Geological exploration in Murzuq Basin: the Geological Conference on Exploration in the Murzuq Basin held in Sabha, September 20-22, 1998

513 Pages·2000·12.968 MB·English

by  M A Sola, D Worsley, Muʼassasah al-Waṭanīyah lil-Nafṭ (Libya), Jāmiʻat Sabhā

PREFACE
Continuing the successful series of conferences on the geology of Libya sponsored by the
National Oil Corporation, the University of Sabha hosted 'The Geological Conference on
Exploration in Murzuq Basin' in September 1998. In spite of Sabha's remote position, 700 km
south of Tripoli, not alleviated by international sanctions to air travel to and from Libya, over
400 delegates attended the conference. This good response, combined with presentations of
almost 40 papers on all aspects of the geology of Murzuq Basin and surrounding areas, resulted
in a stimulating meeting. A concluding panel discussion also covered future exploration
challenges - both in Murzuq Basin itself and in other frontier exploration areas such as Kufrah
Basin. The conference was also a great social and cultural success thanks to the efforts of the
staff of NOC and Sabha University. Following the conference there were two highly popular
excursions, one to Wadi Tanezzuft and Ghat on the western margins of Murzuq Basin, the other
to Wadi ash Shati on the basin's northern flank.
This volume contains 24 of the papers presented at the conference. We have not grouped these
contributions into specific themes, believing that readers will thereby better appreciate the
breadth of topics covered- from palaeontology and biostratigraphy to geophysics, from ore
geology and petroleum exploration to water resources, ranging over the entire geological
column from the Precambrian to the Present. The book concludes with a bibliography covering
all geological aspects of this challenging area.
Many papers herein naturally relate to ongoing hydrocarbon exploration and production.
These contributions give an excellent overview of present status, although not yet fully
answering the challenges we still face in order to better understand the ultimate petroleum
potential of this vast province. The effects of late Ordovician glaciation on reservoir distribution
and- not least- the subsidence, uplift and thermal history of the basin and the consequent
timing and history of hydrocarbon generation and migration still have to be satisfactorily
defined. We have also noted that many workers still attribute local tectonic events to distant and
often irrelevant orogenies in Europe and North America: further exploration in coming years
will demand much more precise analyses of the timing, causes and mechanisms of local tectonic
processes.
We have tried- not always successfully- to maintain a consistent use of geographical and
stratigraphical nomenclature in our editing and have followed the National Atlas of Libya and
the geological map series of the Industrial Research Centre. As noted in the closing panel debate
at the conference, there is an acute need for the adoption of a unified stratigraphical
nomenclature in this area and throughout northern Africa. We thank all authors for willingly
revising their contributions to meet our demands, both in scientific, orthographical and other
respects. Scientific reviews have involved colleagues from many institutions and countries - we
are grateful to all who have taken the time to help ensure the high standard of the resultant
papers.
Our special thanks to Ms. Rosalind Waddams for her scientific and technical assistance in the
entire editing process and not least in the final production of text and graphics - without her we
would not have reached our goals. Editing this volume has been a demanding but enriching task:
we hope that the final product will stimulate the next stage of exploration in this challenging.
The Editors
Tripoli, June 2000
xi
ACKNOWLEDGEMENTS
The organising committee are most grateful to the National Oil Corporation and the University
of Sabha for sponsoring and co-organising the conference held in Sabha in September 1998.
Special thanks also to the National Oil Corporation for its generous support in production of this
resultant volume, entitled 'Geological Exploration in Murzuq Basin'.
Our sincere thanks to Mr. Abdullah Salem E1-Badri, the then Secretary of the General
People's Committee for Energy, Mr. Hamouda Mohammed E1-Aswad, the then Secretary of the
People's Committee of the National Oil Corporation and Chairman of the Conference and Dr.A.
1E Faakhry, Secretary of the People's Committee of Sabha University and Co-Chairman of the
conference for their continued support to ensure its success. Thanks also to the staff of NOC and
Sabha University whose efforts secured the success of this significant international event, and to
all members of the various committees for their work from the early planning phases of the
conference through to the publication of this volume. The Technical Committee contributed
greatly to the success of the conference - as reflected in the number and quality of the papers
presented and produced herein. Last but not least, our sincere thanks also to the Excursion
Committee for its significant concluding contribution to this successful meeting.
The various companies, research institutions, and universities working in Libya also deserve
our thanks for their scientific contributions and logistical support, all of which contributed
greatly to the success of the conference - in particular Agip Oil Company, Arabian Gulf Oil
Company (AGOCO), Industrial Research Centre (IRC), North African Geophysical Exploration
Company (NAGECO), Petroleum Research Centre (PRC), Repsol Oil Operations, Sirte Oil
Company, Umm al-Jawaby Oil Service Company, Veba Oil Operations, Waha Oil Company and
Zueitina Oil Company. Also to Saga Petroleum Mabruk who generously gave D. Worsley time
and facilities to participate in the editing process.
.rD Mustafa Sola, the Secretary of the Conference and Editor-In-Chief of this volume, has
ensured the success of this whole project; our sincere thanks to him and to all his assistants who
worked willingly and efficiently in the conference secretariat. We also greatly appreciate the
Editorial Committee's efforts to ensure the high scientific standard of this volume.
If Allah wills, this conference and the proceedings presented herein will have contributed to
the ongoing exploration of our country and we look forward to further meetings on the
sedimentary basins of Libya.
E.A. 1E Hamyouni
Member of the People's Committee and
General Manager, Joint Ventures Division
National Oil Corporation
GLOSSARY OF ARABIC AND LOCAL NAMES
abu father of maghreb NW Africa in
abyar wells general
aqirah/t mudflat marabat tomb, shrine,
awlad sons, family holy man
ayn, awaynat spring, springs marsa small port, cove
bab door, gate, pass minqar point, ridge, spur
bahr sea nahr river
bani children or nakhla palm tree
descendants of naqazzah/t terrace
bin son of oued wadi
bir, bi'r, well qabilah tribal area
birkah/t temporary pond qabr tomb
buhayrah/t small cultivated qararah/t low, flat-topped
area(s), lake(s) hill(s)
dalou bucket used in old qaryah/t village, villages
wells qasr fort, palace
darari children ramlah/t dunes, dune area
jinn, jenoun devil, devils ra's promontory, spur
dur hills around or sabkhah/t arid evaporitic
beside a plain flat
ehi peak, rocky hill sahel shore, coastal area
eilat family of sahra desert
emi mountain, massif saniyah/t small farm
fesh-fash soft, loose soil sarir gravel plain
fonduq yard, hotel~ostel sawani small farms
ghareb west shati, shatt beach, shore
gilf escarpment suq market
halaq small wadi tadrart mountain, massif
hamadah/t rocky desert tanezzezzeft alum, black dye
plateau tarso high plateau
hasy shallow well, tassili barren plateau
waterhole tawil/ah/t long
ibn son of tmad shallow well,
idhan sand sea waterhole
ilwah/t hill wadi valley,
irq, erg sand sea watercourse
jabal mountain waha oasis
jaemmah mosque washkah/t palm scrub/bush
kabir big, large, great waw crater
kaf cliff, ridge, spur zahr escarpment,
karkur wadi, valley plateau
kharrubah/t carob tree zawiyah/t religious
kharmah/t, kharaymah/t pass, passes education centre
ix
(cid:14)9 2000 Elsevier Science B.V. All rights reserved.
lacigoloeG Exploration ni Murzuq Basin
M.A. Sola and D. Worsley, editors.
CHAPTER 1
Groundwater Salinity Variations in the Cambro-
Ordovician Aquifer of Eastern Jabal al Hasawnah, the
Great Man-made River Project, Libya
A. BINSARITI and FAWZI S. SAEED 1
ABSTRACT
In December 1994 an exploratory drilling programme was initiated by the Great Man-
made River Authority (GMRA) to investigate groundwater quality deterioration in the
vicinity of exploration well No. 29 which taps the Cambro-Ordovician aquifer of
northeastern Jabal al Hasawnah, a region currently being developed as a groundwater
resource of the Great Man-made River Project. In this region the main Cambro-Ordovician
aquifer is overlain by a shallow carbonate aquifer of Paleocene/late Cretaceous age -
herein referred to as the Zimam aquifer, with a basal aquitard composed predominantly of
marly limestone, clay and shale.
The present investigation reveals that downward vertical leakage from the saline Zimam
aquifer to the underlying Cambro-Ordovician sandstone aquifer can occur through fracture
zones in similar conditions to those now confirmed to exist in the vicinity of exploration
well 29A/94 and possibly in the locality of production well No. 125 on line D5 of the
northeastern Jabal al Hasawnah Wellfield. However, leakage of saline water from the
Zimam aquifer into the Cambro-Ordovician aquifer is not evident in the exploration wells
drilled at three locations north and northeast of well 29A/94.
Water quality data from the newly drilled production wells confirm the existence of
increased salinity at the southern extremities of well production lines A1, B ,1 C3 and C5.
This increased salinity is attributed to groundwater flow of lower quality waters emanating
from basinal rim zones (platform limestone) that are known geologically to be very
susceptible to fracturing, a feature which facilitates vertical downward flow from the
overlying Zimam aquifer. Elsewhere in the Cambro-Ordovician aquifer vertical leakage is
less significant as the Zimam basal aquitard is sufficiently thick.
INTRODUCTION
In December 1994 an exploratory drilling programme was initiated by The Great Man-made
River Authority (GMRA) to investigate ground-water quality deterioration encountered in the
vicinity of exploration well 29/89 tapping the Cambro-Ordovician aquifer of northeastern Jabal
al Hasawnah, a region currently developed for the extraction of 2.5 MCM/day of water to be
conveyed to the Jeffara Plain (Fig. .)1 Besides borehole 29A/89, three additional exploration
1 Great Man-made River Project, RO. Box 641-9468, Benghazi, Libya. Fax: [phone].
A. Binsariti and F.S. Saeed
wells were drilled as shown in Fig. 2 (denoted as RE-2DA, RE-4 and RE-3D). Exploration well
29A/94 was drilled 50 m north of exploration well 29/89 in order to check out any possible
faulty completion of that well.
Figure .1 Regional overview showing study area.
Chapter 1 3
Objective of this Study
The objective of this study is to assess the salinity distribution extent and its origin in the
wellfield areas, using the results obtained in 1994 exploration activities combined with the water
quality data obtained from the drilling of 104 production wells in the northeastern Jabal al
Hasawnah Wellfield.
erugiF .2 Location map of the eastern and northeastern Jabal la Hasawnah wellfields.
A. Binsariti and ES. Saeed
THE GREAT MAN-MADE RIVER PROJECT (PHASE II)
Location
Phase II is known locally as the Western Jamahiriya System (WJS). The wellfields are situated
to the east and northeast of Jabal al Hasawnah (a region with no aquifer zone, Fig. .)1 The
wellfield layout is illustrated in Fig. 2. This layout consists of two wellfields; the northeastern
field is bounded by longitudes 14013'00 '' and 14~ E and by latitudes 29042'00 " and
29016'00 '' N. This wellfield is almost entirely situated in the northern confined zone and is
predetermined to provide a production rate of about 0.5 MCM/day. The second wellfield is
situated to the east of Jabal al Hasawnah and is predetermined to provide the remaining rate of
2.0 MCM/day.
The wellfield configurations were optimised to minimise the effects of waterlevel drawdown
in the wellfields and in surrounding agricultural development projects already in the region.
Wellfield discharge was initially planned at a rate of 2.0 MCM/day and subsequently increased
to 2.5 MCM/day. Individual well discharges were predetermined as 45 l/s and 56 l/s, and well
spacing of 1500 m was assumed in the optimising scheme. Predicted model heads after 50 years
of abstraction were judged acceptable.
GEOLOGY
The dominant feature of the study area is the Gargaf Uplift, a huge broad anticlinal structure of
Caledonian-Hercynian origin with an E-W trending axis (Ftirst and Klitzsch, 1963). This
structure constitutes the northern boundary of the Murzuq Basin, which is filled by Palaeozoic
and Mesozoic sediments of marine and continental origin. The Gargaf Uplift consists essentially
of Cambro-Ordovician rocks with occasional minor basement exposures and Tertiary volcanic
flows or plugs. Upper Cretaceous and Paleocene rocks of the Zimam Formation unconformably
overlie the northern flank of the Gargaf Uplift. The geological succession relevant to eastern and
northeastern Jabal al Hasawnah is given in Table .1
The Zimam Formation
This formation includes the Lower Tar Member (Maastrichtian), which consists of claystone,
marl and gypsum with intercalations of sandy calcarenite. The Lower Tar Member is overlain by
the Upper Tar Member (Paleocene), which consists of sandstones with a unit of conglomerate
elbaT .1 Geological succession in the Gargaf Uplift region
Age Aquifer Lithology Thickness )m(
Recent~leistocene Sand dunes - sabkhas - basalt
Paleocene/late Cretaceous Zimam Clay, limestone, marl, dolomite 0-250
and gypsum
Ordovician-Cambrian Palaeozoic Sandstone and quartzite alternating 300-800
with silts and shales
Precambrian Metamorphic and granitic rocks
Chapter 1 5
and sandy clay. The overlying Had Member (also of Paleocene age) consists of dolomites and
interbedded calcilutites forming the Zimam Aquifer, whose basal aquitard is formed by the
Lower Tar Member.
The Cambro-Ordovician (Hasawnah Formation)
This formation consists of quartzitic sandstone. Devonian sandstones and the intervening
Silurian shaly aquitard have both been eroded away from eastern and northeastern Jabal al
Hasawnah, leaving only the Cambro-Ordovician as a single sandstone aquifer. The lithological
range is from coarse to fine-grained sandstones with minor siltstone interbeds. The sandstones
are variably poorly to well cemented with quartz overgrowths and there are occasional friable
fine-grained sandstone beds.
Hydrogeologically, it is important to note the presence of a continuous 50 to 75 m thick
quartzitic unit in the uppermost Cambro-Ordovician succession. This horizon is believed to
result from long-lasting pre-Devonian weathering (Hea, 1971; Dubay, 1980) and is also
associated with the development of confined aquifer conditions in the Cambro-Ordovician
aquifer. Such conditions have also been observed in the area of Wadi Tanezzuft on the
southwestern margin of the Murzuq Basin (Dubay, 1980).
General Tectonic Framework
The Cambro-Ordovician rocks were structurally disturbed by the Caledonian orogeny, which
produced a significant NNW-SSW discontinuity trend and associated small-scale faults.
Tectonic movements in the late Palaeozoic and through the Mesozoic resulted in an ENE-WSW
discontinuity trend comprising the Gargaf Uplift with its associated faults. In this study, special
consideration is given to the system of faults developed on the basin margins as a result of
differential basinal subsidence that occurred subsequent to the deposition of the Upper
Cretaceous and Paleocene sediments.
HYDROGEOLOGY
The water quality distribution pattern in the study area is dominated by the existing geological
structure, which allows hydraulic interconnection between aquifers with variable water quality.
The Cambro-Ordovician Aquifer
This is considered to be the main aquifer in the Gargaf region and consists of fractured sandstone
with intergranular porosity (average core porosity of 20%). The aquifer is developed over a large
area and shows a transmissivity ranging from 1500 to 2000 mZ/day. The field storativity ranges
from 2.2 x 10 5- to 2.0 x 10 3- in the confined areas while in the unconfined areas it may be as high
as 6.4 x 10 -2. Because of extensive local erosion of the Cambro-Ordovician succession,
combined with basinal rim (platform) tectonic effects, hydraulic connections are developed with
other aquifers in the upper sequence in areas north of the Gargaf Uplift.
The Zimam Aquifer
Particular reference is made in this study to the Paleocene-Cretaceous Zimam aquifer, which is
considered to be in a perched position in relationship to the main Cambro-Ordovician
A. Binsariti and ES. Saeed
groundwater body since the aquifer has poor hydraulic properties as well as high salinity. Figure
3 is an isopach map of the Zimam Formation, showing that the formation and its basal aquitard
wedge out to the southwest. The following structural elements mapped by Jurak (1978) in the
study area are relevant to this work:
Exploration well 29/89 (14~ '' E, 28~ '' N) is located on the intersection of two
concealed faults as shown in Fig. .3 There are several faults in the vicinity of the production well
D5-125 and D5-126 in the northwestern part of Fig. .3
The presence of these faults, combined with information on water quality variations obtained
during well testing, confirms the establishment of hydraulic connections between the Zimam
and Cambro-Ordovician aquifers.
Figure .3 Isopach map of the Zimam Formation (metres).
Chapter 1 7
Idrotecnico (1982), in their hydrogeological modelling study of Wadi ash Shati -A1 Jufrah
and Jabal al Hasawnah, assumed a NW-SE trending fault extending from the southeast, located
between exploration well 23/76 and well 29/89 to the northwest, passing approximately 6 km
east of well WS-8. The fault was assumed to justify a reduction of transmissivity necessary to
increase the local hydraulic gradient resulting from model calculations. It was not possible
otherwise to reproduce the steady state piezometric configuration between the Murzuq Basin
and northern Fezzan. Later, in 1983 the two exploratory wells 31/83 and 32/83 were drilled to
ascertain the presence of this assumed fault and the low transmissivity zone. This exploratory
drilling did not confirm the presence of this fault. Subsequent modelling studies of the Western
Jamahiriya Hydrogeological System (WJS) have modified the original Idrotecnico (1982)
geological concepts, including the elimination of the assumed Gargaf fault.
Groundwater Heads
The Cambro-Ordovician forms an extensive regional aquifer, which is assumed to have
continuity from the Murzuq Basin in the south to the 1A Hamada 1A Hamra Basin in the north.
Flow direction from the Murzuq Basin is mainly to the north and northeast.
The steady state piezometry in the east and northeast of Jabal al Hasawnah was measured in
exploration wells and piezometers prior to the start of any important abstractions. The quasi-
steady state hydraulic head distribution is illustrated in Fig. 4. This figure indicates a relatively
flat hydraulic gradient of 1 to 5000 in the south (north of Wadi ash Shati), which can be
interpreted as an indication of relatively high aquifer transmissivity. Northwards, the hydraulic
gradient shows a uniform and steeper pattern (2 to 5000). Regional groundwater flow direction
as indicated by the piezometry is mostly to the northeast, where the aquifer discharges into the
large coastal sabkhas of Tawarga. However, some of the flow north of Jabal al Hasawnah is
diverted to the Kiklah and Upper Cretaceous aquifers higher in the stratigraphic succession.
RESULTS OF EXPLORATION ACTIVITIES
Exploration Well 29A/94
This borehole, with its associated shallow and deep piezometers, confirms the high salinity of
the Cambro-Ordovician aquifer at this location (TDS = 2292) and the relatively low specific
capacity value of 1.47 1/sec/m. In addition, a single-stage pumping test proved conclusively that
the salinity encountered at this location is entirely due to vertical leakage from the intensively
fractured Zimam aquifer. The drawdown in the shallow piezometer measured a total of 0.5 m
and the specific electrical conductivity of the pumped Zimam aquifer amounts to 7352 txs/om,
corresponding to a value of specific electrical conductivity of 3850 Ixs/om at 28~ of the
pumped water of the Cambro-Ordovician aquifer.
A pumping test performed on piezometer 29ZA gave a specific yield estimate of 0.044 1/sec/
m, indicative of low transmissive characteristics of the Zimam aquifer. In this location, the
Zimam basal aquitard amounts to 34 m of clay and shales.
Exploration Well 2DA
This well, located about 25 km northeast of well 29A/94, has a total depth of 480 m and a
Zimam Formation thickness of 72 m. The total dissolved solids in the Cambro-Ordovician