Tectonic history and structural development of the Zallah-Dur al Abd Sub-basin, western Sirt Basin, Libya

Journal of Structural Geology 73 (2015) 33e48

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Tectonic history and structural development of the Zallah-Dur al Abd Sub-basin, western Sirt Basin, Libya Khalifa M. Abdunaser a, *, Ken J.W. McCaffrey b a b

Libyan Petroleum Institute, Libya Durham University, DH1 3LE, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 July 2014 Received in revised form 27 January 2015 Accepted 19 February 2015 Available online 26 February 2015

The Zallah-Dur al Abd Sub-basin area lies in the western part of the Sirt Basin of Libya. 2D seismic data covering an area of about 32,000 km2 were studied along with the formation tops from 240 wells. We mapped a complex network of normal and probable strike-slip faults, generally striking NNW-SSE that control the asymmetry of the basin. Subordinate NEeSW structures acted as transverse faults controlling local depocentres that segment the Zallah-Dur al Abd Sub-basin. A number of active faults in the intra-basin area have been identified in seismic sections with generally moderate to high dip angles, and displaying evidence for positive and negative flower structures. The bordering extensional fault (the Gedari fault) passes at depth into a moderately SW-dipping structure crossing most of the Upper Mesozoic to Cenozoic stratigraphic section. Thickness variations adjacent to other major faults suggest also an original extensional system where inherited high-angle faults were reactivated throughout this time. A detailed analysis of the available seismic reflection and drill hole data shows that an obliquely rifted, multi-cyclic, NNW-SSE trending basin developed during the complex Upper Mesozoic Cenozoic rearrangement of Mediterranean tectonics. Multiple phases of rifting can be observed in the study area affecting a number of different horizons from Upper Cretaceous to Eocene. In the study area, the basin was initiated as a result of a Tethyan oblique extensional rift system that began in the Early Cretaceous and peaked in the Late Cretaceous. The basin reached its rift maturation phase during the Upper Cretaceous as a result of the continuing extensional tectonics on the marginal bounding NNW-SSE trending normal growth faults. During the Alpine-related tectonic pulses of Middle eLate Eocene the Sirt Basin underwent compression resulted in northward tilting of the basin, causing abrupt subsidence in the north and uplift on the basin southern shoulders, possibly driving a late stage of minor regional subsidence. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Sirt Basin Rifting phases Subsidence Gedari fault

1. Introduction The Zallah-Dur Al Abd Trough (Fig. 1) is an extensional sub-basin formed during Cretaceous and Cenozoic rifting in the western Sirt Basin. Cenozoic rock units are well exposed and the subsurface data acquired during a long history of oil exploration makes this a suitable area to study continental rift environments onshore. A segmented fault pattern composed of NWeSE and NeS to NNESSW striking fault systems formed at both the rift borders and within the basin (Abdunaser and McCaffrey, 2014). In general, the

* Corresponding author. E-mail address: [email protected] (K.M. Abdunaser). http://dx.doi.org/10.1016/j.jsg.2015.02.006 0191-8141/© 2015 Elsevier Ltd. All rights reserved.

NWeSE faults controlled the asymmetry of the basin whereas the NEeSW structures act as transverse faults controlling local depocentres that divide the area into a number of elevated blocks and depressions. This study concerns the Zallah-Dur al Abd sub-basin, in the northwestern part of the Sirt Basin, Libya which lies principally between latitudes 28
000 N 30
000 N and longitudes 16
300 E 18
000 E (Fig. 1). The aim of this study was to reconstruct the detailed structural evolution of the basin primarily using seismic data interpretation supported by geological data gathered from about 240 deep wells that uniformly cover the study area. Hence this study provides more detail and spatial resolution than previous studies including that presented by Abdunaser and McCaffrey (2014).

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Fig. 1. Generalized tectonic map of the Sirt Basin as located in the north central Libya showing the major structural elements and oil and gas fields (Modified after Mouzughi and Taleb, 1981 and Abadi, 2002). Dashed box marks the study area with its structural domains referred to in the text. Inset illustrates the Libya map.

Our findings are set into a regional tectonic context of the Sirt Basin to show a complete picture of the Late Cretaceous to present tectono-stratigraphic evolution. 1.1. Datasets 1.1.1. 2D seismic data Structural reconstructions were based on the interpretation of 100 2D seismic lines that have been shot in a NEeSW dip direction and a NWeSE strike direction in order to image the basin-fill and structures clearly. Fig. 2 illustrates the location of the seismic lines and also shows the relationship between the Zallah-Dur al Abd trough and the bounding eastern and western platforms as well as the major internal basin-bounding faults. The 2D seismic reflection data with various spacings were recorded over a broad time interval (1968e1998) from Thumper and dynamite sources (Fig. 2) and are now owned by Teknica Oil Company which has reprocessed the images used in this study. These data are of good quality, in the shallow areas but are increasingly difficult to interpret below 2500 ms. The seismic data have been interpreted to create a series of  (locations on Fig. 2) geological cross sections, AeA0 , BeB0 and CeC oriented NEeSW perpendicular to the main strike of fault arrays in the basin. These sections demonstrate the geometry of the key seismic reflectors and minor faults mapped in the footwalls and hangingwalls to the main fault systems. A further section (DeD0 ) is aligned approximately NWeSE parallel to the strike and shows the overall tilting of the structural blocks. Together these profiles cover more than 32,000 km2 of study area and extend into the adjoining tectonic elements.

1.1.2. Borehole data Borehole records were used to correlate the seismic dataset with the wells and the combined dataset for seismic analysis and was interpreted in the Landmark Seisworks/2D software. Formation top data for about 240 wells and 4 sonic logs collected by Teknica were imported into the study area database. Synthetic seismogram correlations from the 4 sonic logs (Fig. 2 for location) and their corresponding nearest seismic traces created by Teknica have been used to produce well to seismic correlations. Several reflectors, chosen for their prominence, continuity, and geological significance, were identified in the logs and were then mapped in the seismic data throughout the study area. At each stage in the interpretation all the available information were integrated and cross-checked to ensure the interpretation agreed with all the data sources. The interpretation of 2D seismic data with various spacing; combined with boreholes data, has resulted in: 1 mapping of several seismic horizons of regional extent along with identified faults; 2 clearer definition of the boundaries of the major tectonic features; 3 definition of the complete Late Cretaceous to Late Eocene tectono-stratgraphic column across the study area. By constructing regional structure maps, we gain a better understanding of the geological history of this part of the Sirt Basin and can make an evaluation of the hydrocarbon potential its subsurface rocks.

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35

Fig. 2. Illustrates the major faults interpreted from seismic data overlaid on a simplified tectonic elements map of the study area. It also showing location of oil fields, wells, and seismic profiles used in this study.

2. Geological setting This study focuses on a northwest trending portion of the western Sirt Basin system, Libya located on the northernmost African margin. The Sirt Basin is a wide Mesozoic to Cenozoic rift Basin (Fig. 1) composed of several troughs separated by intrabasin highs deepening towards the east. Generally two contrasting groups of models have been postulated for the tectonic evolution of the Sirt Basin (Abdunaser and McCaffrey, 2014). In the first group of models, the basin formed in an intra-plate setting. In the second group of models, the Sirt Basin formed at a plate margin under strike-slip or oblique slip deformation. In these models, Early Cretaceous rifting reflected east-west sinistral shear zones (strikeslip) along the North Africa plate margin and strongly controlled clastic deposition in the Sarir arm (Anketell, 1996), however Ambrose (2000) alternatively proposed that dextral shear forces

dominated this period of deformation. Both Gras (1996), and Guiraud and Bosworth (1997) suggested that dextral shear forces dominated Late Cretaceous tectonism, in the eastern part of Sirt Basin. Recently Capitanio et al. (2009) suggested that the origin and evolution of the Sirt Basin resulted from slab pull forces in response to the development of the Hellenic subduction system. They believe that the Hellenic extension grew initially during the accumulation of slab material on the transition zones, creating differential faulting in Sirt Basin between ~72 and 55Ma, that then abruptly increased at ~50Ma, inducing large extension in the Sirt Basin as well as in the Hellenic Orogen. The surface geology (Fig. 3) of the study area is dominated by Eocene to Recent sedimentary rocks and comprises mainly limestones, shales and gypsum with rare terrigenous sands in the Oligocene (Vessely, 1985; Jurak, 1985). The stratigraphic succession is characterized by several carbonate-clastic alternations and

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secondary transgressiveeregressive cycles, where thickness and subsidence variations suggest several important pulses in tectonic activity resulted in fault movements and basin rifting (Guiraud et al., 1992; Bosworth, 1992). The stratigraphic successions constitute one of the main elements of the Late Mesozoic-Tertiary petroleum systems of the western Sirt Basin (Ghori and Mohammed, 1996; Mansour and Magairhy, 1996; Macgregor and Moody, 1998; Ambrose, 2000; Hallett, 2002). The occurrence of oil is closely linked with the tectono-stratigraphic history of the area, which has created multiple reservoir and seal combinations. Most of the oil fields that have been discovered in the study area appear to be associated with structural hinge zones and adjoining highs or are stratigraphic traps (i.e., Satal carbonate bank).

3. Structure of the western Sirt Basin 3.1. Subsurface stratigraphy The subsurface stratigraphic succession based on formation top data comprises a variety of lithologies ranging from Pre-Cambrian crystalline basement to recent sedimentary rocks (Fig. 4). Total sedimentary thickness exceeds 3500 m and is mostly of Cretaceous and Cenozoic age in the deepest parts of the troughs, diminishing to 1200 m on the regional highs (Abdunaser and McCaffrey, 2014). The sedimentary lithologies are extremely variable, reflecting the complex tectonic and structural evolution of the Sirt Basin. Our work and that of previous workers concerning the evolution of the NW part of the basin shows that the Zallah-Dur al Abd Sub-basin is

Fig. 3. Simplified surface geologic map of Zallah-Dur al Abd Trough shows the exposed rock units, wells, and oil fields compiled after Vesely (1985) and Jurak (1985) and Anketell and Kumati, (1991b).

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37

Fig. 4. Subsurface columnar stratigraphy section of the study area (after Schroter, 1996; Barr and Weegar, 1972).

an elongated multi-cyclic NNW-SSE trending basin, developed within the northern Libya continental shelf which began in the Early Cretaceous and peaked in the Late Cretaceous (Harding, 1984; Gras and Thusu, 1998; Ambrose, 2000), as a rift/sag sub-basin (Jongsma et al., 1985; Guiraud and Maurin, 1992; Anketell, 1996; Anketell and Kumati, 1991a; b; Abdunaser, 2012; Abdunaser and McCaffrey, 2014). In detail:1) The principle structural control is by the Gedari fault, a major NNW-SSE trending normal fault which vertically displaces the whole sedimentary Upper Cretaceous to Eocene sequence. In detail it comprises several faults that show downthrows to the east, of the order of 300e500 m (Vesely, 1985). The Gedari fault system separates the main subsiding basinal block from

the relatively elevated eastern and western platforms (Az Zahrah-Al Hufrah Platform and Al Qargaf Arch as shown in Fig. 2). 2) In the study area, sedimentary thickness ranges between 1000 m and 3500 m depending on the well location in the study area (see Fig. 2 for well locations). In the central east part, wells N1-11 and L1-11 contain more than 3000 m, of sedimentary rocks, well G1-11 contains about 2900 m in the central east part, and there is 2850 m reported in well H1-78 to the south. In contrast the Az Zahrah-Al Hufrah Platform which forms the eastern margin of the trough contains less than 2000 m of sediments in Wells A1-32 and LL1-32. The southern part of the Zallah-Dur al Abd Sub-basin (Fig. 2) is floored with basement volcanics and the northern part with quartzites (Hallett and El-

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3)

4)

5)

6)

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Ghoul, 1996). The pre-Hercynian subcrop is composed entirely of Cambro-Ordovician rocks (Hallett, 2002). The sedimentary section overlying the Cambro-Ordovician sequecnces and in places the Precambrian basement complex is considered to be the first syn-rift sediment in the trough. It comprises a series of unsilicified Nubian Sandstones of Lower Cretaceous age (Fig. 2) that vary considerably in thickness (up to 550 m at Well N1-11) and lithology. Sedimentary thickness thins toward the eastern and western areas, and thickens in the central portion. During the Upper Cretaceous, (Fig. 3) a thick marine formation formed above the Nubian Sandstones (up to 1250 m in Well A3NC131 towards the south-central part of the trough) compared with less than 100 m (wells J1-32, K1-32, B2-32 and C1-32) on the adjacent Az Zahrah-Al Hufrah Platform. This suggests a rapidly subsiding basin floor along the controlling Gedari fault and is evidence for timing of the mature rift phase of the basin. The Paleocene sequence of the study area (Fig. 2) consists of open shelf carbonates and deep marine shales that reached a total thickness of 750e1000 m (950 m at well HH1-32). The patterns closely mimic the underlying structure with thickening in troughs and thinning on platforms indicating that faulting along trough edges was still contemporaneous with deposition. Spatially, carbonates are confined to the structurally higher platform areas while shales occupy the troughs. An extensive evaporite basin developed in the Zallah-Dur al Abd Sub-basin during (Fig. 2) the Eocene including formation of the Hon member of the Gir Formation which reaches over 470 m thickness in several wells (Well HH1-32) (Fig. 3). Alpine-related tectonic pulses marked the end of the major extensional events that occurred during MiddleeLate Eocene and the Sirt Basin became contractional resulting in northward tilting of the basin. This caused renewed abrupt subsidence in the north and uplift on the basin southern margins (van der Meer and Cloetingh, 1993). These stresses terminated subsidence of the basin along the bounding Gedari fault.

3.2. Surface geomorphology Tectonically the area is characterized by seven major geomorphological features or geo-tectonic domains. These are: 3.2.1. Zallah Trough The Zallah Trough (Fig. 1) is situated in the southern central part of the study area and is an asymmetric graben which connects northwards to the Dur al Abd Trough, southwards to the Abu Tumayam Trough and forms a more or less as a continuous NWeSE trending low embayment. It is bounded by the Waddan Uplift to the northwest, the Al Qargaf Arch to the southwest and the Az ZahrahAl Hufrah Platform to the east. Due to down-faulting, the deepest part of the trough lies on the NE side of the axial Al Hulayq Ridge and the southwestern Az Zahrah-Al Hufrah Platform contains the distinctive Gattar Ridge. A series of major fault scarps mark a normal fault zone defining the western edge of the Gattar Ridge and this forms the SW fringe of the Az Zahrah-Al Hufrah Platform. The fault zone extends for approximately 180 km along strike in the study area. 3.2.2. Dur al Abd Trough The Dur al Abd Trough (Fig. 1) represents a more or less continuous, elongated low of three troughs, including Dur al Abd, Zallah and Abu Tumayam Troughs from north to south respectively which runs from the Sirt embayment coast in the north to the southern (shelf) boundary of the Sirt Basin. Some authors believe

that the Zallah Trough narrows northwards and becomes shallower to form the Dur al Abd Trough (Hallett, 2002). The Dur al Abd Trough separates the Az Zahrah-Al Hufrah Platform from the Waddan Uplift in the area of the Mabruk field. Further north it becomes difficult to define as it loses identity in the faulted northeast flank of the Waddan Uplift. In general, NWeSE faults controlled the asymmetry of the basin whereas the NEeSW structures act as transverse faults controlling the local depocentres that divide the Zallah-Dur al Abd sub-basin into a number of elevated blocks and depressions. These are the Facha Graben, Ar Ramlah Syncline, Gattar Ridge, Zallah sub-basin, Al Hulayq Ridge and Ma‘amir Graben (Schroter, 1996; Johnson and Nicaud, 1996) (Fig. 1). Each block is asymmetric, bounded on one side by a major NW trending border fault system with large throws from approximately 300 to 1000 m and a predominant stratal dip direction toward the border fault system. 3.2.3. Az Zahrah-Al Hufrah platform The Az Zahrah-Al Hufrah Platform (Fig. 1) is located in the eastern part of the study area between the Dur al Abd and Zallah Troughs to the west and the Maradah Trough to the east and is bounded to the south by the NNE-SSW trending Kotlath Graben which separates it from Al Bayda Platform (Fig. 1). Along its southwest margin, the Gattar Ridge formed as a result of downfaulting to the WSW (Figs. 1 and 3). The platform overall dips towards the northeast with a northern boundary that is poorly defined and may be affected by wrench-faulting close to the present coastline (Anketell, 1996). The western platform boundary fault in the SW region is characterized by a complex series of en echelon faults indicating sinistral strike-slip movement, with associated riedel shears forming small-scale horst and graben structures during the Eocene (Anketell and Kumati, 1991b). This western platform boundary fault is referred to as the Gedari fault (Anketell and Kumati, 1991b; Anketell, 1996; Jerzykiewicz et al., 2002) as shown in Fig. 1. 3.2.4. Waddan Uplift The Waddan Uplift (Fig. 1) is a gently north-northeast tilted block, approximately 250 km long with a width of 105 km in the south, narrowing to 75 km in the north before it disappears below the coastal plain. The uplift terminates in the south against the northeastern extension of the Al Qargaf Arch. The dominant fault trend is NNW-SSE with a subsidiary orthogonal set trending ENEWSW (Fig. 1). The northeastern margin of the Waddan Uplift is extensively faulted due to dextral shear along a WSW-ESE basement fault (Anketell, 1996). The southern end of the uplift has a convex morphology to the south and is characterised by a discrete fault-bounded margin with downthrow to the southwest, south and southeast that has resulted in the formation of the highest topographical point of the Waddan Uplift. The western boundary is marked by the eastern boundary fault of the Hun Graben (Abdunaser and McCaffrey, 2014) (Fig.1). 3.2.5. Gattar Ridge The Gattar Ridge appears as a terrace adjacent to the southwestern boundary fault of Az Zahrah-Al Hufrah Platform and projects into the eastern side of the Zallah Trough (Fig.1). The western edge of the fault block is bounded by a fault zone up to 1 km wide, consisting of overlapping parallel normal faults that have consistently downthrown to the west (Abdunaser and Reeh, 2007). The fault panel is bounded to the north-northwest by an ENE-WSW trending transverse fault (Figs. 1 and 3). To the north-northwest the Gattar Ridge dies out into an area of close spaced faults with a NWeSE trend. The dominant fault trend on the ridge is NNW-SSE,

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however subsidiary EeW and NEeSW faults sets are also developed. 3.2.6. Al Qarqaf Arch The Al Qarqaf Arch (Fig. 1) is an EeW trending Hercynian structure located on the southwestern edge of the study area and exposes Palaeozoic strata (out of the study area) onlapped by Oligocene cover rocks. Regionally the structure forms a WSW-ENE topographic high which separates the Murzuq and Ghadamis Basins (Fig. 1). The structural grain of the Al Qarqaf Arch is dominated by ENE-WSW trending faults and resistant ridges (possibly silicified relict fault planes and/or igneous dykes). Subsidiary WNW-ESE to NWeSE and NeS trending faults possibly also occur. 3.2.7. Al Hulayq Ridge The Al Hulayq Ridge is a NNW-SSE trending narrow basement ridge or horst block which protrudes into the southern part of the Zallah Trough (Figs. 1 and 2). The western margin of the high is structurally complex and associated with a series of drag folds developed within Paleocene-Eocene strata. The folds form hydrocarbon traps and may represent part of a flower structure created by dextral shear during the Oligocene (Knytl et al., 1996). 3.2.8. Tertiary volcanics Tertiary volcanic rocks are well-exposed in the study area forming the northerly continuation of the basaltic flows of Jabal Al Haruj Al Aswad (Fig. 1) that are present in the southern part of the Sirt Basin. Their extrusion was accompanied by post-rift tilting related to Alpine tectonic pulses and the locations appears localised at the intersection of a NWeSE to NNW-SSE trending Pan-African lower Palaeozoic structural arch (Al Qargaf) and the WSW-ENE trending Hercynian Late Palaeozoic to Mesozoic uplifts (TripoliTibesti arch). The location also coincides with the intersection of NNE-SSW and NNW-SSE extensional structures which themselves are likely to be controlled by the earlier fabrics (Abdunaser and McCaffrey, 2014). This volcanic episode was widespread in east and northwest Africa (Abadi et al., 2008) and has a range of age dates, suggesting Early Eocene to Pliocene (Wilson and Guiraud, 1998). 4. 3D configuration of the basin 4.1. Correlation of seismic and geological cross sections Geological cross sections constructed from the borehole data have led to a better understanding of the structural evolution of the study area. The geological cross sections were used to inform the direction that the seismic cross sections were drawn in order to provide an optimum view of the present day basin structure and/or structural trends. The sections also allowed interpretation of the rock unit thickness variations and facies changes, the position of depocenters, depositional trends and/or paleo-structures (e.g. paleo highs). Both seismic and geological cross sections aid recognition of the structural and stratigraphic controls on petroleum accumulation in the area. The borehole sections are drawn subparallel to and follow the same geometric rules as the seismic cross sections (see Fig. 2 for location). The 2D seismic data are ambiguous deeper than the Late Cretaceous (base Kalash) age due to the vintage of the data acquisition and the digitization process of scanning paper sections. The seismic data were however helpful in interpreting the horizontal variations between boreholes during construction of the geological cross sections. It is generally desirable to have data points relatively evenly spaced along the line of section, wherever this is permitted by well control.

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Three sections were created with perpendicular orientations to the main structural trends to give the most representative view of fold and fault geometry and one section was created parallel to these trends to show the tilting of the structural blocks (Fig. 2). 4.1.1. Structures perpendicular to the regional strike of the basin (NEeSW trending profiles) 4.1.1.1. Cross-section AeA0 . Two seismic lines were combined to create cross-section AeA0 with an aim to show the structural configuration of the Waddan Uplift and Dur al Abd Trough in the northwestern part of the study area (Fig. 5a). There is a large gap of z18 km over the fault hinge zone between the Waddan Uplift and Dur al Abd Trough (at columns B and C Fig. 5a) that can be further constrained by data from wells B1-25 and F1-17 (Fig. 5b). It is clear from this seismic profile that the main faults are normal structures and while some extend from the base of the interpretable section (base Kalash) into the Lower Tertiary others faults only cut the Lower Tertiary. Most of the horizons above the near base Kalash show a thicker stratigraphic section in the troughs compared to adjoining highs, which is confirmed by the geological cross sections. Section AeA0 shows at least five, large down-to-basin (i.e. to the NE) faults with apparent normal throws of 10e200 ms (the displacement cannot be seen clearly on this seismic line due to resolution) at the southeastern end of the Waddan Uplift. This fractured area includes the eastern fringe of the Waddan Uplift as well as the hinge zone between this horst and the Dur al Abd Trough to the east. Horizons in this part of the section are clearly dipping to the east. Most of the sequence confirmed by borehole data is encountered from the Cambro-Ordovician Gargaf sediments to the Lower Eocene Gir Formation (Hon Mb) with the Satal Formation being the thickest part of this megasequence indicating that Waddan Uplift was low during Lower Paleocene compared with the rest of the neighbouring areas. It is unclear whether absence of the Middle Eocene strata in this area was due to continued emergence and non-deposition, or post-Middle Eocene erosion. In any event, erosion of the Middle Eocene strata on parts of the current Waddan Uplift indicates that most of the uplift has occurred since the Middle Eocene. The northeast side of the Waddan Uplift where it adjoins the Dur al Abd Trough has sufficient seismic data and there is evidence for major faults to support a boundary in this area. It is supported by well data (Fig. 5b) which show an increase in thickness of the Tertiary-Cretaceous interval from the centre of the high (i.e., about 1036 m in well A1-44) towards the northeast to about 1463 m in well D1-17 and 1555 m in well A1-91). The well data by itself does not strongly support the presence of a hinge zone or boundary in this area. Within the Dur al Abd Trough (columns C,D,E,F, and G, Fig. 5a) the reflecting horizons are for the most part horizontal with some secondary faulting (throws z5e40 ms) concentrated at middle to shallow depths (down to 1.0 s), above the Base Kalash/Top Sirte reflector, and below Dahra reflector. There are at least 3 major faults that extend below the seismic section and these major faults branch out into smaller faults reminiscent of a ‘flower structure’ suggesting that the Dur al Abd Trough has been the focus of some large-scale wrench tectonics. The displacement is distributed among several fault strands that are seen to coalesce at depth. This image is comparable with other examples of continental transform faults (e.g. Ben-Avraham, 1992) and is typical of a flower structure associated with strike-slip faulting (Harding, 1985). The F1-17 and K1-17 wells constrain the geological cross section (Fig. 5b) and intersect the Dur al Abd Trough. The area seems to have undergone at least three tectonic events, commencing with possible rifting during the Upper Cretaceous (not penetrated by any

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Fig. 5. a. Seismic cross section AeA0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location). b. Geological cross section AeA0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location).

boreholes in the central part of the trough), followed by a large amount of subsidence during the Paleocene and Eocene before a final rifting occurred during the post-Eocene as evidenced by very thick sediments of this age. The interpreted faults from the seismic data along the southern part of the Waddan Uplift indicate potential for several faultbounded closures. Moreover, this structurally complex area is favourably situated for migration of hydrocarbons from the Zallah Trough. 4.1.1.2. Cross-section BeB0 . This profile is comprised of five seismic lines, crossing the Zallah Trough (columns B, C and the first quarter of column D in Fig. 6a), the Al Halayq Ridge (the middle point between columns D and E Fig 6a) and the Dur al Abd Trough (columns E and F Fig. 6a). Finally to the east, the profile crosses the Az ZahrahAl Hufrah Platform (columns G, H, I, J and the first quarter of column

K Fig. 6a). This profile is well supported by borehole data used to construct the geological cross section (Fig. 6b). At the SW end of the section, NE and SW dipping normal faults (at about column C in Fig. 6a) delimit a synclinal axis defining the deepest part of the Zallah Trough sub-basin as evidenced by more than 1067 m of post-Eocene sediments. An absence of Mesdar Member of the Gir Formation compared with Dur al abd Trough and Az Zahrah-Al Hufrah Platform during the same time suggest that this part of the Zallah Trough was elevated during the Late Early Eocene. Some of the normal faults have throws that increase with depth indicating repeated displacement. To the east of the Zallah Trough, a high structure has been identified which possibly corresponds to the northern tip of Al Halayq Ridge (including its flanking hinge zones). Its crestal zone is located at between columns D and E (Fig. 6a). Both sides of the crestal zone are flanked by large, SW- and NE-dipping, normal

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Fig. 6. a. Seismic cross section BeB0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location). b. Geological cross section BeB0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location).

faults and to the north east of these flanking faults (in an area of good seismic data quality), a synclinal axis forms the western edge of the Dur al Abd Trough within which the horizons are interpreted to have been downthrown by a total of z400 ms from the Al Halayq Ridge at the deepest horizon level (Gargaf). A heavily fractured area in the Dur al Abd Trough (centred on column F) displays a number of NE- and SW-dipping faults, that do not show systematic displacement profiles with depth and this can be interpreted as another zone of wrench tectonics. To the east of this area, the hinge zone between the Dur al Abd Trough and the Az Zahrah-Al Hufrah Platform (the first quarter of column G Fig. 6a) is characterized by a large fracture zone in which the majority of the faults appearing to be down to basin normal faults with smallerscale antithetic faults. This hinge zone fault boundary corresponds to the Gedari fault, which shows an overall vertical reflector displacement of c. 750 ms at the deepest interpreted seismic horizon. A considerable monoclinal flexure (at the middle point between column F and G Fig. 6a) forms the hanging wall to the Gedari fault. The monocline involves all interpreted reflectors that are located above the fault. The structure dips to the SW, away from the fault and is bounded on its SW by synclinal and anticlinal fold axes. To east of the Gedari fault system, the Az Zahrah-Al Hufrah Platform is characterized by relatively unbroken post-Cretaceous sediments that generally dip gently and uniformly toward the northeast. There is evidence for very sparse structural deformation that is mainly

limited to deep faults with small throws that are rooted in the basement, but disappear upwards in the Paleocene and Eocene strata. The Satal Formation which is composed of carbonate reef build-up seems to predominate in this part of the section. 4.1.1.3. Cross-section CeC0 . The first seismic line for this cross section starts from the eastern edge of the Al Qargaf Arch and crosses the hinge zone between this and the Zallah Trough. The next seismic line crosses the Al Halayq Ridge (columns F, G, H, and I Fig. 7a) and finally the Abu Tumayam Trough (column J Fig. 7a). At the Al Qargaf Arch, the reflector sequence appears to be an eastward dipping (and thickening) sequence of undulating sediments that are intermittently broken by down-to-basin (and a few down-to-margin) faults. Most of these faults appear to extend from below the deeper reflectors to the surface and have an approximately constant throw along their entire length. Displacement is distributed among several fault strands that are seen to coalesce at depth suggesting the presence of a ‘flower structure’ located at the middle point of column A (Fig. 7a). There is an apparent fault (at the middle of column C Fig. 7a) located in a poorly imaged part of the section that shows potentially a very big offset (c. 1000ms) of the blue and pink horizons (in the web version) however it can not to be resolved clearly on the geological cross section (Fig. 7b) due to a lack of wells in this area. This fault seems to be bound the deepest part of the Zallah Trough in the region from columns C to E (Fig. 7a) eastward.

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Fig. 7. a. Seismic cross section CeC0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location). b. Geological cross section CeC0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location).

The Abu Tumayam Trough forms a downwarped structure which is interpreted from the seismic line 57-83-85 east of the Al Halayq Ridge. This trough appears to be the southern extension of Dur al Abd Trough across an EeW trending barrier. The deepest point of this trough is at the middle point of column J (Fig. 7a) and within it the sediments are gently bowed downwards, with no apparent thickening, towards the trough's centre. 4.1.2. Structures parallel to the regional strike of the basin (NWeSE trending profile) 4.1.2.1. Cross-section DeD0 . Here six seismic lines were interpreted in the northeast part of the study area lying along a 80 km NWeSE strike direction the traverses the centre of the Az Zahrah-Al Hufrah Platform and crosses most of the oil fields that have been discovered on this platform (Fig. 2). This strike cross section is characterized by relatively unbroken post-Cretaceous sediments and show mostly uniform horizontal bedding (Fig. 8a). In general, throughout the whole of the section, most of the reflectors are generally flat-lying and show approximately constant separation. Faults mainly occur below the baseKalash horizon with exception of the areas imaged at columns C and D (Fig. 8a) where some faults are located above this horizon which means that minor tectonic activities during the postCretaceous on this platform. The highest structural point of the line is located between columns E and F of Fig 8a and this appears to

be the “crestal block” of this portion of the Az Zahrah-Al Hufrah Platform. Both the Az Zahrah and Al Hufrah oil fields are located in this area. Moving southeast of the “core block” (columns G to L Fig. 8a) below base Kalash horizon (Upper Cretaceous) sediments show indications of small highs (horsts?) and lows (grabens?) generally bounded by normal faults with upward diminishing throws. The axis of the first syncline is located at column G (Fig. 8a) and the amplitude of the low decreases upwards with basin fill and is best expressed at the Base Kalash/top Sirt (light blue)(in the web version) reflector level. Examples of these troughs can be seen on columns J, K, and L (Fig. 8a). Each of these troughs (and their separating grabens) appear to be bounded by normal faults that are rooted deeper and diminish in throw upwards to end in the Late Cretaceous at the base Kalash/top Sirte (light blue reflector)(in the web version).

5. Fault geometry analysis 5.1. The major fault systems A segmented fault pattern comprised of NWeSE and NeS to NNE-SSW striking extensional fault structures that are tens of kilometres in length has formed at both the rift borders and within the basin (Fig. 2). These trends coincide with the orientation of the

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Fig. 8. a. Seismic cross section DeD0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location). b. Geological cross section DeD0 view of present day structure across the study area from the southwest to the northeast (See Fig. 2 for location).

main tectonic elements in the Sirt Basin. The deformation resulted in a complex structural history that led to the formation of several fault-bounded elevated blocks and depressions (Fig. 1) that displays a characteristic rhomboidal or zigzag fault pattern. The main boundaries in the study area are defined by the Gedari fault, the Qarat Ash Shush fault, east Dur al Abd Trough fault, east Az Zahrah-Al Hufrah Platform fault, east Mabrouk Area fault, east Waddan Uplift fault, west Al Hulayq Ridge fault, and the Western Range fault (Fig. 2). In this paper we will focus on the main Gedari and Western Range faults which bound the Zallah eDur al Abd Subbasin on the east and west respectively.

5.1.1. Gedari fault The Gedari fault is a well-known fault that is related to the oil fields of Az Zahrah, Al Hufrah, Bahi, and Mabruk farther northwestwards (Klitzsch, 1970; Abdunaser and McCaffrey, 2014) and extends along the entire eastern boundary of Zallah-Dur al Abd Troughs adjacent to Az Zahrah-Al Hufrah Platform. The Gedari fault is considered as one of the major structures that has shaped the western part of the Sirt Basin. This fault was initiated in Pre-Cretaceous time with a downthrow to the SW; however, it was reactivated during Cretaceous and Tertiary times. The Gedari fault is highly segmented and dips steeply towards the SW and defines the western edge of the Gattar Ridge and the SW fringe of the Az Zahrah-Al Hufrah Platform. The fault grew from the linkage of a number of shorter fault segments

and the growth of the fault controlled the distribution and character of syn-rift sediments (Abdunaser and McCaffrey, 2014). Stratigraphic thickness variations across the fault indicate several episodes of syn-rift deposition (Abdunaser and McCaffrey, 2014). Such syn-tectonic stratigraphic thickness variations are most evident in the Late CretaceousePaleocene succession on the interpreted seismic sections therefore the Gedari fault was a growth fault during this time (Fig. 9). The fault cuts the entire straigraphic section and the younger layers (Upper Eocene) which on the eastern side of the Gedari fault are flat whilst are gently inclined on the western side of the fault (Fig. 10 seismic line). The maximum throw of the Gedari fault was most likely during deposition or immediately post-deposition of Sirte Shale (Upper Cretaceous) and this has caused maximum displacement at the near top of the Bahi Formation as shown in Fig. 11. A fault throw minimum is recorded during the Eocene but however throw increased again during Late Middle Eocene times as recorded by a large fault throw of the Mesdar Limestone. This means that the Gedari fault movement rate increased again following deposition of the Mesdar Limestone.

5.1.2. The Western Range fault The Western Range fault delimits the northwest part of our study area and comprises several faults that show downthrows to the east, of the order of 300e500 m (Vesely, 1985) (Fig. 2). The sediments exposed along the western side of the Zallah-Dur al Abd

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Fig. 9. Shows growing fault attributed to the fault which is active during the sedimentation.

Fig. 10. Shows zoomed part of seismic section as shown in Fig. 9.

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Fig. 11. A non bacstripping, seismic interpretation showing the maximum and minimum fault throw (displacement) of the Gedari fault.

Trough are gently tilted to the northeast at an angle of 2e3
which indicates less vertical displacement along this fault zone compared with the Gedari fault system to the east. This fault system is offset in the west central part by a north downthrowing normal fault zone belonging to the Al Qargaf Arch and by E to ESE striking transverse faults, which belong to the periphery of the Hun Graben (Fig. 1). The maximum offset (Fig. 12) is of the near top Sirte Shale and there is evidence for reactivation during the Upper Eocene following deposition of the Hon Evaporites, as the fault throw increased during the Late Middle Eocene after deposition of the Mesdar Limestone. In summary, the decrease in maximum fault throw from older to younger horizons across the Western Range fault compared with Gedari fault could be explained by either a shorter duration of fault activity or a slower rate of slip on this fault. Consistent thickness of the horizons across the fault suggests little fault-related sedimentation in the Cretaceous-Eocene period.

6. Discussion Generally speaking and as illustrated by the cross sections, analysis of the seismic and well data together strengthens the interpretation of both. Subtle, uninterpreted faults indicated by the well data can be interpreted on the seismic data, and changes in the patterns in the seismic data can influence the stratigraphic interpretation of the well data. Based on all the interpreted seismic and well-based geological cross sections the study area seems to be more tectonically and structurally complicated to the south. The resulting increased number of the faults reflects the presence of more tectonic blocks to the south. The Waddan Uplift, and the Dur al Abd Trough and the Az Zahrah-Al Hufrah Platform formed on the northern part of the area. To the south, the Waddan Uplift, Zallah Trough, the Dur al Abd Trough, and Az Zahrah-Al Hufrah Platform are present in the central part of the area. The Al Qagaf Arch, Zallah Trough, Al Halayq Ridge, Abu Tumayam Trough and southwestern

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Fig. 12. A non bacstripping, seismic interpretation showing the maximum and minimum fault throw (displacement) of the W. Range fault.\

fringe of Az Zahrah-Al Hufrah Platform appear in the south of the area. Conversely, the strike-parallel cross section shows that most of the reflectors are generally flat-lying with approximately constant separation. In general, the structure of the study area is controlled by normal faults that are symmetrically distributed around the main structural elements. Although they extend up to the surface our inference is that the main phase of active extension was in the Late Cretaceous. Many of these faults clearly offset or disrupt most of the sedimentary sequence indicating that they penetrate deep into the section, probably into Precambrian basement. The basement generally underlies the base Kalash horizon directly in the footwalls. Some folds have been identified in the study area parallel to the major faults and are interpreted to have been either generated by compaction in the troughs or they may have been connected to subsidence movements during the filling of the Sirt Basin. The Gedari fault zone extends below the seismic sections and branches out into smaller faults reminiscent of ‘flower structures’ where it crosses the eastern edge of the Dur al Abd Trough suggesting that the this edge of the trough has been the focus of the large scale wrench tectonics as shown in seismic cross section AeA’. It is thought that much of this deformation originated in the postOligocene period of Sirt Basin. This accompanied inversion and compression that reactivated the area's horst and graben system and added a complex of strike-slip wrench faulting generated anticlines and synclines (Jerzykiewicz. et al., 2002; Abdunaser and McCaffrey, 2014). The geological cross sections show that in this area there are significant differences in the stratigraphic record between the downthrown and upthrown side of the Gedari fault zone. Drilled wells on both sides of the Gedari fault Zone in addition to seismic reflection section show that the total thickness of the Cretaceous and Tertiary strata on the downthrown side of the fault is over twice that compared to the upthrown side (Abdunaser and McCaffrey, 2014). The Gedari fault seems to be the master fault in the study area as it vertically displaces the whole sedimentary sequence of the study area from the Lower Cretaceous until the Eocene. It is clear that the vertical displacement of Gedari fault is dominant in the rift development. The Western Range fault in terms of the magnitude of displacement was most active during the Early Paleocene which caused displacement of the near top Sirte Shale. Based on what has been interpreted and presented for the seismic reflection data used in this section; we can confirm the following points:

1. All the identified faults vertically displace the sedimentary sequence with different throw and heave values at different time intervals. 2. It was noted that most of the identified faults had high slip rates in the early part of their history, which then started to decline as we move to more recent times. These decreases, which are accompanied by thickening of sedimentary layers in the hanging walls of the faults, are due to syn-sedimentary fault growth. 3. Additionally, the vertical displacement values may not be representative of the real displacement along the whole fault because of its strong variation due to fault segmentation. 4. There is a characteristic syntectonic growth stratum in the hanging-wall of the Gedari fault for all time periods constrained by the reflection data. 5. It seems to be that most of the faults initiated at the study area boundaries and then propagated towards the centre of the area. 6. In the study area, this propagation coincides with accelerating rifting (a syn-rift stage) and rapid subsidence in the troughs resulting in thick deep marine sediments (e.g. shales of Campanian age) that are missing or very thin on the platforms and marks the peak of intraplate, African rifting in the Campanian (Guiraud et al., 1992; Bosworth, 1992). This rift climax phase or time was the time of maximum fault displacement rate when sedimentation was outpaced by subsidence. 7. Inspection of the structure maps reveal that the trend of major producing oil fields in the study area follows the fault trends discussed earlier in this study and is dominated by a NNW-SSE structural grain in the vicinity of the Gedari fault in the western part of Az Zahrah-Al Hufarh Platform. In addition, other larger fields generally are located along the internal horsts within the Zallah-Dur al Abd Sub-basin near the deepest part of the basin suggesting that the location of productive fields is largely governed by the thickness and maturity of synrift source rocks. The source rocks are principally the Sirte Shale, and possibly within the Hagfa Shale.

6.1. Structural and tectonic development of the study area The complex Mesozoic and Cenozoic extensional tectonics that became established across northern Libya have obviously influenced the structural evolution of Zallah-Dur al Abd Sub-basin during the time span of the Alpine Orogeny that had resulted in the opening and then the subsequent closing of the Tethyan Sea along the northern margin of the African plate. This produced several different tectonic movements such as: 1) A left lateral drift of the African plate relative to the European plate in the Middle Jurassic during the early phases of the Alpine Orogeny resulting in the spreading of the central portion of the Atlantic Ocean, (Wood, 1984). This was generally responsible for the opening of the Tethys and for the initiation of Zallah-Dur al Abd Sub-basin as a part of the Sirt Basin. (2) The extensional regime reached its peak during the Upper Cretaceous, and must have been so active that the floor of the Zallah-Dur al Abd Sub-basin started a regional homoclinal rotation along the major NWeSE trending master fault (Gedari fault), leading to the deepening and broadening of the basin and bringing it into a well-pronounced petroleum maturation phase. (3) From Turonian times onward, the continued opening of the Central Atlantic Ocean was obscured by a more rapid opening of the North Atlantic Ocean that led to a reversal of the

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relative motion between North Africa and Europe to dextral and leading to the initial stage of the closing of Tethys (Wood, 1984). As a result of this right-lateral transcurrent movement which led to the end of major extensional events during the MiddleeLate Eocene, the Sirt Basin underwent compression that were responsible for the death and declination of the Zallah-Dur al Abd Sub-basin as a basin and resulted in northward tilting of the basin causing abrupt subsidence in the north and uplift on the basin southern shoulders. This possibly drove the latest stage of regional minor subsidence (van der Meer and Cloetingh, 1993) and terminated subsidence along the bounding fault. 7. Conclusions In the present study, geological information from 240 deep wells and 100 seismic sections in the western Sirt Basin have been used in a study of the structural setting and tectonic evolution of the Zallah-Dur al Abd sub-basin of the Sirt basin. Analysis of seismic, boreholes and published geological data have revealed that the Zallah-Dur al Abd sub-basin is bounded from the east by a fault hinge zone separating it from Az Zahrah-Al Hufrah Platform and from the northwestern side by another fault hinge zone separating it from Waddan Uplift with the majority of the faults appearing to be down to basin normal faults. The data also showed that our study area is an extensional, elongated, multicyclic, NNW-SSE trending structurally-controlled sedimentary basin developed within the northern portion of the African plate as a consequence of the left-lateral drift of Northern Africa relative to Europe. This led to subsidence under extension at various rates from Middle Jurassic times until the end of the Eocene, but most markedly during the Upper Cretaceous when a thick sedimentary infill formed mainly in the hangingwall to the Gedari fault. The post-Eocene dextral drift of Africa relative to Europe created compressive stresses in the northern portion of the African Plate and resulted in uplift of the Zallah-Dur al Abd Sub-basin characterized by a northeastward tilting of the basin causing more subsidence in the north than the south portion. The cross sections used in this study demonstrated that the study area seems to be more structurally complicated to the south. The resulting increasing number of the faults has formed more tectonic blocks in the south. The Gedari and Western faults are two major moderate dipping growth faults that bound the main basinal block and control its subsidence with respect to the eastern and western platforms. From the western platform basinward, the sedimentary sequence dips gently eastwards and also thickens gradually towards the basin bounding Gedari fault. A complex network of numerous branching normal and strikeslip faults, generally dominated by a NNW-SSE structural grain formed in response to the Cretaceous and later extensional tectonics superimposed on pre-existing NeS trending Pan-African basement structures and ENE-WSW trending Hercynian structures. Acknowledgements The authors are indebted to the Management of Libyan Petroleum Institute and National Oil Corporation, Libya for their support throughout the study and for allowing us to use the seismic and well data, funding, and the permission to publish this paper. We would also like to extend our thanks to Richard Hobbs, (Durham University, UK) for all of his help, especially when he managed to

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get well data tied to the nearest seismic lines. We thank Gary Wilkinson for seismic data loading.

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