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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( prop) 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 approximately 320 fields in these basins and about 80% of these fields were discovered prior to 1970.
The Ghadames Basin is one of these fields, it's a large intracratonic basin that formed during Palaeozoic - Mesozoic time, located on the North African Platform and extends over parts of Algeria, Tunisia and Libya(R. Underdown and J. Redfern 2007) , and extending over 350000 km2 (K. ECHIKH 1998). It is bordered by the Sirt Basin on the east to the east, the Edjeleh anticline along the Libyan -Algerian border to the west, the Gargaf (Al Qarqaf) Arch to the south, and the Nafusa high to the north.
The Ghadames Basin is separated from the Murzuq Basin to the south by the east-west trending Gargaf Arch and its SW subsurface continuation, the Awbary Saddle (Tawadros, 2001). The Ghadames and Murzuq are separated by Gargaf Arch was probably uplifted and erosion during the Late Silurian Caledonian orogeny (Rusk, 2002).
The Ghadames Basin continues from west of Libya into Algeria where it opens into the Illizi Basin (Tawadros, 2001).
The Ghadames Basin represents a different depositional environment, 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.
The geophysical data is useful for studying the subsurface geology. It will be used in this research to study the structural framework of the Ghadames Basin. ( prop)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.
( prop) Sedimentary basins are generally associated with low gravity values due to the lower density of the sedimentary infill. ( prop) 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. ( prop) 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.
Mesozoic and Paleogene sediments crop out around the periphery of the Basin. Paleozoic sediments are mostly sandstones and shales in the south, and they include continental, shallow marine and glacio-marine sediments. They grade into deltaic and shallow marine sediments in the north. Dolomites and evaporates are common in the upper of the section. Mesozoic sediments are predominantly continental sandstones in the south. They thicken northward and become marine sandstone, carbonates and evaporates. In the north, they are overlain unconformably by Tertiary sediments.
The present structure of the basin is the combined effect of the Taconian, Caledonian, Hercynian and Alpine orogenies. Its history follows closely that of the Murzuq Basin.
Located on the Mediterranean coast of North Africa (figure, 1), 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 Ghadames Basin is located in the western part of the country, located in northwest, and limited to the north by the Jabal Nafusa and to the east by the Jafar Plain. To the southwest and southeast, it's separated by Al Qarqaf Arch from the Murzuq Basin. This research will be focused on the Ghadames Basin, which bounded by the coordinates of 27o - 32o 00' N and 10o 00' - 15o 00' E. In order to understand the structural geological of the area, it is very useful to understand the petroleum system and hydrocarbon flows in the basin.
Figure 1: Location map of the Ghadames Basin (after Shah et al., 1993).
1.3. Problem definition
The Ghadames Basin was formed in response to the tectonic motion between the African and Eurasian plates. The configuration of the Ghadames Basin is governed by the basement structures which were formed during the Early Palaeozoic and contains up to 6000 m of Palaeozoic and Mesozoic sediments, and gave to raise 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. ( prop) 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
( prop) The main objective is to use the geophysical data, particularly the gravity data to reveal the structural features within the Ghadames Basin and compare their configuration with the geophysical anomalies. It is order to understand the subsurface behaviour of the Ghadames 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 Ghadamis Basin.
1.4.2. Specific objective
( prop) To estimate the thickness of a potential sediments infill in the Ghadames 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 Ghadames Basin?
What is the relation between the geophysical anomaly expressions with the geological features of the Ghadames Basin?
The relationship between the structural features and hydrocarbon migration.
What is the extent of the Ghadames Basin boundary?
Is the structural framework control the hydrocarbon cumulative.
The data sets will be used in this study are: gravity dataset, well data, and regional tectonic map of Libya.
Literature review to understand the geology of the area.
Prop Interpolate the gravity data base using the minimum curvature with grid cell in order to produce gravity anomaly map.
Display the gravity grid and magnetic grid in colour shaded map.
Prop Georeferencing all available data set using Universal Transverse Mercator (UTM) projection.
Prop The total field gravity anomaly map and from the total magnetic map to be used in qualitative interpretation.
Two Cross sections will constructed from the gravity dataset in order to use them in the 2D forward modelling.
2D forward modelling will used to correlate the gravity anomaly expressions with the geological model of the Ghadames Basin which consists of a series of troughs and platforms.
2. Literature review
2.1. Ghadames Basin
The Ghadames Basin is a large intracratonic basin on the North African Platform, formed during the Early Palaeozoic. It covers an area of 350 000 km2, across parts of Libya, Tunisia and Algeria, and contains up to 6000 m of Palaeozoic and Mesozoic sediments. The Palaeozoic section is separated from the overlying Mesozoic deposits by a major regional unconformity of Permo-Carboniferous (Hercynian) age.
Prop Burial history has significant implications for hydrocarbon exploration in the area. Prop In particular, it has a critical effect on the timing of hydrocarbon maturation of the main source rock intervals and their expulsion history. Prop A critical aspect of basin modelling is estimation of eroded overburden. (Boote et al. 1998), (Dardour et al. 2003), (Boote & Dardour 2004) and (Hallett 2002) have all highlighted the importance of uplift and erosion associated with the Hercynian, Austrian and Alpine tectonic events.
Prop Although they identified the importance of the Late Eocene Alpine Orogeny across the basin, the exhumation and thickness of missing section was not quantified. Prop (Malla et al. 1998), (Yahi et al, 2001) and (Makhous & Galushkin 2003) provide estimated values for uplift ranging from 3000 to 4000 m over the western and northern margins of the basin, 300-1300m towards the south, and 400-1500m over the central basin.
2.2. Geological setting
2.2.1. Structural setting
The structural configuration of the Ghadames Basin is represented by the structural contour map for the latest Ordovician (Fig. 2). The map shows the highlights of the major tectonic elements bounding the basin, at the west bounded by the Amguid-El Biod Arch, at the northeast by the Nafusah High, and at the east by the Qarqaf Uplift, at the southern is complicated because of second-order structural highs, Tihemboka and A1- Hamra Highs (ECHIKH, 1998).
The Ghadames Basin developed in three phases:
(1) Initiation through reactivation of Pan-African fault systems of a subsiding Palaeozoic basin.
(2) Uplift and erosion of much of the basin during the Hercynian phase.
(3) A northwest tilting and superimposition of a Mesozoic extensional basin.
The present structural framework of the Ghadames Basin was produced by the successive effects of tectonic movements related to the Taconic, Caledonian, Hercynian and Austrian phases. As a consequence, there is a wide variety of structural styles, fault patterns and structural trap types present, and depocentres have tended to migrate with time.
Palaeozoic tectonic events
Early Ordovician time was characterized by a tectonic instability (Attar 1987) indicated by the absence of the Cambrian over the main uplifts. Peak activity occurred during short time, when there was substantial activity, particularly on the southern part of the Ghadamis Basin and close to the Qarqaf Uplift. Active fault uplifts caused erosion and the creation of a series of overlapping deep erosional troughs, which were later filled by periglacial deposits. The folds created at this time are broad, with active faulting controlling thickness and facies distribution in post tectonic formations.
The Taconic unconformity is illustrated by a cross-section though the Dahar High (Fig. 2), where successive units of the Early Ordovician are seen to be overlain by the Late Ordovician Microconglomeratic Shale.
The presence of volcanic layers deep troughs were formed close to the Qarqaf Uplift (Echikh 1992) and were filled with the periglacial facies of the Memouniat Formation. These unconformably overlie older Ordovician strata (Fig. 2). An erosional phase is also noted in southern Tunisia (Chandoul 1992).
Figure 2: Geological cross-section: Ordovician formations (ECHIK, 1998).
A significant Caledonian tectonic event was as a result of the collision between West Africa and North America. This caused the uplifting and erosion of the south-western and southern flank of the Ghadamis Basin, where the Lower Devonian (Tadrart) is seen to directly overly the Upper Silurian (basal Acacus).
The Early Permian Hercynian movements initiated the uplifting of the Nafusah Highs, resulting in the intensive erosion of Palaeozoic rocks, in some cases as deep as the Cambrian. The effects of the earliest movements of the main Hercynian phase on structure are well illustrated in the basin (Attar 1987).
Mesozoic tectonic events
An extensional event affected the area in the Triassic-Liassic, related to the rifting of Tethys and the opening of the Atlantic. This led to the development of a series of en echelon normal faults and tilted blocks, with associated volcanism, in the north western part of the Ghadamis Basin and southern Tunisia. These fault sets can be demonstrated to control thickness and facies changes within Triassic sediments.
Tertiary tectonic events
Latest Eocene movements, which are significant in the Atlas area, affect the Saharan platform with less intensity. The distal effects of this compressive event led to accentuation and remodelling of Austrian structures (Boudjema 1987), inversion of Hercynian or Liassic normal faults (Fig. 3).
Figure 3: Tectonic map of Libya; modified from (Abadi 2002).
Local structural styles and ages
Analysis of seismic data over the Ghadames Basin indicates the existence of a wide variety of structural types of different ages. Many structures and traps have grown successively throughout geological time. It is therefore difficult to classify structures in discrete timing categories, but the following broad groupings can be made:
(1) Structures which grew in pre-Silurian times only; these include traps related to Early Ordovician glacial palaeotopography (Taconic phase).
(2) Structures which grew in pre-Devonian and Devonian times only.
(3) Palaeozoic structures which continued to grow until the Hercynian event.
(4) Structures produced by Liassic extensional movements.
(5) Structures formed during the Austrian phase during the latest Barremian, often by inversion of earlier faults.
(6) Structures formed by Tertiary (Pyrenean, Alpine) structural phases, again often by inversion. Implications of each of these for the formation of structural traps will be discussed later.
It can be noted that in many cases, structural culminations have shifted with time. The Austrian event in particular played a significant role in causing lateral migration of the culminations of Palaeozoic aged structures. It can be noted that the location of structural culminations can consequently differ at the levels of the Liassic anhydrite seismic marker and the Triassic reservoir. In the northern part of the basin, it is possible to identify early structures on which closure has been destroyed through structural tilting related to Mesozoic subsidence. The failure of many dry wells in the basin can be related to the resulting problems of accurately locating structural crests at reservoir level from mapping on higher seismic markers.
2.2.2 Sequence stratigraphic
The Upper Ordovician to base-Carboniferous succession in the Libyan Ghadames Basin can be divided into two distinct intervals separated by the Caledonian unconformity:
(i) A second-order sequence of late Ashgillian -Silurian age (Carr 2002); and (ii) the lower part of a composite Devonian- Carboniferous super-sequence, terminated by the Hercynian unconformity (Belhaj 1996, 2000). Their general character and internal stratigraphic architecture are described as follows:
A. Late Ordovician-Silurian:
Although poorly constrained in the Libyan Ghadames basin, the unconformity at the base of the late Ordovician-Silurian super-sequence is well described in the northern part of the Murzuk (Aziz 2000; Echikh and Sola 2000) and Illizi Basins (Hirst et al. 2002) to the south and SW. The unconformity surface in this area has very pronounced topographic expression. Incised drainage systems with deep erosional valleys, separated by isolated erosional inselbergs and broad platforms, cut deeply into the underlying Haouaz Sandstones in response to a global sea-level fall and initial growth of a short-lived Hirnantian (upper Ashgillian) ice sheet across much of central and southern Africa (Brenchley et al. 1994; Sutcliffe et al. 2000).
This topography was gradually in filled by a very heterogeneous periglacial low stand clastic facies during several cycles of glacial advance and retreat (Fekirine and Abdallah 1998; Davidson et al. 2000). Regional projections suggest similar although perhaps more distal facies in the Ghadames region further north.
As sea level rose at the end of the glacial episode, the North African platform was flooded and blanketed by a transgressive marine-shale dominated facies (Keeley 1989). A brief intra- Rhuddanian anoxic event is marked by deposition of thin but regionally extensive organic-rich shales. (Luning et al. 2000) identified a maximum (second order) flooding event at the Rhuddanian-Aeronian boundary (early-middle Llandovery) with regressive highstand sedimentation commencing in late Llandoverian time.
This continued through to the end of the Silurian with the deposition of a northerly-prograding marine shelf to coastal fluvio-deltaic system, represented by the Tanezzuft Shale/Acacus Sandstone Formations (Bellini and Massa 1980). This larger second-order composite sequence can be subdivided into several higher frequency sequences:
- Second-order lowstand systems tract:
Regional stratigraphic extrapolations combined with limited well control allow the Late Ordovician in the Libyan Ghadames Basin to be subdivided into at least three high-frequency periglacial sequences or parasequence sets (PSS):
Melez Chograne Formation (PSS S1):
The Melez Chograne Formation is dominated by marine shales with thinner interbedded sandstones, generally constrained to the deeper parts of the incised erosional topography of the bounding unconformity surface. The sequence is characterised by matrix-supported pebbles and boulders of striated granite, gneiss and quartzite with common subaqueous slump, liquefaction and turbidity flow structures, interpreted here as a short-lived transgressive event following the initial glacial advance.
Memouniat Formation (PSS S2): Heterogeneous sand and gravel lithofacies:
Dominate the overlying Memouniat Formation, representing a more proximal periglacial setting. Weakly-defined stacking patterns in both well and outcrop control points suggest at least two unconformity-bound high frequency valley-fill cycles, deposited during glacial retreat episodes.
- Second-order transgressive systems tract (PSS S3):
The initial high frequency sequence of the late Ordovician-Silurian transgressive system tract is represented by the Bir Tlacsin Formation and "Argiles Micro- Conglomeratiques" facies equivalent in Algeria.
The Bir Tlacsin Formation:
The Bir Tlacsin Formation is typically composed of black marine shales with common granules and pebbles, passing up into a more sand-rich facies in some areas. Although rather sporadically developed in the northern Murzuk and southern Libyan Ghadames region, it increases significantly in thickness towards the Berkine area of eastern Algeria, where it infills quite marked erosional topography cutting down through the Memouniat Formation into the Cambro-Ordovician succession below (Echikh 1998). Its precise age is poorly constrained and it has been assigned to both the Late Ordovician and Early Silurian (Aziz 2000). However its diamictite-like appearance and association with incised valley systems suggest it is a high frequency per-glacial facies deposited during the final retreat of the Ordovician ice cap.
ii. Tanezzuft "hot" Shale:
The anoxic event which gave rise to the basal Tanezzuft Formation organicrich, radioactive "hot" shales was very brief, confined to two or three graptolite zones in the Rhuddanian stage of the lower Llandovery series (Luning et al. 2000). The facies is present over much of western Libya and appears to extend south into Niger, southern Algeria and Mali (Keeley and Massoud 1998; Luning et al. 2000). However it is locally confined to still under-filled incised valley systems in the northern Murzuk Basin and residual paleo-topography on the Late Ordovician sequence boundary is thought to influence its distribution and thickness in the Libyan Ghadames region as well.
- Second-order highstand systems tract:
Second-order maximum flooding at or near the Rhuddanian-Aeronian transition was followed by the Tanezzuft and Acacus Formations highstand system tract of late Llandoverian to Pridolian age, described by Bellini and Massa (1980). A very precise graptolite zonation allowed these authors to define a strongly diachronous succession of outer shelf Tanezzuft shales and shelf-coastal/deltaic Acacus sandstones, prograding northwards across western Libya during the mid to Late Silurian. It has been possible to refine their interpretation in the Libyan Ghadames region using detailed well correlations supported by limited biostratigraphic control and subdivide it into several high frequency sequences or parasequence sets:
High frequency (PSS) S4:
The early highstand system tract of this sequence is dominated by Tanezzuft Formation shales of mid-Llandoverian to Wenlockian age. Several shelf sandstone dominated parasequence are present to the south and stacking patterns suggest a generally pro-gradational depositional style with coastal fluvio-deltaic equivalents confined to the Murzuk Basin.
High frequency (PSS) S5:
Depositional style appears to change quite dramatically in the late Wenlockian. Log profiles in the lower part of the Acacus Formation suggest a rapid outbuilding and basin ward shift of shallow shelf ("chattery" to cleaning-up profiles) and coastal fluvio-deltaic (blocky profiles) parasequence during the early Ludlovian with more a gradational stacking architecture.
High frequency (PSS) S6:
A regionally extensive shale unit of late Ludlovian age appears to represent a short-lived higher frequency flooding event, separating the S5 and S6 parasequence sets. This was followed by the rapid advance of coastal to fluvio-deltaic (blocky log profiles) dominated parasequence in the upper part of the Acacus Formation during the late Ludlovian to Pridolian.
B. Devonian higher frequency sequences of the Devonian-Carboniferous:
The Devonian-Carboniferous succession in the Libyan Ghadames Basin represents a complex shelf dominated second-order sequence, interrupted by several erosional unconformities of variable importance. The most significant of these are the basal Caledonian and mid-Eifelian events associated with uplift of the Gargaf Arch and separation of the Libyan Ghadames from the Murzuk basin to the south. The super-sequence comprises an Early Devonian (Lochkovian-Pragian) lowstand system (Tadrart Formation), overlain by retrograding transgressive shallow-marine cycles of Emsian to Givetian age (Ouan Kasa Formation and Aouinet Ouinene). A second-order maximum flooding event is recognized within the Frasnian (Aouinet Ouinene III/basal IV). Strunian (upper Famennian) sandstones mark the onset of renewed northwards regression, passing upwards into thick shallow-marine coastal deltaic and carbonate dominated high frequency sequences of the Carboniferous highstand systems tract (El Rweimi 1991). The lower, Devonian part of the supersequence consists of several higher frequency third- to fourth-order cycles:
- Lower Devonian (third-order) composite sequence:
Although more subdued than in the adjacent Murzuk Basin, an intra-Eifelian unconformity and its correlative conformity is considered sufficiently important to be recognized as a third-order sequence boundary, separating a Lochkovian (Gedinnian) lower Eifelian third-order composite sequence from the overlying Middle to Upper Devonian. Two high frequency parasequence sets can be distinguished within this lower interval.
Tadrart Formation (PSS D1):
The Lochkovian- Pragian Tadrart Sandstone Formation rests directly upon the Caledonian unconformity. It is dominated by a braided fluvial sandstone facies, composed of at least two parasequences, which backstep towards the south. Facies character, stacking patterns and stratigraphic position directly above a major regional unconformity suggest the formation may represent the waning part of a second-order lowstand systems tract, more fully developed further outboard to the north of the Ghadames Basin.
Ouan Kasa Formation (PSS D2):
The Ouan Kasa (Emsian-lower Eifelian) Formation is composed of several higher-frequency shallow-marine dominated cycles. These initially backstep (transgress) towards the south but pass up into more progradational parasequence in the upper part of the interval.
- Middle-Upper Devonian second-order transgressive to early highstand systems tract:
Aouinet Ouinene I, II and III (PSS D3 & D4):
The lower part of the Aouinet Ouinene Formation is represented by two high-frequency sequences of stacked neritic to coastal deltaic cycles. In contrast to the underlying Ouan Kasa, the succession is dominated by marine shales and represents the late transgressive systems tract of the Devonian-Carboniferous super-sequence.
Aouinet Ouinene III, IV & Tahara Formations (PSSD5):
An organic-rich mid-Frasnian horizon, the Cues Limestone, is present in the Libyan Ghadames Basin, stratigraphically equivalent to the "Argile Radioactive" of the Berkine region and Dabdab Formation in the Murzuk Basin (Echikh 1992; Hallett 2002). There has been some confusion in the literature about the boundary between the Aouinet Ouinene III and IV, which has been variously positioned above (Massa and Moreau-Benoit, 1976; Parizek et al., 1984; Gundobin, 1985; Hallett, 2002) and below the unit (Echikh 1992). In this analysis, we have placed it within the high-frequency sequence (PSS) D5 sequence set. The apparent regional stratigraphic continuity of the organic-rich interval, and associated well log-defined stacking patterns, suggest that it may be associated with a major flooding event marking the transition into the highstand system tract of the super-sequence. It passes up through a shale-dominant succession into the shallow-marine Tahara sandstones of late Fammenian (Strunian) age. This in turn is bounded by the locally insignificant Acadian unconformity and Carboniferous strata above.
The main reservoirs in Ghadames Basin are the Acacus Formation (Upper Silurian) and the Tadrart Formation (Lower Devonian) and Kasa Formations (Figure 4) (Rusk, 2001). The Acacus net sandstone thickness ranges from approximately 150 m to 400 m (Figure 5). The Acacus average porosity is at least 16%. The Tadrart and Kasa Formations should have a net sandstone thickness of 100-200 m and an average porosity of 14-15% in the study area. These formations, which are a more or less continuous stratigraphic succession, are at depths between 2400 and 4000 m (Figure 6). Only eight exploration wells, most of which were in the north, reached these objectives in the study area. Three other sandstone reservoirs are valid objectives, but because of their shallower depths, they have been the subject of more exploratory drilling than the above formations. They are the Middle Devonian Uennin sandstone (equivalent of the F3 in Algeria), with a thickness range of 0 to 90 m; the Upper Devonian Tahara Formation, with a net sand range of 15 to 60 m ; and the Triassic Ras Hamia Formation, with a net sandstone thickness of 60 to 200 m . All of these sandstones have very good porosity, averaging 14-18%.
Figure4: Generalized stratigraphic chart of Ghadamis and Murzuq Basins, showing source and potential reservoir intervals (Source, Rusk 2001).
Figure5: Generalized stratigraphic chart of Ghadamis, showing source and potential reservoir intervals (Source, Rusk 2001).
Figure 6: Structure map on the top Acacus Formation, Ghadamis. Modified from Masera Corporation (1992), (Source, Rusk 2001).
Generally, there is an effective Acacus shale seal above the sandstone. Where it may be absent, however, the overlying Tadrart will form a combined objective with the Acacus sandstone. Shale horizons consistently provide adequate seals for Tadrart, Kasa, and Tahara sandstones. Throughout most of the area, there are effective shale, carbonate, or evaporite seals for the Ras Hamia sandstone. Because of a dominant continental siliciclastic facies above the Ras Hamia in the southern part of the area, however, a seal may be lacking.
126.96.36.199. Source rock, timing, and migration:
There are two world classes, type II source rocks distributed throughout the entire basin: the Lower Silurian Tanezzuft and the Middle to Upper Devonian Uennin Formations. The two shale formations have an average TOC of 3-5% and are approximately 300-600 m thick in this prime study area.
Depths to the base of the Tanezzuft and Uennin in the area are 3500 - 4500 m and 2500 -3500 m, respectively.
The main stage of oil expulsion from the Tanezzuft source probably occurred from the Late Triassic to Early Cretaceous. Oil expulsion from the Uennin source probably occurred from Early to Late Cretaceous.
In this central basin sector, structural traps were essentially established during Hercynian events, although some early development most likely occurred during the Caledonian orogeny. It is unlikely that the Albian Austrian event or the Eocene Pyrennian events, which affected major highs and coastal areas in the region, caused any significant structural modification to this sector. Consequently, traps were in place prior to migration. Conditions for migration were optimum, in view of the short distance and vertical and lateral carrier systems from the two sources to the multiple reservoirs.
The expected trap types are low-relief, simple, and faulted anticlines; drape anticlines over paleotopographic relief or faulted structures; unconformity truncation of the Tahara sand in the northern part of the study area; and pinch-outs of the Uennin F3 equivalent sand.
2.2.4. Petroleum systems
The most important reservoirs are summarized below for each geological period as showing in figure 7.
The Ordovician section is seen to have a significant sand content in the marginal and structurally highest parts of the basin. A few penetrations in the deepest part of the basin indicate the presence of a more shale facies.
The Cambro-Ordovician has been penetrated by only a few wells, with most of these terminating at the top of the Memouniat. The main producing reservoir lays in the Memouniat quartzitic sands. The facies distribution passing from sandy periglacial deposits in the south, close to the Qarqaf Uplift, to a marine shaly sequence in the central and northern parts of the basin.
The main regional reservoir of Silurian age is the Acacus Formation. The Acacus has been subdivided into three formations, Lower, Middle and Upper, by (Massa 1988), which are well represented in the northern parts of the basin. On the southern margin, Caledonian erosion (Echikh 1984, 1992) has removed the upper two members and only the Lower Acacus is present. In the basin centre, the Silurian has yet to be reached by drilling. The Lower Acacus and equivalent reservoirs show maximum sand development on the southern flank, passing progressively into a more shaly facies in central and north western areas. This trend is sometimes broken by the incoming of sands orientated along SW-NE trends, related probably to an intra-basin high that was active in the Caledonian. The Acacus sands show good petro-physical properties in the productive areas in southern and central parts of the basin, with porosities of 20-25%, rising locally to 30%. Deterioration in petro-physical proprieties occurs from the south towards the north-western part of the basin, where porosities do not exceed 12-15%.
The Lower Acacus has proven to be a prolific producer, with flowing oil in the central and north-western parts of the basin. Twenty-two oilfields and three gas fields have been found in this region.
Lower Devonian reservoirs
There are two discrete producing reservoirs in the Lower Devonian, the Tadrart and Ouan Kasa Formations. The Tadrart reservoir can be correlated throughout the basin, and consists in practically all areas of clean, medium- to coarse-grained sandstones deposited as widespread channelled sheets. The Ouan Kasa Formation can be subdivided into two members, Lower and Upper. The Lower Member shows substantial facies variation, passing from a very sandy succession in the south to shaly with carbonate layers in the northern part of the basin. The Upper Member, in contrast is of fairly uniform composition throughout the basin, consisting of an alternation of sand, silt and shale. It is of markedly less significance as a producing reservoir.
Middle and Upper Devonian reservoirs
Reservoir quality in the Middle and Upper Devonian is considerably more irregular than in the Early Devonian, with reservoir units thinner and generally associated with local highs. The Middle-Upper Devonian reservoirs also generally do not show good petro-physical characteristics, except in some areas of shallow burial and proximity to source areas close to the Tihemboka, Ahara and A1 Hamra Highs.
Sand, often non-marine in origin, is present at Triassic level. This is termed the Ras Hamia Formation. This form the highest-quality and most prospective reservoir in the northern parts of the basin.
The Triassic is characterized by significant vertical and lateral facies variations, which can be related to the topography developed on the Hercynian unconformity (Benrabah et al. 1991). Triassic sediments were supplied from the major paleo-highs such as Jabal Nafusah. In the north-eastern part of the Ghadames Basin, conditions of deposition change to a meandering fluvial system flowing SW to NE.
Further to the NE, the depositional environment passes into fluvial-deltaic and sand development becomes highly irregular.
The Triassic reservoirs are prolific oil and gas producers, particularly over the western flank of the Basin, where a number of more recent oil discoveries have been made.
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