الفهرس 
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Abstract
The present study concerned with the Late Cretaceous sediments, in October Field, central Gulf of Suez. The Gulf of Suez is a rift basin almost 350 km long and 50-80 km wide and trending NNW-SSE. It was initiated in the Late Eocene to Oligocene, as the African and Arabian plates began to separate. October Field covers about 226.2 Km2, located in the northern part of the central area (Belayim province) of Gulf of Suez between latitudes 28° 48ʹ 54.9670ʺ to 29° 15ʹ 39.96ʺ and longitude 32° 47ʹ 33.5165ʺ to 33° 9ʹ 37.2688ʺ. It was discovered and began production in 1977. The Late Cretaceous Matulla and Wata reservoirs have been discovered in 1978 and began production in September 1980. The field was estimated in 1996 to contain 1998 million barrels of oil in-place (MMBO), of which 41% (or 1644 MMBO) was assigned to the Paleozoic-Cretaceous Nubia Sandstone, 13% to the Upper Cretaceous Nezzazat group and 5% to the Miocene Nukhul Formation. The 1996 estimate of field ultimate recovery (EUR) of 556 MMBO has been exceeded by cumulative production, which had reached 819 MMBO from the Nubia Sandstone alone by 2010. 1082 MMBO of the Nubia STOIIP is medium-gravity oil and 562 MMBO is heavy oil, the latter acting as a barrier to bottom-aquifer drive. The oil column within the massive fluvial sandstones of the Nubia is 1092 ft thick. Light oil in the Nezzazat group is trapped within mixed carbonate-clastic deposits of the Matulla, Wata and Raha formations, which have net pay thicknesses of 8-155 ft. Field production rose throughout the 1980s, but water breakthrough and pressure decline in the Nubia led to the implementation of gas lift in 1986. Field production peaked at ~172,000 BOPD in 1991. Continuing pressure decline was reversed by water injection and by the drilling of dedicated Nezzazat group producers, but production generally continued to fall.
An early estimate put the stock total oil in-place (STOIIP) in the Nezzazat group as 250 MMBO. A 15% recovery factor was assumed by Egyptian general petroleum corporation in 1996, giving EUR of 37.5 MMBO. So, the increasing in recovery factor by minimum 1% will give gain 2.5 million barrels of oil which is greater than exploring new areas with current exploration risks in Gulf of Suez. Based on that this study focus on addressing the stratigraphic reservoir heterogeneity factors to support the enhancing oil recovery development projects.
The main objective of this study is to conclude the petrographic elements and the successive depositional events reflecting the heterogeneity of the Turonian to Santonian succession of the Wata and Matulla formations. Also, petrofacies interpretation and reservoir characterization controlling the fluid flow and fluid capacity of these reservoirs were estimated. In order to unlock the reservoir heterogeneity in the study area, it is necessary to study the facies analysis, depositional history, sequence stratigraphic analysis of the Late Cretaceous sediments of Nezzazat Group. To achieve the goals of the study, five representative lithologic sections of cored wells were studied in detail including; GS 197-2, OCT-G11 St1, OCT-B2, OCT-A8St1 and GS 160-2 wells. Detailed core description and laboratory analyses were applied and the following conclusions were achieved:-
5.1. Lithostratigraphy (Cenomanian to Campanian)
Lithologically, the Late Cretaceous sediments in October Field is divided into five rock units; namely from older to younger: Raha Formation, Abu Qada Formation, Wata Formation, Matulla Formation and Sudr.
5.1.1. The Raha Formation (Cenomanian)
Raha Formation is the lowermost rock unit in the studied succession. It is unconformably overlies the Late Cretaceous Nubia Sandstone and conformably overlain by Abu Qada Formation. Raha Formation ranges from 350 feet thick at GS 197-2 well to 610 feet thick at GS 160-2 well toward the north of October Field. Lithologically, it was subdivided into two units; the lower is Abu Had Member and the upper is Mellaha Sand Member. Generally, Abu Had Member consists of shales interbeded with argillaceous limestone and sandstones, whereas Mellaha sand Member is entirely composed of sandstones with good reservoir potentiality. Most of the shales are fossiliferous with Cenomanian foraminiferal and ostracod assemblages This assemblage includes Late Cenomanian Thomasinella gauricoensis, Daxia cenomana, Veenia jezzinensis, Cytherella sulcate, Amphicytherura sexta. In addition to The Late Cenomanian calcareous nannofossil Watznaueria barnesae CC10 a,b subzones.
5.1.2. The Abu Qada Formation (Late Cenomanian-Early Turonian)
Abu Qada Formation overlies the Raha Formation and underlies the Wata Formation. It attains a thickness of 110 ft at GS 197-2 well and 200 ft at GS 160-2 well in the most norther part of the field. It consists of calcareous black to dark gray shales and argillaceous limestones. Most of these shales and limestones are fossiliferous including abundant planktonic foraminifera, ostracods, In addition to, Heterohelix globulosa. Based upon this faunal assemblage, Abu Qada Formation is assigned Late Cenomanian to Early Turonian age.
The Cenomanian-Turonian global boundary of oceanic anoxic event (OAE2) has been detected in this study by using. (1) sharp positive excursion in the bulk δ13 C profile for sample analyzed in Abu Qada and Raha formations, (2) Abrupt increase in organic geochemical parameters such as the Total Organic Carbon (TOC), pyrolysis derived S2 and the hydrogen index values, and (3) Remarkable biostratigraphic turnover in the biostratigraphic content (nannoplankton, formainferal, palynological and ichnological). All of these parameters have been detected in Abu Qada Formation and correlated with global paleoenvironmental turnover and global atmospheric CO2 turndown. Based on this the Cenomanian-Turonian OAE2 detected for the first time in Gulf of Suez in this study as material based chronostratigraphy global boundary.
This study presented the dark gray shale of Abu Qada Formation as source rock oil potential in October Field according to the geochemical analytical results.
5.1.3. The Wata Formation (Middle-Late Turonian)
The Wata Formation overlies Abu Qada Formation and underlies the Matulla Formation. The Wata Formation is generally composed of a thick succession of two limestone bodies separated by alternating sandstone and shale. The middle siliciclastic unit didn’t record in any published literature along Gulf of Suez and Sinai Wata sediments. Therefore, this study adds the middle siliciclastic unit of Wata Formation (unit 2) for the first time to the Wata sediments in Egypt. In addition, it one of the main reservoirs in October Field. The Wata Formation attains thickness of 300 ft at GS 197-2 well and 510 ft at GS 160-2 well northwest. The formation is generally poor in macrofossils due to subsurface drilling conditions. Some undefined bivalves and rare deformed benthonic foraminifera are encountered; however, Discorbis turonicus fauna and Nezzazatinella aegytiaca of Middle to Late Turonian age have been reported. The Turonian ostracod Neocyprideis vandenboldi / Cythereis madaourensis assemblage zone dated the Wata Formation as Middle-Late-Turonian.
5.1.4. The Matulla Formation (Coniacian-Santonian)
The Matulla Formation unconformably overlies the Late Turonian Wata Formation by a Turonian-Coniacian sequence boundary and conformably underlie the Campanian-Maastrichtian Sudr Formation. The Matulla Formation attains thickness 300 ft at GS 197-2 well and 450 ft toward the northern west at GS160-2 well. The formation is mainly composed of thick sandstones and shales with limestones intercalations. It can be lithologically subdivided into three lithologic units: the lower siliciclastic dominated unit, consists of sandstone/shale intercalations with subordinate limestone thin interbeds, the middle shale and sandstone bars dominated unit with carbonate interbeds, composed of frequent limestone beds and the upper carbonate and shale dominated unit with few sandstone interbeds. The siliciclastic of the lower and middle units act as the main reservoir in Late Cretaceous sediments in October Field.
The Matulla Formation dated as Coniacian-Santonian in the subsurface wells by the presence of Ceratobulimina aegyptica of Coniacian age (lower Matulla Formation) and the Discorbis turonicus and Ostracods of Ovocytheridea producta of Santonian age. The Coniacian/Santonian invaluable chronostratigraphic boundary was recognized in this study by the presence of Ephedripites multicostatus, Ephedripites sp and the Tricolpites giganteus that related to the Coniacian age and Early Santonian nannofossil of Reinhardtites anthophorus Zone CC15.
5.2. The Microfacies analysis and depositional trends
The recognized microfacies associations in each rock unit are discussed as follows:-
5.2.1. The Raha Formation
Seven microfacies were recognized in this rock unit. Three of them are non-clastic microfacies association (dolomitized wackestone, Ostracodal wackestone, and foraminiferal wackestone) and four clastic association (dolomitic quartz arenite, siliceous quartz arenite, calcareous quartz arenite, and glaucony rich siltstone). The microfacies trends of the Raha Formation proved deposition under gradual marine deepening conditions related to a gradual sea-level rise of a subtle advancing sea started with the Cenomanian. The deposition started within the realm of shallow supratidal and intertidal sub environments. The deepest marine setting in this episode lies with the depth of intertidal to shelf lagoons. The continuous Cenomanian sea-level rise resulted in the evolution of deeper sub environments of shelf bays and reached the maximum sea-level rise during inner-shelf environment. Toward the top of Raha Formation, a marked sea-level DROP and land-ward shift of the seashore occurred marked by the deposition of intertidal environment.
5.2.2. The Abu Qada Formation
Four microfacies were recognized in this rock unit (sparite foraminiferal wackestone, molluscan foraminiferal packstone, planktonic packstone, and calcareous shale). The shallow supratidal, intertidal and shelf lagoonal marine settings were encountered at the early stage of this formation.
Deep marine facies were recognized (shelf bays to inner-shelf marine settings) with remarkable sea level rise at the Cenomanian-Turonian boundary at the middle part of the formation. Finally, gradual shallow marine fluctuations continued to the upper Wata Formation during the generally advancing sea-level fall.
5.2.3. The Wata Formation
Seven microfacies were recognized in this rock unit. Four non-clastic microfacies association include dolomitized wackstone, bioclastic packstone, peloidal grainstone, and oolitic grainstone. In addition to, three clastic association include calcareous quartz arenite, feldspathic and glauconitic quartz arenite and laminated glauconitic siltstone. The formation represents a depositional episode of relatively fluvio-marine to deep marine environments related to a remarkable sea-level fall and rise. The deposition started with the lower carbonate unit of Wata Formation under deep marine mid/outer shelf conditions related to high sea-level settings. These conditions changed with marked DROP in sea level during deposition of fluviatile and intertidal microfacies associations. The falling in sea level continued to the end of middle Wata-2 unit. General rise in sea level was evidenced by deposition of middle to outer shelf carbonate facies of the upper unit of Wata Formation.
5.2.4. The Matulla Formation
Nine microfacies were recognized in this rock unit. The non-clastic microfacies association include phosphatic dol-wackestone, bioclastic packstone, oolitic grainstone and rudstone. The clastic association include feldspathic glauconitic quartz arenite, laminated ferrugineous siltstone, calcareous litharenite, laminated mud shale and fossiliferous phosphatic glauconitic quartz arenite. The Matulla Formation represents a phase of unstable sea-level conditions where, marked sea-level fluctuations were dominating the entire depositional time of the formation in October Field. The deposition of Matulla Formation started during a phase of sea-level DROP associated with land shift sediments of the lower unit resulted in the dominance of shallow intertidal, tidal channels and shelf lagoonal siliciclastic reservoir facies. Toward the middle parts of the formation, a marked sea-level rise took place accompanied with relatively deep marine sedimentation, where shelf bays and middle to outer marine shelf facies were deposited. These relatively deep marine conditions were occasionally intervened with short-lived shallow intertidal episodes. These deep marine conditions came to close toward the end of the upper parts of the formation, where a marked sea-level DROP took place accompanied with shallow intertidal sedimentation.
5.3. Sequence stratigraphic analysis
The detailed core description, wireline logs and paleo-environmental investigations have enabled subdivision of the Late Cretaceous sediments into seven third order depositional sequences, each represents a separate depositional event, related to an independent marine cycle.
5.3.1. The Depositional Sequence (SQ-1)
5.3.1.1. Distribution
SQ-1 is the lowermost depositional sequence of the Late Cretaceous succession in the October Field. It encompasses the entire sediments of the Cenomanian Raha Formation. The sequence overlies the Early Cretaceous Nubia Sandstone, while underlies the depositional sequence ”SQ-2”. The SQ-1 generally consists of a mixed siliciclastic/carbonate units and display almost uniform thickness ranging between 207 ft in the south at GS 197-2 well, and 55 ft in the north of GS160-2 well. The age of this sequence is generally Early-Middle Cenomanian due to the presence below the Late Cenomanian-Early Turonian Abu Qada Formation.
5.3.1.2. The sequence boundaries
The lower sequence boundary (Sb-1) of the present sequence displays a remarkable change from entirely subaerial continental fluvial sedimentation of the Malha Formation into a shallow coastal marine sub-environments related to the earliest Cenomanian marine transgression. The Sb-1 sequence boundary considered as type-1 sequence boundary. The upper sequence boundary (Sb-2), on the other hand, is a conformable transitional surface marked by the passage from the warm supratidal shallow marine conditions of SQ-1, into relatively deeper shelf bay marine environments of SQ-2 and considered as type-2 sequence boundary.
5.3.1.3. The Transgressive Surface (ts-1)
The transgressive surface (ts-1), of SQ-1 is recorded all-over the October Field. It is delineated here on the basis of the marked depositional facies changes and parasequences depositional architecture. Along ts-1 in the study area a remarkable facies change from short-lived intertidal depositional environments into deeper open shelf to inner shelf marine conditions.
5.3.1.4. The maximum flooding surface (mfs-1)
The mfs-1 of SQ-1 is recorded all-over the study area. It is delineated when the deep inner-middle marine shelf facies were successively prograded by relatively shallower intertidal facies.
5.3.1.5. Systems tract
5.3.1.5.1. Low-stand Systems Tract (LST-1)
The LST-1 is relatively thin unit, varies in thickness from 50 ft in the south to about 42 ft in the north of GS 160-2 well. It is generally siliciclastic unit; while it consists of sandstones in the south, and more shales with minor thin calcareous intervals in the north.
The tract consists of relatively thin retrogradational parasequences depositional succession that successively onlap the underlying Malha Formation through Sb-1.
5.3.1.5.2. Transgressive Systems Tract (TST-1)
TST-1 ranges in thickness from 88 ft in the south to 92 ft in the north of October Field. It is generally consisting of a mixed siliciclastic/carbonate unit with rather enrichment in limestones toward the upper parts. The tract is composed of relatively thick retrogradational/aggradational parasequence depositional successions with laterally extensive, weakly undulating transitional contacts. Due to the continuous sea level rise, the accommodation volume was gradually increased and become greater than that of sediment-supply (A>S), thus relatively thick deeper inner-middle marine shelf parasequences were developed.
5.3.1.5.3. Highstand Systems Tract (HST-1)
The HST-1 is delineated all over the October Field. It ranges in thickness from 60 ft in the south at GS 197-2 well to 50 ft in the north at GS 160 well. Lithologically, the tract consists of a mixed shale/limestone unit, especially at the upper half. It is composed of relatively thin prograding parasequences with flat planar contacts. The rapid basin land-ward shift accelerated the rate of deposition than that of the accommodation volume (S>A). Thus, shallow intertidal quartz arenites were prevailed. Numerous intervals of thin fissile shales with ferruginous bands are reported at the top parts of this tract marking the maximum sea-level DROP during SQ-1, just prior to Sb-2.
5.3.2. The depositional sequence (SQ-2)
5.3.2.1. Distribution
SQ-2 is recorded in the studied successions following upward SQ-1. It encompasses the sedimentary succession of Abu Qada Formation and lower Wata Formation (Wata-3 unit). The age of this sequence ranges from Late Cenomanian to Late Turonian. It ranges in thickness from 180 ft in the south to 230 ft in the north. This thick sequence consists of general siliciclastic/carbonate intercalations with noticeable enrichment of carbonate limestones in the middle parts.
5.3.2.2. Sequence boundaries
The SQ-2 overlies the Sb-2 and overlies by Sb-3 sequence boundary that show remarkable fluvial dominance at the base of middle Wata unit. This Sb-3 matched with global sea level fall (about 75 m) and correlated with sequence boundaries recorded globally in the Eastern Desert.
5.3.2.3. The maximum flooding surface (mfs-2)
The maximum flooding surface (mfs-2) is delineated throughout the October Field marking the maximum sea-level rise and remarkable anoxic event (OAE2) at Abu Qada Formation.
5.3.2.4. Systems Tract
5.3.2.4.1. Transgressive Systems Tract (TST-2)
The TST-2 overlies sequence boundary Sb-2 having the characters of type-2. The below LST does not recorded in any lithofacies along October Field or Sinai previous literatures. The recorded tract is ranging in thickness from 80 ft in the south to 120 ft in the north. The depositional successions were initially developed having a retrogradational/aggradational architecture that were developed within dominantly shelf bay and inner marine shelf sub-environments.
5.3.2.4.2. Highstand Systems Tract (HST-2)
The HST-2 is generally carbonate units ranging in thickness from 40 ft in the south to 62 ft in the north. It consumes the stratigraphic unit of upper Abu Qada and the Wata-3 carbonate unit. The progressive evolution of HST-2 was influenced by successive sea-level fall and reduction of the accommodation volume (S>A) at the top of Wata 3 unit, thus, the deposition was dominated by progradational parasequences as the shoreline gradually shifted landward.
5.3.2.5. The depositional sequence (SQ-3)
5.3.2.5.1. Distribution
SQ-3 is recorded in the examined successions following upward SQ-2. It encompasses the sedimentary succession of middle clastic and upper carbonate of Wata Formation. The age of this sequence ranges between Late Cenomanian to Late Turonian. It ranges in thickness from 250 ft in the south to 305 ft in the north.
5.3.2.5.2. The sequence boundaries
The SQ-3 overlies the Sb-3 type-1 sequence boundary that recorded in the cores and electric logs. The upper sequence boundary Sb-4 marks the maximum sea-level DROP at the end of SQ-3, associated with the dominance of intertidal depositional episode of Coniacian-Santonian Matulla Formation. This sequence boundary (Sb-4) is sharp and clearly defines at the top of Wata Formation by radical lithological changes and the faunal content.
5.3.2.5.3. The transgressive surface (ts-3)
In the study area, the transgressive surface (ts-3) is recorded in the October Field with almost the same characters. It marks the actual advance of the sea level rise when the middle-outer shelf facies overlap the shallower fluvial and intertidal parasequences below.
5.3.2.5.4. The maximum flooding surface (mfs-3)
The mfs-3 recorded by the outer shelfal deeper facies that prograded by shallow inner shelf facies above. The shoreline transgression associated sometimes with subaqueous erosion surfaces during the sea level rise known as ravinement surface. The transgressive lags and the Glossifungites burrows filled by reworked sediments that associated with the landward margin of transgression and maximum flooding surfaces.
5.3.2.5.5. Systems tract:
5.3.2.5.5.1. Lowstand Systems tract (LST-3)
In October Field the lowstand systems tract supported lithologically and depositionally by fluvial channel system and intertidal shoals. It is thin unit varies in thickness from 100 ft in the south to about 130 ft in the north. The submarine fan and lowstand wedge of the lowstand systems tract subdivision are not recorded in October Field and may deposited basin ward in the north of Gulf of Suez.
5.3.2.5.5.2. Transgressive Systems tract (TST-3)
This tract is delineated along the October oil Field and achieving the deepest marine conditions ever recorded in the study area. The tract varies in thickness from 160 ft in the north to 135 ft in the south and presented in Wata-1 unit. It is generally thick carbonate unit with minor siliciclastic intercalations especially in the southern parts of October Field at GS 197-2 well.
5.3.2.5.5.3. Highstand Systems tract (HST-3)
This is the topmost tract in SQ-3, reported all over the study area at the topmost part of Wata Formation and the lowest part of Matulla Formation. The HST-3 is generally siliciclastic/carbonate unit ranging in thickness from 17 ft in the south to 22 ft in the north. The progressive evolution of HST-3 was influenced by successive sea-level fall and reduction of the accommodation volume (S>A), thus, the deposition was dominated by progradational parasequences as the shoreline gradually shifted landward.
5.3.4. The Depositional Sequence (SQ-4)
5.3.4.1. Distribution
This sequence is recorded all-over the October Field and the surrounding area in Sinai and Eastern Desert. It follows the succession of SQ-3 without depositional break, otherwise a hiatus has been recorded at the upper sequence boundary known as Intra Coniacian sequence boundary. This sequence is the main oil producing zone along Matulla Formation (Matulla-3 unit).
5.3.4.2. The Sequence Boundaries
The upper sequence boundary Sb-5 (Intra-Coniacian sequence boundary) is recorded for the first time in Gulf of Suez wells. The Sb-5 is marked by the presence of fine-crystalline highly dolomitized wackstone and highly oxidized dolomitic sandstone with slightly burrowing near the top of lower Matulla unit (Matulla-3). The high resolution biostratigraphic analysis reported in GS 197-2 and OCT-B2 wells manifested the intra Coniacian sequence boundary. This boundary assigned by Eiffelithus eximius nannofossil Zone, Dicarinella primitiva and D. concavata foraminiferal zones which belong to Early Coniacian age.
5.3.4.3. The maximum flooding surface (mfs-4)
The mfs-4 is recorded when the sea level reached the maximum high and deposited the subtidal sediments below of glauconitic laminated mud-shale and then decrease to deposit the intertidal laminated siltstone and very fine sandstone above. The high concentration of the dark green large pellets of glauconite reveals that it belongs to autochthonous glaucony.
5.3.4.3. Systems Tracts:
5.3.4.3.1. Transgressive Systems Tract (TST-4)
The SQ-4 is onlap on the Turonian/Coniacian sequence boundary. The deposition started with shale and claystone transgressive facies that coupled with remarkable acceleration in sea level rise (A>S). TST-4 recorded slight thickness variation in the study area from 114 ft at the north to 100 ft at the south. These sediments composed of aggradational to progradational parasequences of deep subtidal and intertidal rippled sandstone of good porosity and high textural and compositional maturity.
5.3.4.3.2. Highstand Systems Tract (HST-4)
It is generally calcareous/siliciclastic unit ranging in thickness from 72 ft in the south to 220 ft in the north. By the close of TST-4, a strong sea-level DROP took place in an accelerating rate with intense basin landward-shift, especially in the northern parts at GS 160-2 well. Accordingly, the volume of the accommodation zone was successively reduced against a relatively increasing sedimentation rate (S>A).
5.3.5. The Depositional Sequence (SQ-5)
5.3.5.1. Distribution
This sequence recorded along the study area of October Field and the nearby Coniacian-Santonian exposed sections at Gebel Ekma and Gebel Nezzazat. The marker sea level falls at the lower Intra-Coniacian sequence boundary of Sb-5 by the onset depositional of fine crystalline dolostone.
5.3.5.2. Sequence boundaries
The SQ-5 overlies the Sb-5 having the characters of type-1 sequence boundary. The upper boundary is the type-2 Coniacian/Santonian sequence boundary (Sb-6) that detected for the first time in subsurface wells by this study. The Sb-6 has no evidence of subaerial activities or coastal emergence. It seems that the landward shift of the shoreline never exposed the shelf area of SQ-5. So that, this boundary considered as a type 2 sequence boundary. The Sb-6 is relying also on the biostratigraphic criteria that have been recorded by high-resolution biostratigraphic report of GUPCO (2014).
5.3.5.3. The maximum flooding surface (mfs-5)
The mfs-5 has delineated when the sea level reached its maximum level and deposited the deep subtidal sediments and black shale facies that recorded in OCT-B2 well. These facies were replaced by laminated mud shale in GS 197-2 well toward the south.
5.3.5.4. Systems Tracts
5.3.5.4.1. Transgressive Systems Tract (TST-5)
The TST-5 record retrogradation to aggradational facies association start with dolostone in GS 197-2 well and deep subtidal shoal sandstone in OCT-B2. The facies change between the two wells is related to that GS 197-2 well has shallower marine affinity and closer to the fresh water source than OCT-B2 well. The ravinement surface is a subaqueous erosion formed when the sedimentation temporarily ceased after the initial transgression phase immediately start after sea level fall at the beginning of transgressive systems tract. The ravinement surface is characterized by abundant Glossifungites burrows which are vertical shaped or branched traces occur in not firm lithified carbonate or siliciclastic intertidal and shallow marine setting.
5.3.5.4.2. Highstand Systems Tract (HST-5)
The HST-5 thickness is about 140 ft in the north and 100 ft in the south of October Field, this high thickness signifies the high sediment supply with consistent subsidence rate. The siliciclastic deposits in the HST-5, which presented as a good reservoir prograded because of normal regression due to increase of sediment supply influx. The HST-5 shows a remarkable increase in the carbonate deposits which reflect the still stand stage that increased the accumulation rate of the carbonate productivity.
5.3.6. The Depositional Sequence (SQ-6)
The SQ-6 is recorded along all the study area of October Field and the studied sections of west and east central Sinai. This sequence presents the Santonian sediments of Matulla Formation (Matulla-1 unit). The SQ-6 bounded below by the Coniacian/Santonian sequence boundary and above by the Santonian/Campanian boundary.
5.3.6.1. Sequence boundaries
SQ-6 overlies type 2 Sb-6 sequence boundary and underlies Sb-7 Santonian/Campanian sequence boundary. The Sb-7 sequence boundary is easily to track along the study area in Gulf of Suez and along Sinai. The Sb-7 can be detect by remarkable change in the facies and faunal content. Sb-7 is picked by the appearance of the chalky and brown limestone of the above Campanian Sudr Formation and its faunal content of Bulimina prolixia and Bolivina incrassata. This boundary is enriched with bioturbation sediments because of low sedimentation rate.
5.3.6.2. The maximum flooding surface (mfs-6)
The maximum flooding surface at the top of the lower TST-6 is marked by the shallower lagoonal rudistid bivalve build up and shell hash, above the lower subtidal shale.
5.3.6.3. Systems Tracts
5.3.6.3.1. Transgressive Systems Tract (TST-6)
TST-6 thickness is about 90 ft at the south and increases toward the north to become 110 ft. It records the rise of the sea level and the deeper depositional settings at the time of Santonian. The TST-6 is marked mainly by carbonate and shale facies with siliciclastic inputs of glauconitic quartz arenite and ferrigenous siltstone and very fine sandstone. The laminated mud shale, packstone and rudstone facies of the TST-6 act as fluid flow barrier for the clastic reservoir below of Matulla 1 and 2 and the source rock of the Campanian brown limestone above.
5.3.6.3.2. Highstand Systems Tract (HST-6)
The HST-6 overlies the thick retrogradation and progradational transgressive systems tract (TST-6). The HST-6 thickness is about 30 ft in the south and 65 ft in the north of October Field. The HST-6 record shifts from deep below to the shallow facies of lagoonal rudstone above the mfs-6 and prograded tidal channel sandstone in the south at GS 197-2 well.
5.3.7. The Depositional Sequence (SQ-7)
Since this sequence consumes a duration episode of 18.5 Ma and generated due regional tectonic instability resulted in significant sea-level fluctuations, it is herein considered as second-order depositional sequence. The SQ-7 is the topmost depositional sequence in the Late Cretaceous succession of October Field. The sequence is entirely encompassing the succession of the Sudr Chalk. It ranges in thickness from 170 ft in the south to 300 ft in the north. SQ-7 is bounded on lower Sb-7. The Sb-7 contact with SQ-7 is recorded by unconformity surfaces related to regional tectonic events recorded throughout Sinai. The lower limestone sequence of (SQ-7) is very rich in organic carbon in its lower part (Brown limestone) having TOC content ~1-5%, up to 8% and average 2.6% and considered the main source rock of oil in Gulf of Suez.
5.4. Reservoir characterization analysis
The reservoir characterization defined as the methodology that characterizes the reservoir based on its ability to store and produce fluids. Therefore, the optimum reservoir characterization is that integrate three main geological categories (1) the petrological properties, (2) the sedimentological characteristics and (3) the petrophysical properties of the reservoir to address the reservoir characteristics.
(1). The petrological properties or petrofacies discriminate the reservoir based on the parameters and factors control the rock permeability depending on the compositional and textural maturity of the sedimentary unit. In this study, the reservoir petrofacies addressed as the tool that control the reservoir storage and flow capacity (porosity and permeability, respectively), in order to figure out the reservoir heterogeneity that control the hydrocarbon flow. The petrofacies analysis in reservoir characterization is a crucial method in developing and managing reservoirs in the brown fields such as October Field.
(2). Sedimentological characteristics of Wata and Matulla formations are the main factors addressing their reservoir heterogeneity. The changes in depositional facies that resulted from sea level fluctuations lead to deposition of the sand bodies within different sequence stratigraphic framework. Also, lead to change in the sand-bodies geometry and control the diagenetic elements.
(3). The petrophysical properties include the study of the physical characteristics of the rock and their communication and interactions with fluids. The target of reservoir characterization is the three-dimensional image of petrophysical properties. The petrophysical properties of Wata and Matulla rocks were identified by the lab measurements of porosity, permeability and fluid saturations in addition to the formation evaluation of the log curves. The petrophysical evaluation categorized in this study into formation evaluation and neural network of the interpreted petrofacies.
The geological rock typing defines facies (petrofacies), sedimentological and diagenetic properties of the reservoir in a porosity-permeability quantitative framework. This integrated reservoir characterization is critical for successful drilling, production, stimulation and simulation for the exploration and development phases of the field. The reservoir geology studies in this context show the effect of facies and sedimentological properties on reservoir potentiality and address the variation of reservoir variables.
5.4.1. Wata and Matulla petrofacies
The Nezzazat group consisting of mixed siliciclastic and carbonate facies. The Wata and Matulla reservoirs are represented by the siliciclastic middle unit of Wata Formation (Wata-2), lower and middle units of Matulla Formation (Matulla-3 and Matulla-2, respectively).
The petrographic analysis of Wata and Matulla formations have been discriminated their reservoir into 10 petrofacies types based on the factors controlling the permeability. The petrofacies G, P and L are the higher quality petrofacies. The petrofacies M is the lowest quality unit and the main parameter reduced the permeability is the presence of clay matrix. The highest quality petrofacies were found in the Wata channelized quartz arenite. Therefore, it is recommended to use specific polymers to reduce the permeability of these petrofacies and avoid the losses of the injected stimulated fluids.
5.4.2. Wata and Matulla sedimentological characteristics
The hydrocarbon accumulation and production are controlled mainly by the lithological and facies factors resulted from depositional and sedimentological characteristics. The previously described facies analysis and the interpreted petrofacies find that the best reservoir quality among the siliciclastic is the quartz arenite. This arenite is characterized by high textural and compositional maturity in addition to the interconnected primary pores and high permeability that reach 1000 mD. The best reservoir quality also found in the point bar of the fluvial channel of Wata-2 unit and the tidal channel system of Matulla-3. The very poor reservoir was found in the argillaceous sandstone and the sandy limestone facies in Wata-2 and Matulla-2 (petrofacies B). The changes in facies laterally reflect the reservoir performance heterogeneity meanwhile vertical change reflect the reservoir quality within the well. In addition, the tidal channels are more preserved in the southern wells (GS 197-2, OCT-G11 St1 and OCT-B2) than in the northern wells, which indicate that paleoshorline exist south of October Field. The Wata Formation show different performance in lateral and vertical connectivity, where the depositional environment extends laterally from fluvial channel to crevasse in Wata-2 reservoir that reflected by the same pressure performance. The Wata-2 transgressive surfaces (ts-3) acts as vertical permeability barrier that separates Wata-2 reservoir from Wata-1unit. The fluvial influx of Wata-2 unit above the shelfal limestone of Wata-3 increases the reservoir quality by creating vuggy and intercrystalline porosity. The enhancement of the reservoir quality of Wata-3 unit suggests the unconventional potentiality of Wata Formation, it may address increase the oil reserve, and should to consider in the development plan of the field. Therefore, dolomitized zone recommended to be tracked along the fluvial influx within the channel system.
5.4.3. Wata and Matulla reservoir rock typing
A full set of conventional core data (bulk density, grain density, helium porosity, vertical and horizontal permeabilities) and borehole well logs (gamma ray, neutron, density, sonic and formation resistivity) have been used as inputs in the mathematical and statistical petrofacies prediction. The core data quality assurance is the first step to characterize the reservoirs, by removing the main outlier in data reading. The overburden depth correction of the core data have been performed to remove the effect of measuring at ambient conditions. Gathering the porosity and permeability data of the five wells into a composite one pseudo well is important. This allow computing the data in field specification rather than well specific.
The formation evaluation is the first step in petrophysical reservoir characterization of the studied wells to address the reservoir properties and fluid saturations of Wata and Matulla formations. The Wata Formation petrophysical evaluation show different potentiality, where the Wata channelized sandstone of Wata-2 middle unit is the higher potential zone of 15% porosity, 47% to 92% net to gross and 18-45% water saturation. The top or near top of Wata-3 carbonate unit show effective potentiality and proposed to be perforated to increase the recovery factor. The Wata-3 unit has been deposited as carbonate platform of shelfal setting, the quality and potentiality showed to be increased with absence of the point bar sandstone as recorded in OCT-10, OCT-B2 and GS 197-2 wells. The Matulla Formation units have been evaluated to find out two main reservoirs i.e. the lower Matulla-3 and middle Matulla-2. The main Nezzazat oil accumulation produced from these two units based on their high net pay to gross thickness ratio and the amalgamation of good quality quartz arenite tidal channels. The Matulla-1 (limestone and shale) deposited in deeper shelfal setting of limestone and shale, thin intervals of calcareous quartz arenite deposited as storm or washover fans to form good reservoir with high pressure but with limited production and rapid depletion due to its limited continuity such as OCT-A10 well.
The reservoir rock typing in this study used the technique of flow zone indicators (FZI), which is a unique parameter, defined the variation in the pore geometries based on the depositional and diagenetic controls in a distinct zone known as hydraulic unit (HU). The modified FZI equations indicate that for any hydraulic unit a plot of reservoir quality index (RQI) that equal to (K is the permeability and Φ is the porosity) versus the normalized porosity (Φz) which is equal to Φ/(1-Φ) should form a straight line with distinctive unit slope.
The interpreted petrofacies used as hydraulic unit defined from the core description and petrographic analysis. These hydraulic units used to define the flow zone indicator (FZI) for each petrofacies by using reservoir quality index (RQI) versus normalized porosity. Ten representative mathematical regression equations have been derived for the ten petrofacies. According to Amaefule (1993) method, this representative FZI used to create a unique equation for predicting the permeability from porosity for each petrofacies rock type. These equations applied to a log derived porosity for optimum log derived permeability.
At this stage, the rock types have been interpreted in the cored intervals. In order to create a continuous rock type across all zones, a correlation between the rock types and the log curves have been created. The bulk density, the compressional slowness (sonic), the deep resistivity, the gamma ray and the neutron porosity curve used in the five wells as inputs. The outputs are the predicted petrofacies rock types in the cored and un-cored intervals by learning from the correlation of the inputs. The mathematical correlation has been created by using the artificial neural networks (ANN) can learn and simulate the interrelationship between the different parameters of inputs and outputs (desired) data. This technique enables us to predict the petrofacies flow and hydraulic units of Wata and Matulla formations by more than 80% convergence. The predicted permeability has been validated by using the lab measurements data of the cored wells and good convergence found between the measured and derived permeabilities. The Wata-2 unit has the higher quality and the higher predicted permeability as well as the Matulla-3 unit and this conclusion has been confirmed by the production data. Therefore, the permeability predicted in the un-cored well (OCT-B3) used as example to apply this technique along October Field.