Ghadames Basin Research Articles

Depth to basement analysis from gravity field over the Guelb Ahmer horst (Ghadames petroleum province, Southern Tunisia, North Africa)

The Exploration complex geological structures like crystalline basement, significant to hydrocarbon exploration, is complicated in seismic interpretation. Gravity data were analyzed at a Guelb Ahmer horst situated to northern border of Ghadames basin (Southern Tunisia). The average radial spectrum from an area of 40x40 km of the gravity data grid reveals the deep and intermediate basement sources depths. 3D Euler deconvolution technique illustrates also the limits of the crystalline basement sources running essentially between 2000 and 3000 meter deep. The geometries of Mesozoic rocks, Paleozoic strata and Precambrian basement rocks are estimated by five 3D gravity models. These models give a significant support for the determination of potential structural traps related to the Guelb Ahmer horst.

BURIAL AND THERMAL HISTORY MODELLING OF THE MURZUQ AND GHADAMES BASINS (LIBYA) USING THE GALO COMPUTER PROGRAMME

The GALO computer programme was used to model the thermal and burial histories of the Murzuq and Ghadames Basins, Libya. The model was based on recent drilling and seismic data from the basins, and used published deep temperature and vitrinite reflectance measurements. The model provides more accurate results than previous studies which were based on constant geothermal gradients during the basins' histories, and variations in heat flux at the base of the sedimentary cover were chosen so that calculated values of vitrinite reflectance coincide with those observed. The Murzuq Basin and Libyan part of the Ghadames Basin contain similar source rock units but have different burial histories. In the Murzuq Basin, maximum present‐day burial depths of Cambrian sediments range from 2200 to 2800 m and only locally reach 3000 – 3600 m; in the Ghadames Basin, however, burial depths can exceed 4–5 km. The burial history of the Murzuq Basin includes several periods of intense erosion and lithospheric heating which produced significant lateral variations in thermal maturity, leading in places to unexpected results. For example, relatively shallow‐buried Lower Silurian source rocks in the A‐76 area on the flank of the Murzuq Basin have a thermal maturity of Ro = 1.24% which is higher than the maturity of the same interval in more deeply buried areas (wells D1‐NC‐58 or J1‐NC101). In the central part of the Ghadames Basin, the modelling suggests a higher level of thermal maturity for organic matter in Silurian strata (Ro 0.8 to 1.3%), confirming the generation potential of Lower Silurian “hot shales”. Significant hydrocarbon generation began here in the Late Carboniferous and continues at the present day. Modelling of the Late Devonian (Frasnian) Aouinat Ouinine Formation “hot shales” suggests limited hydrocarbon generation depending strongly on burial depth, with the main phase of hydrocarbon generation taking place during the final episode of thermal activation in the Cenozoic. In the wells studied in the Ghadames Basin, the “oil window” extends over a considerable part of the present‐day sedimentary column.

Lineament analysis of South Jenein Area (Southern Tunisia) using remote sensing data and geographic information system

Accurate geological and lineament mapping is a critical task for structural analysis and tectonic interpretation in stable platform domain. Efforts in structural mapping and tectonic interpretation in the South Jenein area and its surrounding are hindered by the difficult access to the outcropping features, where some of the stratigraphic sequences and structures are buried under the sand of the “grand Erg oriental”. This study involves the use of spot Landsat images and Digital elevation model (DEMs) for lineament and fracture analysis. The obtained lineament results showed two major directions of faults: the main lineament direction was 035–065° (NE–SW) whereas the secondary lineament direction was 110–130° (NW–SE). The other directions were roughly E–W and N–S. These results were confirmed by the measurement and analysis of fracture and lineament on the field of the studied area.These structures were the origin of the tilted blocks delimited by fracture and folding of the Upper Cretaceous series (i.e., Senonian). An extensive structural analysis was carried out to precise the kinematic and geodynamic contact in terms of stress tensor in a domain that was considered as a stable platform for longtime.The results will be compared with the surface data in the Mesozoic series of the Tataouine basin and the subsurface data of the northern border of the Palaeozoic Ghadames basin. The fracture measurements and their distribution provided a good opportunity to characterize the petroleum reservoir in this area.

Basement structure of southern Tunisia as determined from the analysis of gravity data: implications for petroleum exploration

Gravity data were analysed in the Ghadames Basin and surrounding regions in southern Tunisia in order to determine the basement structure of the region and its relationship to petroleum exploration in relatively unexplored basins. The analysis included the construction of regional Bouguer gravity anomaly and horizontal gravity gradient maps. These maps indicate that the Ghadames Basin is not a simple sag basin but consists of a series of sub-basins and uplifts. The northern boundary of the basin which we call the Telemzan–Ghadames transition zone is marked by a NE-trending high amplitude gravity gradient anomaly which decreases in amplitude toward the east and breaks into a series of north–south- and east–west-trending anomalies implying a more structurally complex region. When the known petroleum fields are overlain on to the gravity gradient anomaly maps, the fields mostly occur along or next to linear alignments of horizontal gravity gradient maxima. We interpret the correlation of the petroleum fields and horizontal gravity gradient maxima to indicate that the basement was involved in forming the petroleum traps. This study illustrates that a regional gravity analysis can be useful in determining where additional exploration can be applied in relatively unexplored basins.

Source Rock Characterization and Petroleum Systems in North Ghadames Basin, Southern Tunisia

AbstractThis paper presents geochemical analysis of drilled cutting samples from the OMZ‐2 oil well located in southern Tunisia. A total of 35 drill‐cutting samples were analyzed for Rock‐Eval pyrolysis, total organic carbon (TOC), bitumens extraction and liquid chromatography. Most of the Ordovician, Silurian and Triassic samples contained high TOC contents, ranging from 1.00 to 4.75% with an average value of 2.07%. The amount of hydrocarbon yield (pyrolysable hydrocarbon: S2b) expelled during pyrolysis indicates a good generative potential of the source rocks. The plot of TOC versus S2b, indicates a good to very good generative potential for organic matter in the Ordovician, Silurian and Lower Triassic. However, the Upper Triassic and the Lower Jurassic samples indicate fair to good generative potential. From the Vankrevelen diagram, the organic matter in the Ordovician, Silurian and Lower Triassic samples is mainly of type II kerogen and the organic matter from the Upper Triassic and the Lower Jurassic is dominantly type III kerogen with minor contributions from Type I. The thermal maturity of the organic matter in the analyzed samples is also evaluated based on the Tmax of the S2b peak. The Ordovician and Lower Silurian formations are thermally matured. The Upper Silurian and Triassic deposits are early matured to matured. However, Jurassic formations are low in thermal maturity. The total bitumen extracts increase with depth from the interval 1800–3000 m. This enrichment indicates that the trapping in situ in the source rocks and relatively short distance vertical migration can be envisaged in the overlying reservoirs. During the vertical migration from source rocks to the reservoirs, these hydrocarbons are probably affected by natural choromatography and in lower proportion by biodegradation.

Integrated remote sensing data utilization for investigating structural and tectonic history of the Ghadames Basin, Libya

This study was initiated to constrain the geological structure of the Ghadames Basin in northwest Libya. Detailed analysis was based on digital integration of surface data, including SRTM DEM, ETM+ and geologic maps with subsurface data, including well logs and potential field data. Integrated analysis of remotely sensed data of the SRTM and ETM+ were utilized to identify geologic lineaments in the area. Ground-based verification of the remote sensing data was achieved with field work. Interpretation and analysis of the lineaments indicate that the Ghadames Basin is controlled by four main fault systems that trend WNW, NNW, NW and ENE. Well logs and potential field data were used to delineate a detailed picture of the subsurface structure. The potential field data reveal two NE and NNE trending sedimentary basins. The depth of the basement inside the main basin ranges from 2 to 6km. A two-dimensional (2-D) schematic model shows that the basin gradually deepens towards the southwest. The applied data integration gave new insight into the tectonic and structure patterns of the Ghadames Basin and the adjacent areas of northwest Libya.

Aliphatic and aromatic biomarkers from Gondwanan sediments of Late Ordovician to Early Devonian age: An early terrestrialization approach

Twenty one core samples of Late Ordovician to Early Devonian age from sections in southern Tunisia, North African Platform (Gondwana) and containing marine and terrestrial organic matter with microbial input were investigated to link the aliphatic and aromatic hydrocarbon distributions with the terrestrial and marine palynomorph content (e.g. acritarchs, prasinophytes, chitinozoans, cryptospores and trilete spores). In addition to several biomarkers of algal/bacterial origin, long chain n-alkanes that might be derived from land plants, as well as the terrestrial diterpane, norabietane, were found. Several land plant-derived biomarkers, such as retene, cadalene, simonellite, tetrahydroretene and C 19 isohexylalkyl naphthalene were observed in the aromatic fractions. While these terrestrial biomarkers could be clearly recognized in the middle Silurian–lower Devonian samples, their presence in the upper Ordovician–lower Silurian sediments is more doubtful, because of much lower relative abundances. The land plant biomarkers show a fairly good correlation with the occurrence and abundance of cryptospores and trilete spores, derived from bryophytes and tracheophytes, which covered the emerged areas around the Ghadamis Basin during the Silurian and Devonian. The early tracheophytes (e.g. Cooksonia, lycophytes and zosterophylls) are therefore suggested as a new terrestrial source for most of the saturated and aromatic biomarkers found in sediments of Middle Silurian to Early Devonian age.

Diverse assemblages of Mid Devonian megaspores from Libya

A restudy of Devonian palynological assemblages of samples from the A1-69 borehole, drilled in the Ghadamis Basin, western Libya, North Africa has led to the discovery of numerous well-preserved megaspores amongst abundant miospores, rare acritarchs and rare chitinozoans. Thirteen samples from base to top, 1496 ft (456 m) up to 965 ft (294 m), contain the richest Devonian megaspore assemblages from northwestern Gondwana. The section is dated by a rich diverse miospore assemblage indicating the presence of the upper part of the AD pre-Lem Biozone up to the lower part of the TCo Oppel Zone, ranging from a latest Eifelian or earliest Givetian age up to a latest Givetian or an earliest Frasnian age. Seventeen megaspore taxa have been identified. Among them, two new species ( Biharisporites lugardonii and Lagenicula milleri) and four new varieties ( Corystisporites acutispinosus var. acutispinosus and var. bullatus, Heliotriletes longispinosus var. longispinosus and var. radiatus) are described. One new possible megaspore species is also described: Verruciretusispora labiosa sp. nov. One core sample (1293 ft, 394 m) contains several specimens of very large megaspores (ca. 1 mm), the largest known from Devonian localities (except for one specimen observed in the Givetian from Belgium). Among the 17 megaspore taxa from Libya, 6 are present on the Euramerican Continent. This observation is in accordance with previous palynological conclusions that favour the absence of palaeogeographic barriers between the Euramerican and Gondwanan continents that are suggested to be grouped together in a “Pre-Pangea” land mass.

Natural hydrocarbon accumulation related to formation overpressured interval; study case is the Saharan platform (Southern Tunisia)

Organic geochemistry analysis and formation fluid overpressure study have been performed on several oil well data located to the northern part of the Ghadames Basin (southern Tunisia Saharan platform). The main results of this work highlight the important role of overpressured interval in hydrocarbon accumulation. In fact, the analysis of the Paleozoic–Mesozoic petroleum systems in Southern Tunisia shows that the hydrocarbons generated by Ordovician and Silurian source rocks migrate upward following fault systems or through the major Hercynian unconformity to finally accumulates in structural or stratigraphic traps. These migration pathways provide favorable conditions to fill up the Silurian, Devonian, or Triassic reservoirs. The overpressured zone located above Triassic reservoir constitutes a continuous and efficient seal to hydrocarbon trap structures. So the overpressure plays the role of hydraulic seal advantaging oil and gas accumulation in the Ghadames Basin associated with the impermeable (evaporites) and then stopping any upward migration of natural hydrocarbon.

Diisopropyl naphthalenes: Environmental contaminants of increasing importance for organic geochemical studies

Diisopropyl naphthalenes (DIPNs) have distinctive mass spectra, including a base peak at m/z 197 and M +. at m/z 212 and, on the apolar columns typically used in organic geochemical applications, are resolved into five main peaks, although there are ten possible isomers. DIPNs have been recognised in rock matrix and solid bitumen samples from Oklo (Gabon) and subsurface samples of Silurian–Devonian clastic sediments from southern Tunisia (borehole MG1, Ghadamis Basin). They have synthetic/anthropogenic origins and are used as plant growth regulators and in the manufacture of printed paper. The most likely source of the DIPNs in these organic geochemical samples is tissue paper used to wrap the samples. It is suggested that they are preferentially adsorbed by solid bitumen and so are not released during solvent extraction of surfaces. Although they have been reported in environmental and food samples, this appears to be the first report of their occurrence in organic geochemical samples.

Timing of early to mid-cretaceous tectonic phases along North Africa: New insights from the Jeffara escarpment (Libya–Tunisia)

The Jeffara escarpment spans 400 km from southeastern Tunisia to sorthwestern Libya, and marks the northern edge of the Berkine–Ghadames Basin. Its horseshoe shape provides a good 3D control of regional-scale depositional architecture. Historically, the political border between Tunisia and Libya hindered the integration of studies over its entirety, which led to the establishment of separate litho-biostratigraphic frameworks. Field-work undertaken on both sides of the border has allowed the unification of lithostratigraphic schemes developed in both countries. Published stratigraphic ages of the different formations and members are compared and integrated in order to propose a unified bio-lithostratigraphic framework. The correlation of serial sections along the Jeffara escarpment shows that two major tectonic unconformities divide the Early Cretaceous sedimentary pattern. The first one is dated as Late Aptian and is commonly associated with the European “Austrian” tectonic phase. The second, which has previously not been recognized as a regional significant surface in Libya, occurs during the Middle Albian and marks moreover the transition from a siliciclastic to a carbonate-dominated sedimentation regime. These two important regional unconformities form the lower and upper boundaries of the Kiklah–Aïn el Guettar Formations, and can be associated with intra-plate deformation linked with the opening of the central segment of the South Atlantic and the Equatorial Atlantic oceans.

Laser Raman micro-spectroscopy of Proterozoic and Palaeozoic organic-walled microfossils (acritarchs and prasinophytes) from the Ghadamis Basin, Libya and Volta Basin, Ghana

Laser Raman microspectroscopy was used as a microchemical analysis technique to characterize the wall chemistry of organic-walled microfossils (acritarchs and prasinophytes) extracted from Proterozoic (Tonian: ca. 900 Myr) and early Palaeozoic (Silurian: ca. 420 Myr) marine sediments in the Volta Basin of Ghana, and the Ghadamis Basin of Libya, respectively. Raman spectra of Proterozoic acritarchs show spectral features characteristic of kerogenous compounds at ~1350 and ~1600 cm−1, consistently with previously published reports. In addition, spectra from prasinophyte algae from the Silurian sample also show an interesting spectral feature at ~1707 cm−1indicative of carbonyl moieties.Broadly speaking, shape and position of Raman bands appear to depend on the nature of the specimen considered, suggesting that laser micro-Raman analysis can potentially be used to establish phylogenetic relationships (high-rank taxonomy) among the main groups of pre-Cambrian to Palaeozoic palynomorphs.

Laser Raman micro-spectroscopy of Proterozoic and Palaeozoic organic-walled microfossils (acritarchs and prasinophytes) from the Ghadamis Basin, Libya and Volta Basin, Ghana

Laser Raman microspectroscopy was used as a microchemical analysis technique to characterize the wall chem- istry of organic-walled microfossils (acritarchs and prasinophytes) extracted from Proterozoic (Tonian: ca. 900 Myr) and early Palaeozoic (Silurian: ca. 420 Myr) marine sediments in the Volta Basin of Ghana, and the Ghadamis Basin of Libya, re- spectively. Raman spectra of Proterozoic acritarchs show spectral features characteristic of kerogenous compounds at ∼1350 and ∼1600 cm −1 , consistently with previously published reports. In addition, spectra from prasinophyte algae from the Silurian sample also show an interesting spectral feature at ∼1707 cm −1 indicative of carbonyl moieties. Broadly speaking, shape and position of Raman bands appear to depend on the nature of the specimen considered, suggesting that laser micro-Raman analysis can potentially be used to establish phylogenetic relationships (high-rank taxonomy) among the main groups of pre-Cambrian to Palaeozoic palynomorphs.

DIAGENETIC EVOLUTION OF INCISED CHANNEL SANDSTONES: IMPLICATIONS FOR RESERVOIR CHARACTERISATION OF THE LOWER CARBONIFEROUS MARAR FORMATION, GHADAMES BASIN, WESTERN LIBYA

This paper assesses the diagenetic history of potential fluvial hydrocarbon reservoir rocks deposited within incised valley systems of the Lower Carboniferous Marar Formation in western Libya. Outcrop data were collected in the Tinedhan Anticline, located at the southern margin of the Ghadames Basin. Four discrete intervals with channelized sandstones were identified in a section dominated by alternating offshore mudstones and shallow‐marine clastics. The incised channels were cut during major sea‐level lowstands, and filled by fluvial sandstone packages up to 50 m thick. Fifty‐eight samples from four different localities, representing three lowstand systems tracts, were analysed to obtain a statistically meaningful mineralogical and compositional dataset.In addition to burial compaction, three main diagenetic events influenced the reservoir quality of the sandstones. Firstly, early eodiagenesis involved kaolinitization of plagioclase grains. This began before subsequent calcite cementation, probably as a result of flushing by meteoric pore‐waters. The deformation of kaolinite during later compaction resulted in the formation of pseudomatrix which further reduced porosity and permeability. Kaolinite is commonly transformed to illite at temperatures above 140°C in the presence of K‐feldspar. Although K‐feldspar was recorded in the samples, no illite was observed, suggesting that the Lower Carboniferous strata in the study area were not buried in excess of approximately 3.5 km.The second diagenetic phase was the precipitation of calcite cement, present either dispersed throughout the sandbodies or as concretions up to 2 m across, in both cases reducing reservoir quality. The high intergranular volumes (IGV) of calcite‐cemented sandstones (ranging between 35% and 40%) suggest that cementation occurred at burial depths of <500 m. Sandstones without calcite cement have lower IGV of between 17% and 25% as a result of mechanical and chemical compaction. Stable C and O isotope analysis of the calcite cement also supports precipitation at shallow burial depths, indicating a meteoric pore‐water source for the calcite. The third and final diagenetic stage was partial chloritisation of kaolinite during meso‐diagenesis. The elevated temperatures required for this transformation indicate burial to a minimum depth of approximately 2.5 km, which is consistent with the compaction data.Despite these diagenetic effects, the fluvial sandstones have an average porosity of 12%, with a range from 0.5% up to 25%. Permeability measurements on four sandstone samples indicate that the development of pseudomatrix did not reduce permeability significantly.

Sedimentological and sequence stratigraphic analysis of Carboniferous deposits in western Libya: Recording the sedimentary response of the northern Gondwana margin to climate and sea-level changes

Detailed sedimentological and sequence stratigraphic analysis of Carboniferous (Tournaisian to Moscovian) strata exposed in the north-western Murzuq Basin and southern Ghadames Basin, western Libya, provides new insights into the sedimentary response of the northern Gondwana margin to climate and sea-level change. The Lower Carboniferous Marar and Assedjefar Formations can be divided into five depositional sequences of 3rd order. In total 27 facies types are defined, grouped into four facies associations: offshore shales, shallow marine clastics, fluvial sandstones and marine carbonates. The bulk of the Lower Carboniferous interval is dominated by an alternation of offshore shales and shallow marine clastics, which were deposited during the transgressive and highstand systems tracts. The clastic deposits mostly consist of laterally persistent coarsening and thickening upward cycles with a common succession from basal hummocky cross-stratified sandstones to ripple-laminated sandstones, capped by multidirectional cross-stratified sandstones. Within the lowstand systems tracts, lenticular sandbodies have been identified, which vary in thickness from 1.5 m (ca. 40 m wide) to 50 m (ca. 1.5 km wide). These are interpreted to be fluvial channel complexes based on their geometry, erosive base, and presence of thick stacked sandstones with unidirectional planar and trough cross-bedding, the absence of bioturbation and occurrence of land plant fragments. These channel complexes mostly cut down into offshore shales, and are interpreted to be bound at the base by sequence boundaries. Palaeogeographic maps generated for each lowstand system show the location and palaeoflow direction of these fluvial channel complexes. They are interpreted to represent large incised valleys filled with thick fluvial sandstones. Their identification and distribution indicates repeated exposure of large areas of western Libya, most-likely controlled by major eustatic sea-level changes. The Assedjefar Formation exibits a gradual decrease in coarse clastic sediment supply throughout the Serpukhovian and by the Bashkirian and Moscovian during the deposition of the Dembaba Formation a carbonate depositional system was established. Limestones are dominantly made up of a heterozoan fauna (brachiopods, crinoids, bryozoans, gastropods) and are mostly preserved as shallow marine storm and coastal deposits. It is hypothesised that a local increase in aridity and/or the gradual erosion and decreasing topography of the hinterland mountains, with the resulting reduction in discharge, controlled this shift from clastic to carbonate deposition. Available data indicate that the Murzuq Basin was interconnected with the Ghadames Basin at this time and is a postdepositional basin with respect to the Carboniferous interval. The sedimentological and sequence stratigraphic data from the present study offer new insights into the depositional setting and facies distribution in the Carboniferous, and the recognition of major incised fluvial systems has significant implications in the search for potential Carboniferous hydrocarbon reservoirs in the Murzuq, Ghadames, and Illizi Basins.

Palynostratigraphy and vegetational changes in the Siluro-Devonian of the Ghadamis Basin, North Africa

This study, based chiefly on cryptospores and trilete spores from North Africa (MG-1 borehole, Ghadamis Basin, southern Tunisia), contributes to the understanding of the terrestrialization process, its timing and its palaeoenvironmental consequences across the Silurian–Devonian transition in Western Gondwana. The investigated section comprises, in ascending stratigraphic order, the Tanezzuft, Acacus and Tadrart formations. Four well preserved and diversified miospore assemblages are recognized and attributed respectively to the late Gorstian–Ludfordian, Lufordian–early Přídolí, Přídolí and early Lochkovian, based on correlation with previously established palynozonations, and independent age evidence from chitinozoan faunas. The lithostratigraphical boundary between the Acacus and the Tradart fms. corresponds to the Silurian–Devonian transition. In this time span, the Ghadamis Basin was the site of an extensive regression. The middle Silurian–Lochkovian Tunisian miospore assemblages do not differ significantly from coeval associations both from Gondwanan and Laurussian localities, hence they do not provide evidence for a pronounced palaeobiogeographical differentiation. This similarity could reflect: a) palaeogeographical proximity of the two palaeocontinents and b) the low diversity of primitive vegetation in its early stages of evolutionary development. Minor but distinct variations in taxonomic composition of assemblages belonging to the same palaeogeographic domain, are thought to reflect varying environmental or physiographic conditions at regional and/or local scale.

Palynology, organic geochemistry and carbon isotope analysis of a latest Ordovician through Silurian clastic succession from borehole Tt1, Ghadamis Basin, southern Tunisia, North Africa: Palaeoenvironmental interpretation

Palynological and palynofacies analyses combined with organic carbon isotope measurements have been performed on terminal Ordovician through Silurian clastic sediments from the North African margin of Gondwana (southern Tunisia). A first carbon isotopic signal (δ 13C org) from Gondwanan Silurian sedimentary sequences is presented, showing interesting correlation with existing coeval isotopic curves from other areas. Changes in lithology, palynofacies characteristics, palynomorph diversity, carbon isotope developments, and organic geochemistry parameters appear to be all causally linked, and to reflect changes in palaeoceanographic conditions. In particular, the detailed chronostratigraphic correlation and the observed similarities in patterns of carbon isotopic developments and (palyno- and litho-) facies changes through the study section has permitted to identify the supposedly global earliest Wenlock (“Ireviken Event”) and late Ludlow (“Lau Event”) isotopic excursions for the first time in high-latitude Gondwana. This confirms that these excursions were linked to global changes in the oceanic system. The present results suggest that from Rhuddanian to early Wenlock times, an extended period of black shale deposition occurred over the North African Gondwanan margin, progressively transgressing from basin palaeodepressions to basin palaeohighs. Palynofacies and organic geochemistry support a coastal upwelling-promoted productivity increase during this interval, associated to a decrease in diversity of the marine microplanktonic communities. The earliest Wenlock strong positive isotopic shift appears associated with this protracted period of massive black shale deposition, and thus of organic carbon burial, on continental platforms located in high-latitude settings. This could well explain the apparent paradox between excessive carbonate deposition simultaneous to a carbon isotopic shift towards lighter values observed in low palaeolatitude localities (Laurentia and Baltica). the strong, distinct isotope excursion occurring in late Ludlow times is possibly linked to the well know Lau Event. Evidence for extensive organic carbon burial is lacking to explain this strong excursion. Some significant changes in the marine palynomorph communities are recorded in connection to the increase in stable isotope values (δ 13C org). Additionally, a strong correlation between abundance of terrestrially derived palynomorphs (miospores) and δ 13C org development is recorded which seems to support increased clastic input from a terrestrial source.

Petroleum generation and migration in the Ghadames Basin, north Africa: A two-dimensional basin-modeling study

The Ghadames Basin contains important oil- and gas-producing reservoirs distributed across Algeria, Tunisia, and Libya. Regional two-dimensional (2-D) modeling, using data from more than 30 wells, has been undertaken to assess the timing and distribution of hydrocarbon generation in the basin. Four potential petroleum systems have been identified: (1) a Middle–Upper Devonian (Frasnian) and Triassic (Triassic Argilo Greseux Inferieur [TAG-I]) system in the central-western basin; (2) a Lower Silurian (Tannezuft) and Triassic (TAG-I) system to the far west; (3) a Lower Silurian (Tannezuft) and Upper Silurian (Acacus) system in the eastern and northeastern margins; and (4) a Lower Silurian (Tanezzuft) and Middle–Upper Devonian (Frasnian) system to the east-southeast. The Lower Silurian Tanezzuft source rock underwent two main phases of hydrocarbon generation. The first phase occurred during the Carboniferous, and the second started during the Cretaceous, generating most hydrocarbons in the eastern (Libyan) basin. The Frasnian shales underwent an initial, minor generative phase in the central depression during the Carboniferous. However, the main generation occurred during the Late Jurassic–Cenozoic in the western and central depression. The Frasnian shales are currently only marginally mature in the eastern part of the basin. Modeling indicates that the Alpine (Eocene) exhumation of the eastern (Libyan) basin margin had a significant control on the timing of hydrocarbon generation from the Lower Silurian source rock. The preferred burial-history model calibrates source rock maturity data by incorporating late exhumation and reduced subsidence prior to the Hercynian (Carboniferous) orogeny. As a result, the Tannezuft shales preserve their generative potential into the Mesozoic–Cenozoic, with renewed hydrocarbon generation during subsequent reburial, which can migrate to post-Hercynian (Carboniferous) traps, hence favoring the preservation of hydrocarbon accumulations.

Constraining the burial history of the Ghadames Basin, North Africa: an integrated analysis using sonic velocities, vitrinite reflectance data and apatite fission track ages

ABSTRACTConstraining the burial history of a sedimentary basin is crucial for accurate prediction of hydrocarbon generation and migration. Although the Ghadames Basin is a prolific hydrocarbon province, with recoverable oil discovered to date in excess of 3.5 billion bbl, exploration on the eastern margin is still limited and the prospectivity of the area depends on the identification of effective source rocks and the timing of hydrocarbon generation. Sonic velocity, apatite fission track (FT) and vitrinite reflectance analysis offer three complementary methods to determine burial history and provide independent analytical techniques to evaluate the timing and amount of exhumation. The results indicate that two phases of tectonic activity had the biggest influence on basin evolution: the Hercynian (Late Carboniferous–Triassic) and Alpine (Late Mesozoic/Cenozoic) tectonic events. Exhumation during the Hercynian tectonic event increases from the SE, where an almost complete Palaeozoic section is preserved, towards the NW. This study quantifies the significant regional Alpine exhumation of the southern and eastern margins of the basin, with important implications for the timing of hydrocarbon maturation and expulsion, particularly for the Silurian source rock interval. Incorporating elevated Alpine exhumation values into burial history models for wells in the eastern (Libyan) part of the basin allows calibration with available maturity (Roeq) data using moderate values of Hercynian erosion. The result is preservation of the generation potential of Silurian (Tanezzuft) source rocks until maximum burial during Mesozoic/Cenozoic time, which improves the chance for preservation of hydrocarbon accumulations following entrapment.

The importance of constraining regional exhumation in basin modelling: a hydrocarbon maturation history of the Ghadames Basin, North Africa

Estimation of eroded overburden at unconformities is essential to accurately constrain burial histories and predict the timing of hydrocarbon maturation. In the Ghadames Basin, three independent techniques, palaeo-isopach construction, sonic velocity and vitrinite reflectance analysis, were employed. The resultant basin model suggests that only the two most significant unconformities, the Hercynian (Late Carboniferous) and Alpine (early Eocene), have a major control on timing of hydrocarbon charge. Modelling indicates only one period of generation from the Lower Silurian shales on the western margin of the basin, with 95% hydrocarbon generation prior to Hercynian exhumation. The central basin and southern margin experienced maximum burial during Eocene time. The Middle–Upper Devonian mudstones are the main source rocks and they did not generate significant volumes of hydrocarbons over the basin centre until Cretaceous time; they are currently within the peak oil generation zone. In Libya, on the eastern/northeastern flank of the basin, results indicate Cenozoic maximum burial, followed by up to 2000 m of Alpine exhumation. The magnitude of this exhumation has not been recognized previously, although it is a critical component of the basin model as it has a major impact on potential hydrocarbon charge in this area. Maturation models predict that the Lower Silurian source underwent two generative phases: (1) pre-Hercynian (Carboniferous) generation; and, significantly, (2) post-Hercynian (Late Jurassic–Cenozoic) generation. The identification of late hydrocarbon generation offers potential for oil and gas to migrate into post-Hercynian traps. Over the western, northern and eastern flanks, the Devonian source rocks remain immature/ marginally mature at present day.