ABSTRACT                                    

The Lower Silurian shales in Murzuq Basin, SW Libya are termed Tanezzuft Formation, they are underlained by Ordovician sandstones of the Mamuniyat or Hwaz Formations and overlained by the Upper Silurian sandstones of Akakus Formation - The present study of well E1-NC174 is based on the description and petrographic analysis of the lower silurian succession as well as on the lithological description of the Tanezzuft Formation, which has been subdivided into two main facies.

Facies A occur in the Lower Part of the Tanezzuft Formation between interval 7237 ft to 7291 ft, which called (Hot Shale Member). Facies B occurs into the upper part of the Tanezzuft Formation between interval 6272 ft to 7237 ft, which called(Cold Shale ).The Lower Part of Tanezzuft Formation is characterized by well parallel horizontal lamination (Hot Shale Member) from the core section from E1-NC174, reflects low energy of the marine sedimentary environment. The dark gray to black coloured siltstone with shale lamina, and the presence of the pyrite nodules throughout this facies, indicate that the sediments were deposited in reducing environment. The presence of Graptolites in the facies A indicates to a marine environment.

مستخلص الدراسة:

هذه الدراسةبعنوان (وصفية للجزء السفلي من العصر السيلوري شمال غرب حوض مرزق جنوب غرب ليبيا).

يعتبر حوض مرزق أحد اهم الأحواض الرئيسية في ليبيا يقع جنوب غرب ليبيا، والذي تقدر مساحته بحوالي 350.000 كيلو متر مربع. هذا ولقد اقتصرت هذه الدراسة على الجزء السفلي للعصر السيلوري وهذا التكوين مايسمى (بتكوين التنزفت) (Tanezzuft Formation) ولما لإهمية هذا التكوين حيث يعتبر من أهم الصخور المصدرية المكونة للنفط في حوض مرزق والأحواض المجاورة. وبناء على هذا فلقد تمت دراسة هذا التكوين دراسة مستفيضة حيث اشتملت على احد الابار الاستكشافية بالمنطقة وهو بئر (E1-NC174) وكان العمق الذى تم دراسته يتراوح ما بين 6272 قدم الى 7291 قدم، دراسة وصفية وذلك لتحديد البيئة الترسيبية. وهدفت الدراسة إلى:

1.اختبار خصائص الدراسة الوصفية والمعملية لهذا التكوين.

2. دمج الدراسة الوصفية مع الدراسة المعلمية.

3. تقييم الصخر المصدري الرئسي لهذا التكوين في هذا البئر كدراسة وصفية ومبدئيا.

وتوصلت الدراسة إلى النتائج التالية:

1. تم تقسيم هذا التكوين الى قسمين:-

• القسم السفلي وهذا الجزء يتراوح ما بين عمق 7237 الى 7291 قدم ويسمي هذا الجزء (Hot Shale) ، والجزء العلوي يبدا من 6272 قدم الى 7237 قدم ويسمي هذا الجزء ((Cold Shale.

2. تبين من خلال الدراسة الوصفية ان الصخر يحتوي على ترققات جيدة متوازية، دليل على أن الصخر ترسب في بيئة بحرية دو طاقة تدفق هادئ.

3. اللون الرصاصي الغامق و اللون الأسود مع تبادلات من الطين الصفحي والغرين في وجود عقد من معدن البيريت دليل على أن الترسبات ترسبت في بيئة ينقصها الأكسجين، أي بيئة مختزلة تسمح بحفظ المادة العضوية.

4. من خلال الوصف اتضح وجود احفورة تسمى Graptolites وبكثافة عالية والتي لاتتواجد إلا في البيئة البحرية.

3. من خلال الدراسة الوصفية والدراسة المعملية فلقد تم التاكيد على أن الجزء السفلي هو الجزء القادر على الإنتاجية وله القدرة الجيدة على إنتاج النفط، وذلك لانه غني بالمواد العضوية ونقص الأكسجين، ولهذا فإنه يعتبر ذو درجة عالية لحفظ الماد العضوية، والتي بدورها أعطته قدرة فعالة لانتاج النفط, ولهذا يعتبر هو الضخر المصدري بحوض مرزق.

وأوصت الدراسة بما يلي:

• زيادة التوسع في المضاهاة بين هذه الدراسة ودراسة آبار أخرى مجاورة في نفس المنظقة.
• زيادة التوسع خارج حدود نطاق الدراسة (NC-174) لمحاولة تعاقب توزيع التكوين كصخر مصدري (Source Rock) وكصخر غطاءSeal Rock)).

I. Geological settings:

1.1. Introduction:

Libya is situated on the southern coast of the Mediterranean Sea of the North Africa, and it has an area about 1,775,500 km² (685,524 miles²); the country extends about 1,525 km (950 miles) east and west and as much as 1,450 km (900 miles) north and south (Fig.1.1) Except the northernmost parts of the country lies entirely within the Sahara. Difficulties of travel and survival may cause to remain the country unmapped, and the geological information has been acquired slowly (Fello, 2001).

1.2. Geological Setting:                                                             

Libya lies on the centre part of the North African Margin and has endured a polyphase tectono-stratigraphic history that has been controlled by adjacent plate tectonic processes since the Pan African Orogeny. The Palaeozoic Basins are Ghadames, Murzuq and Kufra lie on NW, SW and SE Libya respectively, while the late Mesozoic to Cenozoic within Sirt-Pelagian system and the Cyrenaica Platform occupy the northern coastal fringe (Fig. 1.2) (Thomas 1995).

 

 

1.3. Current Study:

1.3.1. Introduction:

The main study of this project will be investigates the Sedimentological characteristic of the Lower Silurian source rock on the northern-west flank of the Murzuq Basin, SW Libya. The Lower Silurian shales, particularly the Lowermost unit characterized by rich origin matter and called Hot Shale Member, it’s origin and considered as 80-90% of Palaeozoic-sourced hydrocarbons in North Africa, where they charge the intra-Palaeozoic reservoir (Boote et al.,1998). According to Boote et al. (1998), these shales are the most important petroleum source rock within the entire Saharan Platform, and have total organic carbon contents (TOC) of up to 17%. The lower Silurian shales are also a major source rock within the Arabian Peninsula, a fact which was discovered only in the last two decades (Alsharhan and Nairn, 1997).

This prolific basal Silurian source rock level not only occurs over wide areas of North Africa and Arabia, but source rocks are also present at a similar level in the Interior basins of the United States, the Amazon , and on the Russian platform (Klemme and Ulmishek, 1991; Macgregor, 1996). Globally, Silurian organic-rich shales account for 9% of the world’s petroleum reserves (Klemme and Ulmishek, 1991).

In most cases, the shales were deposited directly above upper Ordovician (peri-) glacial sandstones during the initial early Silurian transgression that was a result from melting of the late Ordovician icecap. Deposition of the main organic-rich shale unit in the North Africana and Arabian region was restricted to the earliest Silurian Rhuddanian stage (acuminatus, atavusand probably early cyphus graptolite biozones). During this short period 1–2 My, a favorable combination of factors existed which led to the development of exceptionally strong oxygen-deficiency in the area. In most countries, the post-Rhuddanian Silurian shales are organically lean and have not contributed to petroleum generation. The distribution and subsurface thickness of the Lower Silurian “Hot Shales” have been mapped in detail for the whole North African region, using logs from some 300 exploration wells in Libya, Tunisia, Algeria and Morocco. In addition, all relevant, accessible published and unpublished surface and subsurface data of the Lower Silurian shales in North Africa and Arabia have been reviewed, including sedimentological, biostratigraphic and organic geochemical data. The Lowermost Silurian Hot Shales of northern Gondwana are laterally discontinuous and their distribution and thickness were controlled by the early Silurian palaeorelief which was shaped mainly by glacial processes of the late Ordovician ice age and by Pan-African and Infracamberian Compressional and extensional tectonism. The thickest and most extensive Lower Silurian organic-rich shales in North Africa occur in Algeria, Tunisia and western Libya, while on the Arabian Peninsula they are most prolific in Saudi Arabia, Oman, Jordan and Iraq. The Hot Shales were not deposited in Egypt, which was a large palaeohigh at that time. (Sebastian, et al., 2000).

1.3.2. Aims of the Study:

     The main objectives of the study are as follows:

1. To examine the Sedimentology and petrography characteristics of the Lower Silurian, in order to identify texture, mineral composition, diagenetic features of the Lower Silurian Succession.
2. To provide the main source rock interpretation and petrographic characteristics of the key well E1-NC174 within Elephant Oil Field for entire thickness of Lower Silurian, approximately 340 m (1019 ft).

1.4. Location of Study Area NC 174 Concession:

1.4.1. Introduction:

The NC174 Concession is located on the Northern west flank of the Murzuq Basin, SW Libya, approximately 700 km south-west Tripoli. The NC174 covers an area approximately 11,300 km² , and the northern part of the NC174 bounded by an escarpment up to 1,200 feet high. This escarpment is blocked by a boulder field which slopes gently down to the southeast, and which is dissected by several wadies. The southeastern corner of the block is covered by large sand dunes, separated from the boulder field by an area of flat sand. More than fifty exploration wells have been drilled in the entire of Murzuq Basin, most of them are located on the NC101 area to the east of NC174, and in the NC115 area to the northwest and also NC186 Concession to the North of study area. In addition the main Oil has been discovered in the Cambro-Ordovician Mamuniyat and Hawaz sandstones within several wells into NC101 and NC115 Concessions. Most of these discoveries are relatively small, but four major Oil Fields within NC115 Concession have been discoveries and identified by combined reservoir styles. These discoveries are being developed as the A, B, H and M Fields (Final well report of E1-NC174, LASMO Internal Report).

The entire block of study area NC174 concession has acquired 2,758 km of new seismic data, of which 2,543 km are within NC174. The acquisition can be divided into three phases. Phase 1 comprised 193 km of data acquired on the gravel plain north of the escarpment. Data quality from this phase is very good. Phase 2 comprised just 93 km of data acquired in the sand dunes in order to tie the D1-NC 58 well, Data quality from this phase is good. Phase 3 comprised 2,472 km of data acquired in the field boulder field; Data quality over this phase is difficult terrain varies from good to moderate (Final well report of E1-NC174, LASMO Internal Report).

In addition to the seismic acquisition, a total of 1,456 km of existing seismic data has also been reprocessed, of which 1,065 km are within NC174. The E1-NC174 well is the study well and it’s the fifth exploration well being drilled by Lasmo in the NC174 block. The first well A1-NC174 located on the north Scorpion prospect, and was suspended as a non-commercial oil discovery. The second well B1-NC174 located on the Tuareg prospect, and was abandoned as dry hole with some oil shows. The third well C1-NC174 located on the Camel prospect, and was suspended as a non-commercial oil discovery. The fourth well D1-NC174 located on Scorpion prospect, and was abandoned as a dry hole. The Study well E1-NC174 is located on the central field of the NC174 block (Fig. 1.3). This prospect is developed on the central, hanging wall block between two opposing, roughly N-S trending high angle reverse faults. it is approximately 10 km long parallel to the fault and up to 4 km wide. There are two targets; a primary one comprising Cambro-Ordovician Mamuniyat sandstone, and a secondary one comprising Devonian sandstone. (Final well report of E1-NC174, LASMO Internal Report).

 

II. SEDIMENTOLOGY AND PETROGRAPHY:

2.1. Introduction:

This section attempts to describes and interprets about 1019 ft (310 m), including the Cores (54 ft) and Cutting (965 ft) samples from the Exploration well E1-NC174, which is belong to Lasmo Company and was drilled in July 1997. The main objectives from the sedimentological study is to investigate and analysis the Lower Silurian Succession (Hot Shale Member and Cold Shale). Based on the lithological characteristic of the Lower Silurian Succession, has been subdivided into two main facies, and these facies described below:-

• Facies (A) is belong to the Lower part of Lower Silurian Sucession (Core Samples about 54 ft) (Hot Shales Member), the main interval ranging between (7237 ft to 7291 ft)(Table 2.1).
• Facies (B) is belong to the Upper part of Lower Silurian Sucession (Cutting samples about 965 ft) (Cold Shale), the cutting interval ranging between (6272 ft to 7237 ft) (Fig. 1.5).

Table.2.1. Showing the studied core interval.

Core No.	Core Interval		Recovery %		Formation	Date
	From	To	Ft	%
*1	7237 ft	7297 ft	54	90%	Tanezzuft	1997
2	7360 ft	7410 ft	50	100%	Mamuniyat	1997

* studied core.

2.2. Facies:

The term “Facies” was introduced into geology by Gressly (1838). It meant the entire aspect of a part of the earth’s surface during a certain interval of geological time (Teichert, 1958). The word itself is derived from the latin facia or facies, implying the external appearance, or look of something. The modern usage was introduced by Jamieson (1860), who used the term to imply the sum total of the lithological and palaeontological aspects of a stratigraphic unit.

2.2.1. Sedimentological Features of the Lower Silurian Succession NC174 Concession:

Based on the lithology characteristic of the Lower Silurian Succession within Exploratin well E1-NC174. The entire Succession has been subdivided for two main facies, and these facies described below.

• Facies (A) is belong to the Lower Part of Lower Silurian Sucession (Core Samples 54 ft) (Hot Shale Member), the main interval ranging between (7237ft to 7291ft).
• Facies (B) is belong to the upper part of Lower Silurian Sucession (Cutting samples 965 ft) (Cold Shale), the cutting interval ranging between (6272ft to 7237ft).

2.2.1.1. Facies (A): Lower Silurian (Hot Shale Member):

The Facies (A) is defined from slabbed core, and based on Lithology, sedimentary structure, wire-line logs (Fig 2.1). This facies started from Lower part of the Lower Silurian Succession (Hot Shale Member) the main interval ranging between (7237 ft to 7291 ft) In this facies the entire thickness of study interval is about (54 ft, 16 m), . A core sample has been taken as frequency. In order to collect the sample every three feet, the total number of core samples is eighteen.

2.2.1.1.1. Core Samples (A):

Description: This sample (Figure 2.2,2.3,2.4,2.5) represented by cored interval from 7237 ft to 7291 ft, it’s comprises of Shale with interbedded of Silt, light grey to black and dark coloured, homogeneous, medium hard, blocky, fissile, very thin horizontal lamination, medium to high Graptolites content, micro mica, presence of the pyrite nodules throughout this facies and non calcareous.

 

2.2.1.2. Facies (B): Lower Silurian (Cold Shale):

This facies occurs in the Upper Part of the Lower Silurian Succession and called (Cold Shale), occurred in the E1- NC174. it makes up the bulk of the uppermost shales and represent by the cutting interval from 6272 to 7237 ft, In this facies the entire thickness of study interval is about (965 ft, 294 m), which is called the (Cold Shale), the samples of this facies have been taken irregularly from different intervals by using the cutting samples every (30-50 ft), (Fig 2.1). which have been washed and described by microscope. The total number of cutting samples is (eighteen) and will be described as following.

2.2.1.2.1. Cutting Samples (B):

Description: These samples represented by Cutting interval from 6280 ft to 7237 ft, it’s Comprises of Shale with silt and mica, medium gray to brown, homogeneous, blocky, medium hard, subfissile, Occasionally interbedded with sandstone, quartz, fine to very fine grains, subangular, non calcareous.

2.3. Sedimentary Petrography:

     The main objectives from the petrographic study are to determine the mineral composition of the Lower Silurian Succession within the Exploration well E1-NC174. Based on previous description of the Facies A and Facies B. The petrography analysis has been illustrated the entire of mineral composition within Lower Silurian Succession. Eleven samples have been selected from E1-NC174 based on gamma ray response during different intervals.

2.3.1. Facies (A): Lower Silurian (Hot Shale Member):

This Facies is restricted to the Lower part of the Lower Silurian Succession, particularly in type well E1-NC174. This facies is represented by Core interval from 7237 ft to 7291 ft in type well E1-NC174, and its called (Hot Shale Member), based on Gamma ray response by using the radioactive content.

2.3.1.1. Core Samples A:

Description: This sample (A1) is represented by the Cored interval from 7242 ft to 7243 ft, and characterized by Shale contains 3% Silt grains, 2% Organic Matter, and about 95% shale. With very thin lamination structure.

2.3.2. Facies (B): Lower Silurian (Cold Shale ):

This Facies is restricted to the Upper part of the Lower Silurian Succession,, particularly in type well E1-NC174. This facies is represented by interval from 6272 ft to 7237 ft in type well E1-NC174, and its called (Cold Shale), based on Gamma ray response by using the radioactive content.

2.3.2.1. Cutting Samples B:

Description: This sample (B1) is represented by the Cutting interval from 6430 ft to 6440 ft and characterized by Argillaceous Sandstone contains about 60% of monocrystalline quartz grains (Fig. 2.11A), in the upper part of Facies B characterized by quartz grains are very fine upper in grain size Sand, and appear as moderately sorted, the grains shape mostly subangular to subrounded, also the contact between the grains is straight to concave-convex contacts (Fig. 2.11A). The Silt grains estimated as low percent of monocrystalline quartz grains about 3%. The Clay consist of 25% which supporting the detrital grains. The total autogenic minerals are estimated in this sample about 2% Pyrite (Fig. 2.7 – 2.8), The Carbonate cement reached at 10% and replaced to unstable mineral.

 

Fig. 2.7 . Photomicrograph of the Pyrite nodule (D 3-4).

 

Fig. 2.8. Photomicrograph of the Pyrite nodulus (A-4).

2.4. Summary:

The well parallel horizontal lamination of the cored deposits reflects low energy of the marine sedimentary environment, the light gray to black coloured siltstone with shale laminas and presence of the pyrite nodules throughout this facies, indicates that the sediments were deposited in commonly oxygen deficient (reducing) environment (Allen,1965 ). Graptolites lived exclusively in marine environments and the presence of Graptolites indicate that it’s Marine environment of deposits (pers. comm. Fello & Luning, 2006).                                                        

Referring to the previous description and petrographic interpretation of the entire Facies A and B for the Lower Silurian succession within E1-NC174; the concluding of these interpretations are; the well parallel horizontal lamination of the cored deposits (Hot Shale) within E1-NC174 reflects low energy of the marine sedimentary environment. Presence of Graptolites indicate that it’s Marine environment. The dark gray to black coloured siltstone with shale laminas and presence of the pyrite nodules throughout this facies, indicates that the sediments were deposited in commonly oxygen deficient (reducing) environment. In addition, the main and major mineral composition of the Lower Silurian Succession within E1-NC174; including these mineral (very fine to fine monocrystalline quartz grains, Siltstone, mica, Clay, and the pyrite occur as nodules).

4.2. Recommendations for Future work:

This study including a major attempted to build up and intergrated the geological model of the Lower Silurian Succession by using sedimentological Study The results of this study suggest that more work needs to be done in future along the following lines:

• More and establish details correlation between E1-NC174 and nearby development wells of the Lower Silurian Succession.
• Extend this work outside the boundary of NC174 to try and establish the distribution of the lower Silurian Succession as Source and Seal rock.

REFERENCES:

1. ALLEN, J. R. L, l965. Late Quaternary Niger Delta, and adjacent areas: sedimentary               environments and lithofacies. Amer. Assoc. Petroleum Geologists Bull., Vol.49, p.547-600.

2. Alsharhan, A. S. Nairn, A.E.M, 1997. Sedimentary basins and petroleum geology of the Middle East. Elsevier, 978 p.

3. AZIZ, A., 2000. Stratigraphy and hydrocarbon potential of the Lower Palaeozoic Succession of License NC-115, Murzuq Basin, SW Libya.

4. Behar, V. Beaumont and Penteado, H.L. De B. 2001. Oil and Gas Science and                          Technology – Rev. IFP, Vol. 56 (2001), No. 2, p111-134.

5. Bellini, E. and Massa, D, 1980. A stratigraphic contribution to the Palaeozoic of southern basins of Libya. In: Salem M. J. and Busrewil M. T (Eds.). The Geology of Libya. Academic press, London, Vol. I, P. 3-5.

6. Burollet, P. F, 1960.Lexique Stratigraphique international, 4, Libya, Conger’s   shoreline International Commission de stratigraphic, Recherché, ScientifiqueParis, 62 pp.

7. Collomb, G. R, 1962. Etude geologique de jebel Fezzan et de sabordure palaeozoique. Notes Mem. C. F. P., 1, 36 pp.

8. Conant, L. C. and Goudarzi, G. H, 1967. Stratigraphic and tectonic framework of Libya. American Association of Petroleum Geologists Bullet. Vol. 51, P. 719-730.

9. Desio, A., 1939-1953. Annuli del. Museo Libico distoria Natural. Vol. 1, 431 pp, 1939; Vol. 2, 366 pp, 1940; Vol. 3, 280 pp, 1942; Tripoli, Vol. 4, 1953, Rome.

10. Fello, N. M, 2001. Depositional environments, diagenesis and reservoir modelling of Concession NC115, Murzuq Basin, SW Libya (PhD thesis, University of Durham, England) (unpublished) p. 1-336.

11. Fello, N. M. and Turner B. R, 2004. Depositional environments of the Upper                       Ordovician Mamuniyat Formation, NW Murzuq Basin, Libya. Proceedings of the 3rd International Symposium on Geophysics, Tanta, April 2004, p.166-182.

12. Fello, N. M. Lüning, S. J. Petr Štorch and Jonathan Redfern, 2006. Identification of early Llandovery (Silurian) anoxic palaeo-depressions at the western margin of the Murzuq Basin, (southwest-Libya) based on gamma-ray spectrometry in surface exposures, GeoArabia, Vol. 11, No. 3, Gulf PetroLink, Bahrain. p.101-118.

13. Grubic, A. Dimitrijevic. M. Galecic. M. Jakovljevic, Z. Komarnicki. S. Protic. D. Radulovic. P. and Roncevic. G, 1991. Stratigraphy of western Fezzan, SW Libya. In: Salem, M. J. and Belaid, M. N (Eds.). The geology of Libya, Academic Press, London, New York, vol. IV, p. 1529-1565.

14. Hunt, M. 1981. Source rock characterization by thermal distillation and pyrolysis. In G. Atkinson and .1.3. Zuckerman (eds.), Origin and chemistry of petroleum. Oxford: Pergamon Press, pp. 57-65.

15. Klemme, H.D. Ulmishek. G.F, 1991. Effective petroleum source rocks of the world: Stratigraphic distribution and controlling depositional factors. AAPG Bull., 75: 1809-1851.

16. Klitzsch, E, 1969. Stratigraphic section from the type areas of Silurian and Devonian strata at Western Murzuq Basin, Libya. In: J. J. Williams (Ed.). Geology Archaeology and Prehistory of the southwestern Fezzan, Libya. Petrol. Explore. Soc. Libya, 8^th Annu. Field Conf., p. 83-101.

17. LASMO, 1997. Final well report of E1-NC174(Internal Report).

18. Lüning, S. J. Craig, D.K. Loydell. P. Štorch and W.R. Fitches, 2000. Lowermost Silurian ‘hot shales’ in North Africa and Arabia: regional distribution and depositional model. Earth Science Reviews, v. 49, p. 121-200.

19. Macgregor, D.S, 1996. The hydrocarbon systems of North Africa. Marine and      Petrol. Geol., 13: 329-340.

20. Pierobon, E. S. T, 1991. Contribution to the stratigraphy of the Murzuq Basin, SW Libya. In: Salem, M. J. and Belaid, M. N (Eds.). The geology of Libya, Academic Press London, vol. v, p. 1767-1784.

21. PETERS. K. E, 1986. Guidelines for evaluating petroleum source rocks using programmed pyrolysis. Bull.Am. Assoc. Petrol. Geol. 70. p. 318-329.

22. PETERS. K. E., CASSA. M. R, Magoon, L. B, and W. G. Dow, eds., 1994. The petroleum system-from source to trap: AAPG Memoir 60, p.93-120.

23. Selley, R. C., 1976. An Introduction to Sedimentology, Academic Press Inc. London, 2^nd edition, 408 pp.

24. Sikander. A. H. Basu. S. and Rasul .M, 2000, The Geology of Northwest Libya (Ghadams, Jifarah, Trabulus and Sabratah Basins), VolumeIII, Second Symposium on the Sedimantary Basins of Libya, p. 3-53.

25. Tissot, B., and D. H. Welte. 1984. Petroleum formation and occurrence. 2nd ed. Heidelberg: Springer Verlag.

26. Thomas, D., 1995. Geology, Murzuq oil development could boost SW Libya prospects. Oil and Gas Journal, March 6, p. 41-46.

27. Teichert, C., 1958. Concepts of facies. American Association of Petroleum Geologist Bulletin, vol. 42, p. 2718-2744.