Central Libya Research Articles

Geochemical Evaluation of Campanian Sirte Shale Source Rock, Arshad Area, Sirt Basin, Libya

The present geochemical study deals with evaluation of Sirte Shale (Campanian) source rock in Arshad area which is located in the central Sirt Basin in north central Libya. Sirte Formation is composed mainly of shale, sandy shale, being partially sandy and calcareous facies indicating presence of possible source rocks. It contains enough amount of organic matter (>0.5 wt. %) to generate and expel hydrocarbons under suitable thermal maturity level in the area. The recorded kerogens belong to types III and IV. The pyrolysis results indicate deficient of hydrogen content and poor potential source rocks in all samples. The HI values of the analyzed samples prove predominance of type III (gas-prone) and type IV (non-hydrocarbon potential) kerogens. In general, pyrolysis results and derived parameters indicate poor generating potential for the Sirte Formation (Campanian) in the area and mature kerogen in the oil-window zone. The thermal burial history curves of selected wells in Arshad area indicate that Sirte Shale had been buried in the top peak oil generation or oil window (Late maturity, Ro: 0.9-1.35%) since 4 to 13.5 M. Y. ago at depth range 9100'-9600'. Time of hydrocarbon expulsion has started in the area since 15 M. Y. nearly during the Middle Miocene time (Serravallian age). In Sirt Basin, oil has migrated from the Campanian Sirte Shale source rock which has reached the oil-window at western flank of the South Ajdabiya Trough to the Mesozoic and Paleozoic reservoirs in Zaltan Platform and to the Upper Cretaceous Arshad sandstone reservoir in Arshad oil field as well. The migrated hydrocarbons most likely are entrapped in the structural closures of the different structural traps (folds and fault-blocks) in Zaltan Platform. Likewise, westward up-dip direction of gas migration took place from the Campanian Sirte Shale source rock as buried in the gas-window in the deep Wadayet (NW Ajdabiya Trough) to the Mesozoic and

Unraveling landscapes with phytogenic mounds (nebkhas): An exploration of spatial pattern

Phytogenic mounds (nebkhas) often are symptoms of desertification in arid regions. Interactions among nebkhas and between nebkhas and their environment are however poorly examined. To this end, three main hypotheses of nebkha pattern formation were evaluated in this study. These state that nebkha patterns are either shaped by: (i) biologically induced recruitment inhibiting zones, (ii) biologically induced recruitment encouraging zones, or (iii) by the spatial distribution of abiotic factors which are not biologically driven. Contrasting nebkha landscapes were examined: a highly dense New Mexican mesquite (Prosopis glandulosa) and snakeweed (Gutierrezia sarothrae and Gutierrezia microcephala) ecosystem, and a low-density mixed Tamarix aphylla and Calligonum comosum field in central Libya. Spatial second-order statistics of strategically chosen nebkha subpatterns were compared with those of null models in which observed patches were spatially randomized without overlap. Null model deviations were assessed with goodness-of-fit tests, and interpreted in terms of hypothesized mechanisms of nebkha pattern formation. Our results suggest that biologically induced recruitment inhibiting zones surround adult mesquite nebkhas. The configuration of Calligonum and Tamarix nebkhas may be driven by spatial dynamics of abiotic microsites which are not caused by nebkha interactions. Hence we conclude that both biotic and abiotic drivers can shape nebkha spatial patterns.

Climate change projections of precipitation and reference evapotranspiration for the Middle East and Northern Africa until 2050

ABSTRACTThe Middle East and North Africa (MENA) region can be considered as the most water‐scarce region of the world. The Intergovernmental Panel on Climate Change projects strong changes in climate across MENA, further exacerbating pressure on available water resources. The objective of this study is to undertake a climate change assessment for 22 MENA countries in order to quantify the problems these countries may encounter up to 2050. To evaluate climate change in MENA, nine global circulation models representing two future periods (2020–2030 and 2040–2050) were statistically downscaled and compared with a current climate, defined as the period 2000–2009. Besides precipitation only this study also focuses on change in water demand by vegetation reference evapotranspiration (ETref). It was found that for both future periods the annual precipitation sum will decrease for the majority of countries, with decreases of 15–20% for the latter period. For some countries, e.g. Djibouti and Yemen, an increase in annual precipitation of 15–20% was found. The annual ETref shows an increase for all countries for both future periods, with the strongest increases for the latter period. For the extreme situation, it was found that the minimum monthly and annual precipitation sum does not become smaller in the future climate. It in fact increases. In contrast, the maximum monthly and annual ETref increases for all countries. This indicates that projected changes in demand are likely to have a more adverse effect than changes in supply. Spatial analysis showed that the largest precipitation decreases are to be found in southern Egypt, Morocco, central and coastal Algeria, Tunisia, central Libya, Syria, and central and eastern Iran. A case study for Morocco revealed that the potential water deficit, which is already apparent for the current climate, becomes even larger for the future climate.

New Apterodontinae (Hyaenodontida) from the Eocene locality of Dur At-Talah (Libya): systematic, paleoecological and phylogenetical implications.

The African Hyaenodontida, mainly known from the Late Eocene and Early Oligocene Fayum depression in Egypt, show a very poor diversity in oldest Paleogene localities. Here we report new hyaenodontidans found in the late Middle Eocene deposits of Dur At-Talah (Central Libya), known to have recorded the earliest radiation of African anthropoids. The new hyaenodontidan remains are represented by dental and postcranial specimens comprising the historical material discovered by R.J.G. Savage in the last century and that of the recent Franco-Libyan campaigns. This material includes two apterodontines, in particular a subcomplete skeleton of Apterodon langebadreae nov. sp., bringing new postcranial elements to the fossil record of the genus Apterodon. Anatomical analysis of the postcranial remains of Dur At-Talah suggests a semi-aquatic lifestyle for Apterodon, a completely unusual locomotion pattern among hyaenodontidans. We also perform the first cladistic analysis of hyaenodontidans including apterodontines: Apterodon and Quasiapterodon appear close relatives to “hyainailourines”, in particular to the African Oligo-Miocene Metasinopa species. Apterodon langebadreae nov. sp. could be the most primitive species of the genus, confirming an African origin of the Apterodontinae and a further dispersion event to Europe before the early Oligocene. These data enhance our knowledge of early hyaenodontidan diversification into Africa and underline how crucial is the understanding of their evolutionary history for the improvement of Paleogene paleobiogeographic scenarii.

Paleogeography of the Eosahabi River in Libya: New insights into the mineralogy, geochemistry and paleontology of Member U1 of the Sahabi Formation, northeastern Libya

A large paleo-river of Miocene age traversing Libya from south to north has been previously demonstrated by seismic, stratigraphic, paleontological, and remote sensing data. The depositional environment of As Sahabi Formation in north central Libya is this large and now extinct Eosahabi River. However, the source of this major African river has remained controversial. Dark-colored sedimentary material with magnetic properties suggested a source from the basaltic Haruj as Aswad massif in south central Libya. To test this hypothesis, mineralogical and geochemical study of clayey sediments from three localities, P25, P28 and P96c, from fossiliferous Member U1 of the As Sahabi Formation, were carried out. Results strongly indicate very mature and re-processed sediments of continental origin, and felsic sources with no basaltic contribution. Thus the origin can be traced to an upland area of outcrop of rocks of Precambrian continental origin in northeastern Chad. An alternative scenario is that the Sahabi sediments originated from Precambrian outcrops in Ethiopia through an east–west river connection with the Nile. We consider this hypothesis unlikely, since it is based only on remote sensing data, and lacks any time control or geological supporting evidence. The abundant vertebrate fauna from the Sahabi Formation shows strong similarities with penecontemporaneous fossil faunas in Chad and is supportive of an origin of the Eosahabi River in Neogene mega-Lake Chad.

New insight into the sedimentology and stratigraphy of the Dur At Talah tidal-fluvial transition sequence (Eocene–Oligocene, Sirt Basin, Libya)

The Dur At Talah escarpment is exposed in the Abu Tumayam Trough at the southern part of the Sirt Basin, central Libya. The cliff (∼145m high and ∼150km long) is oriented along an E–W axis and faces southward. Only a few field studies have been previously carried out in this area, and these were mainly focused on the succession’s famous vertebrate fossil-content. The reconstruction of the depositional environments, which is the purpose of this paper, remained poorly documented. In this study, the uppermost Eocene rock succession composing the Dur At Talah escarpment is divided into two stratigraphic units: the New Idam Unit at the base composed of highly bioturbated fine sand/claystone alternations, and the Sarir Unit at the top dominated by medium to very coarse grading sometimes to microconglomeratic sandstones. This complete succession is built up of shallow marine (New Idam Unit) to fluvial (upper part of Sarir Unit) deposits passing through a “marine/fluvial” transition zone (lower Sarir Unit). The stratigraphic succession suggests a global regressive trend. The marine part of the New Idam Unit is dominated by deposits attributed to tidal depositional environments including tidal flat, tidal channel and tidal bars as well as biostroms of oyster shells at the base of the unit. The lower part of the Sarir Unit appears to be deposited in a fluvial influenced, tide-dominated environment. The upper part of the Sarir Unit, made of coarse-grained to microconglomeratic sandstones interbedded with paleosoil horizons, is interpreted as being fluvial.

New Hystricognathous Rodents from the Early Oligocene of Central Libya (Zallah Oasis, Sahara Desert): Systematic, Phylogenetic, and Biochronologic Implications

ABSTRACT Four fossil rodent taxa are described from a new locality in the Oligocene Continental and Transitional Marine Deposits outcropping in the vicinity of Zallah, Sirt Basin, central Libya. These rodents belong to the infraorder Hystricognathi Tullberg, 1899, and are distributed amongst four genera (Metaphiomys Osborn, 1908; Phiocricetomys Wood, 1968; Talahphiomys Jaeger et al., 2010; and Neophiomys , new genus) that include one new species, Phiocricetomys atavus, and one new combination, Neophiomys paraphiomyoides (Wood, 1968), formerly Phiomys paraphiomyoides Wood, 1968. The specimens described here have profound implications for the phylogenetic and systematic status of early hystricognathous rodents and paleogeographical debates regarding their origin. Based on a cladistic analysis, Waslamys attiai Sallam et al, 2009, is transferred to the genus Protophiomys Jaeger et al., 1985, as Protophiomys attiai (Sallam et al, 2009), new combination. That species is the type species of the monobasic genus W...

An integrated remote sensing and GIS analysis of the Kufrah Paleoriver, Eastern Sahara

A combined remote sensing (optical and radar imagery) and GIS (hydrologic network delineation) analysis allows mapping of the Kufrah Paleoriver of Libya and sheds light on its geomorphic evolution during the Neogene. The Kufrah system, which is now largely buried beneath the windblown sands of the Eastern Sahara, drained an area of about 236,000 km 2 in central and southern Libya. The river discharged across a large inland delta to the Al-Jaghbub depression in northern Libya, and ultimately through the Sirt Basin to the Mediterranean Sea. Radar imagery reveals buried features of the landscape including drainage divides, locations of possible stream capture, deeply-incised valleys, and the distal margins of the inland delta. Previous studies have shown that the Kufrah Paleoriver is the successor of the Sahabi River, which drained most of central Libya during the late Tertiary. Satellite imagery supports the concept of large-scale drainage rearrangement in the Quaternary, driven by tectonic subsidence that diverted streamflow and sediment discharge away from the Sahabi basin toward the inland delta of the lower Kufrah basin. Paleochannels crossing the delta suggest that at various times during the Quaternary, the Kufrah Paleoriver either drained externally through the deeply-incised Sahabi Paleochannel to the Mediterranean Sea, or drained internally to paleolakes in the Al-Jaghbub depression. Thick alluvial deposits on the delta and lake margins likely provided a major sediment source to build the Great Sand Sea, which covers the region today. The southwestern branch of the Kufrah drainage is aligned with an elongated trough that connects to the Amatinga River system in Chad. Thus the Kufrah watershed may have served as an outlet from Megalake Chad to the Mediterranean Sea during humid phases of the Neogene. If so, the combined Amatinga/Kufrah system may have served as one of the proposed natural corridors used by human and animal populations to cross the Sahara during the Pleistocene. These findings hold promise for modeling past lake levels and paleoclimates, locating groundwater sources in the region, and exploring for reservoirs of oil and natural gas in the region.

Impact of structural setting on diagenesis of fluvial and tidal sandstones: The Bahi Formation, Upper Cretaceous, NW Sirt Basin, North Central Libya

This petrographic, mineral chemical, fluid inclusion, and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the Upper Cretaceous Bahi Sandstones in the rift Sirt Basin, NW Libya varies systematically along a series of closely-spaced, dominantly normal faults between the basin margin and more basinward-located areas.Shallow eogenetic modifications resulting from the percolation of meteoric waters, include infiltration of grain coating clays, kaolinitization of detrital silicates, and cementation by dolomite and K-feldspar overgrowths. Mesogenetic alterations (>70 °C, >2 km) include feldspar albitization, illitization of infiltrated clay and kaolinite, conversion of kaolinite into dickite, and cementation by quartz overgrowths (Th 112 °C–134 °C), barite (Th 145 °C–158 °C) and Fe-carbonates. The restriction of barite and Fe-carbonate cements to the basinward-located sandstones suggests formation by hydrothermal fluids along the faults. Extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters. Results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and, by extension, reservoir quality in analogous epicratonic rift basins.

Late middle Eocene epoch of Libya yields earliest known radiation of African anthropoids

Reconstructing the early evolutionary history of anthropoid primates is hindered by a lack of consensus on both the timing and biogeography of anthropoid origins. Some prefer an ancient (Cretaceous) origin for anthropoids in Africa or some other Gondwanan landmass, whereas others advocate a more recent (early Cenozoic) origin for anthropoids in Asia, with subsequent dispersal of one or more early anthropoid taxa to Africa. The oldest undoubted African anthropoid primates described so far are three species of the parapithecid Biretia from the late middle Eocene Bir El Ater locality of Algeria and the late Eocene BQ-2 site in the Fayum region of northern Egypt. Here we report the discovery of the oldest known diverse assemblage of African anthropoids from the late middle Eocene Dur At-Talah escarpment in central Libya. The primate assemblage from Dur At-Talah includes diminutive species pertaining to three higher-level anthropoid clades (Afrotarsiidae, Parapithecidae and Oligopithecidae) as well as a small species of the early strepsirhine primate Karanisia. The high taxonomic diversity of anthropoids at Dur At-Talah indicates either a much longer interval of anthropoid evolution in Africa than is currently documented in the fossil record or the nearly synchronous colonization of Africa by multiple anthropoid clades at some time during the middle Eocene epoch.

New rodent assemblages from the Eocene Dur At-Talah escarpment (Sahara of central Libya): systematic, biochronological, and palaeobiogeographical implications

In this paper, we describe four fossil rodent taxa from two new localities situated in the Idam Unit ('Bioturbated Unit) of the Dur At-Talah escarpment in central Libya These rodents belong to the family Phiomyidae (Hys tricognathi) and are distributed amongst three genera (Phiomys, Protophiomys, and Talahphiomys gen. nov.) that include three new species (Phiomys hammudai sp. nov., Protophiomys durattalahensis sp. nov., and Talahphiomys libycus sp. nov.). Although some of these new species are inorphologically close to certain phiomyids from the latest Eocene and Oligocene of the Jebel elQatrani Formation of the Fayum in Egypt, the Idam rodent faunas lack the abundant and somewhat morphologically derived Fayum phiomyids (such as: Metaphiomys, Gaudearnus, Paraphiornys, Phiocricetomys), thereby excluding a similar age for the Dur At-Talah rodent assem blages. More resemblance is shared with the phiomyid (Protophiomys algeriensis) of the Nementcha locality in Algeria, for which a late middle Eocene age is presently admitted. Protophiomys is a primitive representative of the phiomyid African radiation and it is represented at Dur At-Talah by a slightly more derived species (Pr. durattalahensis) than that of Nementcha, thereby suggesting a younger age for Dur AtTalah. As a result, the new rodent assemblages suggest a late middle Eocene age for the Idam ('Bioturhated) deposits of the Dur At-Talah escarpment. This age hypothesis is substantiated by other mammals (especially Proboscidea), which occur in the same sedimentological unit. Interestingly, the dental pattern of Protophiornys and that of Talahphzomys have somewhat stronger affinities with South Asian hystricognath baluchimyines than with Fayum phiomyids. It is clear that haluchimyines and phiomyids have a common ancestry, and that dispersal occurred between Asia and Africa during the middle of the Palaeogene. However, it is not clear if both groups can be strictly separated in two distinct natural groups inasmuch as some haluchimyines (e.g Lophthaluchia, Bugtirnys, Hodsahibia) appear to be phioinyid4ike, and some early members of phiomyids (e.g. Protophiomys, Talahphiomys) are baluchimyine4ike. South Asia and North Africa represent two centres of adaptive radiation of early hystricognathous rodents. The strong dental resemblances between early Asian and African forms are perhaps the result of subsequent convergent evolution after an initial dispersal from Asia, Otherwise, the systematics of these rodents has to be entirely revised, or we must consider that their historical biogeography is much more complex. (C) 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160, 195-213.

Gaudeamus lavocati sp. nov. (Rodentia, Hystricognathi) from the early Oligocene of Zallah, Libya: first African caviomorph?

A new African species of hystricognathous rodent, Gaudeamus lavocati sp. nov., is described herein from the early Oligocene deposits of Zallah locality (Sirt basin, Central Libya). The dental morphology of this species is very close to that of some earliest South American caviomorphs. It allows a reinterpretation of molar crest homologies among earliest caviomorphs, pentalophodonty being confirmed as the plesiomorphic molar condition in Caviomorpha. This morphological resemblance argues for close affinities between Gaudeamus and earliest South American hystricognaths. Cladistic analysis supports Gaudeamus lavocati sp. nov. as the first known African representative of Caviomorpha, implying that its ancestors were part of the African phiomyid group that crossed the South Atlantic by a direct immigration route. Alternatively, the series of derived dental features of Gaudeamus could also be interpreted as evolutionary synchronous convergences of an African hystricognath lineage towards the specialized pattern of some caviomorphs. However, the high level of similarities concerning teeth morphology and enamel microstructure and the similar age of fossiliferous strata on both continents make this interpretation less probable. The phylogenetic position of this taxon is of considerable importance because it represents an enigmatic component of the phiomorph-caviomorph radiation in Africa and appears as a new clue toward the understanding of caviomorph origins.

Between past, present and future: The Sudan's role as a middle-stream country

ABSTRACT Extending over nearly one million square miles and being the biggest country in Africa in terms of geography, the Sudan occupies a middle position in the Nile Basin, between the upstream countries and Egypt. All of the Nile countries except Tanzania, Rwanda and Burundi, are neighbours of the Sudan. Other Sudan non-riparian neighbours are the Central African Republic, Chad and Libya. The purpose of this article is to explain the factors related to the Nile waters that developed during the colonial period, the post-independence era and the present, that prevented the Sudan from playing a crucial central role in the Nile Basin. The article argues that the 1959 bilateral Nile Waters Agreement between the Sudan and Egypt, in addition to civil wars and the economic and political instability that the Sudan has witnessed since independence in January 1956, has prevented it from playing a crucial role as a middle-stream country. The article takes the form of a description of a historical process with analytical insights.

Petrogenesis of the eastern part of the Al Haruj basalts (Libya)

The eastern part of the Al Haruj volcanic field (central Libya) is dominated by lava flows with subordinate dykes as well as scoria and spatter cone deposits which formed around 4–0.5 Ma. These rocks are olivine- clinopyroxene- and rarely plagioclase-phyric alkaline to transitional basalts with intersertal to intergranular groundmass. They are fairly primitive rocks (MgO and Ni contents ∼10 wt.% and >130 ppm, respectively). A positive correlation between MgO contents and Ni and Cr concentrations is suggestive of olivine (±spinel, ±Fe–Ti-oxide) fractionation. The rocks show LREE- and incompatible trace element enriched patterns on chondrite- and primitive mantle-normalized spider diagrams, respectively. Geochemical modeling revealed that the composition of most East Al Haruj basalts can be reproduced by ∼5% melting of an undepleted, garnet-bearing peridotitic source, similar in composition to primitive asthenospheric mantle and typical of within-plate setting. The most incompatible element enriched samples could either represent extremely small degrees of melting of a similar source or, more probably, magmas derived from a more enriched, (lithospheric?) source. East Al Haruj magmatism occurred from predominantly asthenospheric-derived melts which formed and transported along fundamental sub-lithospheric faults of the Tibesti-Sirt basement. These faults were likely related to the late-stage evolution of the Sirt Basin characterized by NW–SE compression and perpendicular tension.

Preliminary study on the import of lavic millstones in Tripolitania and Cyrenaica (Libya)

We present the results of the first archeometric study of some Roman millstones and mortars made of volcanic rocks found at the Libyan archeological sites of Leptis Magna and Cyrene. Two millstones from the first site dating from the III century AD and one from the second, also dating from the II–III centuries AD, were found to made from leucite–phonolite lavas from the quarrying area near Orvieto, in the Vulsini Volcanic District (Roman Volcanic Province). These findings provide further confirmation of the central role played by Orvieto in the Roman production and export of millstones. Mugearite lavas from Etna, transitional basalt from the island of Pantelleria, alkaline basalt, probably from Al Haruj-Al Aswad (Central Libya), and high-K calc-alkaline andesite of uncertain geological origin were the other rocks used for the millstones investigated in this study. This evidence clearly indicates a trade in millstones between Tripolitania, Cyrenaica and Central-South Italy.

The importance of remote sensing techniques in surface geologic mapping

The role of remote sensing is well known and can be successfully applied as an initial step in surface geologic exploration. The aim of this study, by means of Landsat TM 7 image interpretation, is to assist photogeologic map compiling, which will serve as a core document for an upcoming field mapping exercise. The mathematical background and the wide variety of math-method possibilities offered by a satellite image interpretation software were used for this purpose. In this way the requested preliminary photogeologic map was compiled, based on the spectral properties of the surface geologic formations. Fortunately the areas of interest were in a desert area (Central Libya), so there was no limitation by vegetation coverage to hinder the application of spectral properties to geologic formations. After the geocoding of the images the display of different compositions of the Landsat TM 7 registration channels was begun. Contrast enhancement and histogram fitting were applied, and further operations to highl...

Palaeovalleys and fault-controlled depocentres in the Late-Ordovician glacial record of the Murzuq Basin (central Libya)

Abstract In the Gargaf area, the stratigraphic architecture of the Late-Ordovician glacial drift results from successive glacial erosion events, with the location of the main glacial valleys partly controlled by inherited Panafrican structural trends, and by the existence of glacio-isostatically induced fault-related depocentres. Four laterally discontinuous, depositional units correspond to the filling of palaeovalleys. Each of the corresponding basal bounding surfaces was incised during a major ice front advance, reaching at least the northern Gargaf (>28°S). The bulk of the glacial record is made up of fluvial to shallow-marine sediments deposited in relatively distal glacial environments. Each unit, which recorded a glacial-interglacial climatic cycle, can be used for correlation throughout the Murzuq Basin and even at the scale of the North Gondwana platform. To cite this article: J.-F. Ghienne et al., C. R. Geoscience 335 (2003).

The Vischer family archives

AbstractUntil the end of the 19th century, the camel road from Kukawa near Lake Chad in Northern Nigeria to Tripoli in Libya was essentially a slave route. Only the most robust slaves survived the desert march which usually ended in the great slave market of Murzuk in central Libya. Prior to John Hare in 2001/2002, the last foreigner known to have followed the road in its entirety was the Swiss-born, British national Hanns Vischer, in 1906.This article, which quotes from the Vischer family archive, highlights and compares the differences that have arisen in undertaking this journey with camels after a lapse of 100 years. It reveals how Vischer was attacked by Tuaregs at Tajirhi in southern Libya and his encounters with imprisoned Turkish and Bulgarian revolutionaries and miscreants in Murzuk. It also highlights the dangers to a foreigner of undertaking this journey at a time when the Ottoman empire was crumbling and the French and the British were striving to carve out their colonial territories in the vast Saharan and sub-Saharan regions of Africa.

Petrography and palynology of quartzites in the Sirte Basin, central Libya

Petrographic and palynological analysis and wireline log correlations of the so-called pre-Upper Cretaceous ‘quartzites’ in the central Sirte Basin permits, for the first time, the recognition of the development of six types of quartzites and sandstones of Late Cambrian, Late Devonian, Devonian-Carboniferous and Early Cretaceous age. The Late Cambrian section age, dated by the presence of associated marine acritarchs Vulcanisphaera turbata and Retisphaeridium dichamerum, comprises two shallow marine quartzite units separated by a varicoloured argillaceous feldspathic sandstone unit (Red Beds). The two quartzite units show tidal bedding and mud couplets. They were probably deposited in a subtidal environment. They form fractured reservoirs in the giant Attahaddy Gas Field. The varicoloured sandstone unit (Red Beds) is rich in Skolithos and was probably deposited in an intertidal environment. A Late Devonian (Strunian) shallow marine to continental sandstone unit age dated by the marine acritarch Gorgonisphaeridium solidum and the land-derived spore Verruciretusispora cf. famenensis has been identified in wells V1-59 in the southwest of the study area and EE1-59 in the southeast. A Carboniferous (Visean) sandstone unit age dated by the land-derived spores Aratrisporites saharaensis and Calamospora sp. has been identified in wells C3-6, KKKK1-6, BBBB2-6 and V1-59. The Early Cretaceous continental fractured quartzite unit dated by tricolpate pollen produces oil in the Wadi Field. The study reveals that the Devonian and Carboniferous sandstones, with good primary porosity, previously considered Nubian and Cambro-Ordovician, respectively, form an attractive hydrocarbon-bearing trend in the southern part of the study area.

PHYSICOCHEMICAL CHARACTERIZATION OF CRUDE OIL FRACTIONS BY ARTIFICIAL NEURAL NETWORKS

ABSTRACT This paper presents a novel approach to the problem of characterization of petroleum fractions. An artificial neural network consisting of a three-layer perceptron is used; to predict volume and weight yields, viscosity, specific gravity and sulphur content. The network was trained using assay data relative to crude oils from central Libya and south-west Iran. After training, the predictive capabilities of the perceptron were tested on systems not included in the learning set. In addition, a comparison was made with the estimates provided by a widespread crude-oil evaluation procedure. The results obtained indicate that accuracies can be achieved that are even better than those derived from current estimation methods.