Sirte Basin Research Articles

Regional geological and tectonic structures of the Sirt basin from potential field data

The area involved in this study is Sirt basin, which is located in the north central part of Libya within the bounder's 27N-33N and 16E-22E. The study involves the analysis of gravity and magnetic data to delineate structures and faults and to locate any major structures. The produced Bouguer gravity map shows prominent NW-SE and N-NW trends. Isostatic residual map is characterized by a dominant NW– SE trend in the study area. This is clearly evident in the isostatic residual. The main trending anomalies are in the northern and southeastern parts of the study area with NW-SE orientation. A strong NW-SE trend is truncated by E-W trending in the southeastern and southwestern parts of the area. This is consistent with change of tectonic zones. The magnetic expression in the northern part of Ajdabiya trough is characterized by NW-SE trending structures which coincide with late Cretaceous structures of the Sirt basin, while the southern part is characterised by NE-SW trending features which coincide with a late Paleozoic trend. The northern part of the Ajdabiya trough is separated from the southern part by a prominent NE-SW lineament that is expressed in both the gravity and magnetic data. It is interpreted as a basement fault, which separates a thicker southern crust from a thinner northern crust. The high gravity anomaly within the northern part of the Ajdabiya trough is interpreted as a result of mantle upwelling which caused thinning of the continental crust beneath the northern part of the Ajdabiya trough. The Total horizontal derivative results of Gravity, 3D Euler Deconvolution of gravity and magnetic data magnetic anomalies produced features trending similar to the positions of tectonic and geological information from the Sirt basin. High gradient values delineate NNW-SSE to N-S and NW-SE trends which mark the faulted southwestern, southern, northern and central boundaries of the basin, respectively. New faults with orientations NNW-SSE trends along the southwestern flank of the Sirt basin and is truncated by E-W faults dividing it into segments. Strong N-S lineaments occur over the southern and central part of study area and are well indicated by the 3D Euler Deconvolution. From this study the 3D Euler Deconvolution provides very useful information of the rift structures.

Seismically driven characterization of vuggy porosity and fractures in a carbonate field, Sirte Basin, Libya

Mahdi Hammad, Abdulhamed Shahlol, Sobhi Hajaj, Amares Aoues, Ahmed Ouenes, Hamed El Werfali, Fawzi BuArgoub and Anthony Kirkham describe a workflow that fully utilizes pre-stack seismic attributes to derive reliable geologic and fracture models for a carbonate field in the Sirte Basin, Libya, validated by blind well testing and actual wells drilled after the study.

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.

Use of biomarker distributions and compound specific isotopes of carbon and hydrogen to delineate hydrocarbon characteristics in the East Sirte Basin (Libya)

Biomarker ratios, together with stable carbon (δ 13C) and hydrogen (δD) isotopic compositions of individual hydrocarbons have been determined in a suite of crude oils ( n = 24) from the East Sirte Basin to delineate their sources and respective thermal maturity. The crude oil samples are divided into two main families (A and B) based on differences in source inputs and thermal maturity. Using source specific parameters including pristane/phytane (Pr/Ph), hopane/sterane, dibenzothiophene/phenanthrene (DBT/P), Pr/ n-C 17 and Ph/ n-C l8 ratios and the distributions of tricyclic and tetracyclic terpanes, family B oils are ascribed a marine source rock deposited under sub-oxic conditions, while family A oils have a more terrigenous source affinity. This genetic classification is supported by the stable carbon isotopic compositions (δ 13C) of the n-alkanes. Using biomarker maturity parameters such as the abundance of Pr and Ph relative to n-alkanes and the distribution of sterane and hopane isomers, family A oils are shown to be more thermally mature than family B oils. The contrasting maturity of the two families is supported by differences between the stable hydrogen isotopic compositions (δD) of Pr and Ph and the n-alkanes, as well as the δ 13C values of n-alkanes in their respective oils.

Modelling of paleo-saltwater intrusion in the northern part of the Nubian Aquifer System, Northeast Africa

A numerical groundwater model of the Nubian Aquifer System was established to prove the influence of rising seawater levels on the groundwater salinity in northern Egypt over the last 140,000 years. In addition, the impact of a groundwater recharge scenario for these 140,000 years, involving climatic change, on the saltwater/freshwater interface was investigated. Saltwater intrusion induced by rising water levels of the Mediterranean Sea led to salinisation from the Mediterranean Sea to the Qattara depression. This modeling approach was supported by a density-driven model setup and calculation. The modelled saltwater/freshwater interfaces partially fitted the observed ones, especially in the southern half of the Qattara depression. In other parts of the northern Nubian Aquifer System, the ingression of salt water was modelled adequately, but in the west, small regions of the measured interface were not. The development in the Qattara depression (Egypt) and Sirte basin (Libya) were investigated in more detail. The different behaviour in the Sirte basin may be due to high evapotranspiration rates in some former periods, salt solutions from the pre-Quaternary layers or saltwater infiltration from sabkha-like recent salt-bearing sediments.

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.

Archaeology in the Sirte Basin: Preliminary Results of Mitigation Surveys carried out for Shell 2007–2009

AbstractThis paper presents in summary form the results of three years of remote sensing and ground-truthing of archaeological sites falling within concession areas held by Shell Libya in the Sirte Basin. This area spans the frontier of Tripolitania and Cyrenaica, ranging from Wadi ben Jawwad in the west to Ajdabiyah in the east and south to Jebel Zelten. The work was commissioned by Shell to inform them of archaeological sensitivities during their ongoing operations in the region. The results presented here include the identification of thousands of previously unknown sites of all periods, dating from the Lower Palaeolithic to the Second World War. Prehistoric discoveries include significant numbers of Middle Palaeolithic sites, particularly at Jebel Zelten, and large numbers of Pastoralist Neolithic sites in the coastal hinterland and on palaeoshorelines around sabkhas further north. Traces of Libyan tribes of the first millennium BC have been found in the form of large numbers of pastoralist desert campsites. Extensive evidence for Roman and late antique settlement has been found in the coastal hinterland on both sides of the Gulf of Sirte, as well as evidence for indigenous ‘long hut’ settlements along the desert fringe.

From Neoproterozoic to Early Cenozoic: exploring the potential of older and deeper hydrocarbon plays across North Africa and the Middle East

Abstract As the traditional exploration plays in the main productive basins of North Africa and the Middle East become more ‘mature’, attention is increasingly focusing on more challenging, older and deeper plays in the main producing basins and on high-risk, but more conventional, plays in under-explored frontier areas. This shift brings with it a range of technical and commercial challenges that must be addressed, if exploration in the region is to remain an attractive proposition. Exploration in North Africa and the Middle East has traditionally focused on the prolific Mesozoic- and Cenozoic-sourced petroleum systems of the Nile Delta, the Sirte Basin, the Pelagian Shelf, and the Arabian Plate and on the Palaeozoic-sourced petroleum systems of the Berkine, Ghadames, Illizi, Ahnet and Murzuq basins, the Central Arabian Basin, the Qatar Arch and the Rub Al Khali Basin. Together these form one of the most prolific petroleum provinces in the world and, as a consequence, there has been little commercial incentive to invest in exploring more challenging and riskier plays in these areas. However, as the need to find new reserves becomes imperative, attempts are increasingly being made to test new play concepts and to extend already proven plays into new areas. Key recent developments in this regard include the recognition of the hydrocarbon potential of the Neoproterozoic to Early Cambrian (‘Infracambrian’) sedimentary section lying below the traditionally explored Palaeozoic succession in many basins in North Africa. In some areas, particularly the Berkine Basin in Algeria, the Nile Delta in Egypt and the Rub Al Khali Basin in Saudi Arabia, attention is also increasingly being focused on developing deeper gas plays, both in new areas and beneath existing producing fields. The technical challenges associated with these deeper gas plays are immense and include difficult seismic imaging of deep prospects, low porosity and permeability, high temperature and pressure and a critical need to identify ‘sweet spots’ where either locally preserved primary reservoir characteristics or secondary enhancement of reservoir quality through palaeo-weathering and/or fracturing allow commercial rates of gas production to be achieved. Despite these challenges, it is clear that the future for exploration in many of the more mature basins of North Africa and the Middle East will increasingly lie in evaluating such older and more deeply-buried plays.

From thin sections to discovery: the Upper Satal in the central Sirte Basin (north-central Libya) a success story

From thin sections to discovery: the Upper Satal in the central Sirte Basin (north-central Libya) a success story

Reflection images from ambient seismic noise

One application of seismic interferometry is to retrieve the impulse response (Green’s function) from crosscorrelation of ambient seismic noise. Various researchers show results for retrieving the surface-wave part of the Green’s function. However, reflection retrieval has proven more challenging. We crosscorrelate ambient seismic noise, recorded along eight parallel lines in the Sirte basin east of Ajdabeya, Libya, to obtain shot gathers that contain reflections. We take advantage of geophone groups to suppress part of the undesired surface-wave noise and apply frequency-wavenumber filtering before crosscorrelation to suppress surface waves further. After comparing the retrieved results with data from an active seismic exploration survey along the same lines, we use the retrieved reflection data to obtain a migrated reflection image of the subsurface.

The opening of Sirte basin: Result of slab avalanching?

North Africa's Sirte basin opening is an enigmatic feature in the complex Meso-Cenozoic rearrangement of Mediterranean tectonics. New borehole data inversion constrains its deformation history showing a stretching event starting ~ 70 Ma and terminating in a further abrupt increase at ~ 50 Ma, rapidly fading afterwards. The timing of this event hardly reconciles with the Mesozoic major plates reorganization following the spreading of the Atlantic, and the Neogene Central Mediterranean tectonics, active at different times. Here, we propose that Sirte rifting could have been driven by the pull exerted by the Hellenic subduction. Reconstructions of Hellenic convergence and slab deep subduction, as constrained by plate kinematics and tomography, show that large slab mass accumulates in the upper mantle by late Cretaceous–Paleogene, eventually forcing further sinking in the lower mantle, coeval to the growing strain evolution recorded in Sirte. Furthermore, the ~ 20 m.y. phase of large pull here recorded, the consequent rapid growth over ~ 10 m.y. and following decay are compatible with the dynamics of slab avalanche as revealed by numerical models, showing that the Sirte basin opened in response to the large pull force developed during the mass flush, and transferred from deep slab to surface.

Multilateral Horizontal Well Improves Oil Recovery in a Mature Field in Libya

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 112125, "Multi Lateral Horizontal Well Application for Improving the Oil Recovery of a Mature Field, Intisar N. Field, Concession 103 - Libya," by M.M. Gharsalla and M.B. Elghmari, Zueitina Oil Company, originally prepared for the 2008 SPE North Africa Technical Conference and Ex hib ition, Marrakech, Morocco, 12-14 March. The paper has not been peer reviewed. The full-length paper examines the performance of a multilateral completion in a mature thin carbonate reservoir. The initial field development with two vertical wells resulted in a poor recovery of only 9% of the original oil in place. A reservoir-simulation study in 2003 indicated that drilling vertical wells in a very thin reservoir of poor quality was not cost-effective and recommended drilling horizontal wells. The stabilized oil-production rate of the multilateral horizontal well was approximately three times higher than that of the vertical well. Introduction Intisar N field is in Concession 103 in Libya approximately 360 km south of Benghazi in the prolific Sirte basin. The field was discovered in 1968 by drilling Well N1 to a total depth of 10,318 ft in the Lower Sabil. The Intisar N field comprises two separate carbonate reservoirs, the Upper Sabil (Zelten) at a depth of 9,230 ft and Lower Sabil (Heira) at 9,820 ft, approximately 600 ft apart. Fig. 1 shows a map of formation tops of the Upper Sabil and Lower Sabil formations. Both reservoir structures are composed of two anticlinal saddles believed to be separated by a fault. The field is relatively small in size, with initial oil in place of 19.0 million STB of which 14.4 million STB is contained in the Zelten (Upper Sabil) reservoir and 4.6 million STB in the Heira (Lower Sabil) reservoir. Stratigraphy and Trapping Mechanism The general stratigraphy and structure in the Concession 103 area has been controlled by the shelf edge in Upper Paleocene times between the Marsa Brega Trough and the Gialo/Amal high. Reefal bioherms developed on this shelf edge through the Upper Paleocene, after which there was a transgression or subsidence in the very late Paleocene resulting in the deposition of the Kheir marls and shale. During the Eocene, there was predominantly shelf carbonate deposition, with the grain size determined by provenance and water depth. Locally, the sedimentation has been influenced by drape over the reef, resulting in locally shallow water depths in the immediate area of the reef, resulting in rather gentle homoclines providing up to 50 to 70 ft of closures. The oil accumulations at Concession 103N in the Zelten and Heira reservoirs are trapped by the homoclines. Petrophysical Properties The reservoir quality varies laterally significantly in both the Upper and Lower Sabil; the quality improves toward the crest and deteriorates toward the flanks. Most parts of the Upper Sabil formation have fair porosity, and good solution porosity is locally present. The oil systems in the Upper and Lower Sabil reservoirs are light and highly undersaturated.

Infracambrian sediments in Libyan sedimentary basins

Abstract Infracambrian sediments are widely distributed in Libya, outcropping on the eastern and western margins of Al Kufrah Basin and the eastern margin of Murzuq Basin. The sediments have been penetrated in the Central Cyrenaica Platform, Concession 10, NW Sirte Basin, and Block NC115, NW Murzuq Basin. There are two main subdivisions. The first is metamorphosed due to local volcanism in the Murzuq and Al Kufrah basins. The second is unaltered and has been penetrated in the NW Sirte Basin. It occurs as outcropping limestone on the eastern margin of the Murzuq Basin. These sediments generally show lateral thickness variability, with the thickest section, approximately 991–1067 m, in the Cyrenaica Platform. Individual units show thinning towards the Precambrian basement highs, and both fining-up and coarsening-up successions. Two-dimensional seismic data acquired by the operating companies AGIP and AGOCO in the southern Al Kufrah Basin image strata presumed to be Infracambrian. The Infracambrian sediments were probably deposited as lens-shaped bodies in palaeo-lows (graben, half-graben and troughs) alongside Precambrian basement highs, trending NW–SE in the Cyrenaica platform and NE–SW in the Al Kufrah and Murzuq basins. These sediments show lateral facies changes and their nomenclature differs across the basins. Palynological and palaeontological studies suggest a Late Riphean age.

Neoproterozoic-Early Cambrian (Infracambrian) hydrocarbon prospectivity of North Africa: a synthesis

Abstract Despite the existence of proven Neoproterozoic–Early Cambrian (‘Infracambrian’) hydrocarbon plays in many parts of the world, the Neoproterozoic Eon, from 1000 Ma to the base of the Cambrian at 542 Ma, is relatively poorly known from a petroleum perspective. The so-called ‘Peri-Gondwanan Margin’ is one region of the Neoproterozoic world that is exciting particular interest in the search for ‘old’ hydrocarbon plays, mainly due to exploration success in time-equivalent sequences of Oman. The ‘Infracambrian’ succession in North Africa is widely accessible, and is already emerging as a hydrocarbon exploration target with considerable potential and with proven petroleum systems in different areas. The Taoudenni Basin (Mauritania, Mali, Algeria) in western North Africa is an underexplored basin, despite the Abolag-1 well (Texaco 1974) gas discovery. New palynological data have recently provided the first definitive Late Riphean age dates for the stromatolitic limestone reservoir sequence in Abolag-1. The widespread presence of stromatolitic carbonate units of potential reservoir facies in many parts of North Africa has been confirmed by new fieldwork in the Taoudenni Basin, in the Anti-Atlas region of Morocco and in the Al Kufrah Basin of Libya. Similar biostratigraphic age constraints have also been obtained from subsurface sequences of the Cyrenaica Platform bordering the East Sirte Basin of Libya, many of which have been traditionally assigned an ‘unconstrained’ Cambro-Ordovician age on the basis of lithological characteristics. Besides the proven, producing, weathered-granite reservoir in East Sirte Basin, the hydrocarbon potential of Neoproterozoic–Early Cambrian sequences developed in structural troughs bordering the south Cyrenaica Platform is still being evalutated. Neoproterozoic–Early Cambrian organic-rich strata with hydrocarbon source rock potential are widespread along the Peri-Gondwanan Margin. Some of the black shales encountered on the West African Craton may be as old as 1000 Ma and predate the Pan-African orogenic event. The Late Ordovician–Early Silurian systems in North Africa and the Middle East may form a good analogue for post-glacial source rock depositional systems in the Neoproterozoic, where black shale deposition may also have been triggered by post-glacial sea-level rise.

Mapping of a major paleodrainage system in eastern Libya using orbital imaging radar: The Kufrah River

Over the last few decades, remote sensing has revealed buried river channels in a number of regions worldwide, in many cases providing evidence of dramatic paleoenvironmental changes over Cenozoic time scales. Using orbital radar satellite imagery, we mapped a major paleodrainage system in eastern Libya, that could have linked the Kufrah Basin to the Mediterranean coast through the Sirt Basin, possibly as far back as the middle Miocene. Synthetic Aperture Radar images from the PALSAR sensor clearly reveal a 900 km-long river system, which starts with three main tributaries (north-eastern Tibesti, northern Uweinat and western Gilf Kebir/Abu Ras) that connect in the Kufrah oasis region. The river system then flows north through the Jebel Dalmah, and forms a large alluvial fan in the Sarir Dalmah. The sand dunes of the Calanscio Sand Sea prevent deep orbital radar penetration and preclude detailed reconstruction of any possible connection to the Mediterranean Sea, but a 300 km-long link to the Gulf of Sirt through the Wadi Sahabi paleochannel is likely. If this connection is confirmed, and its Miocene antiquity is established, then the Kufrah River, comparable in length to the Egyptian Nile, will have important implications for the understanding of the past environments and climates of northern Africa from the middle Miocene to the Holocene.

Tectonics and subsidence evolution of the Sirt Basin, Libya

A backstripping analysis of 225 wells located within the Sirt Basin, Libya, provides new constraints on the development of the Sirt Basin. Four tectonic phases are identified from Late Jurassic to present. The presentation of contour maps of subsidence and crustal stretching allows spatial and temporal variations in stretching to be visualized. A close match is observed with stretching phases documented for other African basins, consistent with discrete phases in the opening of the Tethys and Atlantic. Rifting and reactivation appear to be primarily controlled by the orientation of the basin and the underlying basement structure with respect to stress directions. The tectonic subsidence curves have also been forward modeled with an automated modeling technique to quantify the variation in timing and the magnitude of rifting. The tectonic subsidence history of the Sirt Basin is characterized by periods of stretching, alternating with periods of relative tectonic quiescence and thermal subsidence. Stretching started at the centers of the troughs and migrated toward the platform crests.

Palaeohydrology of the Fazzan Basin and surrounding regions: The last 7 million years

We have integrated information on topography, geology and geomorphology with the results of targeted fieldwork in order to develop a chronology for the development of Lake Megafazzan, a giant lake that has periodically existed in the Fazzan Basin since the late Miocene. The development of the basin can be best understood by considering the main geological and geomorphological events that occurred thought Libya during this period and thus an overview of the palaeohydrology of all Libya is also presented. The origin of the Fazzan Basin appears to lie in the Late Miocene. At this time Libya was dominated by two large rivers systems that flowed into the Mediterranean Sea, the Sahabi River draining central and eastern Libya and the Wadi Nashu River draining much of western Libya. As the Miocene progressed the region become increasingly affected by volcanic activity on its northern and eastern margin that appears to have blocked the River Nashu in Late Miocene or early Messinian times forming a sizeable closed basin in the Fazzan within which proto-Lake Megafazzan would have developed during humid periods. The fall in base level associated with the Messinian desiccation of the Mediterranean Sea promoted down-cutting and extension of river systems throughout much of Libya. To the south of the proto Fazzan Basin the Sahabi River tributary know as Wadi Barjuj appears to have expanded its headwaters westwards. The channel now terminates at Al Haruj al Aswad. We interpret this as a suggestion that Wadi Barjuj was blocked by the progressive development of Al Haruj al Aswad. K/Ar dating of lava flows suggests that this occurred between 4 and 2 Ma. This event would have increased the size of the closed basin in the Fazzan by about half, producing a catchment close to its current size (~ 350,000 km 2). The Fazzan Basin contains a wealth of Pleistocene to recent palaeolake sediment outcrops and shorelines. Dating of these features demonstrates evidence of lacustrine conditions during numerous interglacials spanning a period greater than 420 ka. The middle to late Pleistocene interglacials were humid enough to produce a giant lake of about 135,000 km 2 that we have called Lake Megafazzan. Later lake phases were smaller, the interglacials less humid, developing lakes of a few thousand square kilometres. In parallel with these palaeohydrological developments in the Fazzan Basin, change was occurring in other parts of Libya. The Lower Pliocene sea level rise caused sediments to infill much of the Messinian channel system. As this was occurring, subsidence in the Al Kufrah Basin caused expansion of the Al Kufrah River system at the expense of the River Sahabi. By the Pleistocene, the Al Kufrah River dominated the palaeohydrology of eastern Libya and had developed a very large inland delta in its northern reaches that exhibited a complex distributary channel network which at times fed substantial lakes in the Sirt Basin. At this time Libya was a veritable lake district during humid periods with about 10% of the country underwater.

Cyrenaican “shock absorber” and associated inversion strain shadow in the collision zone of northeast Africa

Far-field compressional stresses resulting from arc collisions with the northeast coast of Africa-Arabia propagated across the African plate in the Santonian (ca. 84 Ma), and later similar pulses occurred through much of the Cenozoic. Waves of inversion-related features including folds, thrusts, and igneous activity developed during the compressional events. Structures generated were uneven in intensity and distribution, and were commonly concentrated in existing rifts. Because of the irregular shapes of the North African continental margin and associated inboard basins, Cyrenaica underwent the most severe shortening, although deformation extended eastward from Libya to the Levant. Areas south and southeast of Cyrenaica (the Sirt Basin and the far Western Desert of Egypt) underwent very little shortening because they occupied a regional strain shadow. The eastern region of the Western Desert and Sinai, which were not sheltered by the stress-absorbing Cyrenaica inversion, recorded strong compressional deformation synchronous with that in Cyrenaica. As closure of the Mediterranean continues, Cyrenaica will become an indenter comparable to those of other orogenic belts. The detailed tectonic evolution of individual northeast African basins has depended on their positions in relation to the Cyrenaican strain shadow. No single subsidence and paleostress history reflects the complete regional development of the converging Mediterranean–North African margin.

Grapsoid crabs (Crustacea: Decapoda: Brachyura) new to the Sirte Basin, southern Mediterranean Sea—the roles of vessel traffic and climate change

Grapsoid crabs (Crustacea: Decapoda: Brachyura) new to the Sirte Basin, southern Mediterranean Sea—the roles of vessel traffic and climate change

Practical Approach To Achieve Accuracy in Sanding Prediction

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 100944, "Practical Approach To Achieve Accuracy in Sanding Prediction," by K. Qiu, SPE, J.R. Marsden, SPE, J. Alexander, SPE, and A. Retnanto, SPE, Schlumberger, and O.A. Abdelkarim, SPE, and M. Shatwan, Agoco, prepared for the 2006 SPE Asia Pacific Oil & Gas Conference and Exhibition, Adelaide, Australia, 11–13 September. Sand production can reduce oil production, cause erosion in downhole and surface facilities, require additional separation and disposal, and lead to significant economic loss. Precautionary, but unnecessary, sand prevention will result in unwarranted reduction in productivity. Overestimating or underestimating sanding risk increases the chances of serious economic loss. Reliable sanding-prediction analysis provides a basis for designs that achieve appropriate sand-management strategies and maximize economic production. Introduction The giant Messla field is in the southeast portion of Sirte basin in Libya, approximately 500 km southeast from Benghazi. The field has been producing for more than 30 years, and since the mid-1980s, some wells have suffered massive sanding, while others have not. A geomechanics and sanding study was initiated in 2005 to investigate this variation, to evaluate the severity of sanding risk in other wells, and to provide information and interpretations needed to design appropriate completions, maximize economical production, and optimize future reservoir management. Sanding analyses were conducted by use of a proprietary sanding-prediction application. This application incorporates a novel analytical model that integrates a simple linear elastic analysis and then accounts for rock failure, plasticity effects, and scale effects. Information is from simple laboratory tests and the results of previous extensive numerical modeling and field validation. Significant factors contributing to sand production include stresses, rock strength, draw-down and depletion, and completion type and geometry so that the analysis achieves practicability and fit-for-purpose accuracy. The implementation allows modifying the analytical elastic calculations to achieve a more realistic and accurate prediction. The remaining major uncertainties in the predictive capacity of the model arise from uncertainties of the input parameters rather than from the model itself. Therefore, to achieve the desired accuracy in the analyses and predictions, attention should focus on decreasing uncertainties of the input parameters. Sanding-Prediction Approaches There are three main sources of uncertainty in sanding prediction: the predictive model itself, the input data for the model, and processing uncertainties such as truncation and round-off errors. Reliable analyses are obtained by generating a well-constrained mechanical Earth model (MEM) to minimize uncertainties in the input data, by accounting for all the main influences, and by simplifying processing. Empirical Approach. This popular approach relies on data collected from the field, possibly supplemented by laboratory experiments on cores. In most instances, only one or two parameters are used to assess sanding risk and to establish cutoffs for conditions of sanding or no sanding. For example, a cutoff depth or a cutoff compressional-slowness criterion might be used to determine whether or not sand control might be needed in a particular field.