geology

Wednesday, 27 April 2011

petroleum geology about Sudan

About Sudan

Sudan is a vast country of approximately 2.5 million square kilometres and a population of about 30 millions. Khartoum is the capital of Sudan lies at the confluence of the White Nile and the Blue Nile, both forming the river Nile, the longest river in the world, which flows into Mediterranean Sea. Sudan has common borders with nine countries in addition to the Red Sea coastline, which connects the country to the Middle East and Asia. As such Sudan is considered as an important gate to Africa. The climate in Sudan ranges from equatorial frost in the south to the great desert in the north.
Port Sudan is the main port of the country assisted by the smaller port of Suakin. Recently an oil export terminal of Bushier was opened in August 1999, see Fig (1).
The geology of the Sudan is extremely diverse and its interior basin evolution (Late Jurassic-Early Cretaceous) is related to the Central Africa Shear Zone.
Exploration activities started in the late fifties in the Red Sea and in the early seventies in the south central Sudan. Several hydrocarbon fields have been discovered offshore and onshore.
With the exception of the limited exploration efforts which have been put to evaluate the hydrocarbon potential of south central and west Sudan, most areas are considered to be largely unexplored.
The infrastructures for petroleum industry in the Sudan are in a good position now and it is continuously progressing.

Exploration History
Hydrocarbon exploration activities in Sudan date back to the late fifties, when the Italian Agip was granted the first exploration permit in the Red Sea. The company drilled 6 wells; non-of them considered a discovery.
The most important episodes in the history of exploration in the Red Sea basin are those of Chevron Oil Company (1975 – 1977) and the International Petroleum Corporation (IPC 1991-1996) with notably:
  * Three wells drilled by Chevron, two of them were considered dry Gas and wet Gas/condensate discoveries.
  * 1061 Kms. of 2D marine reflection Seismic lines were conducted by Geco Prakla for IPC, which enabled imaging of deep structures beneath thick salt and evaporite layers.
In 1975, Chevron signed a production sharing agreement with Sudan government to explore for oil and gas over a concession area (516000 sq. Kms.) in central and south central Sudan. After conducting extensive geological and geophysical works, Chevron started drilling operations, which let to the first oil discovery at Abu Gabra in 1979.
In 1980, Chevron discovered Unity oil field, followed by Heglig, Talih, Sharaf, Adar and Yale oil fields.
In 1980, Total Oil Company signed an EPSA with the Government for oil exploration in block (B). The company conducted geological and geophysical work and defined prospects for exploration drilling. Due to the break out of civil war in the southern Sudan, Total Company suspended its operations up to date.
In 1982, Sun Oil Company was awarded a concession in central and north central Sudan. The company drilled six wells, however, no significant hydrocarbon discoveries were encountered, hence Sun terminated its contract in 1990.
In August 1993, State Petroleum Company (subsidiary of Arakis) signed an EPSA with the Government over blocks 1, 2 and 4. The Company discovered very significant oil accumulations in Toma South, El toor, El nar and Um sagura fields.
In August 17, 1995, The government and Gulf Petroleum Company signed a Production Sharing Agreement for the development of Adar- Yale oil field (Block 3D).
In September 1995, the Government and the China National Petroleum Corporation signed EPSA over Block (6). The company has drilled three successful appraisal wells based on 3D seismic work, two of them in Abu Gabra field and one in Sharaf field.
In March 1996 a consortium group, which includes CNPC, PETRONAS, SUDAPET and SPC signed agreements for conducting upstream and downstream operations in blocks # 1, 2 and 4 originally acquired by State.
In Oct. 1998,Tilsman has taken the share of State Petroleum Company in the consortium.
In March, 12, 2000 a consortium of Gulf Petrolum Company (Sudan) Ltd., Melut Petroleum Company Ltd. and Sudapet Ltd. signed production sharing agreement over blocks 3D, 3E and 7E.
  FIG(6) shows part of  the petroleum infrastructures in the Sudan, which include the existing and under construction refineries, the crude oil pipeline route (L = 1610 KM., DIA. = 28 Inch, MAX. CAP. = 450000 BBLS/DAY) and  transportation means. In addition to these there are good communication means, airports, airstrips..etc.
The Sudanese Petroleum Data Center (SPDC) and the Sudanese Central Petroleum Laboratory (SCPL) are at their final touches to start furnishing services, to all oil companies working in upstream and downstream sectors, by the end of this year (1999).
The Sudanese Petroleum Corporation (SPC) and the Bureau of Geophysical Prospecting (BGP) have formed a joint seismic processing company, under the name the Blue Nile Processing Company Ltd. (BPC), and registered under the Sudanese laws.
The company is set up with objective of providing qualified 2D and 3D processing services for clients both in and outside Sudan. The annual processing capacity of BPC is up to 6000 Kms. Of  2D and 500 Km² of  3D, or simply 10,000 Km of  2D.

Out Line of Geology of Sudan
Structural Setting
The interior Sudan basins have been related in its evolution to the west and central Africa rift system, which was formed in Late Jurassic to Early Cretaceous. The shear stresses, which have been released as the result of the Global tectonic, which led to the separation of Africa from south America and the subsequent opening of the south Atlantic basin, has been transferred along the central Africa Fault Zone. The structural development of the Muglad basin is marked with three major rift cycles and sagging characterized with coarsening upward sequences of clastic sediments, those are:
·        Early Rifting; began in Late Jurassic – Early Cretaceous (130 – 150 MYBP). It is the strongest and lasted until near the end of Albian.
·        Intermediate Rifting; Senonian – Turonian. Development of rift lakes and deposition of lacustrine and flood – plain fine sediments (Aradeiba Fm.). It is associated with minor volcanism.
·        Late Rifting; Late Eocene – Oligocene. It is equivalent to the opening of the Red Sea and East African Rift.
·        Sagging; Middle Miocene, gentle subsidence and little faulting.  
The age estimates are poorly constrained by sparse palynological data.The recognition of the three rift cycles is based on identification of three regionally correlative depositional cycles which are particularly evident in the Muglad basin. Each cycle boundary is locally expressed by a subtle angular unconformity.
These movements led to the creation of a complex system of linked  extensional and transtensional sub-basins. The sub-basins typically have a half-graben geometry that was modified by subsequent reactivation during younger rift cycles. These grabenal structures trending generally NW – SE and arranged in an echelon pattern.
Muglad, Melut and Blue Nile are the three major sedimentary basins
Geological Setting.
 The geology of the Sudan Fig. (2) is extremely diverse with a variety of metamorphic, igneous and sedimentary rocks. Three metamorphic belts have been recognized, these are:
·         South Equatoria.Imatong Mountain.
·         Central Sudan and Nile Valley.
·         Red Sea.
 The intensity of deformation and metamorphism varies from place to place. Extensive studies were carried out by several workers, proved that the central Sudan and Nile Valley metamorphic belt include relic structures of ancient basement which date back to Lower Proterozoic or even Archean.
The oldest sedimentary rocks encountered so far in Sudan are of Cambro - Ordovician age? . These occur within narrow grabens formed by rifting, which preceded consolidation of Pan African structures in north and north-western Sudan, or as thin sedimentary cover (Sassa plain – Red Sea Hills) elsewhere entrapped within shallow underlying basement rocks.
Fig (2) shows the distribution of sedimentary basins in Sudan. Much attention has been given to explore the rift related basin systems, south central and central Sudan, while those defined by surface mapping and regional gravity work in central north and northern Sudan received less attention.
The gravity, seismic and drilling data acquired in interior Sudan basins indicated that more than 30000 feet of clastic sediments occur within the deepest central trough of the three major rift basins.
Fig(3), shows generalized stratigraphic columns for the Kaikang trough, which runs along the western flank of the Muglad basin and that of Unity in the Eastern flank, whereas, Fig(4) shows the stratigraphy of the Melut basin.
The sediments are interbedded sandstones, claystones, siltstones, mudstones and shales. Intrusive rocks (Sills) were encountered in some wells such as in
Garad – 1, Sobat – 1, Tabaldi - 1…etc.
Changes in lithofacies primarily reflect variations in the subsidence rates of various sub-basins. The lack of significant magmatism during active rifting, despite 20 to 30 KM of extension, is attributed to the shallow depth of fault detachments at a brittle – ductile transition of 12 to 16 KM.
Abu Gabra Formation is the main source rock, consists of dark lacustrine shales containing a typically waxy kerogen and proved to be a reservoir in block # 6. Bentiu formation, Darfur group sandstone members, Amal and Tendi formations are the principal reservoirs. Shales and claystones within
Darfur Group as well as shales and claystones within Abu Gabra Formation act as seal to underlying oil bearing horizons.
None of the wells drilled in the Melut basin, has encountered the source rocks, however, based on the crude oil biomarker distributions and characteristics, it is believed that the source rock in Melut basin is equivalent to the Abu Gabra Formation fount in the Muglad basin. Yabous Formation is the main reservoir in Melut basin.
Time of oil migration is uncertain, but it seems to be during mid to late Tertiary in the Muglad basin and during mid – Cretaceous to Late Cretaceous time in the Melut basin.
The present temperature gradients over these basins are normal and no over pressure zones have been recorded so far.

·         The Petroleum Resources Act 1998 and the Petroleum Regulations of 1973 regulate exploration and production of oil and / or gas accumulations in Sudan both onshore and offshore.
·         Major amendments have been introduced in the petroleum resource Act which give the Minister of Energy and Mining, with the approval of the Board of Petroleum Affairs, the right to conclude agreements for the exploration and production of oil and / or gas in the Sudan.
·         All the Agreements signed by the Sudan Government with the international oil companies are Exploration and Production Sharing Agreements (EPSA).The EPSA embodies the entire rights and obligations of the Government and the Oil Company.
·          The Oil Company, under the EPSA, recovers its operations cost from an agreed and negotiable percentage of the total production. The profit oil is divided between the Government and the Oil Company according to certain percentage to be agreed upon by both parties.
·         Equipment of the Oil Company used in petroleum operations are exempted from custom duties.
·         The oil discovered and exported by the Oil Company is exempted from any export taxes.
·         Exchange rights are provided for in the EPSA and the Oil Company is given the right to sell its share of oil and / or gas and keep the proceeds outside Sudan.
LICENSE POSITION IN SUDAN
The International oil companies operating now in Sudan includes:
·         Greater Nile Petroleum Operating Company (GNPOC).
The shareholders are CNPC (40%), Petronas(30%), Talisman (25%) and Sudapet (5%).
·         China National Petroleum Company (CNPC), 100% owner.
·         International Petroleum Corporation (IPC).The shareholders are IPC (40.375%), Petronas (28.5%), MOV AG(26.125%) and Sudapet (5%).
·         Gulf Petroleum Company – Sudan (GPC); 46%, Melut Petroleum Company 46% and Sudapet 8%.
·         Total EXPLORATION - Sudan, the shareholders are
Total Exploration (32.5%), Marathon Petroleum (32.5%), Kuwait Foreign Petroleum Exploration Co. (25%) and GPC (10%).
  The Ministry of Energy and Mining has adopted the blockage system. Since 1992, accordingly the Sudan basins were divided into a number of blocks based on the amount of geophysical, geological and drilling works done Fig (5) and table (1&4). Most of these blocks are free for contracting with any company or a group of companies, which are capable financially and technically to do the work. All data for free blocks are available in the Sudan.


Block No.
Block Name
Sub - Block
Area(SQ.km)
Operator
1
UNITY
1(a)
7173.26
GNPOC


1(a)
154.88
GNPOC
2
HEGLIG
2(a)
8628.62
GNPOC


2(a)
370.92
GNPOC
3
ADAR
3(D)
140.8
GPC,MELUT,SUDAPET


3(E)
10361

4
KAOKANG

32586.08
GNPOC
5
SUDD
5(a)
29885
IPC


5(B)
14643
C
6
ABU GABRA

38468
CNPC
7
MELUT
7(E)
61918.73
GPC,MELUT,SUDAPET
8
BLUE NILE

65856
F
9
KHARTOUM

128088
F
10
GEDARIF

57604
F
11
HUMRA

119124
F
12
MURDI

42888
F
13
DUNGUNAB

24600
F
14
ABYAD

135020
F
15
TOKER

28100
F
16
HALAIB

10300
IPC
17
C

65750
F
18
B

118.586
TOTAL

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