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The Precambrian basement: A major reservoir in the rifted basin, Gulf of Suez
Journal of Petroleum Science and Engineering 19 Ž1998. 201–222
The Precambrian basement: A major reservoir in the rifted basin, Gulf of Suez M.G. Salah ) , A.S. Alsharhan
1
Faculty of Science, UAE UniÕersity, P.O. Box 17551, Al-Ain, United Arab Emirates Received 13 May 1996; accepted 10 June 1997
Abstract The expected instructions from an exploration manager, to stop drilling and abandon a well where the bit hits the Precambrian basement, no longer applies. The fractured and altered Precambrian basement rocks are the most prolific reservoirs in the southern Gulf of Suez and the northern Red Sea rifts where hydrocarbons are produced from 8 fields, with porosity and permeability values up to 15% and 300 millidarcy, respectively. The surface and subsurface Precambrian basement rocks are related to the final stages of the tectonic–magmatic cycle of the Arabo-Nubian Shield and are composed of quartz–diorite, granodiorite, syenogranite, alkali granites and andesite porphyry, dissected by means of dykes, fractures and joints. Three main directions of fractures, northwest–southeast, northeast–southwest and east northeast–west southwest have been detected in the study area. The porosity and production rates of this reservoir, as well as the oil–water contact movement, depend mainly on the age, intensity and direction of the fractures, diagenetic processes and the dip and direction of the dykes and brecciated zones. The alteration processes reach their maximum intensity in the topmost section, known as the basement cover, where the solution and leaching has led to the enlargement of the fractures and vertical communications. The underlying fracture zone has been affected by differential alteration processes, creating zones of high and low vertical porosity and permeability. Thus, the reservoir potential of the Precambrian basement has been greatly underestimated. q 1998 Elsevier Science B.V. Keywords: Gulf of Suez; Egypt; petroleum geology; reservoir characterization; Precambrian basement; fractured reservoirs
1. Introduction Hydrocarbons are produced from granitic andror volcanic reservoirs in more than twenty basins all over the world. Such basins are found in Algeria, Argentina, China, Great Britain, Indonesia, Russia, Ukraine, Venezuela, Yemen and many others ŽSalah, 1994.. The area that forms the scope of this study ) 1
Corresponding author. E-mail: [email protected] E-mail: [email protected]
lies in the southern Gulf of Suez and northern Red Sea, Egypt at the triple junction of the main rifts between the Red Sea, Gulf of Aqaba and Gulf of Suez ŽFig. 1.. The most characteristic topographic features of the study area are the exposures of the Precambrian basement massifs at four localities: Ž1. the Esh Mellaha range to the southwest, Ž2. Shadwan island to the southeast, Ž3. the Sinai massif to the northeast and Ž4. the Jebel Zeit to the northwest ŽFig. 2.. The Esh Mellaha range and the Jebel Zeit have a more or less complete pre-Miocene Žprerift. se-
0920-4105r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 0 - 4 1 0 5 Ž 9 7 . 0 0 0 2 4 - 7
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Fig. 1. Regional structural elements and geometry of the Gulf of Suez region showing the surface geology, major hinge zones, and regional dips in the subbasins.
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Fig. 2. Location map showing the onshore geology of the study area, distribution of the oil fields that produce hydrocarbons from the fractured basement and the wells used in his study.
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quence on its western side. The former is onlapped to the south by a Miocene reef complex at Jebel Abu Shaar plateau ŽFig. 2.. This work provides a comprehensive study on the reservoir characteristics of the Precambrian basement in this rift basin, including composition, nature and genesis of pore spaces, fracture pattern analysis, and factors controlling production. The prolific oil-producing Gulf of Suez basin contains more than 70 oil fields, and reservoirs range from the Precambrian basement to the upper Miocene. Some commercial oil andror gas fields such as Zeit Bay, Shoab Ali, Hilal, East Zeit and Geisum have already been discovered in the study area. The oldest discovery in the southern Gulf of Suez, being the first in the Middle East, was made in 1886 when crude oil seeped into tunnels which were dug for extracting sulphur in the Gemsa area on the western coast of the Gulf of Suez ŽFig. 2.. The recent Hareed oil and gas field was discovered in 1988 by Conoco, Esso and Gulf Canada oil companies ŽFig. 2.. The present analyses of the Precambrian basement in the southern Gulf of Suez and northwestern Red Sea are based on the interpretation of both geophysical and geological data, including subsurface information from more than 55 wells drilled in the study area, surface outcrops and aerial photograph examination of the structural configuration of the region. In addition, electrical, fractures, and production logs were used to help in detecting the reservoir characterization of the Precambrian basement and its fracture patterns. As the Zeit Bay and Geisum oil fields have the largest production from the fractured basement, they will be considered as case studies. For almost ninety years, the genesis and classification of the Precambrian basement rocks in Egypt with special interests on the Sinai and Eastern Desert basement rocks, have been the subject of numerous investigations. Among past and recent papers covering different aspects of the origin, development and classification of the Precambrian basement are those by El Ramly and Akkad Ž1960., El Shazly Ž1964., Hashad Ž1980., Akkad and Noweir Ž1980., El Bay-
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oumi Ž1984., Hassan and Hashad Ž1990. and El Gaby et al. Ž1990.. The Precambrian basement attracted the attention of the oil explorationists after the discovery of the Zeit Bay oil field in 1981. The reservoir characteristics of this additional potential reservoir in the Gulf of Suez oil basin were described by Askary Ž1986., Shaheen and Darwish Ž1986., Zahran and Ismail Ž1986., Zahran and Askary Ž1988., Abu Taleb et al. Ž1990., El Wazeer et al. Ž1990., Ismail and El Moula Ž1992., and Alsharhan and Salah Ž1994.. 2. Stratigraphic framework The definition of the lithostratigraphic units in the Gulf of Suez and the Red Sea has been studied by many authors, such as Hume Ž1911., Abdallah et al. Ž1963., EGPC Stratigraphic Committee, 1964 and EGPC Stratigraphic Sub-Committee Ž1974., Darwish and El-Araby Ž1993. and Alsharhan and Salah Ž1995.. The stratigraphic section of the study area ranges in age from Precambrian to Recent and can be classified into two megasequences: prerift Žpre-Miocene. and synrift ŽMiocene to Recent., as summarized below. 2.1. Prerift megasequence This section ranges in age from Cambrian to early Oligocene. It was penetrated by few wells drilled in the study area and is exposed to the west of the Esh Mellaha range and Jebel Zeit ŽFig. 2.. The prerift megasequence reflects a number of tectonic events that affected the area. Continental- to shallow-marine sandstones interbedded with thin shales and limestones were deposited from Cambrian to Early Cretaceous time ŽPre-Cenomanian., and are known as the Nubia Sandstones. Uplift and erosion preceded a widespread transgression from the north at the beginning of the Cenomanian. A southward-directed onlap resulted in overstepping of the Cenomanian, Turonian and early Senonian sequence, known as Nezzazat Group. This group consists of four formations, Raha, Abu Qada, Wata and Matulla, and consists of interbedded sandstones, shales and carbonates.
Fig. 3. Major tectonic elements map of the southern Gulf of Suez showing the main high trends, troughs ŽSource Kitchens., and migration pathways.
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During Senonian time there was a transition from mainly clastic facies in the lower unit ŽMatulla Formation. to dominantly carbonate facies in the upper unit ŽDuwi and Sudr Formations.. The Paleocene Esna Formation was deposited before a period of severe erosion. This event was simultaneous with the Syrian Arc system and led to the thinning of the Esna Formation over the whole area with complete erosion in some places. A widespread transgression occurred during Eocene time and the Esna Formation was overlain by the carbonates of the Thebes Formation. 2.2. Synrift megasequence This section represents late Oligocene to PostMiocene time and rests unconformably on the prerift section. Upper Oligocene sediments, known as the Tayba Formation, were not penetrated in the study area but are present only on the central sector of the Gulf of Suez. This formation consists mainly of mixed clastic and volcanic rocks. The early Miocene Nukhul Formation, deposited in different settings, reflects environmental heterogeneity due to the different tectonic setting of the separate fault-blocks. The Nukhul Formation consists of conglomeratic and brecciated sandstones with thin shale interbeds, in the lower member ŽShoab Ali., and is influenced by the unconformity between the Miocene and the PreMiocene sequences. The upper member ŽGhara. consists of anhydrites, limestones, shales and sandstones, deposited in restricted, shallow-marine and continental settings.
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As the maximum extensional stresses resulted in rapid subsidence, major half-graben troughs were formed, with deposition of the Rudeis Formation. The shoulders of the rift began to rise and coarse sands and conglomerates were deposited near the margins of the subsiding rift during the middle Miocene. Fine-grained sediments accumulated in deeper-marine settings, in addition to some prolific reservoirs consisting of submarine turbidites and coarser sandstones. During the middle to late Miocene, the climate in the Gulf of Suez and the northern Red Sea changed and marly evaporite cycles with sandstones and conglomeratic beds were deposited at localized areas, forming the Kareem and Belayim Formations. The culmination of the evaporitic sequence, South Gharib and Zeit Formations, occurred during Late Miocene times. The main constituents were salt and anhydrite with thin interbeds of shale. Some beds of sandstone are recorded at the marginal areas. During the Post-Miocene to Recent, two lithologies were deposited Žcarbonate and clastic. known as the Shagra and Wardan Formations, respectively. Palaeontological evidence suggest a connection to the Indian Ocean through the Red Sea during the Pliocene.
3. Structural analysis The study area was the subject of intensive surface and subsurface geological and geophysical in-
Plate 1. ŽA. Quartzdiorites Žsidewall core of well: Hareed-2; depth: 2530 m. ŽPPL s plane polarized light.. Phanerocrystalline, inequigranular, fine to medium and subhedral to anhedral plagioclase, with fine to medium and anhedral quartz grains. Grains are largely open packed and some of the intergranular pores are filled with dolomite and nonferroan calcite Žbottom center.. Note the high porosity due to intense fracturing. ŽB. Granodiorite Žwell: Hareed NB-1; depth: 2470 m. ŽXPL s cross polars.. Coarsely crystalline, fresh or locally sericitised plagioclase feldspars with unstrained and anhedral quartz crystals. Alkali feldspars occur as albitic coronas on the plagioclase crystals Žtop center.. Note the open fractures induced by sidewall coring processes. ŽC. Granodiorite ŽWell: Hareed-2; depth: 2503 m. ŽPPL.. Euhedral, intensely sericitised plagioclase feldspar with anhedral quartz crystals. The dark opaques comprise pyrite replacement of biotite. Note the cloudy sericitised plagioclase Žcenter right. and the flow alignment ŽCenter.. ŽD. Alkali granite Žwell: Estakosa-1; depth: 2665 m. ŽPPL.. Poorly to moderately-sorted and angular to surrounded lithic grains of granite float in a groundmass of aligned feldspars, opaques and green chlorite. Note the dolomite texture and the poor porosity. ŽE. Alkali granite Žwell: Estakosa-1; depth: 2636 m. ŽXPL.. Granitic fragment of quartz Žtop left. and sericitised potassium feldspar Žcenter. showing strong replacement by calcite Žtop center.. Note the very poor porosity. ŽF. Lithic fragments Žwell: Hareed NB-1; depth: 3002 m.ŽXPL.. Poorly-sorted sandstone with perthite rich granitic fragments Žtop right and bottom centre. contained in a ground mass of dolomite. Note the common patches of anhydrite and the kaolinite filled vugs. ŽG. Granodiorites Žwell: Hareed-2; depth: 2642 m. ŽPPL.. Intensively fractured coarse plagioclase feldspars with excellent porosity Žup to 21%. Žleft. with coarse crystalline quartz aggregate Žtop right.. ŽH. Granite ŽWell: Hareed-1; depth: 2650 m. ŽPPL.. Unfractured fragment Žbottom left. displaying no or very poor porosity.
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vestigations for hydrocarbon prospecting and exploration. Among these studies are those by Said Ž1962, 1990., Robson Ž1971., Bayer et al. Ž1988., Evans
Ž1988., Richardson and Arthur Ž1988., Salah Ž1989, 1994., Hughes et al. Ž1991., Hammouda Ž1992. and Salah and Alsharhan Ž1996..
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The Gulf of Suez rift represents a northern extension of the Red Sea. It constitutes a large depression which lies below sea level in its axial portion. The Suez rift trends in a NNW–SSE direction separating the African plate from the Sinai microplate ŽFig. 1.. McKenzie et al. Ž1970. matched the coastline of the Arabian Peninsula with that of African Red Sea to detect the precise direction of plate motion. Both reached the same conclusion, namely that the general movement of the Arabian plate relative to the African plate was toward the northeast. The contrasting widths of the northern portion of the Red Sea rift zone and the southern Gulf of Suez suggest that left-lateral movement along the Gulf of Aqaba strike–slip zone has allowed significantly more crustal extension within the Red Sea than has been achieved within the Gulf of Suez. The northwestern sector of the Red Sea has experienced more extension than the other parts of the Gulf of Suez ŽMeshref and Khalil, 1990.. The limits of the Gulf of Suez rift are defined by laterally persistent fault zones trending NW–SE on both sides of the rift system. These bounding faults and basement exposures are clear in the study area and standout as topographic markers outlining the rift geometry ŽFig. 1.. The fault system geometry of the basin suggests an extensional setting. Generally, the Gulf of Suez is subdivided into three subprovinces which are in accordance to the dominant structural dip of the sedimentary cover ŽMoustafa, 1976.. These subprovinces are separated by two major transfer faults or hinge zones. The northern is the Zafarana Hinge Zone and separates the northern sector of the Gulf of Suez with its general dip to the southwest from the central sector which dips to the northeast. The second hinge zone is the Morgan Hinge Zone and separates the central sector of the Gulf from the southern sector which dips to the southwest ŽFig. 1..
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The timing of rifting is dated as late Oligocene to early Miocene. Dating the basaltic igneous activity in the Suez area has placed the rift between 19 to 24 Ma ŽFichera et al., 1992.. Litho- and bio-facies analysis of the stratigraphic sequence have suggested an age of late Oligocene to early Miocene ŽSalah, 1989, 1994.. Cochran and Martinez Ž1988. established the initiation of rifting from the time of evolving the oceanic crust of the Red Sea and indicated that this started about 23 Ma. Moretti et al. Ž1986. concluded that the main rifting phase was in early Miocene time. The interpretation of the data show that the southern sector of the Gulf of Suez consists of elongated troughs that contain several high trends Želongated structural highs.. Both troughs and highs have the same trend as the Gulf of Suez trend ŽNW–SE.. These highs are dissected by some cross elements which trend NE–SW and ENE–WSW. These are looked upon as strike-slip faults dislocating these highs. The distinctive structural and stratigraphic features vary within these subbasins and even within the same subbasin. These subbasins are, from west to east, Gemsa, West Shadwan, East Zeit, Ghara, East Shadwan and Eastern Trough ŽFig. 3.. The major high trends are, from north to south, Eastern, Shoab Ali, Islands, B, Hareed, Geisum, Felefel, Coastal, Jebel Zeit, Ras Bahar and Esh Mellaha ŽFig. 3.. The stratigraphic succession and depth to basement varies from one structural high to another and also varies within the same high. These highs are dissected by major cross-Gulf trending faults named as cross faults by Meshref et al. Ž1988.. 4. Basement petrography and diagenesis The materials used in this part of the study include thirty seven petrographic thin sections that were made from selected intervals in some cores,
Plate 2. ŽA. Closely spaced joints in granite partially filled with chlorite. The top most section is much more fractured than the bottom section. ŽJebel Zeit area.. ŽB. Extensional conjugate open fractures. Please note the directions of the fractures ŽJebel Zeit area.. ŽC. Highly chloritised granite Žleft. in contact with jointed Early Paleozoic Nubia Sandstone Žright.. Please note the impermeable shear zone in between. ŽJebel Zeit area.. ŽD. NW-trending mafic dyke cuts across granite ŽEsh Mellaha Range.. ŽE. NE-trending composite dyke, consists of older green andesite Žbottom. and younger buff colored rhyolite Žtop., cuts across granite ŽJebel Zeit area.. ŽF. Rose diagram showing the surface and subsurface fracture trends of the Precambrian basement in the southern Gulf of Suez.
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side-wall cores, ditch samples and surface outcrops of the Precambrian basement in the study area. These thin sections covered the offshore and onshore parts of the southern Gulf of Suez, including the Hareed, Felefel, Estakoza and Zeit Bay wells and the Jebel Zeit and Esh Mellaha range exposures. The core samples were first described and studied as hand specimens and then by a binocular microscope. Thin sections were made for selected intervals in the studied cores as well as for both side-wall core and ditch samples. The selected samples were examined petrographically after impregnation with blue plastic resin to detect the porosity types. All thin sections were studied petrographically and the diagenetic alterations were elucidated. This petrographic study aims to determine the composition, the main diagenetic processes and the reservoir potentiality of the basement in the study area. The role of diagenesis is interpreted in relation to pore geometry, size and filling material.
4.1. Petrography Igneous, metamorphic and volcanic rocks were recognized in both surface and subsurface basement. The granite and felsic dykes, that intruded the granites, are the main rock type. The volcanics are present in Andesite porphyry form. The granite consists mainly of feldspars and quartz with minor Ž0–5%. constituents of mafic minerals Žbiotite and hornblende.. The feldspars contain orthoclase, perthite and plagioclase with different percentages. The studied samples comprise a framework of flow aligned euhedral plagioclase feldspars, with intercrystalline subhedral alkali feldspars Žorthoclase and perthite. and anhedral quartz and green biotite ŽPlate 1.. Plagioclase feldspars have coronas of albitic composition which show granophyric intergrowths with quartz. The porosity of this granites depends mainly on the diagenetic processes but generally, the alteration processes reach their maximum intensity in the topmost section, known as the basement cover. Here
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solution and leaching has led to the enlargement of the fractures and vertical communication. Following the classification of Akkad and El Ramly Ž1960., these Precambrian basement granitic rocks can be classified into three groups: Quartz diorites, syeno-granites and alkaline granites. Quartz diorites: These were observed in samples from the Hareed and Zeit Bay wells. Framework grains are mainly composed of Ž) 70%. plagioclase; these holocrystalline crystals are fine to medium, subhedral to anhedral and partially sericitised. Other framework grains present are Ž- 10%. anhedral, fine to medium quartz, Ž16%. and nonferroan dolomite occurs as a spary cement infill and Ž3%. ferroan calcite ŽPlate 1A., Granodiorites: These granites contain less plagioclase Ž55–70%. and more quartz grains Ž) 20%. than the previous granite type. The quartz grains are predominantly unstrained, anhedral, single crystals or intergrowths of some crystals ŽPlate 1B.. Alkali feldspars occur as perthites andror albitic coronas on plagioclase crystals ŽPlate 1B.. Some samples showed fresh or locally sericitised Plagioclase ŽPlate 1B., while in others, the plagioclase is intensely sericitised ŽPlate 1C.. Green biotite is present in the form of scattered chloritised flakes or as subhedral to anhedral platy grains, and is locally altered to pyrite. Granodiorites are recorded from the Felefel, and Hareed wells. Alkaline granites: These are granites containing 65% or more potassium feldspars with almost 15% of plagioclase feldspars and no more than 15% of quartz, and are poorly to moderately sorted with angular to surrounded texture ŽPlate 1D and E.. This granite is present in samples from Zeit Bay ŽDarwish and El-Araby, 1993. the Esh Mellaha, Hareed and Estakoza wells and yield poor porosity. The potassium feldspars include orthoclase, perthites and microcline, which, along with plagioclase feldspars, show varying degrees of alteration and replacement by calcite ŽPlate 1E.. Monocrystalline quartz shows undulose extinction and polycrystalline quartz comprises multi-celled examples. Mica is dominantly chlorite.
Fig. 4. Fracture identification log of one of the Geisum wells showing the difference between the open fractures, appearing as dark colours, and the closed fractures which appear as light colours and the tracing of these fractures.
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4.2. Diagenesis Diagenesis of the Precambrian basement has been interpreted on the basis of cementation, as well as pore geometry and pore-filling material. The depositional setting, mineralogy, textures, pore-fluid composition and migration and burial history are the principal factors which have affected the basement diagenesis. Alteration products are varied but include pyrite replacing biotite; and anhydrite, dolomite and calcite replacing feldspars. Fractures are common, often plugged by chlorite, dolomite and calcite, although some intercrystalline porosity may be retained by the dolomite fracture fill. Some of the plagioclase were sericitised. The mafic mineral biotite is sometimes chloritised. The sequential diagenetic processes are: Ž1. crystallization of plagioclase feldspars; Ž2. final replacement of crystals; Ž3. cooling and crystallization of albitic coronas, perthits, quartz and biotite; Ž4. sericitisation of plagioclase feldspar; Ž5. minor chloritisation of biotites; Ž6. fracturing of cold intrusives; Ž7. cementation by dolomite and Ž8. selective alteration and replacement of crystals by pyrite. These diagenitic processes affected the reservoir quality of the Precambrian basement in several ways as discussed in the reservoir potential section in this paper.
5. Fracture geometry The goals of this part of the study are to detect the orientationŽs. of fractures, their porosity, permeability, and the relation between fracture geometry and production behavior. In this study, the borehole compensated neutron log ŽNPHI., the formation density compensated log ŽRHOB., the gamma ray log ŽGR., the formation microscanner tool ŽFMS., the high resolution dipmeter tool ŽHDT., the fracture identification logs ŽFIL. and the production logs ŽPL. were the main tools for enhancing the fracture analysis of the drilled Precambrian basement in the study area. 5.1. Orientations of fractures Both surface and subsurface investigations were carried out to determine the fracture patterns of the basement in the study area.
5.1.1. Surface features The Precambrian basement is exposed in Sinai, Shadwan Island, Esh Mellaha range and Jebel Zeit ŽFig. 1.. The exposed basement of Jebel Zeit and Esh Mellaha Range form an elongated NW–SE blocks parallel to the Gulf of Suez trend Žclysmic trend.. The Jebel Zeit form the main body in this study, being the most accessible one and the closest exposure to the Zeit Bay and Geisum oil fields, contain several mountain peaks that vary from 83 to 454 m in height with general drain eastward to the Gulf of Suez ŽFig. 1.. Four main trends of fractures were recognized ŽPlate 2. and matched with those described by El Wazeer and Ismail Ž1988., Abu Taleb et al. Ž1990. and Ismail and El Moula Ž1992.. These fracture trends are: Ža. NW–SE trend: is the dominant trend in the Precambrian basement and reflects the impact of the Oligo–Miocene rift of the Gulf of Suez basin. This trend is known as the clysmic trend; Žb. NE–SW trend: is the second dominant fracture trend in the study area and reflects the impact of the Gulf of Aqaba rift; Žc. N–S trend and Žd. ENE–WSW trend: these two trends reflect the combination of the Gulf of Suez and Gulf of Aqaba trends. 5.1.2. Subsurface features The FMS acquires conductivity measurements from sixty four electrodes on four image arrays equally spaced around the well borehole. Fractures in the wellbore are especially distinctive on the FMS images as their conductive nature is in sharp contrast to the resistive nature of the nonfractured basement rocks. Fracture anomalies appear on an azimuth plot of the images as portion of high amplitude, dark, sinusoids Žconductive. on the resistive Žlight. background of the image ŽFig. 4.. The orientation of the fractures can be detected by tracing the complete sinusoid of the fracture event on the azimuthal plot and calculating the vertical amplitude of the event and the orientation of the lowest portion of the sinusoid. The fracture dip is then calculated as the arc-tangent of the amplitude divided by the wellbore diameter with the dip azimuth as the orientation of the lowest part of the sinusoid. Zahran and Ismail Ž1986. and El Wazeer and Ismail Ž1988. recognized three major groups of fractures in the Zeit Bay oil Field:
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Fig. 5. Porosity ŽNPHI and ROHB. and gamma ray ŽGR. electrical logs of different rock types within the Precambrian basement. Note the high porosity in the fractured granite and the intrusive dykes.
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Ø from Ø from Ø from
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Fractures that strike N558W with a dip ranges 608 to 708 ŽGulf of Suez trend.; Fractures that strike N158W with a dip ranges 508 to 608 ŽRed Sea trend.; Fractures that strike N608E with a dip ranges 808 to 908 ŽGulf of Aqaba trend.
5.2. Morphological analysis of fractures This analysis includes the fracture numbers, width, and porosity. The degree of fracturing depends mainly on the ductility of the rock ŽStearns and Friedman, 1972.. In the most southern Gulf of Suez and the Northern Red Sea, several dykes Žless ductile. were recorded within the Precambrian basement. The electrical images of the FML can be interpreted easily to differentiate between the open fractures, appearing as dark colours, and the closed fractures which appear as light colours ŽFig. 4.. The interpretation of the Borehole electric imagery shows more fractures within the dykes, enhancing more porosity ŽFigs. 5 and 6.. The electrical image also contains amplitude data, which is correlatable to fracture aperture. The fracture traces on the images were analyzed and showed fracture apertures in the range of 10 m m to 3 q mm, averaging 300 m m ŽFig. 6.. The opening of the fractures could be due to mechanical weathering, chemical activity, erosion and leaching. These processes increase to great extent the porosity and permeability of the fractures and so increase the productivity from the fractured Precambrian basement. This is supported by the conclusions of El Wazeer and Ismail Ž1988. that the production rate from the Precambrian basement in the Zeit Bay oil field increases along the dykes. The porosity of the fractures is affected mainly by the fracture width, area, spacing, surface roughness and filling materials. The electric porosity logs NPHI and RHOB were interpreted to calculate the porosity of the different types of the Precambrian basement ŽFig. 5.. The interpretation of these porosity logs showed that the fractured granite, the metasediments and the intrusive dykes within the basement posses the highest porosities ŽFig. 5.. Also, some Helium, Mercury and Xenon core porosities were measured for some reservoir intervals within the fractured basement in Geisum oil field to check the reality of the porosities calculated from the fracture logs. The
fracture porosity calculated from the fracture logs were correlated with the porosity detected from the electric logs and cores and showed a good match ŽFig. 6.. Errors can occur in measuring the porosity from the electric images. This could be due to the changes in the conductivity of the fractures and the presence of any conductive minerals along the fractures, but overall, porosities calculated by this method give a considerable idea about the porosity in the fractured basement. El Wazeer and Ismail measured the porosity of the Precambrian basement in Zeit Bay oil field using the following equation: Fracture porositys
WF) L) h1 h3
,
where WF is the width of fracture, L is the length of fracture and h is the height of fracture. This equation was used to calculate the fracture porosity in some wells in Geisum and Esh Mellaha oil fields and shows a considerable match with the porosity detected from the electric logs. The fracture summary log ŽFig. 6. display the fracture porosity, fracture width in combination with conventional open hole interpretation. 5.3. Relation between fracture geometry and production behaÕior The fractured Precambrian basement is found to be the major producing reservoir at some oil fields in the study area, e.g., Zeit Bay, with an average production rate of 80,000 bblsrday ŽEl Wazeer and Ismail, 1988.. The production of hydrocarbons from the fractured Precambrian basement depends mainly on the fracture density and number of intersecting fracture sets. Those wells with single set of fracture show less production rate and faster declination of production. To evaluate the production efficiency of hydrocarbons from the fractured Precambrian basement in the southern Gulf of Suez and northern Red Sea, we must keep in mind the wellbore orientation as most of the wells in Geisum, Ashrafi and Zeit Bay oil fields are deviated and fractures are striking parallel to the well deviation direction. It is obvious from the production logs that the top section of the basement suffered more fracturing, wider fractures and posses higher fracture porosity. Hence the flow
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Fig. 6. Fracture summary log displays the fracture porosity, fracture width in combination with conventional open hole interpretation.
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rate increases at the top most section ŽFig. 7.. In some other cases at the Zeit Bay oil field, the production declined rapidly and a considerable- to large-movement up of the original oil water contact was recorded. Such move-up would suggest a marked coning of the water in the surrounding area. The analysis of the fracture geometry have revealed: Ž1. the fracture analyses in the Zeit bay and Geisum oil fields showed more or less the same trends as those detected from the surface exposures, Ž2. the directions of these fractures have the same orientations of the main controlling faults of most of the oil fields in the southern Gulf of Suez. Therefore, they are related to the same tectonic stresses, Ž3. the degree of fracturing relatively increases in the dykes Žless ductile., Ž4. the fracture width range from hairline to significant wide fractures, and Ž5. the open fractures are present in shallower levels, while the deeper zone are characterized by partially filled very fine fractures and microfractures. It should be stated that the presence of fractures in one well does not guarantee the same density of fractures in an offset well. This could be related to the location of the well with respect to the whole structural closure.
6. Hydrocarbon potential of the Precambrian basement The study area has excellent potential for hydrocarbon accumulation, as the rifting of the Gulf of Suez produces favorable conditions for Ž1. organicrich, oil andror gas-prone source rock deposition, Ž2. a suitable maturity regime for generating hydrocarbons, Ž3. the development of fractures in the Precambrian basement, and the accumulation of sandstone and carbonate reservoirs, ranging in age from Precambrian to Miocene, Ž4. the presence of potential fine-grained clastic and evaporite seals and Ž5. several types of traps for accumulation of hydrocarbons ŽSalah, 1994.. The stratigraphic column of the southern Gulf of Suez contains Pre-rift ŽPre-Miocene. and synrift ŽMiocene. potential source rocks. The Pre-Miocene
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source rocks are the Thebes-Esna interval, Duwi-Sudr interval ŽUpper Senonian Carbonates. and the shales of the Lower Senonian Matulla Formation. The Upper Senonian Carbonates ŽDuwi and Sudr Formations. are the richest and primary source rocks in the northern and central Gulf of Suez, however, the thickness and richness of these carbonates are decreasing southwards. Three Miocene intervals have been identified having rich potential source units: the Rudeis, Kareem and Belayim Formations. As this part of the rifted Gulf of Suez basin is characterized by variable crustal thickness, there are some hot spots which gave rise to localized mature source areas even within shallow depths ŽSalah, 1992.. The oilroil and oilrsource correlation indicated that the middle Miocene Kareem and Belayim Formations are the main source for the most southern Gulf of Suez and northern Red Sea oils ŽAlsharhan and Salah, 1994, 1995.. 6.1. ReserÕoir potential The Precambrian basement form one of the most important reservoirs in the southern Gulf of Suez and north Red Sea subbasins, and produce andror test oil from eight oil fields, e.g., Zeit Bay, Shoab Ali, Hilal, Sidki, Geisum, Ashrafi, Hareed and Esh Mellaha ŽFig. 1.. The reservoir parameters, net pay thicknesses range between about 10 and 300 m; porosities range from 2 to 15%, and permeabilities from 20 to 300 md. Overall reservoir quality depends on the density and orientation of fracturing, the presence of dykes and brecciated zones and the diagenetic processes such as feldspar alteration and kaolin precipitation. In addition to the petrographic investigation, the open hole electric and production logs were correlated to identify the different rock types of the Precambrian basement in the study area. The porosity logs, neutron ŽNPHI. and density ŽRHOB., the gamma ray ŽGR. and the production logs were used to enhance the correct estimation of its reservoir characterization and production ability. Fig. 5 shows
Fig. 7. Production log for the fractured Precambrian basement in one of the wells in the southern Gulf of Suez. The production is mainly from the upper part of the section as it has the maximum porosity.
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Fig. 8. Variations in reservoir quality in the Zeit Bay oil field due to the difference in their diagenitic histories.
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Fig. 9. Basement relief maps of Zeit Bay and Geisum oil fields showing the structural configuration of these fields.
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the impact of fractures and difference in rock types on responses of the NPHI, RHOB and GR logs. It is obvious that the fractured granite is more porous than that of the tight granite. The metasediments contain more radioactive materials Žuranium. as indicated by the high GR and is characterized by an approximate overlain between NPHI and RHOB ŽFig. 5.. The metavolcanics consists mainly of ferromagnesian minerals and hydrated aluminum silicates and so the density log reads very high. The intrusive dykes contain more radioactive materials and show very high GR and is characterized by a remarkable increase in NPHI log reflecting good porosity ŽFig. 5.. The diagenetic processes and related alteration phases of the basement play an important role in reservoir characterizations. As a result of leaching, some altered parts in the feldspars and micas are removed from the alteration products leaving behind open spaces which are present as intercrystalline and intracrystalline pores and vugs. The filling materials, alteration products Žchlorite, sericite, illite, kaolinite and silica. as well as secondary minerals of sedimentary origin Žcalcite, anhydrite and pyrite. are present in variable amounts in the studied Precambrian basement. Some of the fractures and the intercrystalline pores are partially or completely filled with one or mixture of these minerals. The high ratio of the secondary minerals may explain the direct connection of some of these fractures through open channels to the overlaying sedimentary cover ŽShaheen and Darwish, 1986.. The highly sericitised zones are of good reservoir quality while the kaolinitised zones are of poor reservoir quality. The corrosion of the quartz clusters, specially in the top most part of the basement, is followed by silica and pyrite filling which reduce the porosity to a minimum and can act as a seal. The Zeit Bay oil field is a very good example that shows variable reservoir qualities due to different diagenetic histories ŽFig. 8.. Almost all the oil fields in the southern Gulf of Suez produce hydrocarbons from the upper section of the fractured Precambrian basement. Most of the early drilled wells in Zeit Bay and Geisum oil fields were deepened to improve production from the fractured basement. However, the well Geisum-A4 originally tested 870 BOPD and jumped to 2800 after deepening the well, some water-cut was encountered
and production decreased to 1500 BOPD. The flowmeter data Žflow rate. was correlated with the fracture porosity and showed a good match ŽFig. 7.. 6.2. Trapping mechanism The fine clastics of the prerift sequence form potential vertical seals above the underlying basement reservoirs, and lateral seals when adjacent to these reservoirs across faults. The Miocene evaporites act as seals for the Precambrian basement reservoir on the down-thrown side of major clysmic faults, or downdip from uplifted, tilted fault blocks ŽFig. 9.. However, the magnitude of the throw on the clysmic faults is critical for effective sealing ŽRashed, 1990.. A small throw will site Paleozoic shales on the down-thrown side of the fault against the porous basement section on the uplifted block Že.g. the Esh Mellaha oil field.. A larger throw will bring the Upper Miocene evaporites against the basement reservoirs on the uplifted block as in the Zeit Bay, East Zeit and Hareed oil fields. Generally, two structural mechanisms for hydrocarbon entrapments are recorded in the Precambrian basement in the southern Gulf of Suez. These are: Ža. Faulted structural trap: Both the prerift and synrift reservoirs produce oil from a faulted trap which is sealed vertically by one of the seals and juxtapose a younger seal on the down thrown side of the fault ŽFig. 9.. This trap is sourced from prerift andror synrift sources across synthetic faults, e.g., Geisum and Shoab Ali oil fields, and Žb. Four way dip closures: This trap is present as the hanging wall anticlinal basement reservoirs sealed vertically by intraformational Miocene mudstones or evaporites and sourced across or up faults from prerift andror synrift sources, e.g., Zeit Bay oil field ŽFig. 9..
7. Conclusions The Precambrian basement rocks are composed of quartz–diorite, granodiorite, syenogranite, alkali granites and andesite porphyry dissected by means of dykes, fractures and joints. The main directions of fractures that affect these basement rocks are north-
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west–southeast, northeast–southwest and east-northeast–west-southwest. The reservoir characterization of the Precambrian basement depends mainly on the fractures, the diagenetic processes and the dip and direction of the dykes and brecciated zones. The topmost section, known as the basement cover, yields the best reservoir potential responding to the enlargement of the fractures and vertical communications, and the intensive affect of diagenetic processes. It is fair to say that where basement rocks are encountered in a closure on the crest of a tilted fault block, they possess sufficient porosity and permeability to be an effective potential reservoir.
Acknowledgements The authors are especially grateful to the Egyptian General Petroleum Corporation and Geisum Oil Company for supplying the materials and the necessary data on which the present work is based and the permission to publish it. We thank C.G.St.C. Kendall, University of South Carolina and JPSE reviewers H. Kulhe and A.Y. Huc for their constructive comments which we have used to improve our paper.
References Abdallah, A.M., Adindani, A., Fahmy, N., 1963. Stratigraphy of the Lower Mesozoic rocks, western side of the Gulf of Suez. Egyptian Geol. Surv. and Min. Res. Dept. 27, 23 pp. Abu Taleb, H., Helal, I., Standen, E.J., 1990. Granitic basement fracture study in the Geisum field, Gulf of Suez. Proc. 10th EGPC Explor. Semin. Cairo 1, 651–675. Akkad, M.K., El Ramly, M.F., 1960. Geological history and classification of the basement rocks of central Eastern Desert of Egypt. Geol. Surv. Egypt, 9, 24 pp. Akkad, M.K., Noweir, A., 1980. Geology and lithostratigraphy of the Arabian Desert orogenic belt of Egypt. Bull. Appl. Geol. King Abdul Aziz Univ. Jeddah 3, 127–135. Alsharhan, A.S., Salah, M.G., 1994. Geology and hydrocarbon habitat in rift setting: Southern Gulf of Suez, Egypt. Bull. Can. Petrol. Geol. 42 Ž3., 312–331. Alsharhan, A.S., Salah, M.G., 1995. Geology and hydrocarbon habitat in rift setting: Northern and central Gulf of Suez. Egypt. Bull. Can. Petrol. Geol. 43 Ž2., 156–176. Askary, S., 1986. Layering of basement in Zeit Bay field, Gulf of Suez, Egypt. Proc. 8th EGPC Explor. Semin. Cairo 1, 225–242. Bayer, H.J., Hotzl, ¨ H., Jado, A.R., Ruscher, B., Voggenreiter, W.,
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1988. Sedimentary and structural evolution of the northwest Arabian Red Sea margin. Tectonophysics 153, 137–151. Cochran, J.R., Martinez, F., 1988. Evidence from the northern Red Sea on the transition from the continental to oceanic rifting. Tectonophysics 153, 221–234. Darwish, M.S., El-Araby, A.M., 1993. Petrography and diagenetic aspects of some siliciclastic hydrocarbon reservoirs in relation to rifting of the Gulf of Suez. 1st Int. Symp. Sedimentation and Rifting in the Red Sea and Gulf of Aden. Geol. Surv. Egypt 1, 143–155. EGPC Stratigraphic Committee, 1964. Oligocene and Miocene stratigraphy, Gulf of Suez region. Spec. Publ., 142 pp. EGPC Stratigraphic Sub-Committee, 1974. Miocene rock stratigraphy of Egypt. Egyptian J. Geol. 18, 1–59. El Bayoumi, R.M., 1984. Ophiolites and melange complex of Wadi Ghadir, Eastern Desert, Egypt. Bull. Appl. Geol. King Abdul Aziz Univ. Jeddah 6, 324–329. El Gaby, S., List, L.K., Tehrani, R., 1990. The basement complex of the Eastern desert and Sinai. In: Said, R., 1990. Geology of Egypt. Balkema, Rotterdam, 734 pp. El Ramly, M.F., Akkad, M.K., 1960. The basement complex of X the central Eastern Desert of Egypt between Lat. 24830 and X 25840 N. Geol. Surv. Egypt 8, 35. El Shazly, E.M., 1964. On the classification of Precambrian and other rocks of magmatic affiliation in Egypt. Proc. of the Int. Geol. Conf., India, Sector No. 10. El Wazeer, F., Ismail, F., 1988. Analysis of subsurface fractures in basement Zeit Bay field, Gulf of Suez, Egypt. Proc. 8th EGPC Explor. Semin. Cairo 1, 68–85. El Wazeer, F., Ismail, F., Standen, E., 1990. Fracture geometry and hydrocarbon productivity in the basement rocks of the Zeit Bay oil field, Gulf of Suez, Egypt. Proc. 10th EGPC Explor. Semin. Cairo 1, 579–605. Evans, A.L., 1988. Neogene tectonic and stratigraphic events in the Gulf of Suez rift area, Egypt. Tectonophysics 153, 235– 247. Fichera, R., Giori, I., Milad, G., 1992. Southern Gulf of Suez: Interpretation of seismic, gravity and magnetic data as constraints in the resolution of deep structural setting. Proc. 11th EGPC Explor. Semin. Cairo 1, 31–44. Hammouda, H., 1992. Rift tectonics in the southern Gulf of Suez: Gravity and magnetic contribution. Proc. 11th EGPC Explor. Semin. Cairo 1, 17–28. Hashad, A.H., 1980. Present status of geochronological data on the Egyptian basement complex. Bull. Appl. Geol. King Abdul Aziz Univ. Jeddah 3, 31–46. Hassan, M.A., Hashad, A.H., 1990. Precambrian of Egypt. In: Said, R., ŽEd.., Geology of Egypt. Balkema, Rotterdam, 734 pp. Hughes, G., Varol, O., Beydoun, Z.R., 1991. Evidence for middle Oligocene rifting of the Gulf of Aden and for late Oligocene rifting of southern Red Sea. J. Mar. Petrol. Geol. 8, 354–358. Hume, W.F., 1911. The effect of secular oscillations during the Cretaceous and Eocene periods. Q. J. Geol. Soc. London 67, 118–148. Ismail, F., El Moula, I., 1992. The impact of dykes and brecciated zones on production form fractured basement reservoir Zeit
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Bay, Gulf of Suez, Egypt. Proc. 11th EGPC Explor. Semin. Cairo 1, 206–223. McKenzie, D.P., Davis, D., Molnar, P., 1970. Plate tectonics of the Red Sea and East Africa. Nature 226, 243–248. Meshref, W.M., Abu El-Karamat, M.S., Gindi, M.K., 1988. Exploration concepts for oil in the Gulf of Suez. Proc. 9th EGPC Explor. Semin. Cairo 1, 1–23. Meshref, W.M., Khalil, B., 1990. Evidence for northerly propagation in Suez rift. Proc. 10th EGPC Explor. Semin. Cairo 1, 55–77. Moretti, I., Chenet, P., Colletta, B., 1986. The narrowing of the Suez rift: A combination of stretching and secondary convection. Proc. 8th EGPC Explor. Semin. Cairo 1, 269–308. Moustafa, A.G., 1976. Block faulting in the Gulf of Suez. Proc. 5th EGPC Explor. Semin. Cairo 1, 1–36. Rashed, A., 1990. The main fault trends in the Gulf of Suez and their role in oil entrapment. Proc. 10th EGPC Explor. Semin. Cairo 1, 1–24. Richardson, M., Arthur, M.A., 1988. The Gulf of Suez-northern Red Sea Neogene rift: A quantitative basin analysis. J. Mar. Petrol. Geol. 5, 247–270. Robson, D.A., 1971. The structure of the Gulf of Suez Žclysmic. rift, with special references to the eastern side. J. Geol. Soc. London 127, 247–267. Said, R., 1962. Geology of Egypt. Elsevier, Amsterdam, N.Y., 317 pp.
Said, R. ŽEd.., 1990. The Geology of Egypt. Balkema, Rotterdam, 734 pp. Salah, M.G., 1989. Geology and hydrocarbon potential of the southern sector of the Gulf of Suez. Unpublished M.Sc. thesis, Cairo Univ., 130 pp. Salah, M.G., 1992. Geochemical evaluation of the southern sector of the Gulf of Suez. Proc. 11th EGPC Explor. Semin. Egypt 1, 384–385. Salah, M.G., 1994. Geology of the northwestern Red Sea with special emphasis on the petrology, sedimentology and hydrocarbon potential of the Miocene Kareem Formation. Unpublished Ph.D. thesis, Cairo Univ., 166 pp. Salah, M.G., Alsharhan, A.S., 1996. Structural influence on hydrocarbon entrapment in the northwestern Red Sea, Egypt. Bull. AAPG 80, 101–118. Shaheen, S., Darwish, M., 1986. Petrography, alteration processes and fracture patterns of basement reservoir rocks in Zeit Bay field, Gulf of Suez, Egypt. Proc. 7th EGPC Explor. Semin., Egypt 1, 206–224. Zahran, I., Ismail, F., 1986. Similarity in composition and tectonic style between basement exposed in Jebel El Zeit and Zeit Bay subsurface, Gulf of Suez, Egypt. Proc. 8th EGPC Explor. Semin. Egypt 1, 183–205. Zahran, I., Askary, S., 1988. Basement reservoir in Zeit Bay oil field, Gulf of Suez, Egypt. Proc. 9th EGPC Explor. Semin. Egypt 1, 163–188.
The Precambrian basement: A major reservoir in the rifted basin, Gulf of Suez M.G. Salah ) , A.S. Alsharhan
1
Faculty of Science, UAE UniÕersity, P.O. Box 17551, Al-Ain, United Arab Emirates Received 13 May 1996; accepted 10 June 1997
Abstract The expected instructions from an exploration manager, to stop drilling and abandon a well where the bit hits the Precambrian basement, no longer applies. The fractured and altered Precambrian basement rocks are the most prolific reservoirs in the southern Gulf of Suez and the northern Red Sea rifts where hydrocarbons are produced from 8 fields, with porosity and permeability values up to 15% and 300 millidarcy, respectively. The surface and subsurface Precambrian basement rocks are related to the final stages of the tectonic–magmatic cycle of the Arabo-Nubian Shield and are composed of quartz–diorite, granodiorite, syenogranite, alkali granites and andesite porphyry, dissected by means of dykes, fractures and joints. Three main directions of fractures, northwest–southeast, northeast–southwest and east northeast–west southwest have been detected in the study area. The porosity and production rates of this reservoir, as well as the oil–water contact movement, depend mainly on the age, intensity and direction of the fractures, diagenetic processes and the dip and direction of the dykes and brecciated zones. The alteration processes reach their maximum intensity in the topmost section, known as the basement cover, where the solution and leaching has led to the enlargement of the fractures and vertical communications. The underlying fracture zone has been affected by differential alteration processes, creating zones of high and low vertical porosity and permeability. Thus, the reservoir potential of the Precambrian basement has been greatly underestimated. q 1998 Elsevier Science B.V. Keywords: Gulf of Suez; Egypt; petroleum geology; reservoir characterization; Precambrian basement; fractured reservoirs
1. Introduction Hydrocarbons are produced from granitic andror volcanic reservoirs in more than twenty basins all over the world. Such basins are found in Algeria, Argentina, China, Great Britain, Indonesia, Russia, Ukraine, Venezuela, Yemen and many others ŽSalah, 1994.. The area that forms the scope of this study ) 1
Corresponding author. E-mail: [email protected] E-mail: [email protected]
lies in the southern Gulf of Suez and northern Red Sea, Egypt at the triple junction of the main rifts between the Red Sea, Gulf of Aqaba and Gulf of Suez ŽFig. 1.. The most characteristic topographic features of the study area are the exposures of the Precambrian basement massifs at four localities: Ž1. the Esh Mellaha range to the southwest, Ž2. Shadwan island to the southeast, Ž3. the Sinai massif to the northeast and Ž4. the Jebel Zeit to the northwest ŽFig. 2.. The Esh Mellaha range and the Jebel Zeit have a more or less complete pre-Miocene Žprerift. se-
0920-4105r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 0 - 4 1 0 5 Ž 9 7 . 0 0 0 2 4 - 7
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Fig. 1. Regional structural elements and geometry of the Gulf of Suez region showing the surface geology, major hinge zones, and regional dips in the subbasins.
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Fig. 2. Location map showing the onshore geology of the study area, distribution of the oil fields that produce hydrocarbons from the fractured basement and the wells used in his study.
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quence on its western side. The former is onlapped to the south by a Miocene reef complex at Jebel Abu Shaar plateau ŽFig. 2.. This work provides a comprehensive study on the reservoir characteristics of the Precambrian basement in this rift basin, including composition, nature and genesis of pore spaces, fracture pattern analysis, and factors controlling production. The prolific oil-producing Gulf of Suez basin contains more than 70 oil fields, and reservoirs range from the Precambrian basement to the upper Miocene. Some commercial oil andror gas fields such as Zeit Bay, Shoab Ali, Hilal, East Zeit and Geisum have already been discovered in the study area. The oldest discovery in the southern Gulf of Suez, being the first in the Middle East, was made in 1886 when crude oil seeped into tunnels which were dug for extracting sulphur in the Gemsa area on the western coast of the Gulf of Suez ŽFig. 2.. The recent Hareed oil and gas field was discovered in 1988 by Conoco, Esso and Gulf Canada oil companies ŽFig. 2.. The present analyses of the Precambrian basement in the southern Gulf of Suez and northwestern Red Sea are based on the interpretation of both geophysical and geological data, including subsurface information from more than 55 wells drilled in the study area, surface outcrops and aerial photograph examination of the structural configuration of the region. In addition, electrical, fractures, and production logs were used to help in detecting the reservoir characterization of the Precambrian basement and its fracture patterns. As the Zeit Bay and Geisum oil fields have the largest production from the fractured basement, they will be considered as case studies. For almost ninety years, the genesis and classification of the Precambrian basement rocks in Egypt with special interests on the Sinai and Eastern Desert basement rocks, have been the subject of numerous investigations. Among past and recent papers covering different aspects of the origin, development and classification of the Precambrian basement are those by El Ramly and Akkad Ž1960., El Shazly Ž1964., Hashad Ž1980., Akkad and Noweir Ž1980., El Bay-
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oumi Ž1984., Hassan and Hashad Ž1990. and El Gaby et al. Ž1990.. The Precambrian basement attracted the attention of the oil explorationists after the discovery of the Zeit Bay oil field in 1981. The reservoir characteristics of this additional potential reservoir in the Gulf of Suez oil basin were described by Askary Ž1986., Shaheen and Darwish Ž1986., Zahran and Ismail Ž1986., Zahran and Askary Ž1988., Abu Taleb et al. Ž1990., El Wazeer et al. Ž1990., Ismail and El Moula Ž1992., and Alsharhan and Salah Ž1994.. 2. Stratigraphic framework The definition of the lithostratigraphic units in the Gulf of Suez and the Red Sea has been studied by many authors, such as Hume Ž1911., Abdallah et al. Ž1963., EGPC Stratigraphic Committee, 1964 and EGPC Stratigraphic Sub-Committee Ž1974., Darwish and El-Araby Ž1993. and Alsharhan and Salah Ž1995.. The stratigraphic section of the study area ranges in age from Precambrian to Recent and can be classified into two megasequences: prerift Žpre-Miocene. and synrift ŽMiocene to Recent., as summarized below. 2.1. Prerift megasequence This section ranges in age from Cambrian to early Oligocene. It was penetrated by few wells drilled in the study area and is exposed to the west of the Esh Mellaha range and Jebel Zeit ŽFig. 2.. The prerift megasequence reflects a number of tectonic events that affected the area. Continental- to shallow-marine sandstones interbedded with thin shales and limestones were deposited from Cambrian to Early Cretaceous time ŽPre-Cenomanian., and are known as the Nubia Sandstones. Uplift and erosion preceded a widespread transgression from the north at the beginning of the Cenomanian. A southward-directed onlap resulted in overstepping of the Cenomanian, Turonian and early Senonian sequence, known as Nezzazat Group. This group consists of four formations, Raha, Abu Qada, Wata and Matulla, and consists of interbedded sandstones, shales and carbonates.
Fig. 3. Major tectonic elements map of the southern Gulf of Suez showing the main high trends, troughs ŽSource Kitchens., and migration pathways.
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During Senonian time there was a transition from mainly clastic facies in the lower unit ŽMatulla Formation. to dominantly carbonate facies in the upper unit ŽDuwi and Sudr Formations.. The Paleocene Esna Formation was deposited before a period of severe erosion. This event was simultaneous with the Syrian Arc system and led to the thinning of the Esna Formation over the whole area with complete erosion in some places. A widespread transgression occurred during Eocene time and the Esna Formation was overlain by the carbonates of the Thebes Formation. 2.2. Synrift megasequence This section represents late Oligocene to PostMiocene time and rests unconformably on the prerift section. Upper Oligocene sediments, known as the Tayba Formation, were not penetrated in the study area but are present only on the central sector of the Gulf of Suez. This formation consists mainly of mixed clastic and volcanic rocks. The early Miocene Nukhul Formation, deposited in different settings, reflects environmental heterogeneity due to the different tectonic setting of the separate fault-blocks. The Nukhul Formation consists of conglomeratic and brecciated sandstones with thin shale interbeds, in the lower member ŽShoab Ali., and is influenced by the unconformity between the Miocene and the PreMiocene sequences. The upper member ŽGhara. consists of anhydrites, limestones, shales and sandstones, deposited in restricted, shallow-marine and continental settings.
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As the maximum extensional stresses resulted in rapid subsidence, major half-graben troughs were formed, with deposition of the Rudeis Formation. The shoulders of the rift began to rise and coarse sands and conglomerates were deposited near the margins of the subsiding rift during the middle Miocene. Fine-grained sediments accumulated in deeper-marine settings, in addition to some prolific reservoirs consisting of submarine turbidites and coarser sandstones. During the middle to late Miocene, the climate in the Gulf of Suez and the northern Red Sea changed and marly evaporite cycles with sandstones and conglomeratic beds were deposited at localized areas, forming the Kareem and Belayim Formations. The culmination of the evaporitic sequence, South Gharib and Zeit Formations, occurred during Late Miocene times. The main constituents were salt and anhydrite with thin interbeds of shale. Some beds of sandstone are recorded at the marginal areas. During the Post-Miocene to Recent, two lithologies were deposited Žcarbonate and clastic. known as the Shagra and Wardan Formations, respectively. Palaeontological evidence suggest a connection to the Indian Ocean through the Red Sea during the Pliocene.
3. Structural analysis The study area was the subject of intensive surface and subsurface geological and geophysical in-
Plate 1. ŽA. Quartzdiorites Žsidewall core of well: Hareed-2; depth: 2530 m. ŽPPL s plane polarized light.. Phanerocrystalline, inequigranular, fine to medium and subhedral to anhedral plagioclase, with fine to medium and anhedral quartz grains. Grains are largely open packed and some of the intergranular pores are filled with dolomite and nonferroan calcite Žbottom center.. Note the high porosity due to intense fracturing. ŽB. Granodiorite Žwell: Hareed NB-1; depth: 2470 m. ŽXPL s cross polars.. Coarsely crystalline, fresh or locally sericitised plagioclase feldspars with unstrained and anhedral quartz crystals. Alkali feldspars occur as albitic coronas on the plagioclase crystals Žtop center.. Note the open fractures induced by sidewall coring processes. ŽC. Granodiorite ŽWell: Hareed-2; depth: 2503 m. ŽPPL.. Euhedral, intensely sericitised plagioclase feldspar with anhedral quartz crystals. The dark opaques comprise pyrite replacement of biotite. Note the cloudy sericitised plagioclase Žcenter right. and the flow alignment ŽCenter.. ŽD. Alkali granite Žwell: Estakosa-1; depth: 2665 m. ŽPPL.. Poorly to moderately-sorted and angular to surrounded lithic grains of granite float in a groundmass of aligned feldspars, opaques and green chlorite. Note the dolomite texture and the poor porosity. ŽE. Alkali granite Žwell: Estakosa-1; depth: 2636 m. ŽXPL.. Granitic fragment of quartz Žtop left. and sericitised potassium feldspar Žcenter. showing strong replacement by calcite Žtop center.. Note the very poor porosity. ŽF. Lithic fragments Žwell: Hareed NB-1; depth: 3002 m.ŽXPL.. Poorly-sorted sandstone with perthite rich granitic fragments Žtop right and bottom centre. contained in a ground mass of dolomite. Note the common patches of anhydrite and the kaolinite filled vugs. ŽG. Granodiorites Žwell: Hareed-2; depth: 2642 m. ŽPPL.. Intensively fractured coarse plagioclase feldspars with excellent porosity Žup to 21%. Žleft. with coarse crystalline quartz aggregate Žtop right.. ŽH. Granite ŽWell: Hareed-1; depth: 2650 m. ŽPPL.. Unfractured fragment Žbottom left. displaying no or very poor porosity.
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vestigations for hydrocarbon prospecting and exploration. Among these studies are those by Said Ž1962, 1990., Robson Ž1971., Bayer et al. Ž1988., Evans
Ž1988., Richardson and Arthur Ž1988., Salah Ž1989, 1994., Hughes et al. Ž1991., Hammouda Ž1992. and Salah and Alsharhan Ž1996..
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The Gulf of Suez rift represents a northern extension of the Red Sea. It constitutes a large depression which lies below sea level in its axial portion. The Suez rift trends in a NNW–SSE direction separating the African plate from the Sinai microplate ŽFig. 1.. McKenzie et al. Ž1970. matched the coastline of the Arabian Peninsula with that of African Red Sea to detect the precise direction of plate motion. Both reached the same conclusion, namely that the general movement of the Arabian plate relative to the African plate was toward the northeast. The contrasting widths of the northern portion of the Red Sea rift zone and the southern Gulf of Suez suggest that left-lateral movement along the Gulf of Aqaba strike–slip zone has allowed significantly more crustal extension within the Red Sea than has been achieved within the Gulf of Suez. The northwestern sector of the Red Sea has experienced more extension than the other parts of the Gulf of Suez ŽMeshref and Khalil, 1990.. The limits of the Gulf of Suez rift are defined by laterally persistent fault zones trending NW–SE on both sides of the rift system. These bounding faults and basement exposures are clear in the study area and standout as topographic markers outlining the rift geometry ŽFig. 1.. The fault system geometry of the basin suggests an extensional setting. Generally, the Gulf of Suez is subdivided into three subprovinces which are in accordance to the dominant structural dip of the sedimentary cover ŽMoustafa, 1976.. These subprovinces are separated by two major transfer faults or hinge zones. The northern is the Zafarana Hinge Zone and separates the northern sector of the Gulf of Suez with its general dip to the southwest from the central sector which dips to the northeast. The second hinge zone is the Morgan Hinge Zone and separates the central sector of the Gulf from the southern sector which dips to the southwest ŽFig. 1..
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The timing of rifting is dated as late Oligocene to early Miocene. Dating the basaltic igneous activity in the Suez area has placed the rift between 19 to 24 Ma ŽFichera et al., 1992.. Litho- and bio-facies analysis of the stratigraphic sequence have suggested an age of late Oligocene to early Miocene ŽSalah, 1989, 1994.. Cochran and Martinez Ž1988. established the initiation of rifting from the time of evolving the oceanic crust of the Red Sea and indicated that this started about 23 Ma. Moretti et al. Ž1986. concluded that the main rifting phase was in early Miocene time. The interpretation of the data show that the southern sector of the Gulf of Suez consists of elongated troughs that contain several high trends Želongated structural highs.. Both troughs and highs have the same trend as the Gulf of Suez trend ŽNW–SE.. These highs are dissected by some cross elements which trend NE–SW and ENE–WSW. These are looked upon as strike-slip faults dislocating these highs. The distinctive structural and stratigraphic features vary within these subbasins and even within the same subbasin. These subbasins are, from west to east, Gemsa, West Shadwan, East Zeit, Ghara, East Shadwan and Eastern Trough ŽFig. 3.. The major high trends are, from north to south, Eastern, Shoab Ali, Islands, B, Hareed, Geisum, Felefel, Coastal, Jebel Zeit, Ras Bahar and Esh Mellaha ŽFig. 3.. The stratigraphic succession and depth to basement varies from one structural high to another and also varies within the same high. These highs are dissected by major cross-Gulf trending faults named as cross faults by Meshref et al. Ž1988.. 4. Basement petrography and diagenesis The materials used in this part of the study include thirty seven petrographic thin sections that were made from selected intervals in some cores,
Plate 2. ŽA. Closely spaced joints in granite partially filled with chlorite. The top most section is much more fractured than the bottom section. ŽJebel Zeit area.. ŽB. Extensional conjugate open fractures. Please note the directions of the fractures ŽJebel Zeit area.. ŽC. Highly chloritised granite Žleft. in contact with jointed Early Paleozoic Nubia Sandstone Žright.. Please note the impermeable shear zone in between. ŽJebel Zeit area.. ŽD. NW-trending mafic dyke cuts across granite ŽEsh Mellaha Range.. ŽE. NE-trending composite dyke, consists of older green andesite Žbottom. and younger buff colored rhyolite Žtop., cuts across granite ŽJebel Zeit area.. ŽF. Rose diagram showing the surface and subsurface fracture trends of the Precambrian basement in the southern Gulf of Suez.
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side-wall cores, ditch samples and surface outcrops of the Precambrian basement in the study area. These thin sections covered the offshore and onshore parts of the southern Gulf of Suez, including the Hareed, Felefel, Estakoza and Zeit Bay wells and the Jebel Zeit and Esh Mellaha range exposures. The core samples were first described and studied as hand specimens and then by a binocular microscope. Thin sections were made for selected intervals in the studied cores as well as for both side-wall core and ditch samples. The selected samples were examined petrographically after impregnation with blue plastic resin to detect the porosity types. All thin sections were studied petrographically and the diagenetic alterations were elucidated. This petrographic study aims to determine the composition, the main diagenetic processes and the reservoir potentiality of the basement in the study area. The role of diagenesis is interpreted in relation to pore geometry, size and filling material.
4.1. Petrography Igneous, metamorphic and volcanic rocks were recognized in both surface and subsurface basement. The granite and felsic dykes, that intruded the granites, are the main rock type. The volcanics are present in Andesite porphyry form. The granite consists mainly of feldspars and quartz with minor Ž0–5%. constituents of mafic minerals Žbiotite and hornblende.. The feldspars contain orthoclase, perthite and plagioclase with different percentages. The studied samples comprise a framework of flow aligned euhedral plagioclase feldspars, with intercrystalline subhedral alkali feldspars Žorthoclase and perthite. and anhedral quartz and green biotite ŽPlate 1.. Plagioclase feldspars have coronas of albitic composition which show granophyric intergrowths with quartz. The porosity of this granites depends mainly on the diagenetic processes but generally, the alteration processes reach their maximum intensity in the topmost section, known as the basement cover. Here
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solution and leaching has led to the enlargement of the fractures and vertical communication. Following the classification of Akkad and El Ramly Ž1960., these Precambrian basement granitic rocks can be classified into three groups: Quartz diorites, syeno-granites and alkaline granites. Quartz diorites: These were observed in samples from the Hareed and Zeit Bay wells. Framework grains are mainly composed of Ž) 70%. plagioclase; these holocrystalline crystals are fine to medium, subhedral to anhedral and partially sericitised. Other framework grains present are Ž- 10%. anhedral, fine to medium quartz, Ž16%. and nonferroan dolomite occurs as a spary cement infill and Ž3%. ferroan calcite ŽPlate 1A., Granodiorites: These granites contain less plagioclase Ž55–70%. and more quartz grains Ž) 20%. than the previous granite type. The quartz grains are predominantly unstrained, anhedral, single crystals or intergrowths of some crystals ŽPlate 1B.. Alkali feldspars occur as perthites andror albitic coronas on plagioclase crystals ŽPlate 1B.. Some samples showed fresh or locally sericitised Plagioclase ŽPlate 1B., while in others, the plagioclase is intensely sericitised ŽPlate 1C.. Green biotite is present in the form of scattered chloritised flakes or as subhedral to anhedral platy grains, and is locally altered to pyrite. Granodiorites are recorded from the Felefel, and Hareed wells. Alkaline granites: These are granites containing 65% or more potassium feldspars with almost 15% of plagioclase feldspars and no more than 15% of quartz, and are poorly to moderately sorted with angular to surrounded texture ŽPlate 1D and E.. This granite is present in samples from Zeit Bay ŽDarwish and El-Araby, 1993. the Esh Mellaha, Hareed and Estakoza wells and yield poor porosity. The potassium feldspars include orthoclase, perthites and microcline, which, along with plagioclase feldspars, show varying degrees of alteration and replacement by calcite ŽPlate 1E.. Monocrystalline quartz shows undulose extinction and polycrystalline quartz comprises multi-celled examples. Mica is dominantly chlorite.
Fig. 4. Fracture identification log of one of the Geisum wells showing the difference between the open fractures, appearing as dark colours, and the closed fractures which appear as light colours and the tracing of these fractures.
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4.2. Diagenesis Diagenesis of the Precambrian basement has been interpreted on the basis of cementation, as well as pore geometry and pore-filling material. The depositional setting, mineralogy, textures, pore-fluid composition and migration and burial history are the principal factors which have affected the basement diagenesis. Alteration products are varied but include pyrite replacing biotite; and anhydrite, dolomite and calcite replacing feldspars. Fractures are common, often plugged by chlorite, dolomite and calcite, although some intercrystalline porosity may be retained by the dolomite fracture fill. Some of the plagioclase were sericitised. The mafic mineral biotite is sometimes chloritised. The sequential diagenetic processes are: Ž1. crystallization of plagioclase feldspars; Ž2. final replacement of crystals; Ž3. cooling and crystallization of albitic coronas, perthits, quartz and biotite; Ž4. sericitisation of plagioclase feldspar; Ž5. minor chloritisation of biotites; Ž6. fracturing of cold intrusives; Ž7. cementation by dolomite and Ž8. selective alteration and replacement of crystals by pyrite. These diagenitic processes affected the reservoir quality of the Precambrian basement in several ways as discussed in the reservoir potential section in this paper.
5. Fracture geometry The goals of this part of the study are to detect the orientationŽs. of fractures, their porosity, permeability, and the relation between fracture geometry and production behavior. In this study, the borehole compensated neutron log ŽNPHI., the formation density compensated log ŽRHOB., the gamma ray log ŽGR., the formation microscanner tool ŽFMS., the high resolution dipmeter tool ŽHDT., the fracture identification logs ŽFIL. and the production logs ŽPL. were the main tools for enhancing the fracture analysis of the drilled Precambrian basement in the study area. 5.1. Orientations of fractures Both surface and subsurface investigations were carried out to determine the fracture patterns of the basement in the study area.
5.1.1. Surface features The Precambrian basement is exposed in Sinai, Shadwan Island, Esh Mellaha range and Jebel Zeit ŽFig. 1.. The exposed basement of Jebel Zeit and Esh Mellaha Range form an elongated NW–SE blocks parallel to the Gulf of Suez trend Žclysmic trend.. The Jebel Zeit form the main body in this study, being the most accessible one and the closest exposure to the Zeit Bay and Geisum oil fields, contain several mountain peaks that vary from 83 to 454 m in height with general drain eastward to the Gulf of Suez ŽFig. 1.. Four main trends of fractures were recognized ŽPlate 2. and matched with those described by El Wazeer and Ismail Ž1988., Abu Taleb et al. Ž1990. and Ismail and El Moula Ž1992.. These fracture trends are: Ža. NW–SE trend: is the dominant trend in the Precambrian basement and reflects the impact of the Oligo–Miocene rift of the Gulf of Suez basin. This trend is known as the clysmic trend; Žb. NE–SW trend: is the second dominant fracture trend in the study area and reflects the impact of the Gulf of Aqaba rift; Žc. N–S trend and Žd. ENE–WSW trend: these two trends reflect the combination of the Gulf of Suez and Gulf of Aqaba trends. 5.1.2. Subsurface features The FMS acquires conductivity measurements from sixty four electrodes on four image arrays equally spaced around the well borehole. Fractures in the wellbore are especially distinctive on the FMS images as their conductive nature is in sharp contrast to the resistive nature of the nonfractured basement rocks. Fracture anomalies appear on an azimuth plot of the images as portion of high amplitude, dark, sinusoids Žconductive. on the resistive Žlight. background of the image ŽFig. 4.. The orientation of the fractures can be detected by tracing the complete sinusoid of the fracture event on the azimuthal plot and calculating the vertical amplitude of the event and the orientation of the lowest portion of the sinusoid. The fracture dip is then calculated as the arc-tangent of the amplitude divided by the wellbore diameter with the dip azimuth as the orientation of the lowest part of the sinusoid. Zahran and Ismail Ž1986. and El Wazeer and Ismail Ž1988. recognized three major groups of fractures in the Zeit Bay oil Field:
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Fig. 5. Porosity ŽNPHI and ROHB. and gamma ray ŽGR. electrical logs of different rock types within the Precambrian basement. Note the high porosity in the fractured granite and the intrusive dykes.
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Ø from Ø from Ø from
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Fractures that strike N558W with a dip ranges 608 to 708 ŽGulf of Suez trend.; Fractures that strike N158W with a dip ranges 508 to 608 ŽRed Sea trend.; Fractures that strike N608E with a dip ranges 808 to 908 ŽGulf of Aqaba trend.
5.2. Morphological analysis of fractures This analysis includes the fracture numbers, width, and porosity. The degree of fracturing depends mainly on the ductility of the rock ŽStearns and Friedman, 1972.. In the most southern Gulf of Suez and the Northern Red Sea, several dykes Žless ductile. were recorded within the Precambrian basement. The electrical images of the FML can be interpreted easily to differentiate between the open fractures, appearing as dark colours, and the closed fractures which appear as light colours ŽFig. 4.. The interpretation of the Borehole electric imagery shows more fractures within the dykes, enhancing more porosity ŽFigs. 5 and 6.. The electrical image also contains amplitude data, which is correlatable to fracture aperture. The fracture traces on the images were analyzed and showed fracture apertures in the range of 10 m m to 3 q mm, averaging 300 m m ŽFig. 6.. The opening of the fractures could be due to mechanical weathering, chemical activity, erosion and leaching. These processes increase to great extent the porosity and permeability of the fractures and so increase the productivity from the fractured Precambrian basement. This is supported by the conclusions of El Wazeer and Ismail Ž1988. that the production rate from the Precambrian basement in the Zeit Bay oil field increases along the dykes. The porosity of the fractures is affected mainly by the fracture width, area, spacing, surface roughness and filling materials. The electric porosity logs NPHI and RHOB were interpreted to calculate the porosity of the different types of the Precambrian basement ŽFig. 5.. The interpretation of these porosity logs showed that the fractured granite, the metasediments and the intrusive dykes within the basement posses the highest porosities ŽFig. 5.. Also, some Helium, Mercury and Xenon core porosities were measured for some reservoir intervals within the fractured basement in Geisum oil field to check the reality of the porosities calculated from the fracture logs. The
fracture porosity calculated from the fracture logs were correlated with the porosity detected from the electric logs and cores and showed a good match ŽFig. 6.. Errors can occur in measuring the porosity from the electric images. This could be due to the changes in the conductivity of the fractures and the presence of any conductive minerals along the fractures, but overall, porosities calculated by this method give a considerable idea about the porosity in the fractured basement. El Wazeer and Ismail measured the porosity of the Precambrian basement in Zeit Bay oil field using the following equation: Fracture porositys
WF) L) h1 h3
,
where WF is the width of fracture, L is the length of fracture and h is the height of fracture. This equation was used to calculate the fracture porosity in some wells in Geisum and Esh Mellaha oil fields and shows a considerable match with the porosity detected from the electric logs. The fracture summary log ŽFig. 6. display the fracture porosity, fracture width in combination with conventional open hole interpretation. 5.3. Relation between fracture geometry and production behaÕior The fractured Precambrian basement is found to be the major producing reservoir at some oil fields in the study area, e.g., Zeit Bay, with an average production rate of 80,000 bblsrday ŽEl Wazeer and Ismail, 1988.. The production of hydrocarbons from the fractured Precambrian basement depends mainly on the fracture density and number of intersecting fracture sets. Those wells with single set of fracture show less production rate and faster declination of production. To evaluate the production efficiency of hydrocarbons from the fractured Precambrian basement in the southern Gulf of Suez and northern Red Sea, we must keep in mind the wellbore orientation as most of the wells in Geisum, Ashrafi and Zeit Bay oil fields are deviated and fractures are striking parallel to the well deviation direction. It is obvious from the production logs that the top section of the basement suffered more fracturing, wider fractures and posses higher fracture porosity. Hence the flow
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Fig. 6. Fracture summary log displays the fracture porosity, fracture width in combination with conventional open hole interpretation.
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rate increases at the top most section ŽFig. 7.. In some other cases at the Zeit Bay oil field, the production declined rapidly and a considerable- to large-movement up of the original oil water contact was recorded. Such move-up would suggest a marked coning of the water in the surrounding area. The analysis of the fracture geometry have revealed: Ž1. the fracture analyses in the Zeit bay and Geisum oil fields showed more or less the same trends as those detected from the surface exposures, Ž2. the directions of these fractures have the same orientations of the main controlling faults of most of the oil fields in the southern Gulf of Suez. Therefore, they are related to the same tectonic stresses, Ž3. the degree of fracturing relatively increases in the dykes Žless ductile., Ž4. the fracture width range from hairline to significant wide fractures, and Ž5. the open fractures are present in shallower levels, while the deeper zone are characterized by partially filled very fine fractures and microfractures. It should be stated that the presence of fractures in one well does not guarantee the same density of fractures in an offset well. This could be related to the location of the well with respect to the whole structural closure.
6. Hydrocarbon potential of the Precambrian basement The study area has excellent potential for hydrocarbon accumulation, as the rifting of the Gulf of Suez produces favorable conditions for Ž1. organicrich, oil andror gas-prone source rock deposition, Ž2. a suitable maturity regime for generating hydrocarbons, Ž3. the development of fractures in the Precambrian basement, and the accumulation of sandstone and carbonate reservoirs, ranging in age from Precambrian to Miocene, Ž4. the presence of potential fine-grained clastic and evaporite seals and Ž5. several types of traps for accumulation of hydrocarbons ŽSalah, 1994.. The stratigraphic column of the southern Gulf of Suez contains Pre-rift ŽPre-Miocene. and synrift ŽMiocene. potential source rocks. The Pre-Miocene
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source rocks are the Thebes-Esna interval, Duwi-Sudr interval ŽUpper Senonian Carbonates. and the shales of the Lower Senonian Matulla Formation. The Upper Senonian Carbonates ŽDuwi and Sudr Formations. are the richest and primary source rocks in the northern and central Gulf of Suez, however, the thickness and richness of these carbonates are decreasing southwards. Three Miocene intervals have been identified having rich potential source units: the Rudeis, Kareem and Belayim Formations. As this part of the rifted Gulf of Suez basin is characterized by variable crustal thickness, there are some hot spots which gave rise to localized mature source areas even within shallow depths ŽSalah, 1992.. The oilroil and oilrsource correlation indicated that the middle Miocene Kareem and Belayim Formations are the main source for the most southern Gulf of Suez and northern Red Sea oils ŽAlsharhan and Salah, 1994, 1995.. 6.1. ReserÕoir potential The Precambrian basement form one of the most important reservoirs in the southern Gulf of Suez and north Red Sea subbasins, and produce andror test oil from eight oil fields, e.g., Zeit Bay, Shoab Ali, Hilal, Sidki, Geisum, Ashrafi, Hareed and Esh Mellaha ŽFig. 1.. The reservoir parameters, net pay thicknesses range between about 10 and 300 m; porosities range from 2 to 15%, and permeabilities from 20 to 300 md. Overall reservoir quality depends on the density and orientation of fracturing, the presence of dykes and brecciated zones and the diagenetic processes such as feldspar alteration and kaolin precipitation. In addition to the petrographic investigation, the open hole electric and production logs were correlated to identify the different rock types of the Precambrian basement in the study area. The porosity logs, neutron ŽNPHI. and density ŽRHOB., the gamma ray ŽGR. and the production logs were used to enhance the correct estimation of its reservoir characterization and production ability. Fig. 5 shows
Fig. 7. Production log for the fractured Precambrian basement in one of the wells in the southern Gulf of Suez. The production is mainly from the upper part of the section as it has the maximum porosity.
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Fig. 8. Variations in reservoir quality in the Zeit Bay oil field due to the difference in their diagenitic histories.
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Fig. 9. Basement relief maps of Zeit Bay and Geisum oil fields showing the structural configuration of these fields.
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the impact of fractures and difference in rock types on responses of the NPHI, RHOB and GR logs. It is obvious that the fractured granite is more porous than that of the tight granite. The metasediments contain more radioactive materials Žuranium. as indicated by the high GR and is characterized by an approximate overlain between NPHI and RHOB ŽFig. 5.. The metavolcanics consists mainly of ferromagnesian minerals and hydrated aluminum silicates and so the density log reads very high. The intrusive dykes contain more radioactive materials and show very high GR and is characterized by a remarkable increase in NPHI log reflecting good porosity ŽFig. 5.. The diagenetic processes and related alteration phases of the basement play an important role in reservoir characterizations. As a result of leaching, some altered parts in the feldspars and micas are removed from the alteration products leaving behind open spaces which are present as intercrystalline and intracrystalline pores and vugs. The filling materials, alteration products Žchlorite, sericite, illite, kaolinite and silica. as well as secondary minerals of sedimentary origin Žcalcite, anhydrite and pyrite. are present in variable amounts in the studied Precambrian basement. Some of the fractures and the intercrystalline pores are partially or completely filled with one or mixture of these minerals. The high ratio of the secondary minerals may explain the direct connection of some of these fractures through open channels to the overlaying sedimentary cover ŽShaheen and Darwish, 1986.. The highly sericitised zones are of good reservoir quality while the kaolinitised zones are of poor reservoir quality. The corrosion of the quartz clusters, specially in the top most part of the basement, is followed by silica and pyrite filling which reduce the porosity to a minimum and can act as a seal. The Zeit Bay oil field is a very good example that shows variable reservoir qualities due to different diagenetic histories ŽFig. 8.. Almost all the oil fields in the southern Gulf of Suez produce hydrocarbons from the upper section of the fractured Precambrian basement. Most of the early drilled wells in Zeit Bay and Geisum oil fields were deepened to improve production from the fractured basement. However, the well Geisum-A4 originally tested 870 BOPD and jumped to 2800 after deepening the well, some water-cut was encountered
and production decreased to 1500 BOPD. The flowmeter data Žflow rate. was correlated with the fracture porosity and showed a good match ŽFig. 7.. 6.2. Trapping mechanism The fine clastics of the prerift sequence form potential vertical seals above the underlying basement reservoirs, and lateral seals when adjacent to these reservoirs across faults. The Miocene evaporites act as seals for the Precambrian basement reservoir on the down-thrown side of major clysmic faults, or downdip from uplifted, tilted fault blocks ŽFig. 9.. However, the magnitude of the throw on the clysmic faults is critical for effective sealing ŽRashed, 1990.. A small throw will site Paleozoic shales on the down-thrown side of the fault against the porous basement section on the uplifted block Že.g. the Esh Mellaha oil field.. A larger throw will bring the Upper Miocene evaporites against the basement reservoirs on the uplifted block as in the Zeit Bay, East Zeit and Hareed oil fields. Generally, two structural mechanisms for hydrocarbon entrapments are recorded in the Precambrian basement in the southern Gulf of Suez. These are: Ža. Faulted structural trap: Both the prerift and synrift reservoirs produce oil from a faulted trap which is sealed vertically by one of the seals and juxtapose a younger seal on the down thrown side of the fault ŽFig. 9.. This trap is sourced from prerift andror synrift sources across synthetic faults, e.g., Geisum and Shoab Ali oil fields, and Žb. Four way dip closures: This trap is present as the hanging wall anticlinal basement reservoirs sealed vertically by intraformational Miocene mudstones or evaporites and sourced across or up faults from prerift andror synrift sources, e.g., Zeit Bay oil field ŽFig. 9..
7. Conclusions The Precambrian basement rocks are composed of quartz–diorite, granodiorite, syenogranite, alkali granites and andesite porphyry dissected by means of dykes, fractures and joints. The main directions of fractures that affect these basement rocks are north-
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west–southeast, northeast–southwest and east-northeast–west-southwest. The reservoir characterization of the Precambrian basement depends mainly on the fractures, the diagenetic processes and the dip and direction of the dykes and brecciated zones. The topmost section, known as the basement cover, yields the best reservoir potential responding to the enlargement of the fractures and vertical communications, and the intensive affect of diagenetic processes. It is fair to say that where basement rocks are encountered in a closure on the crest of a tilted fault block, they possess sufficient porosity and permeability to be an effective potential reservoir.
Acknowledgements The authors are especially grateful to the Egyptian General Petroleum Corporation and Geisum Oil Company for supplying the materials and the necessary data on which the present work is based and the permission to publish it. We thank C.G.St.C. Kendall, University of South Carolina and JPSE reviewers H. Kulhe and A.Y. Huc for their constructive comments which we have used to improve our paper.
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