafr El Sheikh Formation.
4.4 The Western Desert
The Western Desert extends over the area, 700000 km², located to the west of the River Nile and its Delta. It approximately involves two-thirds of the total area of Egypt and extends for 1000 km between the Mediterranean Sea to the North and the Sudanese Border in the south with approximate width of 600 km to 800 km between the Nile Valley and the Libyan border.
Fig 4.5 shows the composite stratigraphic column of the Western Desert. The sedimentary section of the Western Desert extends between the lower Paleozoic and Recent. Four major sedimentary cycles are characterized, with maximum southward transgression reported during the Carboniferous, the Upper Jurassic, the Middle and Late Cretaceous, and the Middle Miocene and Pliocene times (Said, 1990) 57.
The subsurface Jurassic deposits of the north Western Deseret are classified into the following units (from bottom to top)
4.6.1 Wadi Natrun Formation
Wadi Natrun Formation is well represented in the northwestern corner of the Western Desert, and consists of lagoonal deposits, which extends alternating with dense limestone, green dolomite and shale, with subordinate interbeds of sandstone and anhydrate (Said ,1990) 57.
4.6.2 Khataba Formation
The Middle-Late Jurassic time is represented by the Khatatba Formation. It is mainly made of sandstone with organic-rich shale and coal seams. It is overlying the Ras Qattara Formation, a non-marine formation of siliciclastic lithofacies , and separated by unconformity, (Said, 1990). The Khatatba Formation marks the first appearance of the Jurassic sediments in the northern Western Desert. The Khatatba Formation is stratigraphically located below the calcareous sediments of the Upper Jurassic shallow-marine Masajid Formation, which is capped by the Cimmerian unconformity that marks a period of tilting, uplift, and partial erosion/karstification of the Jurassic units (Keeley and Wallis 1991) 58.
4.6.3 Masajid Formation
Masajid Formation is composed of a massive limestone sequence of Middle to Late Jurassic. The carbonate sequence is dominantly cherty in the northeast and central parts of the northern Western Deseret. It overlies conformably the clastic Khatatba Formation and underlies unconformably the clastic units of the Lower Cretaceous sequence (Said ,1990) 57.
4.7 The Cretaceous Units
The Cretaceous sediments of the Western Desert is divided into a lower and upper Cretaceous units. The lower Cretaceous units involve an important carbonate bed of the Alamein dolomite that provides the reservoir rock for three oilfields of the region.
4.7.1 The Lower Cretaceous Units
18.104.22.168. Burg El Arab Formation
It is a thick sequence of fine to coarse-grained clastics. This formation is subdivided into four members, from bottom to top: Alam El Bueb, Alamein, Dahab and Kharita (Said ,1990)57.
22.214.171.124 Alam El Bueb member
The Alam El Bueb member involves a sandstone unit and shale interbeds. It falls below the well-marked Alamein dolomite member. It includes units that were given different names such as Matruh Group, Aptian clastics, Dawabis and Shaltut. The environment of deposition was predominantlu sallow marine with significant continental effect, especially toward the south (Said, 1990) 57.
126.96.36.199 Alamein member
Alamein member is made up of light brown hard microcrystalline dolomite with a few thin shale interbeds. This unit is markedly distinct from the other clastics of the Alam El A and Dahab members. The type of section of Alamein sediments occupies the interval between 2489 to 2573 m of the Alamein-I well (30° 36? 39″ N, 28° 43? 52″ E) (Said,1990) 57.
188.8.131.52 Dahab member
It is composed a grey to greenish grey shale unit with thin interbeds of sandstone and siltstone. It rests conformably over the Alamein dolomite and underlies the sandy Kharita member. The Dahab members is relatively thick compared to the Alamein member (Said,1990) 57. The maximum thickness is reported in the subsurface in the type well (174m).
184.108.40.206 Kharita member
Kharita member is composed of fine to coarse-grained sandstone with shale and carbonate interbeds. The carbonate interbeds increase in frequency and thickness toward the northwest. The type section of this member extends between 2501 m and 2890 m in the Kharita-I well. This formation rests conformably over the Dahab member and falls below the Bahariya Formation. The maximum reported thickness is 1100m in the Mersa Matruh-I well (Said, 1990) 57.
4.7.2 The Upper Cretaceous units
The upper Cretaceous of the north-western desert is subdivided into three lithostratigraphic units: Bahariya Formation, Abu Roash Formation and Khoman Formation.
220.127.116.11 Bahariya Formation
The Bahariya formation is exposed along the floor and sides of the Bahariya depression. The exposed section is at least 170m and can be subdivided into three members: Gebel Ghorabi, Gebel Dist and El Heiz. The Bahariya Formation involves Late Cenomanian sediments that first deposited as the fluviatile sediments of the Gebel Ghorabi member, followed by the estuarine sediments of the Gebel Dist member) and finally the lagoonal calcareous mud and grits of the Gebel El Heiz member. The Gebel Ghorabi Member involves the coarse-grained and cross-bedded sandstone that seems predominantly non-fossiliferous. Alternatively, the Gebel Dist clastics involve the ferruginous fine-grained and extensively fossiliferous well bedded sediments. The El Heiz Member comprises dolomite, sandy dolomite and calcareous grits rich in fossils (Said, 1990)57.
18.104.22.168 Abo Roash Formation
Abo Roash formation is composed of the Upper Cenomanian to the Santonian limestone sequence with interbeds of shale and sandstone (Said,1990) 57. It is subdivided into seven lithostratigraphic units by Norton (1967) 59, and is designated from bottom to top: G, F,E,D,C, B and A. The thickness of this formation is about 130 feet, and this formation rests underlain the Bahariya formation and overlain Khoman formation.
22.214.171.124 Khoman Formation
The type section of this unit is located at Ain Khoman located to the southwest of Baharyia Oasis (27° 55? N,28° 30? E). This formation was deposited western desert during the Campanian and Maastrichtian Hantar 1990, Norton,19676059. Lithologically, it consists of two main units: the upper unit is composed of white and occasionally very pale orange limestone intercalated with sandstone and shale. The lower unit involves limestone mostly interbedded with shale. The limestone is massive and changes to white dolomitic limestone, and the shale is dominantly brownish gray in color with medium hardness. It is unconformably overlained by the Apollonia Formation and is unconformably underlained by the Abu Roash Formation.
Figure 4.4 :General lithostratigraphic column of the Nile Delta Area. E.G.P.C., (1994)48
Figure 4.5 :General lithostratigraphic column of the North Western Desert. E.G.P.C,199448