Structural Configuration of the Sirt Basin
STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Tarek Sabri Elakkari March, 2005
STRUCTURAL CONFIGURATION OF THE SIRT BASIN
by
Tarek Sabri Elakkari
Thesis submitted to the International Institute for Geo-information Science and Earth Observation in partial fulfilment of the requirements for the degree of Master of Science in Geo-information Science and Earth Observation, Specialisation: Geological Resources Management and Environmental Geology (GRMEG)
Thesis Assessment Board
Prof. Dr. F.D. van der Meer (Chairman of the Board, ESA Department, ITC, The Netherlands) Prof. Dr. C.V. Reeves (External examiner, Delft, The Netherlands) Dr. S.D. Barritt (1st Supervisor, ESA Department, ITC, The Netherlands) Dr. M. van der Meijde (1st Supervisor, ESA Department, ITC, The Netherlands) Dr. P.M. van Dijk (2nd Supervisor, EREG Programme Director, ITC, The Netherlands)
INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION ENSCHEDE, THE NETHERLANDS
I will not use data used in the thesis that are owned by ITC or third parties for publishing without written permission of the ITC thesis supervisor. I certify that although I may have conferred with others in preparing for this assignment, and drawn upon a range of sources cited in this work, the content of this thesis report is my original work. Signed …………………….
Disclaimer
This document describes work undertaken as part of a programme of study at the International Institute for Geo-information Science and Earth Observation. All views and opinions expressed therein remain the sole responsibility of the author, and do not necessarily represent those of the institute.
Abstract
The Sirt basin is one of the youngest sedimentary basins in Libya and covers an area of approximately 600.000 km² in north central Libya. It is located on the northern margin of the African plate, with approximate coordinates (14°00`-20°00`E) and (28°00`-31°00`N). The northern margin of the African plate was affected by a series of tectonic activities due to interaction of the African and Eurasian plates that formed the structural features of the Sirt basin. The formation of the Sirt basin was associated with series of subsidence events which caused number of troughs along E-W trending basement faults during the Early Cretaceous and NW-SE trending basement faults during the Late Cretaceous. The troughs are separated by a series of platforms. These structures are obscured by a thick sedimentary cover.
Geophysical data including ground survey gravity and aeromagnetic data is very important for studying the subsurface structures within the Sirt basin. It is revealed the NW-SE subsurface structures within the Sirt basin as well as NE-SW structures continuing towards the east of the Sirt basin. The gravity data reveals the difference in the structural trend within the Sirt basin compared to the surrounding area of Al Bottnan basin and Jaghbub uplift on the eastern edge of the Sirt Basin. The low gravity expression of the Murzuq and Ghadamis basins indicates the extent of the Sirt basin to the west. The low gravity anomaly expressions are normally associated with the trough structures, while the high gravity expressions are normally associated with the platform structures within the Sirt basin. The Agedabia trough is one the structural features within the Sirt basin, which is located in the eastern part of the Sirt basin with approximate coordinates (19°10`-20°50`E) and (26°30`-30°00`N). The gravity anomaly expression within the Agedabia trough is varied from high gravity anomaly expression in the northern part of the Agedabia trough to low gravity anomaly expression in the southern part of the Agedabia trough. The magnetic expression in the northern part of Agedabia trough is characterized by NW-SE trending structures which coincide with late Cretaceous structures of the Sirt basin, while the southern part is characterized by NE-SW trending features which coincide with a late Palaeozoic trend. The northern part of the Agedabia trough is separated from the southern part by a prominent NE-SW lineament that is expressed in both the gravity and magnetic data. It is interpreted as a basement fault, which separates a thicker southern crust from a thinner northern crust. The high gravity anomaly within the northern part of the Agedabia trough is interpreted as result of mantle upwelling which caused thinning the continental crust beneath the northern part of the Agedabia trough.
Keywords: African plate, Eurasian plate, Sirt basin, gravity data, aeromagnetic data, crustal thickness.
i Acknowledgements
I would like to express all of my sincere respect to my mother, father, sisters and brothers in Libya for their encouragement and support during my study in Netherlands.
I would like to thank Biruni Remote Sensing Center Tripoli, Libya for giving me this opportunity to study here in ITC. I am very grateful to my supervisors Dr. S.D. Barritt, Dr. P.M. van Dijk and Dr. M. van der Meijde for their encouragements and guidance during the research work. Without their help this work will not be possible.
I would like to thank Abdulbaset Musbah Abadi for his permission to use the gravity data available in ITC. Special thanks to Dr. P.M. van Dijk for providing me the gravity data and the references related to study and also for his encouragement during the thesis period. Special thanks for Dr. S.D. Barritt who provided me by the magnetic data, for her help and guidance during data processing and for her explanation to the geophysical dataset. Special thanks for Dr. M. van der Meijde for his tectonic explanation and guidance during the thesis period.
I extend special thanks to Mrs. Drs. N.C. (Nanette) Kingma for having organized the core module very well, which is the backbone of the study here at ITC and for her encouragement during the final exam.
I would like to thank all of the EREG staffs for their guidance and explaining during the modules and field work in Spain. Special thanks are also dedicated to Christopher Andreas Hecker, Ernst Schetselaar and Frank van Ruitenbeek for their guidance in the specialization and elective modules related to my need.
My thanks are also to my friends Ms. Beatriz Lao Ramos from Cuba, Mr. Zhengquan Lu form China and Muhibuddin Bin Usamah from Indonesia for their friendship and for sharing such good experiences here at ITC. I would not feel homesick with the companion of my fellow Libyans who gave me strength during the hard time. I must say thanks to Khaled Madi, Khaled Abujanah, Ali Atia, Sami and Yousef.
ii Table of contents
1. Introduction ...... 1 1.1. Background...... 1 1.2. Location ...... 2 1.3. Problem definition ...... 3 1.4. Objective of the research ...... 3 1.4.1. Main objective...... 3 1.4.2. Specific objective ...... 3 1.5. Research questions...... 3 1.6. Datasets...... 4 1.6.1. Gravity dataset...... 4 1.6.2. Aeromagnetic dataset ...... 4 1.6.3. Well data...... 4 1.6.4. Regional tectonic map ...... 5 1.7. Methodology...... 5 2. Literature review ...... 7 2.1. Sirt Basin ...... 7 2.2. Geological setting ...... 7 2.3. Stratigraphic units...... 10 3. Gravity data...... 15 3.1. Introduction...... 15 3.2. Gravity dataset ...... 15 3.3. Grid processing...... 16 3.3.1. Gridding...... 16 3.3.2. Contouring...... 17 3.4. Grid colour shaded image...... 18 3.5. Regional-residual separation ...... 19 3.6. Low pass filter ...... 20 3.7. Gravity grid profiles...... 21 3.8. Dynamic links of multi dataset ...... 22 3.9. Qualitative interpretation...... 23 3.9.1. Southern gravity profile...... 24 3.9.2. Northern gravity profile...... 24 3.10. Gravity map interpretation...... 25 4. Modeling of gravity anomalies ...... 29 4.1. 2D forward modeling...... 29 4.2. Import data to potent program ...... 29 4.3. Subset data ...... 31 4.4. Density of sedimentary rocks ...... 32 4.5. The southern and northern profiles...... 33 4.6. Conceptual geological model...... 33 4.7. Gravity models...... 34
iii 4.8. Quantitative interpretation...... 36 5. Aeromagnetic data...... 45 5.1. Introduction...... 45 5.2. Dataset ...... 45 5.3. Basement...... 46 5.4. Grid colour shaded image...... 46 5.5. Upward continuation ...... 46 5.6. Grid profile ...... 47 5.7. Qualitative interpretation...... 48 6. Tectonic implication...... 51 7. Conclusion and recommendations ...... 54 7.1. Conclusion ...... 54 7.2. Recommendations...... 56 8. References ...... 57
iv List of figures
Figure 1 -1.Geological map of Libya (1985), Futyan and Jawzi. (1996) showing the major tectonic elements in Libya ...... 2 Figure 1 -2 Flow chart of the methodology...... 6 Figure 2 -1 Development of African rift systems and closure of the Tethys during Neocomian (144.0- 121 Ma) and mid-Aptian (121-98.9 Ma) (after Gealey, 1988 and Wilson and Guiraud, 1992), source: (Anketell 1996) ...... 8 Figure 2 -2 Tectonic map of Libya, modified from Abadi, 2002...... 9 Figure 2 -3 Stratigraphic- lithologic correlation chart of the Upper Cretaceous and Tertiary succession of the Sirt Basin, from the NW of the Sirt basin on the left to the east of the Sirt basin on the right. From Abadi, 2002 ...... 13 Figure 3 -1 Onshore gravity data where the intervals of the gravity stations are not equally spaced.....17 Figure 3 -2 Bouguer anomaly map with contour interval of 5mgal showing the structural features within the Sirt Basin as well as Al Bottnan basin, Jaghbub uplift and Al Jabal al Akhdar...... 18 Figure 3 -3 RGB shaded Bouguer anomaly map, with the northern and southern grid profile ...... 19 Figure 3 -4 Regional Bouguer anomaly map with low pass filtering 50 km ...... 20 Figure 3 -5 Regional Bouguer anomaly map with low pass filtering 200 km ...... 21 Figure 3 -6 Dynamic links between total field of Gravity map in the right and low pass 50km in the left and their profile. The link is illustrated by grey cursor on the maps as well as on the profile...... 23 Figure 3 -7 Southern gravity grid profile ...... 24 Figure 3 -8 Northern gravity grid profile ...... 25 Figure 3 -9 Gravity bouguer anomaly map from previous work Abadi 2002, shows the previous structures interpretation within the Sirt basin, with the location of the well data which will be used in the gravity modeling...... 27 Figure 3 -10 HSV Gravity anomaly shaded map shows the boundary of the geological features as well as ...... 28 Figure 4 -1 ERmapper Header file showing the gravity components Gz (total gravity field) that we used in the modelling as well as the appropriate coordinate system for the area ...... 30 Figure 4 -2 Gravity contoured map, include the two gravity cross sections...... 30 Figure 4 -3 Plan displaying the rectangular window of the subset 2, which relates to the southern profile with its contour lines. The red and blue rectangular represent the polygon prisms that would be used in the gravity model 1 ...... 31 Figure 4 -4 Plan displaying the rectangular window of the subset 1, which relates to the northern profile with its contour lines. The red and blue rectangular represent the polygon prisms that would be used in the gravity model 2 ...... 32 Figure 4 -5 Conceptual geological model. Source (Abadi, 2002)...... 33 Figure 4 -6 Located Euler deconvolution from the magnetic data...... 35 Figure 4 -7 Structural features within the Sirt basin include Al Braygah intra-trough horst...... 38 Figure 4 -8 Model 1, the southern profile ...... 39 Figure 4 -9 Model 2A, the northern profile. It shows miss fitting between the observed and calculated field ...... 40 Figure 4 -10 Model 2B, the northern profile. It shows good fitting between the observed and calculated field in the western part of...... 41
v Figure 4 -11 Model 2C, the northern profile. It shows the first scenario with respect to the high density carbonate strata ...... 42 Figure 4 -12 Model 2D, the northern profile. It shows the second scenario with respect to the mantle process bellow the lithosphere ...... 43 Figure 5 -1 Aeromagnetic known coverage for Libya ...... 46 Figure 5 -2 Total magnetic intensity map, upward continued up to 1000m ...... 47 Figure 5 -3 Gravity and magnetic grid profile ...... 48 Figure 6 -1 Schematic sketch shows the how African plate subducted underneath the Eurasian plate..51 Figure 6 -3 Gravity data on the left and Magnetic data on the right show the northern and southern parts of the Agedabia trough...... 53
vi List of Tables
Table 1 -1 Gravity dataset ...... 4 Table 1 -2 Aeromagnetic dataset...... 4 Table 1 -3 Well data set (Abadi, 2002)...... 4 Table 4 -1 Densities of sediments and sedimentary rocks (source Telford 1976)...... 32 T able 4-2 Comparison between the well data and the depth that achieved by the gravity data ...... 36 Table 5 -1 Specification of the surveys...... 45
vii STRUCTURAL CONFIGURATION OF THE SIRT BASIN
1. Introduction
1.1. Background Libya is one of the North African countries. It shares its borders with Tunisia and Algeria on the west, Egypt on the east, Niger and Chad on the south, while in the north is located on the Mediterranean coast. It is one of the largest oil and natural gas producing countries in Africa. Most of its production is from oil fields which are distributed in the major sedimentary basins in Libya, in approximately 320 fields in these basins and about 80% of these fields were discovered prior to 1970. In the Sirt basin, the majority content of the known hydrocarbons reserved in Libya was formed during the Mesozoic. The Ghadams and Murzuq basins were formed during Palaeozoic time. Its formation was linked to the plate motion and believed to be related to the opening history of Atlantic Ocean and the convergence between African and Eurasian plates. These basins represent different depositional environments, ranging from continental to marine deposits, which enabled a favourable configuration of different source and reservoir rocks, as well as seals within the basins. The sediment configuration within the basins was controlled by basement structures which are divided the entire basin into a series of troughs and platforms.
Geophysical data is useful for studying the subsurface. It is used in this research to study the structural frame work of the Sirt basin.
Gravity data involves the measurement of the earth’s gravitation field at specific locations on the earth’s surface to determine the location of the subsurface density variations which are caused by some rocks having more (or less) density than others, higher residual gravity values are found in rocks that are denser such as metamorphic and igneous rocks, while lower gravity values are found in the sedimentary rocks that are less dense. The density of sedimentary rocks increases with depth which is mainly due to compaction. Sedimentary basins are generally associated with low gravity values due to the lower density of the sedimentary infill.
Magnetic data involves measuring the variations in the earth’s magnetic field caused by the distribution of magnetic minerals in the rocks that make up the upper part of the earth’s crust, it has application in oil exploration to determine the thickness of a non-magnetic sedimentary section overlaying a magnetic basement, it is usually assumed that the structures of the sediments are controlled by basement topography. They are useful to outline intrusive bodies and to define the location, depth and extend of buried basement by thick sedimentary cover and the fundamental structural trend of a region, which structural boundaries have implication in tectonic studies.
These data with the available geological information are useful for geological mapping, oil exploration, sedimentary basin analysis, regional tectonic implication, mineral exploration and earthquake hazard studies.
1 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
1.2. Location Located on the Mediterranean coast of North Africa, Libya is one of the largest countries in Africa occupying 1.757.000 km² with more than 1800 km long coast line. Most of the country lies in the arid zone of the Great Sahara desert. The major tectonic elements within the country are shown in Figure 1-1. The Sirt basin is located in the northern part of the country, the Ghadamis basin located in northwest, and the Murzuq basin located in the southwest. The last two basins are separated by Al Qarqaf Arch. Furthermore, the Jabal al Akhdar Mountain and Cyrenaica Platform are located in the northeast. Al Kufrah basin is located in the southeast, while the Tibesti Mountains are located in the southern part of Libya. These tectonic elements were formed in different periods. This research will be focused on the Sirt basin, which represents the youngest tectonic element compared with the others, with approximate coordinates (14°00`, 20°00`E) (28°00` -31°00`N).
Figure 1 -1.Geological map of Libya (1985), Futyan and Jawzi. (1996) showing the major tectonic elements in Libya
2 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
1.3. Problem definition The Sirt basin was formed in response to the tectonic motion between the African and Eurasian plates. The configuration of the Sirt basin is governed by the basement structures which were formed during the Late Jurassic-Early Cretaceous (144.2-112.2 Ma), and gave to rise a series of troughs and platforms area. These structures affected the entire sedimentary sequence of the basin, followed by successive phases of subsidence which represented a period of instability in the region. These structures are buried below a thick sequence of sediments. However, it is difficult to map them and to understand their configuration without using geophysical data such as gravity data, magnetic data, and well log data.
1.4. Objective of the research
1.4.1. Main objective The main objective is to use the geophysical data, particularly the gravity data to reveal the structural features within the Sirt basin and compare their configuration with the geophysical anomalies. It is done so in order to understand the subsurface behaviour of the Sirt basin below a thick sequence of sediments using the anomaly signatures of the gravity and magnetic data and link them to the structural features of the Sirt basin.
1.4.2. Specific objective ° To estimate the thickness of a potential sediments infill in the Sirt basin, and compare it to the known thickness from well data. ° To delineate the boundary of geological structures within the survey area beneath the sedimentary section using the geophysical data, and compare their anomaly expressions with the known geological structures. ° To discriminate between the geophysical anomaly expressions within the same geological feature.
1.5. Research questions ° What is the depth of the basement in the Sirt basin? ° What is the relation between the geophysical anomaly expressions with the geological features of the Sirt basin? ° Why the northern part of the Agedabia trough is characterized by high positive gravity anomaly expression, while the southern part is characterized by low gravity anomaly expression? ° What is the extent of the Sirt basin boundary?
3 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
1.6. Datasets
1.6.1. Gravity dataset Table 1 -1Gravity dataset Type of Dataset Source of the Dataset Gravity observation Data coverage Onshore Gravity This data was obtained from Dataset the petroleum Research Centre These data sets consist The data covers (PRC) Tripoli, Libya of 131,016 gravity the area of Offshore Gravity This data was obtained from observation values 14°00´E to 25°00´E
GETECH, Leeds (UK), 25º00´N to 33º00´N Dataset courtesy Prof .D. Fairhead
1.6.2. Aeromagnetic dataset Table 1 -2 Aeromagnetic dataset Type of data set Source of the dataset Grid cell size Aeromagnetic survey grid The African Magnetic 1000 meters grid cell size Mapping Projects (AMMP)
1.6.3. Well data Table 1 -3 Well data set (Abadi, 2002) Names of the structural Thickness of Thickness of Palaeocene Thickness of Oligocene features, penetrated by Cretaceous strata and Eocene strata to recent strata well data In Hameimat trough 2400m 2000m 600m In Rakb High 400m 1500m 900m In southern Agedabia 3000m 2300m 1500m trough In Zelten platform 450m 1500m 550m In Hagfa trough 2000m 2000m 500m In Beda platform 400m 1500m In Kotla graben 1300m 1700m In the Gatar Ridge 400m 1600m
In Zallah trough 1400m 2100m
4 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
1.6.4. Regional tectonic map Regional tectonic map of Libya is used to show the tectonic elements in region, include the structural features of the Sirt basin as shown in Figure 2-2.
1.7. Methodology ° Literature review to understand the geology of the area. ° Interpolate the gravity data base using the minimum curvature with grid cell size 2.5km in order to produce gravity anomaly map. ° Display the gravity grid and magnetic grid in coloure shaded map (REG and HSV) ° Georeferencing all available data set using Universal Transverse Mercator (UTM) projection, zone 33 and Datum WGS84. ° Grid processing was used to enhance the original grid data. (Low pass filter for the gravity data and upward continuation for the magnetic data) from the MAGMAP facility in Geosoft Oasis®. ° Grid profiles were used from the total field gravity anomaly map and from the total magnetic map to be used in qualitative interpretation. ° Two Cross sections are constructed from the gravity dataset in order to use them in the 2D forward modelling, using the potent program. ° 2D forward modelling was used to correlate the gravity anomaly expressions with the geological model of the Sirt basin which consists of a series of troughs and platforms. ° Automatic dynamic link option in Geosoft Oasis® was used between multi source datasets in order to assist in the qualitative interpretation (gravity data, magnetic data, regional tectonic map)
The summary of the methodology for the research is shown in Figure 1-2
5 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
input data
Geophysical Gravity aeromagnetic data database grid file
2-D grid processing Colour shaded minimum curvature grid
Total magnetic Bouguer gravity WGS84,UTM33 intensity map anomaly map analytic signal map
grid processing (MAGMAP) low pass filter Literature upward continuation Review
Enhanched processed Enhanched gravity map grid magnetic map
Qualititive Qualititive interpretation gravity profiles interpretation Map profile
subsurface Gravity modeling interpretation
Final interpretation report writing
Figure 1 -2 Flow chart of the methodology
6 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
2. Literature review
2.1. Sirt Basin The Sirt basin is one of the youngest sedimentary basins in Libya and covers an area of 600.000 km² in north central Libya. It is located along the North African continental margin and bounded by Gulf of Sirt along the Mediterranean coast and extends to beyond lat 26°N south. This basin is characterized by a series of northwest trending platforms and troughs, which stretches 600 km in the east-west direction from Hun Graben in the west to the Cyrenaica Platform in the east. It is bounded to the south by the Tibisti Massif and to the west by Al Qarqaf Uplift and the Ghadamis and Murzuq basins. The topography of the Sirt basin is variable from 330 m above sea level in the Dahra platform to -27.50 m below sea level in the north of the Agedabia Trough. The lowest part is situated close to the coastline near the Gulf Sirt, and coincides with the tectonic subsidence calculated by Abadi (2002) for the most recent phase (49 Ma- present). More than 7 km sediments thickness was accumulated in the deepest part of the basin (Agedabia trough) in the eastern part of the Sirt basin in Mesozoic and Cainozoic age (98.9Ma-0Ma).
2.2. Geological setting The Palaeozoic times in Libya was represented by the Caledonian orogeny (Silurian- Devonian) resulted in the uplift of Tripoli-Tibesti uplift along NW trending, and by the Hercynian orogeny (Permian-Jurassic) resulted in NE trending of Sirt arch. During the Palaeozoic time the area, where the Sirt basin existing now, was an arch (Shaaban and Ghoneimi 2001) which represented one of the NE old Hercynian structures.
During the Late Jurassic - Early Cretaceous (144.0-112.2 Ma) the central Atlantic was opened between NW Africa and North America, which caused west movement for African plate relative to the European plate. E-W trough structures were developed in the Niger and north Cameron (Anketell 1996). During this period continental rifting was active in Africa, affecting NE Brazil-Gulf of Guinea- southern Chad domain, the Sudan, Kenya, N and E Niger and the area of western desert of Egypt (Abu Gharadig basin) and southern Sirt basin in Libya (Guiraud and Maurin 1991). The E-W trending structures of the Sarir and Hameimat troughs in the south east of the Sirt basin are coincided with this period.
This period was attributed to the collapse of the Sirt Arch, as a result of plate movements along a group of basement faults forming, failed triple junction in E-W, NW-SE and NE-SW directions (Shaaban and Ghoneimi 2001). It was associated with deformation in the African plate, which caused development of sedimentary basins within the plate. The tectonic sedimentary evolution of the African basins is characterized by polyphase rifting and assumed to be linked to the opening of the south and equatorial Atlantic. The Sirt basin is one of these basins which were developed in this
7 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
period and is believed to be affected by opening of Atlantic Ocean and Tethys Ocean that was formed due to the movement between Africa and Eurasian plates.
The extension of the opening of the south Atlantic during (119-130 Ma) through the Nigeria and eastern Niger, is possibly the extent of Sirt basin (Fairhead and Green 1989).
The main movement of African plate along the northern margin relative to Euasian plate caused most of the fracture zones in the northern part of Libya, which is expressed in the Sirt basin as series of troughs and platforms. This movement was west movement of Africa plate relative to Eurasian plate in the late Jurassic-early Cretaceous (144.0-112.2 Ma).
During the Late Cretaceous, the NW-SE structural features within the Sirt basin were developed as shown in Figure 2-1. It was related to crustal extension in a direction NE-SW that represents the extent of the West African rift system stretching form the Sirt basin in Libya to the Benue trough in the Niger. Their formation is associated with the opening of the equatorial Atlantic, during the Santonian (89-83.5 Ma) the movement of Africa plate relative to Eurasian plate changed to east movement, with compressional events due to the convergence between African and Eurasian plates (Abadi 2002).
Figure 2 -1 Development of African rift systems and closure of the Tethys during Neocomian (144.0-121 Ma) and mid- Aptian (121-98.9 Ma) (after Gealey, 1988 and Wilson and Guiraud, 1992), source: (Anketell 1996)
The tectonic features of the Sirt basin are represented from east to west by Maragh trough, Agedabia trough, Hameimat trough, Sarir trough, Hagfa trough, Kotla graben, Zellah trough, Dor El Abida trough and finally Hun graben. They are separated by Amal platform, Rakb high, Zelten platform,
8 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Beda platform, Dahra platform, Waddan uplift, as shown in Figure 2-2. These structural features have great significance for the hydrocarbon migration from the troughs along the block basement faults to the platforms where most of the known oil fields are located.
Figure 2 -2 Tectonic map of Libya, modified from Abadi, 2002
The Sirt basin is considered to be a type of continental rifting (extensional) area and is referred to as part of the Tethyan rift system (Ahlbrandt 2001). It is formed during the Early Cretaceous-Tertiary in response to crustal extension causing active subsidence resulting in the collapse of the Sirt Arch (F.D.van der Meer 1993). The tectonic evolution of the Sirt basin was studied by several authors who confirmed different phases of uplifting and subsidence within the basin. Four main phases are encountered within the Sirt basin (Abadi 2002). Phase I is related to the Late Jurassic to the early Cretaceous (144.2-112.2 Ma), which represents the subsidence in the southeast of the basin along an E-W structural trend. Phase II is related to the Late Cretaceous (98.9-65Ma) with five time interval including Cenomanian (98.9-93.5Ma), Turonian (93.5-89 Ma), Coniacian and Santonian (89 Ma- 83.5), Campanian (83.5 Ma-71.3 Ma) and Maastrichtian (71.3-65 Ma). This phase represents the major and rapid subsidence within the basin along the NW-SE structural trend and the main significant petroleum source rocks array of sediments. Phase III is related to the Palaeocene until early Eocene (65-49Ma). Phase IV is related to the Middle Eocene until the present day (49-0Ma). These tectonic phases had an important effect on the stratigraphic units within the Sirt basin. The subsidense phases , and were caused due to fault activates which are related to renewed rifting during these phases. The phase IV was caused by sediments load and thermal relaxation within the basin (Abadi, 2002)
9 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
During the Cenomanian (98.9-93.5 Ma) a marginal marine was developed in the southern part of Sirt basin which continued until the Coniacian (89-83.5 Ma). Shallow marine was developed during the Palaeocene times (86.0-54.8 Ma) in southern Libya with active subsidence affecting most of the Sirt basin troughs (Guiraud and Bosworth 1999).
During the Early Eocene (54.8-49 Ma) the basin was affected by subsidence in most of the troughs and regression causing the presence of evaporites in the southern and western sides of the basin (Gir formation; Hun evaporates). The Late Eocene (37.0-33.7 Ma) was characterized by marine deposits due to a transgression event developed in the Early Oligocene. In this period subsidence was active along NW structure in the Agedabia trough (Guiraud and Bosworth 1999).
Regression event occurred in Post - Middle Miocene times due to rapid fall in the level of the Mediterranean, which is caused of continental deposition. During the Late Neogene, an increase in subsidence can be related to compression events due to the collision between Africa and Europe (Van der Meer and Cloetingh 1993)
2.3. Stratigraphic units The stratigraphic units in the Sirt basin are controlled by basement structures, which is divided the whole basin into discrete structural features. Five sedimentary sequences were encountered within the Sirt basin, a typical of rift complex configuration (Abadi, 2002). Pre- rift sediments, represented by the Palaeozoic and Triassic strata, Syn-rift basin fill represented by late Jurassic-early Cretaceous deposition and characterized by continental-marine siliclastic rocks, Syn-rift basin fill represented by the late Cretaceous deposition and charactarized by marine silclastics and carbonate rocks, Syn-fill basin fill represented by the Palaeogene deposition and charactarized by carbonate and evaporate strata, and finally Post-rift basin fill represented by the Neogene deposition and characterized by continental siliclastic strata. The late Cretaceous strata contain most source rocks in Sirt basin while the Palaeocene strata contain most reservoir and seal rocks in the Sirt basin. The basement rocks in the Sirt basin consist of granites, volcanic and metamorphic rocks, of Precambrian and early Cambrian age, it is exposed only in the Tibesti mountains, Jabel Awaynat and in the central of the Gargaf Arch. ° Pre-rift sediments The Hofra formation represents the pre-rift units of Cambro-Ordovician deposits which consist of quartz sandstone with minor amount of shale, siltstone and conglomerate (Barr and Weeger 1972). ° Syn-rift basin fill The Nubian sandstone represents the early syn-rift of late Jurassic to late Cretaceous, with non marine deposits which cover throughout the Sirt basin, and consists of sandstone, siltstone, shale and conglomerate (Barr and Weeger 1972). It is preserved in a thick section in the grabens of the basin and thin or eroded on the platforms and highs. The Nubian sandstone unconformably overlies (HERCYNIAN UNCONFORMITY) basement rocks or lower Paleozoic formations. ° Syn-rift basin fill The Bahi formation represents upper Cretaceous deposits ranging from Cenomanian-Danian, located in the northwest Sirt basin, and consisting of interbedded sandstone, siltstone, conglomerate and shale
10 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
(Barr and Weeger 1972). The same period the Maragh formation was deposited in the eastern part of Sirt basin, consisting of sandstone, conglomerate, shale with minor carbonate, representing a transgressive cycle. The Lidam formation represents the upper Cretaceous shallow marine deposits consists mainly of dolomite with mixing sands in the lower portion, it is unconformably (SIRT UNCONFORMITY) overlying the Bahi formation, Nubian formation, Paleozoic and basement rocks. The Rakb Group consist of three formations including Argub carbonate, Rachmat formation and Sirt shale in the northwestern, the Argub carbonate represents Turonian marine deposits, and consists mainly of dolomite with interbeded limestone and thin sandy stringers (Barr and Weeger 1972). The Rachmat formation represents Coniacian to Santonian deposits, and consists mainly of shale with minor limestone, sandstone and interbeds dolomite (Barr and Weeger 1972). Most of the formation is deposited in trough areas where it reaches the maximum thickness while being absent in the highs areas. The Sirt shale represents the Campanian deposits, and consists of a shale sequence with thin limestone interbeds (Barr and Weeger 1972).The Kalash limestone represents the Maastrichtian deposits and consists of argillaceous calcilutite with some interbeds of calcareous shale (Barr and Weeger 1972). It is the lateral equivalent of the Waha limestone and lower Satal formation. The lower member of the Satal formation is upper Cretaceous in age, located in the northwestern Sirt basin, and consists of massive argillaceous calcilutite to calcarenite near the top of the portion with few thin dolomite layers (Barr and Weeger 1972). ° Syn-rift basin fill V Palaeocene sediments. The Hagfa shale formation is located throughout the central and western Sirt basin; it represents an open sea environment of Danian deposits, and consists of shale with thin limestone interbeds (Barr and Weeger 1972). The upper member of the Satal formation represents Danian deposits, consists of fine grained calcarenite in association with calcilutite (Barr and Weeger 1972). The Beda formation represents Selandian deposits which consist mainly of various interbedded limestone lithofacies with subordinate dolomite and calcareous shale (Barr and Weegar, 1972) in the southwestern Sirt basin. In the northwestern part of the basin the formation becomes more shaly this formation representing a shallow marine environment of deposition. The Dahra formation represents Thanetian deposits which consist mainly of white to light gray chalky, calcarenite, calcilutite and subordinate tan to brown microcrystalline dolomite and thin interbeds of dark shale with minor anhydrite in the upper part of the formation (Barr and Weegar, 1972). In the southwestern part of the basin the lower part of the formation becomes more shaly, this formation is the lateral equivalent of the lower shale units of the Khalifa formation. The Khalifa formation is located in the subsurface of the western and central basin, and represents the Selandian-Thanetian deposits which consist mainly of argillaceous limestone which were deposited in shallow marine environments. In the upper part of the formation the shale sequence was deposited in open marine conditions in the south part of the basin (Barr and Weegar, 1972). The Jabal Zelten group represents the Thanetian deposits. It is composed of two formations, the Zelten formation in the southern part and Harash in the northern part of the basin. The Zelten formation consists mainly of limestone with amounts of shale, and it is widespread in the western and central part of the basin. The Harash formation consists mainly of white to brown, argillaceous calcilutite and muddy calcarenite with thin interbeds of calcareous, fissile shale in the lower part of the formation (Barr and Weegar, 1972), and it is located in the western and central part of the basin. The Khier formation represents the Thanetian deposits which are characterized by open marine environments; it consists mainly of shale with some clay, marl and limestone. V Eocene Strata
11 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
The Gir formation represents the lower Eocene deposits and it is composed of three members. The Facha member which consists of massive dolomite with minor amounts of anhydrite in the western part of the basin, Hun evaporite member which consists of a sequence of interbedded anhydrites and dolomite with minor shale in the western and central part of the basin, and the Mesdar limestone member which consists mainly of massive limestone with thin beds of shale and dolomite in the central and eastern part of the basin (Barr and Weegar, 1972. The Mesdar member is characterized by an open marine environment. The Gialo formation represents the middle Eocene deposits which are widespread in most of the basin except the northwest part of the basin where the Gedari formation was deposited. It consists of a thick sequence of shallow marine limestone, and the limestone lithofacies include muddy nummulitic calcilutite and calcarenite (Barr and Weegar, 1972) and the depositional environment was characterized by shallow to open marine conditions. The Augila formation represents the upper Eocene deposits, in the southeast and south-central part of the basin the formation is composed of three members: the lower member consists of soft shale with thin argillaceous limestone or dolomite interbeds, the middle member consists of soft, porous, glauconitic quartz sandstone, and the upper member consists of hard, sandy, slightly glauconitic limestone. The lower member is characterized by open marine environments and the upper by shallow marine environments. The upper part of the formation is unconformably overlain by the sandstone of Arida formation (Barr and Weegar, 1972). V Oligocene Strata The Najah group represents the Oligocene deposits located in the eastern and central part of the basin; with Arida as lower formation and Diba as upper formation of the Najah group. The Arida formation is located in the south-central part of the basin, subdivided into a lower unit that consists mainly of sandstone and an upper unit consisting of shale. The formation is represented by a rapid change in the environment of deposition from continental sandstone in southeast to carbonate in the north. The Diba formation is located in the south-central part of the basin and consists of an alternating sequence of thick fine to coarse sandstone units and thin soft shale (Barr and Weegar, 1972). The shale indicative for open marine environment during this period. ° Post-rift Basin fill The Marada formation represents the Miocene deposits within the basin, and consist of multi lithofacies including interbedded shales, sandstone, sandy limestone and calcarenites, the formation contains a number of rapidly changing facies which represents an interfingering of various continental, littoral, and shallow marine environments (Barr and Weegar, 1972) .
12 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 2 -3 Stratigraphic- lithologic correlation chart of the Upper Cretaceous and Tertiary succession of the Sirt Basin, from the NW of the Sirt basin on the left to the east of the Sirt basin on the right. From Abadi, 2002 () Hun graben,() Waddan uplift,() Dor El Abida trough,(V) Bu Tamaym trough
13
STRUCTURAL CONFIGURATION OF THE SIRT BASIN
3. Gravity data
3.1. Introduction Gravity measurements can be obtained by satellite gravity such as GRACE mission or by ground or air borne surveys. The gravity data used in the study is ground survey data, which covers most of the northern and central part of the Sirt Basin as well as the east of Sirt basin. Previous works were done by El-Batrouki and Zentani (1980), and Abadi 2002. They studied the structural features within the Sirt Basin by using the ground survey gravity data and their relation with the tectonic evolution and petroleum exploration. Here we used the ground survey gravity dataset which is available in ITC and data provided by Abadi 2002. To further study about the geological feaures within the Sirt basin, gravity data and its anomaly expressions were used and compared with the known geological features of the Sirt basin.
The variation in the density of the crust and the sedimentary cover are represented on the gravity anomaly map by high or low gravity anomalies. The long and short wavelength and amplitude of the gravity anomalies give an idea about the size and depth of the geological structures, which are buried below a thick of sedimentary section.
3.2. Gravity dataset The ground surveys gravity data was obtained from Petroleum Research Centre (PRC) in Tripoli Libya and includes 86327 stations on land area as shown in Figure 3-1. Most of these stations cover the entire Sirt basin with Cyrenaica platform, Al Bottnan basin and Jaghbub uplift. Bouguer corrections were applied by (Abadi, 2002) in order to obtain the complete Bouguer anomaly map which forms the basis of the interpretation of gravity data on land. Finally the data was combined with marine gravity data which has 44689 readings, obtained from GETECH, Leeds (UK) from Geosat and ERS-1 geodetic missions. The data were obtained as (x, y) observations and z value representing the Bouguer anomaly values. The offshore area (Mediterranean Sea) north of the Sirt basin is shown as high gravity anomaly expression in the gravity anomaly map because there is no free air correction applied on the offshore data that were used. The space between the gravity stations as we mentioned above and shown in figure 3-1 are varied in the density, from high density stations with 700m space between them. In the map, they were shown as black points close to each others, to low density stations with 7000m space between them.
15 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
3.3. Grid processing
3.3.1. Gridding The gridding was the first step in the data processing, which interpolates the Bouguer anomaly values of the data base to a square grid. The term grid refers to the files that contain location (x, y) and data (z) gravity observation values, which are interpolated to create a regular and smoothly sampled representation of the locations and data. The interpolation methods of (x, y, and z) data within Geosoft Oasis montage software (www. Geosoft.com) are: 1) Bi-directional gridding It is designed to interpolate roughly parallel line based data, which is carried out in two steps • The data along each survey line is interpolated to give values at each intersection with the defined grid lines. • The interpolated values along each grid line are extracted from the previous interpolated values then stored and finally interpolated to give the gridded value at each grid cell. 2) Minimum curvature random gridding It interpolates the (x, y, z) data by fitting a two dimensional surface to the (x, y, z) data in this case the curvature of surface is minimized. It is recommended for random distribution data. 3) Kriging It determines the statistically most probable value at the node from the surrounding real data values.
In the preparation for gridding the data, their geodetic coordinates were converted to x-y coordinates using projection UTM, Datum WGS1984, ellipsoid WGS84 and Zone33 which is the same as used for the local maps. The gravity survey stations within the region are not line based and the observation intervals are not equally spaced. For that reason, it is transformed to the grid using computer program Geosoft Oasis based on minimum curvature surface method (Briggs, 1974; Swain 1976), with a grid cell size of 2.5 km to create final bouguer anomaly map as shown in Figure 3-2.
The gravity anomaly map was produced by displaying the gridded data as a colour map. It shows the distribution of the gravity anomalies within the region, which are generally caused by lateral density contrast within the sediments and basements, throughout crust of the earth.
16 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 3 -1 Onshore gravity data where the intervals of the gravity stations are not equally spaced. The space interval between the stations is varying from 700m where the stations are intensive to 7000m where the stations are less intensive.
3.3.2. Contouring The gravity anomaly maps were displayed as contour maps, which were the base of the gravity interpretation before the development of the computer programs. Nowadays, the gravity data is displayed as colour maps, where the colour represents different gravity anomaly form high to low within the gravity map.
The gridded data is taken as input for producing a contour map with a contour interval of 5 mgal. The contour map of the gravity data shows many anomalies of various shapes and size, as well as the trend directions of the geological features in the region. The trend is mainly NE-SW trend patterns in the Al Bottnan basin as shown in Figure 3-2 and mainly NW-SE trend patterns in the Sirt basin, which are related to different tectonic events. The contour density of the gravity anomaly map is useful in the interpretation and used as indication for the locations of fault zones when the contour interval changes rapidly from high to low value. If the contour changed from 15 mgal in the Cyrenaic platform to -15mgal in the Al Bottnan basin, this is used as indicator for structural features, which is caused of the sudden change. The gravity anomalies with their contours were compared with the known geological features of the region to draw a relationship between the total field expressions with the known geology.
17 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 3 -2 Bouguer anomaly map with contour interval of 5mgal showing the structural features within the Sirt Basin as well as Al Bottnan basin, Jaghbub uplift and Al Jabal al Akhdar
3.4. Grid colour shaded image The image coloured tool within the Oasis montage enables to interactively edit and modify the colour zoning of the grid data and provides a set of colour table files to determine the colour palette used to colour a grid data. It allows changing the inclination, declination as well as the brightness and contrast of a colour shaded grid which is useful to displaying the structure features of interest. It was done so in order to be more interpretable for the human eye and the geological features can be extracted as much as possible, to be used finally in the qualitative interpretation. In order to illustrate the geological features in artificial illumination with 45 degree inclination and declination, and to help to correlate the potential field expressions with the platform and trough structures within the region, the normal shading model with RGB table were used and shown in Figure 3-3.
The RGB colour shaded map is used as background to select the appropriate locations for the two gravity profiles to pass most of the geological features within the Sirt basin. The gravity grid profiles will be displayed in curved line, which will be indicative for the structures distribution within the Sirt basin.
18 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
The main reason behind the selection of the two gravity grid profiles is to cross the northern and southern parts of Agedabia trough which is located in the eastern part of the Sirt basin where the gravity anomaly expression changes from high to low along the same trend of the trough.
Figure 3 -3 RGB shaded Bouguer anomaly map, with the northern and southern grid profile
3.5. Regional-residual separation Gravity fields at the Earth's surface contain anomalies from sources of various size and depth. To interpret these fields, it is desirable to separate anomalies caused by certain features from anomalies caused by others. In considering the wavelength of gravity anomalies, the short wavelength anomaly is related to the near surface, while the long wavelength anomaly is related to the deep seated bodies. As Nettleton’s (1954) definition of regional and residual anomaly the regional is the smooth part and is attributed to effects which are too deep or too broad in relief to be a possible expression of structure or other disturbance of interest. The residual is the part left after subtraction.
Based on type of the study, the regional and residual maps will be produced. If the study is intended to the deep crustal structures as this study, the regional gravity map will be useful for such study where the deep structures of the lower crust it will enhanced. The regional grid is often used to enhance the residual anomalies of primary interest as the hydrocarbon explorations, which is produced by subtracting the regional anomaly grid from the original total field.
19 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
3.6. Low pass filter The long wavelength component of the Bouguer gravity field is usually attributed to density variations considered to be deeper than general exploration interest for hydrocarbon and mineral exploration.
The MAGMAP filter design in Oasis montage gives a facility to select the appropriate filter for the original total field data. Low pass filter is used to enhance the deep seated features from the gravity field data in order to produce regional gravity anomaly map as shown in Figure 3-4. During our processing different cut off was used 20, 30, 40 km in order to produce regional gravity anomaly map. Finally 50 km was considered as cut-off to be used for the low pass filter, which will reject the entire wavelength that less than 50 km. That was not considered since we are interested in deep seated structures beneath the sedimentary section of the Sirt basin, which pass the wavelength more than 50 km. The regional gravity anomaly grid could be used to produce the residual gravity anomaly by subtracting the regional grid from the original total field grid. The eastern part of the Sirt basin reflects long wavelength which is indication for deep seated structures. The north part of Agedabia trough is characterized by long wavelength about 75 km wide, which reflects deep seated structure within the northern part of Sirt basin. The small anomalies within the Sirt basin which are related to the near surface structures or small structures are enhanced and disappeared on the regional gravity map. The effect of low pass 50 km filtering compared to the total field will be observed since the two components will be used in the gravity profiles.
Figure 3 -4 Regional Bouguer anomaly map with low pass filtering 50 km
20 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Since we are interested to the deep lower crustal structures, more than that produced by using the low pass 50 km, we selected another different cutoff with 200 km which will reject the entire wavelength that less 200 km and pass the wavelength that more than 200 km as shown in Figure 3-5. That was done since we are interested to the deep lower crustal structures where the interface between the continental crust and the mantle or in other term to reveal the mantle process below the lithosphere.
Figure 3 -5 Regional Bouguer anomaly map with low pass filtering 200 km
Our focused was concentrated for the Agedabia trough in the eastern part of Sirt basin, as result by using the low pass 200 km, the northern part of Agedabia trough still characterized by high gravity anomaly expression response in the regional map above as shown in Figure 3-5 which is indicative that, the northern structural feature of the Agedabia trough is related to the lower crustal deep features that caused by the mantle process below the lithosphere.
3.7. Gravity grid profiles Two profiles were constructed on the gravity anomaly map as shown in Figure 3-3 using different grid components, the total gravity field grid and low pass 50 km grid. From these components the profile will be extracted from the existing map and placed in a line in the current database that was created in the beginning of the process.
21 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
The aim of using different grid components is to attempt to define the signature of the gravity anomalies with different deep levels. It was assumed that the small anomalies are related to near surface structures, while the big anomalies were related to the deep structures. This study tried to link gravity components to the geological structures of the Sirt basin which is characterized by series of troughs and platforms separated by block faults. The separation between the troughs and platforms area is expressed on the gravity map and gravity profiles by steep gradient (or sudden change) which is related to fault structures. The gravity profiles are selected in NE-SW direction, which is perpendicular to the NW-SE strike of the geological features of the Sirt basin. The southern profile shown in Figure 3-7 represents the structural features of central part of the Sirt Basin, while the northern profile shown in Figure 3-8 represents the structural features of the northern part of the Sirt basin. Both of the profiles covered an area with average 400 km. Figure 3-3 in section 3.4 showed the location of the two profiles. The two gravity profiles were selected in order to cross the structural features that show different gravity expressions within the same structure and to cross most of the structural features within the Sirt basin.
As shown in Figure 3-3, Agedabia trough is characterized by two gravity anomaly expressions: high gravity anomaly in northern part of the trough and low gravity anomaly in the southern part of the trough. The Dahra platform is characterized by two gravity expressions: high gravity anomaly on the both sides (the Gatar Ridge to the west and Messlah High to the east) of the platform and low gravity anomaly on the middle of the platform. Based on that, the two gravity profiles were selected.
3.8. Dynamic links of multi dataset The definition of dynamic links is interactive graphic connection that activate between data bases, profiles and any number of maps in the work space. When the active links are active and by selecting any item of the views data, a cursor automatically connects the item with the others of the same location. By using this facility, it gives us the ability to look at multi source data of the same area at the same time since they have the same coordinate system. It is used to link between the grid profiles and their colour maps, which were represented by the total field gravity map and the regional gravity anomaly map as shown in Figure 3-6. The link is made to draw relationships between the potential fields of the gravity data and the geological information obtained from the regional tectonic map. It gives an idea about the relation between the maps represented by colours and their profiles represented by curved lines that reflect the distribution of the anomalies within the map. The link also provides quick quality control, processing and analysis by using all of the available data and information.
22 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Zelten platform Agedabia Low-pass 50km Hagfa Trough Trough Total gravity field
Gravity profile
Figure 3 -6 Dynamic links between total field of Gravity map in the right and low pass 50km in the left and their profile. The link is illustrated by grey cursor on the maps as well as on the profile
As shown in Figure 3-6, it illustrated the link between the gravity anomaly maps and their profile. The Hagfa trough is bounded by Zelten platform on the east of the trough, where the gravity profile is changed rapidly from high gravity anomaly to low gravity anomaly, which reflects the fault zone that separated them. The sudden change of the gravity anomaly is illustrated by grey cursor on the gravity maps and the gravity profile.
3.9. Qualitative interpretation This kind of interpretation was attempted to correlate most of the obvious anomalies, which are shown on the gravity anomaly maps with the regional geological structures. The gravity anomaly expressions within the Sirt basin has elongate pattern followed mainly the NW-SE trend directions as show in Figure 3-3. It reflects the Late Cretaceous structures of the Sirt basin beneath varying layers of Cretaceous to recent sediments.
Both sides of the edges of elongate gravity maxima are a series of anomaly minima produced by structural faults, which interrupted the gravity anomaly and caused the change from gravity anomaly maxima to gravity anomaly minima. These faults are expressed on the gravity maps as steep gradient from high gravity anomaly expressions to low gravity anomaly expressions. On the gravity profiles, they are expressed by sudden change in the gravity curves, which is caused by the changing of the density contrast between the various sedimentary rocks within the basin or by the interface between the sedimentary rocks and the basement below the sedimentary section of the Sirt basin.
23 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
From the gravity data, it was found that the amplitude of the anomaly varied as function of depth and density contrast within the sediments or between the sediments and the basement rocks. The edges of the denser rocks can be associated with the inflection on the gravity curve. The interpretation is based on the visualization by looking at multi maps at the same time with respect to their profiles. .
3.9.1. Southern gravity profile As shows in Figure3-7, the gravity grid profile reflects the distribution of the gravity anomaly sources along the southern profile which are related to structural features of the southern part of the Sirt basin. Started form Maragh Trough in the northeast of the basin and ended by Zalah trough in the south west of the basin, they are separated by series of platforms along block basement fault zones. The separation between them is expressed on the gravity profile by sudden change (sharp change) in the gravity anomaly, which is indicative for the fault zone that separate the troughs form the platforms. The Zelten platform and Beda platform are characterized by a short wavelength anomaly within the platform, which is indicated for near surface structural features that related to the short wavelength. They are enhanced and disappeared on the regional grid component (low pass component). The southern Agedabia trough and Hagfa trough are characterized by low gravity anomaly expressions in all of the components, which are indicative for deep low structural features with average -20mgal, due to low density sediments infill within the troughs. The gravity profile is reflected four low structural features within the Sirt Basin, which are related to Zallah, Hagfa, Agedabia and Maragh troughs. The Beda platform, Zelten platform and Rakb High are characterized by high gravity anomaly expressions compared to the trough structures with average 5mgal, which are related to the uplift of the basement rocks or horst areas which are characterized by high density that caused of the high gravity anomaly expressions.
Beda platform Maragh Zallah Trough Trough Agedabia Rakb Hagaf Zelten Trough High Trough Platform
Figure 3 -7 Southern gravity grid profile
3.9.2. Northern gravity profile As shown is Figure 3-8, the gravity grid profile reflects the distribution of the gravity anomaly sources along the northern profile which are related to the structural features of the northern part of the Sirt basin. Started by Amal platform in the east of the Sirt basin and ended by Dor El Abida trough in the west of the Sirt basin, separated by a series of trough and platform structures.
24 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
The Dahra platform is characterized by high gravity anomalies expressions on the eastern and western parts of the platform, while the middle part is characterized by a low gravity anomaly expression in all of the gravity components that are used in the gravity profile. Those high gravity anomaly expressions are related to the Messlah High and Gatar Ridge on the edge sides of the platform. They are characterized by sudden change (sharp change) in the gravity profile from high to low gravity anomaly on the east and west of the Dahra platform, which is related to the fault zones that separated the platform from the Hagfa trough in east and Dor El Abida in the west of the Sirt basin. Most of the oil fields within the Dahra platform are located on the Gatar Ridge and Messlah High, which maybe due to the high density fringing reefs or carbonate rocks which represent good reservoir within the Dahra platform. The low gravity anomaly within the middle part of the Dahra platform is related to the low density source rocks, where most of the hydrocarbon is occurred. The variation of the gravity anomaly expressions within the Dahra platform is could be even related to the trough structure within the Dhara platform bounded by two uplifted area in the edge sides of the platform.
The north Agedabia trough is characterized by broad and high gravity anomaly expression in all the gravity components, which is different than the southern part of the trough as illustrated in the two gravity grid profiles. The long wavelength of the northern part of Agedabia trough which is about 75 km is revealed deep structural feature within the north of Sirt basin that related to the mantle process below the lithosphere. It is bounded by platform structures on both sides of the trough. In the Dor El Abida trough, the total gravity field curve reflects small structure feature that disturbed the gravity anomaly and it is smoothed in the regional grid during the low pass filter.
Messlah Zelten Amal High Platform Dahra Hagfa Platform Dor El Abida platform Trough Trough Agedabia Gatar Trough Ridge
Figure 3 -8 Northern gravity grid profile
3.10. Gravity map interpretation The HSV table, one of the Oasis facilities is used to create wet look shaded colour map as shown in Figure 3-10. In HSV coloured map, the hue is controlled by the original grid data while the saturation and the value are controlled by the shaded grid. Different angle was used for the inclination and declination in order to enhance the extent of the geological features within the Sirt basin as well as the east of the Sirt basin (Al Bottnan basin, Al Jabal Akhdar uplift and Jaghbub uplift) in the gravity map, to be used for the qualitative interpretation.
25 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
As shown in Figure 3-10 Jabal al Akhdar Uplift and Jaghbub uplift are located in the east of Sirt basin, and separated by Al Bottnan basin. Al Bottnan basin is expressed on the gravity map as elongate linear feature with NE-SW trend direction, it is characterized by low gravity anomaly with minimum value -23.8mgal. It is related to low density sediments infill within the basin. The Jaghbub uplift is characterized by NE-SW trend direction and high gravity anomaly expression with 33mgal which is related to the uplift of the high density basement rocks. The sudden change in the gravity anomaly expression from low gravity anomaly in Al Bottnan basin to high gravity anomaly in the Jaghbub uplift reflects the structural feature that caused the gravity change. They are associated with the NE-SW Hercynian trend that formed in late Palaeozoic time, which is older and different than the late Cretaceous trend features of the Sirt basin.
The Jabal al Akhadar is characterized by high gravity anomaly expression which is related to the high density shallow basement rocks. It is associated with folding of the late Palaeozoic rocks during the Late Cretaceous produced due to dextral transpression across NE Africa (Guiraud, Bosworth, 1999).
The Sirt basin is bounded by NE-SW trend features of the Al Bottnan basin, Al Jabal al Akhdar and Jaghabub uplifts in the east of Sirt basin, while in the southwest is bounded by Murzuq basin which is expressed on the gravity map by low gravity anomaly expression, as shown in Figure 3-10. The eastern boundary of the Sirt Basin was defined based on the different in the trend of the geological features, which are mainly NW-SE in the Sirt basin and NE-SW in the east of the Sirt basin. The western boundary is based on the low gravity anomaly expression of the murzuq and Ghadamis basins.
Most of the northern and central parts of the structural features within the Sirt basin are characterized by NW-SE trend direction, which is related to the late Cretaceous rift that affected the entire Sirt basin. They are represented from east to west respectively by Maragh trough, Amal platform, Rakb High, Agedabia trough, Zelten platform, Hagfa trough, Dahra platform, Zallah trough, Dor El Abida trough, waddan uplift and Hun graben as shown in Figure 3-10. The gravity anomaly expressions within the Sirt basin is varying form high gravity anomaly expression in the northern part of Agedabia trough with maximum value 17.4mgal, to low gravity anomaly expression in the Hagfa trough with minimum value -27mgal. In the gravity anomaly map, the high gravity anomaly expression is not always restricted to the platform structures as we detected in the northern part of the Agedabia trough and the central parts of the Zelten and Dahra platforms where we detected low gravity anomaly within the two platforms. It is related to the structural features within the platforms that caused of the gravity disturbance from high density rocks to low density rocks.
The gravity anomaly expression in southeast of the Sirt basin is characterized by unique E-W trend direction, and low gravity anomaly expression with minimum-20mgal, is related to Hameimat and Sarir troughs, which are formed during the early stage (Late Jurassic early Cretaceous) in the formation of Sirt Basin.
Kotla graben extends from Hagfa trough in northeast to the Al Haruj volcanic in the southwest and separates Beda platform form Dahra platform. It is characterized by elongate linear feature with unique NE-SW trend direction as shown in Figure 3-10. It is expressed on the gravity anomaly map by low gravity anomaly with minimum -22mgal, which reflects low structural feature with low density
26 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
sediments infill within the graben. It is reactivated during the subsidence phases of the Sirt basin in the Late Cretaceous, along the old Hercynian trend due to the weakness zones of the upper crust.
The Agedabia trough represents the deepest trough within the Sirt basin as known form the well data. Figure 3-10 shows the northern part of the trough, which is characterized by long wavelength gravity anomaly with 17.4mgal. It is separated from Zelten platform in the west by NW block fault zone, which is expressed on the gravity map by steep gradient. The southern part of the trough is separated by high gravity anomaly, which is related to the Rakb High in the eastern part of Sirt basin. It is expressed by low gravity anomaly with minimum -20.33mgal, and separated form the northern part by NE basement fault zone as recognised from the gravity data.
Generally, normal faults are one of the continental rift environments, which occur when the hanging block falls downward relative to the foot wall. Based on that, the trough we interpreted as hanging wall where most of the sediments were deposited, while the platform structures we interpreted as foot wall where less sediments were deposited.
Most of the oil fields are located on the edge of the platform structures such as Gatar Ridge, Messlah High, south Zelten platform and Rakb high where most of the high density carbonate strata and fringing reefs are occurred which represent good reservoir rock. The location of the oil fields is associated with the high gravity anomaly expression as shown in Figure 3-9.
Gravity profiles
Figure 3 -9 Gravity bouguer anomaly map from previous work Abadi 2002, shows the previous structures interpretation within the Sirt basin, with the location of the well data which will be used in the gravity modeling
27 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 3 -10 HSV Gravity anomaly shaded map shows the boundary of the geological features as well as
28 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
4. Modeling of gravity anomalies
4.1. 2D forward modeling Anomaly source distribution can be estimated in two ways which are known as forward and inversion modelling. The 2D forward modelling is used to estimate the distribution of the gravity anomaly source for any body that has infinite strike length and takes place when the user manually adjust the model parameters in order to improve the fit between the observed field and the calculated field, while the inversion model takes place when the computer program adjust the model parameters automatically to improve the fit between the observed field and the calculated field.
We applied 2D forward modeling along two constructed profiles, by using the potent computer program which combines the observed gravity field, a geological model, and the calculated field to relate them into a highly interactive interpretation frame work in order to understand the geological structures of, in this case, the Sirt basin. The forward modeling consists of entering the shape and density of one or more geological bodies to the computer program (potent) to calculate their gravity effect at the stations on the surface or above the surface. Since we are dealing with the sedimentary basin with different sediments layers, we represented the sediments within the basin in the geological model by polygon prisms with different block density that related to the Cretaceous and Tertiary strata. After performing the calculation, we compared the calculated result to the observed gravity field in order to get best fit between the observed and the calculated field. Adjustment are required to the shape or density of the bodies and the potent program recalculate the model until the calculated anomaly curve fits the observed field curve, while in some cases the calculated anomaly curve do not fit with the observed field, due to the deep geological process as we will see later.
4.2. Import data to potent program Since the gravity survey isn’t line based and the observation intervals aren’t equally spaced, therefore isn’t possible to import the observed values as location (x, y) and z measurement values in the potent program, in this case we imported the original total field as grid file using the ER Mapper grid (*.grd) as input file. During the import processing the ER Mapper header file is displayed in order to select which component we are going to use and to illustrate the coordinate system of the grid file as shows in Figure 4-1. In potent program we can use more the one component of potential field at the same time, for example we can use the (TMI) total magnetic intensity field with the total gravity field as input components for modeling. In our model we used only the total gravity field component Gz, and finally the total field gravity gridded data is displayed in colored map as shown in Figure 4.2. It shows the varying gravity anomaly expressions within the Sirt basin, and use it to construct the cross sections.
Potent program represents each grid point as (X, Y, Z, F) observation points, where is X, Y represents the horizontal coordinates, Z represents the height of the observations in meter which is zero in our case and F represents the gravity observed field.
29 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 4 -1 ERmapper Header file showing the gravity components Gz (total gravity field) that we used in the modelling as well as the appropriate coordinate system for the area
3400000 Observed - Gz
3300000
Subset 1
3200000
Y(m)
Subset 2 3100000
500000 X(m) 600000 700000 800000 900000 1000000 1100000 Figure 4 -2 Gravity contoured map, include the two gravity cross sections
30 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
4.3. Subset data The subset data is a sample of the observations from a dataset. It consists of all observations that lie within a rectangular window. The subset can be created using each line in x, y, z file or as in our case we used the subset tool ( ) of potent program to define the rectangular window that encompasses the observations which we want to include them in the subset. Two subsets are created regarding to the two gravity cross sections as shown in Figure 4-2 and displayed in plan windows as shown in Figure 4-3 which is related to the subset 2 and Figure 4-4 is related to the subset 1. The two subsets are created in order to illustrate the gravity anomalies that related to the two cross sections with more details.
During the qualitative interpretation for the gravity data within the Sirt basin, we detected variable gravity anomaly expressions which are related to different geological features. As known from the geological information and previous works, the Agedabia trough is opened through the Mediterranean sea and represents the deepest part within the Sirt basin, the gravity expression within the Agedabia trough is variable as shown in the previous chapter 3, where the southern part of the trough is characterized by low gravity expression with -20.33mgal, while the northern part of the trough is characterized by high gravity anomaly with 17.4mgal. Based on the different of the gravity expressions along the trend of the Agedabia trough, we selected our gravity profiles to cross the two gravity anomaly expressions, and then to construct two gravity models along these profiles, in order to investigate the response of this different of the gravity anomaly expression within the same trend.
3240000 Observed - Gz 156217 6 64 593 3556 3200000 2 141 12153027 170 65111897 2 32650 26 2 391 28 3160000 3 342 3 356 Y(m) 4 413 4 308 4479 4 464 3120000 6534 6520 8
760000X(m8) 00000 840000 880000 920000 960000 1000000 1040000 1080000 1120000 1160000 1200000
Figure 4 -3 Plan displaying the rectangular window of the subset 2, which relates to the southern profile with its contour lines. The red and blue rectangular represent the polygon prisms that would be used in the gravity model 1
31 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Observed - Gz 3360000
132189 322276 25 3320000 2 3464 9 10 8 1 213 1313 3280000 1 415 1176 1 189 Y(m) 521 72305 3240000
680000X(m)720000 760000 800000 840000 880000 920000 960000 1000000 1040000 1080000 1120000
Figure 4 -4 Plan displaying the rectangular window of the subset 1, which relates to the northern profile with its contour lines. The red and blue rectangular represent the polygon prisms that would be used in the gravity model 2
4.4. Density of sedimentary rocks In average, sedimentary rocks have lower densities than igneous and metamorphic rocks. The density of the sediments is influenced by the depth below the surface. The density increases with an increase in depth and by their age as well as the degree of compaction and water content. The older sediments are denser than the younger sediments. However, the lithologic types and porosity also may influence the relationship between the density and depth. The density contrasts between the sediments and between the sediments and basement rocks are responsible for the gravity anomaly field. The high density basement may or may not equivalent to the crystalline magnetic basement. Table 4.1 shows densities of sediments and sedimentary rocks.
Table 4 -1Densities of sediments and sedimentary rocks (source Telford 1976) Rock type Range(wet) g/cm³ Range(dry) g/cm³ Clays 1.63-2.6 1.3-2.4 Gravels 1.7-2.4 1.4-2.2 Sand 1.7-2.3 1.4-1.8 Silt 1.8-2.2 1.2-1.8 Soils 1.2-2.4 1.0-2.0 sandstone 1.61-2.67 1.6-2.68 Shales 1.77-3.2 1.56-3.2 Limestone 1.93-2.90 1.74-2.76 Dolomite 2.28-2.90 2.04-2.54
Since we don’t have borehole density reading for sub-surface strata, we attempted to assign the actual density for the basin strata from general literatures review which is not particular for the Sirt basin. For the crustal density we took 2.67g/cc, for the Cretaceous strata we took 2.6g/cc which consists generally from shale. For the Paleocene-Eocene strata we took 2.55g/cc which consist generally from
32 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
carbonate, for the Oligocene to Quaternary strata we took 2.45g/cc which consist generally from mixed carbonate and clastic strata.
4.5. The southern and northern profiles The profiles are selected to cover as much of the gravity stations reading as it is possible to avoid the effect of the interpolation during the gridding process. The southern profile extends from Maragh trough in the northeast to the Zallha trough in the southwest as shown in Figure 4-2 and passing through the following features Rakb high, Agedabia trough, Zelten platform, Hagfa trough, Beda platform, Kotla graben, Gattar ridge, Zallah trough, with distance 450 km, along this profile we construct our gravity model 1 which is based on controlled data (well data) used from (Abadi, 2002). It penetrated the sedimentary section of the Sirt basin, and reached to the lower Cretaceous strata which consists mainly of Nubian sandstone, and represents the Hercynian unconformity that separated it from the underlying granitic, metamorphic and Cambro-Ordovician basement rocks. The northern profile extends from Amal platform in the northeast to the Waddan uplift in the northwest as shown in Figure 4-2 and passing through the following features Amal platform, northern part of Agedabia trough, northern part of Zelten platform, Hagfa trough, Dahra platform including the Messlah High and Gatar Ridge, Dor El Abida trough and Waddan uplift with distance 450 km. Along this profile we constructed the gravity model 2 where in the western part of the profile we have controlled data, while in the northern part of Agedabia trough we don’t have controlled data.
4.6. Conceptual geological model Before starting modeling the gravity data, we should think about the appropriate conceptual geological model that will be correlated with the gravity anomalies and how we will relate the high and low gravity anomalies with this conceptual geological model.
Several geological cross sections were constructed along most of the geological features of the Sirt basin. Figure 4-5 shows one of the geological model from (Abadi, 2002) which is related to the geological structures of the Sirt basin Based on that, we will construct our conceptual geological model and correlate it with the gravity anomaly expressions, along the two gravity profiles.
Figure 4 -5 Conceptual geological model. Source (Abadi, 2002)
33 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
4.7. Gravity models Generally, the continental crust can be considered to be composed of sediment layer (0-5 km thick) underlain by an upper layer (10-20 km thick) and a lower layer (15-25 km thick) of the continental crust.
We started modeling the gravity data along the two profiles by representing the sedimentary section of the Sirt basin as three layers. The sedimentary layers consist of Cretaceous Strata, Palaeocene-Eocene Strata and Oligocene-Quaternary Strata. Appropriate density was assigned to them which is increasing with the depth. The sedimentary layers represent the upper layer of the model, the reason behind representing the upper layer of the gravity model by three sedimentary layer just to make the gravity model easy as much as possible, and if we represented by several sedimentary layers then we need to use the actual density for each layer or in other term for each formation of the Sirt basin which we don’t have it in this research. The middle layer of the model represents the continental crust with density 2.67g/cc, and 35 km thickness. The bottom of the model is represented by the mantle layer with high density 3.3g/cc. The boundary between the crust and the mantle is known as moho or discontinuity after the Croatian seismologist Andrija Mohorovicic (1857-1936) who discovered that the depth is between 25 and 60 km deep beneath the continents crust and between 5 and 8 km beneath the oceanic crust.
With regard to the controlled well data that reached to the lower Cretaceous (Hercynian unconformity) as shown in Figure 3-9, we started model the gravity data along the southern profile by representing the sedimentary strata from the available well data. Then, the adjustment was successively made in the boundary between the earth’s crust and mantle with respect to the well data in order to get the best fit between the observed gravity field and the calculated gravity field, as shown in Figure 4-8.
The northern part of the Sirt basin is opening through the Mediterranean sea and the northern rim of the basement in the Sirt basin steepness rapidly towards the Mediterranean (El-Makhrouf 1996)
In order to construct the model for the gravity data along the northern profile as shown in Figure 4-2, we followed the same steps as mentioned above and the density of the layers is the same as it was in the model 1. Since we do not have controlled data on the northern part of Agedabia trough, it was attempted to use the located Euler deconvolution for magnetic data in order to estimate the depth to the basement for Agedabia trough, as result the northern part of the Agedabia trough is shown as deep trough with more than 10000 m depth as shown in Figure 4-6. Based on that we represented the northern part of Agedabia trough as deep trough in the gravity model, and we assumed that the depth to the lower Cretaceous strata about 8000m. Figure 4-9 shows the model 2A which is regarded to the northern profile, the observed gravity field is not fit well with the calculated field, in this case adjustment in the gradient of the block faults between the trough and platform structures is made in order to have good fit between the observed and the calculated field as shown in Figure 4-10 where the western part of the model 2B shows good fit between the observed gravity field and the calculated field, while in the Agedabia trough shows misfit between the observed gravity field and the calculated field.
34 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 4 -6 Located Euler deconvolution from the magnetic data
During the subsidence phases which are characterized by the regression and transgression of the Sea level, series of high density thick carbonate has deposited in the Sirt basin as shown in Figure 2-3 which is possible caused of the high gravity anomaly within the Agedabia trough. Based on that we built the first scenario with regard to the high gravity anomaly expression within the northern part of Agedabia trough, we assumed that the sedimentary layer of the Paleocene and Eocene strata is characterized by high density with 2.8g/cc value instate of 2.55g/cc, that was only with respect to the northern part of Agedabia trough, Figure 4-11 shows the model 2C which is related to the first scenario. The model 2C still shows misfit between the observed field and the calculated field in the northern of Agedabia trough.
The Sirt basin is type of continental rifts. Generally, the continental rifts are caused by thinning of the crust beneath the rifts. They are characterized by high heat flow as well as geothermal and volcanic activities, regarded to reflect hot asthenosphere at relative shallow depths. The major mechanism behind these rifts is upwelling or diapirism of the mantle and stressed produced by moving plates.
As shown in the regional gravity anomaly map with low pass 200 km in Figure 3-5 the northern part of Agedabia trough still characterized by long wavelength anomaly expression in the low pass 200 km filter, while some of the geological features are disappeared with this filter, it gave us an idea that the long wavelength within the northern part of Agedabia trough is related to the deep seated structures,
35 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
which is thought to be due to the undulations of the crust and mantle interface or in other terms, this long wavelength is related to the mantle process below the lithosphere which allow to the dense mantle material to be in shallow depth, that was the second scenario . Figure 4-12 shows the model 2D which is related to the second scenario, it shows good fitting between the observed gravity field and the calculated field, with regard to the upwelling of the mantle within the northern part of Agedabia trough.
The width of the northern part of Agedabia trough is represented as wide trough in the gravity models 2, that was based on the size and shape of the gravity anomaly expression within the northern Agedabia trough on the gravity anomaly map which approximately 75 km, as well as the magnetic expression within the northern part of Agedabia trough as we will see in the next chapter.
4.8. Quantitative interpretation Gravity models are coinciding with the configuration of the Sirt Basin, which reflects a series of troughs and platforms bounded by basement faults, which reflect a period of instability within the region.
The gravity model along the profile is combined the observed and the calculated field with the conceptual geological model along the southern profile where the low gravity anomaly expressions are related to the trough structures and the high gravity anomaly expressions are related to the platform structures. The thickness of sediments (Cretaceous-recent strata) within the troughs and platforms area, which were obtained from gravity data, is compatible with the well data that were used as controlled data along the southern profile as shown in table 4-2.
4 -2 Comparison between the well data and the depth that achieved by the gravity data
Structure name Well data Gravity model South Agedabia trough 6800m 7500m Hameimat trough 5000m 5500m Hagfa trough 4550m 4500m Kotla graben 3000m 3500m Zallah trough 3500m 3500m Rakb High 2800m 2900m Zelten platform 2500m 2500m Gatar Ridge 2000m 1900m Beda platform 1900m 2000m Waddan uplift 1000m 1500m
When we compared the thickness of the sedimentary section in the eastern part and western part of the model 1 as shown in Figure 4-8, it is increasing towards the east of the Sirt basin, which reflects the most intense periods of subsidence within the eastern part of Sirt basin. The sedimentary section
36 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
of the Oligocene-Quaternary is absent over large scale in the western part, which reflects less intense subsidence in western part of the Sirt basin.
The faults that separated the troughs form the platforms are not vertical in the gravity models where we change the gradient of the faults in order to have good fit between the observed and calculated gravity field. The faults gradient in gravity models are varied from very steep as in the Hagfa trough as shown in Figure 4-12 to semi vertical in the other structures, and we do not have another data, such (seismic data) or field evident to confirm that.
The northern part of the Hagfa trough as shown in Figure 4-12 is wider than the southern part as shown in Figure 4-8. It was interpreted as intensive subsidence which affected in the northern part more than the southern part.
Wennekers et al (1996) referred the northern part of Agedabia trough as a deep, intra-trough horst feature which was identified on both gravity and seismic data. They called the Al Brayqah horst (Hallet 2002), as shown in Figure 4-7, which occurred within the northern profile that we used in the gravity model. Regarding to that, we linked the short wavelength of the gravity anomaly, which is superimposed with the long wavelength of the north Agedabia trough to the Al Brayqah horst as shown in Figure 4-12.
The high positive gravity anomaly within northern part of Agedabia trough as shown in Figure 4-12 we interpreted as: ° Thinning of the continental crust beneath the trough which is thought to be caused by the upwelling of the mantle during the continental rift that formed the Sirt basin due to the tectonic evolution between African and Eurasian plates. During the tectonic evolution between African and Eurasian plate the northern part of Agedabia trough is affected by NE-SW crustal extension which gave possibility to the upwelling of the mantle, the NE-SW crustal extension is activated only within the northern part of Agedabia trough, while the southern part of the Agedabia trough isn’t affected by this NW-SE crustal extension. ° The short wavelength which superimposed with the long wavelength is related to the Al Brayqah intra-trough horst.
Figure 4-12 shows two grey curves which are related to the calculated field for the model layers , the upper curve shows the contribution of the lower layer (mantle) where the interface between the mantle and the lower crust. The lower curve shows the contribution of the bottom sediments where the interface between the basement rocks and sediments.
In the northern Agedabia trough, the grey curves illustrate the relation between the long wavelength gravity anomaly with the northern Agedabia trough. The long wavelength related to the interface between the mantle and the lower crust, while the short wavelength that superimposed with the long wavelength related to the interface between the (Al Brayqah intra-trough horst) basement and the sediments cover.
37 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 4 -7 Structural features within the Sirt basin include Al Braygah intra-trough horst
38 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
8.0 Zelten platform 0.0 Gatar Zallah Ridge Rakb Trough Beda Hagfa High -8.0 Kotla platform Trough Agedabia Hameimat Graben Trough trough
-16.0
-24.0
Gz (mgal) Calculated field Observed field -32.0
P(m) 0 200000 400000 Y 0 3200000
-10000
Oligocene-Quaternary Strata, 2.45g/cc
Palaeocene-Eocene Strata, 2.5g/cc
Cretaceous Strata, 2.6gcc -20000
Approximate well location
Crust 2.67g/cc
-30000
Moho
Z(m) Az = 75.5deg Mantel 3.3g/cc -40000
Figure 4 -8 Model 1, the southern profile
39 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
24.0
Zelten 16.0 Messlah Platform Gatar Dahra High Agedabia Amal 8.0 Ridge Platform Trough Trough Waddan Hagfa 0.0 Uplift Dor El Abida Trough Trough -8.0
-16.0
-24.0 Gz (mgal) -32.0
P(m) 0 300000
Oligocene-Quaternary Strata, 2.45g/cc -10000 Palaeocene-Eocene Strata, 2.5g/cc
Cretaceous Strata, 2.6gcc
-20000
Crust 2.67g/cc -30000
Z(m) Moho
Mantel 3.3g/cc -40000 Az = 78.7deg
Figure 4 -9 Model 2A, the northern profile. It shows miss fitting between the observed and calculated field
40 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
48.0
40.0 Calculated field 32.0 Observed field
24.0 Zelten 16.0 Messlah Platform Gatar Dahra High Amal 8.0 Waddan Ridge Platform Agedabia Trough 0.0 Uplift Dor El Abida Hagfa Trough -8.0 Trough Trough
-16.0
-24.0 Gz (mgal) -32.0
P(m) 0 200000 400000
-10000 Oligocene-Quaternary Strata, 2.45g/cc
Palaeocene-Eocene Strata, 2.5g/cc
Cretaceous Strata, 2.6gcc -20000
Approximate well location
Crust 2.67g/cc
-30000
Moho Z(m) Mantel 3.3g/cc
-40000 Az = 78.7deg
Figure 4 -10 Model 2B, the northern profile. It shows good fitting between the observed and calculated field in the western part of the Sirt basin, while the northern part of Agedabia trough shows miss fitting between the observed and calculated field
41 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
24.0 Calculated field 16.0 Observed field Zelten Messlah Platform Amal 8.0 Gatar Dahra High Agedabia Trough Ridge Platform Trough 0.0 Waddan Dor El Abida Hagfa -8.0 Uplift Trough Trough
-16.0
-24.0 Gz (mgal) -32.0
P(m) 0 300000
-10000 Oligocene-Quaternary Strata, 2.45g/cc
Palaeocene-Eocene Strata, 2.5g/cc
Palaeocene-Eocene Strata, 2.8g/cc within the -20000 northern part of Agedabia trough Cretaceous Strata, 2.6gcc
Crust 2.67g/cc -30000
Z(m) Moho
Mantel 3.3g/cc -40000 Az = 78.7deg
Figure 4 -11 Model 2C, the northern profile. It shows the first scenario with respect to the high density carbonate strata
42 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
48.0
40.0 Calculated field Observed field 32.0
24.0 Agedabia Trough 16.0 Messlah Zelten Gatar Dahra High Platform Amal 8.0 Ridge Platform Al Brayqah Trough 0.0 Waddan Horst -8.0 Uplift Dor El Abida Hagfa Trough Trough -16.0
-24.0 Gz (mgal) -32.0 P(m) 0 0 200000 400000
-10000
Oligocene-Quaternary Strata, 2.45g/cc
Palaeocene-Eocene Strata, 2.5g/cc -20000
Cretaceous Strata, 2.6gcc
Approximate well location Crust 2.67g/cc
-30000
Moho Z(m) Mantel 3.3g/cc
-40000 Az = 78.7deg
Figure 4 -12 Model 2D, the northern profile. It shows the second scenario with respect to the mantle process bellow the lithosphere
43
STRUCTURAL CONFIGURATION OF THE SIRT BASIN
5. Aeromagnetic data
5.1. Introduction Aeromagnetic anomalies occur due to the variation in the earth’s magnetic field caused by the distribution of the magnetic minerals in the rocks that make up the upper part of the earth’s crust.
The features and patterns of the aeromagnetic anomalies can be used to reflect important information about the regional geology including the locations of buried faults, magnetic bearing rocks and the thickness of the sedimentary rocks within the basins which are generally non-magnetic. This information is useful for the sedimentary basin studies, mineral exploration and geological mapping.
Basement crystalline rocks generally contain sufficient magnetic minerals to cause variation in earth’s magnetic field that can be mapped by aeromagnetic survey. Faults on the magnetic maps often cut magnetic bodies and offset magnetic anomalies.
5.2. Dataset The data is provided by African Magnetic Mapping Project (AMMP) as grid data, with grid cell size 1000m. The known coverage of Libya is shown in the figure 5-1. The study area covers the northern part of the Sirt Basin as well as Al Bottnan Basin and Jaghabub uplift with approximate coordinate 28º, 30` N to 31º, 30`N and 18º to 23ºE and total area 223km×260 km.
Table 5 -1 Specification of the surveys Survey number 3084 Client: Western Atlas International. Year: 1960. Line spacing: 5km. Flying height: 457.2m Asl Survey number 1182.03 Client: Shell International Petroleum Mij.b.v. Year: 1970. line spacing 4km Survey number 1176.02 Client: Societe National ELF Aquitaine(Prod). Year 1969. line spacing 4km. Flying hight:609m Asl Survey number 1329 Client: AGIP SpA. Year:1960. line spacing: 5km. Flying height:762m Asl Survey number 2539.03 Client: WAHA Oil Company. Year:1989
45 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Figure 5 -1 Aeromagnetic known coverage for Libya
5.3. Basement The basement is assemblage of rocks that underlies a sedimentary basin and represents the main source of magnetic anomalies. If it contains numerous magnetic rocks such as igneous intrusions or extrusive, magnetic metamorphic or magnetic sediments units, these can provide information on the morphology of the sedimentary basin and its structure. The basement rocks in the Sirt basin consist of granites, volcanic and metamorphic rocks, of Precambrian and early Cambrian age which was recorded by some of drill holes that reached it. It is exposed only to the north of the Tibesti mountain in the southern of Libya, Jabel Awaynat in southeast of libya and in the central of the Gargaf Arch in the eastern part of Libya.
5.4. Grid colour shaded image Colour shaded method is used to display the gridded data in colour shaded map with artificial illumination in order to enhance the boundary of the geological structures within the region. We used the HSV table assigned from Oasis montage to display the grid data in wet look shaded colour image, to be used in the qualitative interpretation.
5.5. Upward continuation The upward continuation smoothes out the high-frequency anomalies relative to low-frequency anomalies. It is used to remove or minimize the effects of shallow source and noise in grids data.
46 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
We used the MAGMAP facility in order to process the original grid data using the upward continuation filter to produce total magnetic map continued up to distance of 1000 m as shown in Figure 5-2, which is up to one grid cell size. It shows the total magnetic intensity data that would be produced if the data were collected at height of 1000m higher than the original survey.
Short wavelength Anomaly
Grid profile
Figure 5 -2 Total magnetic intensity map, upward continued up to 1000m
5.6. Grid profile The profile is selected to cross the northern part of Agedabia trough, zelten platform, Amal platform and Hagfa trough. We used the original grid data of the total field gravity anomaly and the upward grid data of the total intensity magnetic anomaly to construct the profile as shown in Figure 5-3, in order to compare the response of two potential filed dataset over the same area, which is focused on the northern part of Agedabia trough.
47 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Zelten platform Amal Agedabia Platform Hagafa Trough Trough
Figure 5 -3 Gravity and magnetic grid profile, for the location of the grid profile refer to Figure 5-2
5.7. Qualitative interpretation North Agedabia trough is characterized by low magnetic anomaly and is shown as wide trough on the magnetic grid profile as we represented in the gravity model, the prominent trend within the trough is NW-SE basement trend feature which is related to the late Cretaceous structures within the Sirt basin, with 60 km wide as shown in Figure 5-2. It is bounded by Amal platform on the east and Zelten platform on the west. They reflected high magnetic anomalies due to the uplift of the basement rocks, while the southern part of the Agedabia trough is characterized by NE basement trend, which is coincided with the Palaeozoic Hercynian trend. By comparing the gravity and magnetic expressions as shown in Figure 5-3, both of them are characterized by high gravity and magnetic anomaly expressions in the Zelten platform, Amal platform and low magnetic and gravity anomaly expressions in Hagfa trough, while in the northern Agedabia trough the gravity and magnetic expressions are different.
The high gravity anomaly of the northern part of Agedabia trough as we mentioned above is related to the high density material coming from the upwelling mantle into the lithosphere layer. The upwelling process is associated with the high temperature (Curie point 578Cº) where the heat destroys the magnetism and causing melting in the lower part of the crust, due to this process the magnetic properties in the lower crust are lost and have expressed by a low magnetic anomaly in the magnetic data. That was just a theoretical explanation, because we do not have data to confirm that.
Magnetic expression within the Al Bottnan basin is reflected elongate NE-SW trending linear feature which is coincided with the old Hercynian trend of the late Palaeozoic, with -70nT magnetic amplitude, which is related to the sedimentary infill within the basin and even the sedimentary infill within the Al Bottnan basin was recognised in the gravity data where it characterized by low gravity anomaly expression along the basin which is related to the low density sediment infill. It is separated by high magnetic anomaly with average 90nT which is related to the Jaghbub uplift in the south of the Al Bottnan basin. The sudden change in the magnetic anomaly reflected the basement fault, which separated Al Bottnan basin from the Jaghbub uplift.
48 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
The short wavelength anomaly between the NE linear trend and NW linear trend within the Agedabia trough as shown in Figure 5-2 and illustrated by small circle is related to the volcanic activities that occurred during the intersection between the recent NW trending basement structure which caused by the NE-SW crustal extension due to the bending of the continental crust of African plate beneath the Eurasian plate and the old Hercynian NE trending basement structure.
Hagfa trough is reflected low magnetic anomaly with amplitude -45nT, with NW-SE trend structure, which is coincided with the late Cretaceous structures of the Sirt basin.
The Hameimat trough is formed during the early stage of formation the Sirt basin. It is characterized by low magnetic anomaly expression, with NE trend which is indicative for reactivated rifting along the NE Hercynian trend in the Late Jurassic–Early Cretaceous in the early formation stage of Sirt basin.
49
STRUCTURAL CONFIGURATION OF THE SIRT BASIN
6. Tectonic implication
The structure features within the Sirt basin which represented by E-W trend in the southeast and NW- SE trend in the central and northern part of the basin are believed to be related mainly to tectonic motion between Africa, south America, and Eurasian plates.
The Gondwana and Laurasia were formed in the early Jurassic (200 Ma) by the break up of Pangea. The Gondwana was the southern half of Pangea, it had broken apart by 100Ma middle Cretaceous, included most of the land masses which makes today continents of the southern hemisphere including South America, Africa, Australia, Antarctica, India and Madagascar. The Laurasia was the northern half of Pangea, it included most of the land masses which makes today continents of the northern hemisphere including North America, Europe and Asia, and is believed to have broken up about (100 Ma) with the separation of North America from Europe. http://kartoweb.itc.nl/gondwana/gondwana_gif.html
The Sirt basin is located in the North African continental margin, the northern part of the Sirt basin was affected by a successive tectonic process due to the tectonic motion between African and Eurasian plates, during the Mesozoic and Cainozoic times several tectonic events were incounted between African and Eurasian plates, which are charactarized by west and east movement of African plate relative to Eurasian plate with compressional events due to this movement.
African plate Eurasian plate
Slab-pull force
Figure 6 -1 Schematic sketch shows how African plate subducted underneath the Eurasian plate which caused of the structural features within the Sirt basin
51 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Southern Aged abia trough: NE-SW trends basemen t fault possibly reactivated due to flexure of the subducting Afr ican plate beneath NW-SE structural features of the Eurasian plate Sirt basin (horst and graben)
NE-SW crustal extension within the northern part of Agedabia trough due to flexure of the subducting
slab during the tectonic evolution between African and Eurasian plates
Figure 6 -2 Schematic sketch shows the NE-SW crustal extension due to the collision between African And
Eurasian plates which caused of the NW-SE trending features of the Sirt basin, and mainly for the northern
part of Agedabia trough due to the bending of the continental crust of African plate.
Figure 6-2 shows in more details the NE-SW crustal extension which is caused of NW-SW trending feature within the northern part of Agedabia trough due to the flexure of the subducting slab during the tectonic evolution between African and Eurasian plates, while the southern part of Agedabia trough according to our geophysical data isn’t affected by the NE-SW crustal extension and came apart from the northern part by NE-SW basement fault, and the southern part of Agedabia trough is possibly reactivated along the NE-SW trending feature which inherited form the old Hercynian trend.
During the movement of Africa plate relative to Eurasian plate the slab-pull force was the dominant force for continued subduction of African plate relative to Eurasian plate, which was the prominent effect of the northern Sirt basin and expressed as series of trough and platform structures.
Our focus on the Agedabia trough where our geophysical data illustrated this tectonic process between the African and Eurasian plates. The magnetic data clearly shows that the northern part of Agedabia trough is charactarized by NW trending features which are different from the NE trend of the southern part of the trough, where the NW trend structure is related to the formation of the Sirt basin during the Mesozoic and Cainozoic times while the NE trend structure is possibly inherited from older structural features which are related to the Palaeozoic times. From the gravity data, it is clearly shown that the northern part of Agedabia trough is characterized by high gravity anomaly
52 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
expression, while the southern part is characterized by low gravity anomaly expression as we mentioned and explained before as the result of crustal thinning and mantle up welling.
Basement fault
Figure 6 -3 Gravity data on the left and Magnetic data on the right show the northern and southern parts of the Agedabia trough which are came a part by NE-SW basement fault
The African plate was subducted underneath the Eurasian plate during the tectonic process between the two plates, which is affected the northern part of Sirt basin and mainly in the northern part of Agedabia trough, due to that the northern part of Agedabia trough was affected by NE-SW crustal extension due to the bending of continental crust of African plate which is caused NW-SE trend feature within the northern part of Agedabia trough and allowed to the high dense material form the mantle to be in shallow depth as recognized from the gravity data as shown in Figure 6-3 and followed by thinning of the continental crust beneath the northern part of Agedabia trough. The southern part of Agedabia trough was dominated by reactivation along the NE-SW trending structure (Inherited from the Hercynian period), the northern part of Agedabia trough is separated form the southern part by NE-SW basement fault which is recognised from the magnetic and gravity data.
The tectonic process (NE-SW tectonic extension) within the northern Agedabia trough is more recent than southern parts, which is maybe related to the late stage of formation the Sirt basin, due to the collision between the African and Eurasian plates.
53 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
7. Conclusion and recommendations
7.1. Conclusion This study of gravity and aeromagnetic data of the Sirt basin has led to some interesting features. Normally, the NW-SE trending graben areas are characterized by low gravity anomalies and the horsts (platforms) are having high gravity anomaly values.
Generally the low gravity anomaly expressions are normally associated with low structural features (troughs), while the high gravity anomaly expressions are associated with high structural features (platforms). However, some of the geological features within the Sirt basin are characterized by reverse gravity expressions. The platforms are having low gravity expressions and the troughs are having high gravity expressions. For example, the northern Agedabia trough is characterized by high gravity expression. On the other hand, the Waddan uplift, Dahra platform and middle part of the Beda platform are characterized by low gravity expressions.
Agedabia trough is characterized by two gravity anomaly expressions: low gravity anomaly expression on the southern part of the trough with, and high gravity anomaly expression in the northern part, which is characterized by long wavelength. The northern part is characterized by NW trend on both the gravity and magnetic data, while the southern part is characterized by NE trend recognized from the magnetic data. These two areas are separated by a pronounced ENE trending lineament, which is recognized on both the gravity and magnetic data and is interpreted as basement fault. The conclusion is that both sides of the Agedabia trough represent different basement structure.
Most giant oil fields within the Sirt basin are located on the edges of the platforms, near gravity gradients. For example: Dahra and Hofra fields on the edge of the Dahra platform, Waha on the edge of the Zelten platform and Amal and Nafoora on the edge of Rabk high. A typical gravity profile through the platforms shows high gravity anomalies on the edges, and lower values at the centre of the platforms. This could be due to two factors: the deposition of (relatively dense) limestones in fringing reefs, and secondly by the disintegration of platforms in the later stage of the basin development (Abadi, 2002), resulting in tilted fault blocks.
Two gravity models were constructed along two profiles in the Sirt basin in order to discriminate between gravity expressions within the northern and southern parts of the Agedabia trough.
The southern profile is controlled by well data. The gravity model related to the southern profile shows good fitting as result between the observed and calculated gravity field, and the depth to the base of the Cretaceous strata that were achieved by the gravity data is compatible with the well data. The low gravity anomaly across the model is related to the low structural features within the Sirt basin which are mainly due to the low density sediments infill within the troughs. The gravity model shows five low structural features which are related to the Hameimat, Agedabia, Hagfa troughs and Kotla graben as well as Zallah trough from east to west respectively. The southern Agedabia trough is the deepest known structure in the southern part of the Sirt basin.
54 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
Based on the gravity model, the long wavelength and high gravity anomaly of the northern Agedabia trough is thought to be the result of mantle processes below the lithosphere, with the upwelling of high density material which caused of thinning in the continental crust beneath the northern Agedabia trough. The short wavelength which is superimposed on the long wavelength anomaly is related to the Al Brayqah intra-trough horst.
The magnetic data within the Agedabia trough revealed two different trending features: NW trend within the northern part and NE within the southern part. The NE trend is related to the old Hercynian trend while the NW trend is related to the formation of the Sirt basin during the Late Cretaceous. These structures reflect different period of tectonic instability within the Sirt basin, with NE-SW crustal extension within the northern part of Agedabia trough due to the flexure of the subducting slab during the tectonic evolution between African and Eurasian plate, and nothing clearly visible according to our geophysical data that suggest the southern part of the Agedabia trough is considered to be tectonically quite and came apart from the northern part of the trough by NE-SW basement fault, and possibly the southern part of Agedabia trough is reactivated along the older trend during the formation of the Sirt basin where our data didn’t clearly show that as it did within the northern part of Agedabia trough.
The northern part of the Agedabia trough is characterized by a low magnetic expression which we interpreted as result of the mantle upwelling, associated with high temperature. Due to this process melting occurred in the lower crust where the magnetic properties are lost and have expressed by a low magnetic anomaly in the magnetic data.
As shown in Figure 3-5 the long wavelength gravity anomaly with 200 km low pass filter (regional gravity anomaly map) shows the northern part of Agedabia trough is still characterized by long wavelength gravity anomaly expression with this regional component, which gave us an idea that the long wavelength with the northern part of Agedabia trough is related to the deep seated structure due to the mantle process below the lithosphere.
During the tectonic motion between the African and Eurasian plates, the northern part of Agedabia trough was the most affected structure within the Sirt basin due to that tectonic evolution (African plate subducted beneath the Eurasian plate), the NE-SW crustal extension was caused the NW-SE structural feature within the northern part of Agedabia trough and allowed to the high dense material from the mantle to be in shallow depth or in other term, this tectonic extension gave the possibility to upwelling of the mantle and followed by thinning in the continental crust beneath the northern part of Agedabia trough.
Based on the geophysical data that we have, the idea or the result which is related to the upwelling of the mantle is a hypothetical result and there is no extra data to confirm this idea, and further studies are required especially within the northern part of Agedabia trough.
55 STRUCTURAL CONFIGURATION OF THE SIRT BASIN
7.2. Recommendations Further studies are required for the area of Sirt Basin to understand the effects of the tectonic processes on the formation of the Sirt basin, their relation on the crustal thickness, and the occurrence of the hydrocarbon deposits. For further study like this, these following recommendations should be considered: ° Actual sediments density, from well data that penetrated the sedimentary section of the Sirt basin are required to be used for the gravity model ° The data used in this study is random ground survey gravity data, and is not equally spaced. To avoid the interpolation error, which appeared during the gridding process, it is recommended to use the new gravity satellite data from GRACE and GOCE satellite missions to model the crustal thickness in the region and to be able to recognize large scale regional trend. ° Seismic data are required to constrain the gravity data to understand the behavior of the subsurface structures, as well as the shape of the basin. ° Seismological data can contribute to the estimation of the crustal thickness. ° A detailed gravity modeling exercise, aided by seismic and well information, could help in resolving the question related to the high gravity anomaly on the edges of the platforms (Zelten and Dahra platform) where most of the oil fields are located. It could help in distancing between tilted fault blocks created by platform disintegration and/or fringing carbonate reefs, and therefore it will be of significance for oil exploration.
(C.Condie 1989)
(Barr and Weegar 1972) (Bayona Pelaezy September, 1994) (Hinze 1985) (Gibson and S.Millegan..) (Asfaha December, 1990) (Fairhead and Okereke 1987) (Genik 1992) (Telford.W.M. and Sheriff.R.E. 1976; Abadi and Van dijk 1993; Gunn 1997)
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8. References
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Abadi, A. M. and P. Van dijk (1993). "SHORT NOTES AND GUIDEBOOK ON THE STRATIGRAPHY AND TECTONICS OF WEST ZALLAH TROUGH, SIRT BASIN, LIBYA." "First Symposium" Geology of Sirt Basin: 1-52.
Ahlbrandt, T. S. (2001). "The Sirte Basin Province of Libya—Sirte-Zelten, Total Petroleum System." (U.S. Geological Survey Bulletin 2202–F): 29 pp.
Anketell, J. M. (1996). "Structural History of the Sirt Basin and its Relationships to the Sabratah Basin and Cyrenaican Platform, Northern Libya."ELSEVIER" 3: 57-87.
Asfaha, W. (1990). Processing and interpretation of Gravity and Aeromagnetic data, GAMBELLA, SW ETHIOPIA. MSc thesis, ITC, Enschede: 73 pp.
Barr, F. T. and A. A. Weegar (1972). STRATIGRAPHIC NOMENCLATURE OF THE SIRT BASIN, LIBYA, THE PETROLEUM EXPLORATION SOCIETY OF LIBYA (TRIPOLI, LIBYA): 179 pp.
Bayona Pelaezy, D. (1994). INTERPRETATION OF AIRBORNE AND GROUND MAGNETIC SURVEYS IN THE UCAYALI AND MADRE DE DIOS BASINS AND THE ANDAHUAYLAS AREA, PERU, MSc thesis, ITC, Enschede: 72 pp.
Condie, K. (1989). Plate Tectonic & Crustal Evolution, third edition: 476 pp.
El-Makhrouf, A. A. (1996). "The Tibisti-Sirt Orogenic Belt, Libya, G.S.P.L.A.J." ELSEVIER" The Geology of Sirt Basin, volume 3: 107-120.
Fairhead, J. D. and C. M. Green (1989). "Controls on rifting in Africa and the regional tectonic model for the Nigeria and East Niger rift basins." Journal of African Earth Sciences" 8: 231-249.
Fairhead, J. D. and C. S. Okereke (1987). "A regional gravity study of the West African rift system in Nigeria and Cameroon and its tectonic interpretation." Tectonophysics" 143: 141-159.
Genik, G. J. (1992). "Regional frame work, structural and petroleum aspects of rift basins in Niger, Chad and the Central African Republic (C.A.R.)." ELSEVIER": 169-185.
Gibson, R. I. and P.S.Millegan.. "Geological Application of Gravity and Magnetics: Case Histories (AAPG studies in Geology, No.43) and SEG Geophysical Reference Series, No.8)." 162.
Guiraud, R. and W. Bosworth (1999). "Phanerozoic geodynamic evolution of northeastern Africa and the northwestern Arabian platform,." ELSEVIER" 315(1-4): 73-104.
Guiraud, R. and J.-C. Maurin (1991). "Early Cretaceous rifts of Western and Central Africa :an overview." Tectonophysics" 213: 153-168.
Gunn, p. j. (1997). "Regional magnetic and Gravity responses of extensional sedimentary basins." AGSO JOURNAL OF AUSTRALIAN GEOLOGY&GEOPHYSCS 17(NUMBER 2): 115-130.
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Hallet, D. (2002). Petroleum Geology of Libya " ELSEVIER"
Hinze, J. W. (1985). The Utility of Regional Gravity and Magnetic Anomaly maps, Society of exploration Geophysical (Tulsa,Oklahama): 454 pp.
Shaaban, F. F. and A. E. Ghoneimi (2001). "implication of seismic and borehole data for the structure, petrophysical and oil entrapment of Cretaceous-Palaeocene reservoirs, northern Sirt basin, Libya." Journal of African Earth Sciences 33: 103-133.
Telford.W.M., G. L. P. and K. D. A. Sheriff.R.E. (1976). Applied Geophysics: 860 pp.
Van der Meer, F. and S. Cloetingh (1993). "Intraplate stresses and the subsidence history of the Sirte basin, Libya." ELSEVIER" The Geology of Sirt Basin, volume 3: 212-230.
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