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3rd EAGE North African/Mediterranean Petroleum and Geosciences Conference and Exhibition
- Conference date: 26 Feb 2007 - 28 Feb 2007
- Location: Tripoli, Libya
- ISBN: 978-90-73781-42-9
- Published: 26 February 2007
21 - 40 of 100 results
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Large-Scale Physiography of the Murzuq Basin Shelf during Hirnantian Ice-Sheet Final Retreat and Silurian Transgression
Authors J. Moreau, L. Degermann, J.F. Ghienne and J.L. RubinoThe Hirnantian glaciation strongly affected the lower Palaeozoic sedimentary succession of the Murzuq Basin (Libya). After final deglaciation, the remnant topography displays imprints of most of the ice-sheet retreat evolution. One large ice stream pathway, in the form of a wide erosional trough including surbordinate higher areas, has been recognized. Several ice-fronts and related proglacial fan-deltas are localized at basin scale. These architectural elements are closely associated with the position of the Ordovician reservoir relative to the Silurian “Hot Shale”.
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The Upper Miocene Melquart Fm.- A Reefal Barrier South of the Gabes Gulf (Libya)
Authors J.L. Rubino, J. Camy-Peyret and P. LapointeBase on 3D seismic analysis an outcrops occurence an extensive reefal barrier is described in the Gabes Gulf including all the morphological elements that are known in the modern complexes.
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The Numidian Flysch Complex of Onshore Tunisia (Southern Kroumirie Range) - Facies Analysis and Stratigraphic Review
Authors S. Riahi, K. Bou Khalfa, M. Soussi and K. Ben Ismail-LattracheSedimentological analysis and biostratigraphic review of the Oligo-Miocene deposits of northern Tunisia provide new data allowing new stratigraphic and paleogeographic reconstruction. The Kroumirie and Zouza member have the same age and correspond to sand/mud and mud/sand rich systems of a deep sea fan system with a detrital influx supplied from the North. Structural analysis demonstrate that the Numidian flysch has been displaced by a major thrust fault.
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Coastal Plain Depositional Systems of the “Trias Carbonate” in the Berkine Basin, Algeria
By N. SabaouThe Trias Carbonaté overlying the TAGI is present in the Berkine Basin and comprises mainly dolomitic shales, with interbedded dolomite layers. The interval is essentially non-reservoir, although thin channel sandstones (often oil stained) are occasionally encountered. Some fluvial terrigenous input is still important at this stage, with the influence of sea level fluctuations and probably regional tectonic events. Sandstones are fine to medium-grained and very thin in the central part of Berkine Basin (where marine conditions are dominant), but they may become thicker to the south of the basin (Turner et al., 2001). Based on core analysis, these sands are interpreted as high sinuosity fluvial/estuarine channels cutting a coastal plain. The purpose of this study is to identify vertical changes in facies, depositional environments and their distribution, sequence stratigraphy and relative sea level controls (Sabaou, 2003). Correlation of the stratigraphic successions with the regional sequence stratigraphic framework, based on 120 wells (Turner et al., 2001), shows that the vertical change in coastal plain character can be related to relatively high-frequency base level cycles that are expressed as transgressive-regressive marine cycles in downdip areas.
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Sedimentary Evolution of Late Ordovician Sequence in El-Feel Field – An Example of Integrated Sedimentologic, Stratigraphic and 3D Seismic Data
Authors G. Serafini and E. TrinciantiNew data concerning the Mamuniyat and Melaz Shugran Fms. in El-Feel Field. are presented. Integrating palynological analyses with sedimentological and 3D seismic data, the physical stratigraphic framework and the vertical sedimentary evolution of the Cambro-Ordovician sequence were defined. The regionally recognised paleohighs-lows model in Mamuniyat Fm. seems not to be supported at the field scale. What was previously defined as Hawaz Fm. in some wells is now ascribed to Mamuniyat Fm. and, as a consequence, it wouldn’t represent the infilling of the morphological lows among the Hawaz paleohighs. Mamuniyat Fm. might represent the infilling of a large scale physiographic element whose edges are not recognisable within the 3D seismic grid. The Melaz Shugran Fm. is quite organised in terms of vertical stacking pattern showing some differences with respect to the classic shaly sequence found in the Murzuk Basin. The regional map of the Melaz Shugran Fm. facies distribution would suggest the occurrence of an extensive fluvio-deltaic depositional system sourced from the South possibly originated by the melting of the ice front during the Ordovician glaciation.
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El Feel Field – An Innovative Approach to Control Prior Model & PetroAcoustic Data for Seismic Reservoir Characterization
Authors A.I. Marini, E.L. Della Rossa and F. OwainaEl Feel reservoir is characterized by high impedance fluvio-glacial Ordovician sandstone sealed by soft shales. Recognition of seismic responses at Top reservoir cannot provide information on reservoir quality due to simultaneous changes of reservoir and sealing acoustic properties. Acoustic inversion, integrating well and seismic data was therefore a crucial step to derive a 3D seismic reservoir characterization and a robust prior model is required. A limited number of the utilized wells are vertical and the rest are mostly slanted (borehole dips >40°), therefore acquired velocity measurements are mostly biased by shale anisotropy. Also input seismic data fidelity is an issue; it was managed by adopting CRS-stack volume, superior for interpretative and quantitative uses. A geostatistical-based proprietary methodology was developed to integrate seismic and well impedance trends, horizons interpretation, and conditioning wells. Thomsen’s anisotropy parameters estimates from well data were used to reduce velocity measures to vertical wells conditions. The followed approach improved data reliability for Petro-Acoustic analysis, well-seismic tie, prior model generation, and inversion. Impedance-Porosity analyses suggested inversion results calibration feasibility by linear or non-linear (Neural Network) approaches. Output Porosity cube, useful for reservoir modelling, captured sedimentary features highlighted by visualization techniques.
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New Concepts in Stratigraphy for Near-Synchronous Correlation of the Paleocene-Eocene Interval of the Se Sirte Basin
Authors M.G.G. de Jong and S.D. NioThe new method of climate stratigraphy has been used to generate a well-to-well near-synchronous correlation framework with a unifying stratigraphic scheme for the interval from the uppermost Cretaceous to Eocene in the southeastern Sirte Basin of Libya. Climate stratigraphy combines the principles of global cyclostratigraphy with spectral (frequency) analysis of facies-sensitive wireline logs such as GR to identify near-synchronous bounding surfaces. The so-called spectral trend attribute curve (INPEFA_GR) has been developed to show the breaks and trends which are used for correlation. Seventeen regionally important near-synchronous bounding surfaces have been systematically identified in wells. Lateral facies variations between these bounding surfaces as well as regional differences in facies development through time become apparent – a framework for systematic evaluation of potential reservoir-seal pairs has been generated. Problems and uncertainties inherent to lithostratigraphic correlations are largely overcome. Details of the method and of its application to well correlation in the SE Sirte Basin will be given in the presentation.
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Comparative Anatomy of Cretaceous Carbonate Ramps - An Outcrop-Driven Interpretation for a Sirte Basin Field
By J. RushThe use of outcrop analogs for constraining stratigraphic interpretations has often been presented for mature fields with seemingly inexhaustible data and tight well spacing. In contrast, a viable fourth-order stratigraphic interpretation of a mixed carbonate siliciclastic system is proposed for a Sirte Basin field with only twenty wells on 640-acre spacing. The objective was to squeeze as much information regarding the depositional environments from scarce well data, integrate observations within an outcrop analog, and develop a play concept to prioritize a potential field acquisition candidate. The result is an outcrop-driven depositional model and stratigraphic framework that honors observations made from core, thin sections, accommodation trends, and paleogeographic setting. A “best fit” analog was identified from Albian outcrops along the Lower Canyons of the Rio Grande in the Big Bend region of Texas. Lithofacies, fossil assemblages, and accommodation trends were analyzed for comparison and were found to be remarkably similar. Actual and comparative evidence suggests this field was situated atop a shallow, current-swept, intrabasin platform, characterized by localized ramps onlapping basement highs. Clastic-rich, radiolitid buildups and associated grainstones developed along the inner ramp, onlapping basement highs, and grade offstructure into chalky Inoceramus-globigerinid wackestones and shales.
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Chemostratigraphy of Cambro-Ordovician to Silurian Sequences from Block NC186 in the Murzuq Basin, Western Libya
Authors O. Kaabar, J.L. Algibez, T. Pearce and A. KhojaThe characterization and correlation of sedimentary sequences based on changes in inorganic geochemical data. The bulk geochemistry of sediments changes in response to subtle changes in mineralogy caused by source composition, facies, weathering or diagenesis. The analytical techniques employed are highly sensitive and the technique can often see correlative features that cannot be detected using E-log data. In some instances, this enables apparently uniform successions to be correlated with confidence.
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Climate Sequence Stratygraphy Unraveled by the Analysis of the Clay Content Evolution
More LessCase Study - The Triassic Formation of the Hassi R’mel Field (Algeria). In the sedimentary recordings, clays, if inherited can give a fundamental a reliable and fundamental indication on the continental contributions. The examples studied come from central sahara wells of various basins. The triassic shale sandstone formation of the central sahara is about 200 meters thick, made of two series of the shale sandstone separated by volcanic rocks. The clay fraction was studied by using diffractometer and the following results were obtained: - from the point of view of the clay fraction constitution a) in oued mya basin the principal minerals are illite, chlorite and mixed layers; b) on the other hand in the berkine basin, there is an association of illite, chlorite mixed layers and kaolinite -from the point of view of the characteristic of clay: during the interpretation of the diffractogramms and by superimposing them we remark that: the intensity of illite peak (I001) and chlorite peak (I002) vary considerably with the depth. The peak variation does not occur at the random. In fact, as observed the intensity of the illite peak (I001) decrease from the bottom to the top in a metric level or profile. Thus, it was observed that at the bottom of the profile, illite shows a high intensity (I001) peak, with a good cristallinity, and as one goes up in the profile(i.e.a decrease in the depth) the peak intensity becomes weaker and sometimes, nothing can be noticed on the diffractogramm. The explanation of the loss of the intensity is not due to the reduction in the partiules but to the worse cristalline organisation of the crystalline system. Indeed the climate which characterized the triassic period is arid or semi arid, he is marked by alternation of wet and dry period. The state of the clay fraction, which depends of the source of the sedimentary material that is homogenous in the sedimentary environment system. In period of dryness, the evaporation is intense, concentrates the solutions which become more agressive and start to initially deteriorate the sedimentary unit with the contatc water sediment then penetrates in the sedimentary unit according to its permeability. This proves that the substance analysed is amorphous by XRD, revealing thus a high weathering of the clay fraction. This decrease in the intensity of the I001 peak of illite from the bottom to the top of the level is spectacular. This phenomenon is observed both in the clay fraction content in the sand and shale samples. The repetition of this phenomenon in the space et time and at the regional scale shows the regularity of the climate.This evolution of the clay fraction is similar to the present day pedogenetic one. Thus, degradations or tranformations by degradations seem to be an overall mechanism in the meteoric alteration and pedogenetic process. This permits us to define sections in the sequence called genetic sequences.
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Stratigraphic Architecture of the Devonian Succession in Awaynat Wanin Area - Ghadamis Basin, Western Libya
Authors M.M. Rahuma, J.N. Proust and R. EschardDuring Palaeozoic times, the structuration of North Gondwana induced the formation of sub-basins separated by tectonic arches on the N orth African platforms. The slowly subsiding sub-basins were fed by periodically emerging and eroded arches (uplift vs eustatic sea-level drops) that control sediment distribution in space and time. The Devonian successions, located at the boundary between the Qarqaf arch and the subsiding Ghadamis basins in western Libya, represent a good record of this evolution and then an opportunity to discuss the relative influences of parameters that control the sequence architecture and reservoir distribution. The Devonian formations (Tadrart, Ouan Kasa, Awaynat Wanin, and Tahara Formations) in western Libya consist of more than 1000m of siliciclastic sediments. The succession is Pragian (Early Devonian) to Strunian (Late Devonian) in age and lies on Llandoverian (Early Silurian) shales along the Caledonian unconformity. These rocks are among the largest oil reservoirs in N orthern Africa. Most them are stratigraphic rather than structural traps and require detailed sedimentologic and stratigraphic works to improve the results of the exploration activities.This study based on outcrop and subsurface data respectively collected along the arch in Awaynat Wanin area and into the Ghadamis basin can provide some guidelines to fulfil this perspective.
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High-Resolution Chronostratigraphy of Varied Biostratigraphic Data from Sirt Basin Wells - An Example of Graphic Correlation and Discussion of Other Methodologies
Authors T.D. Demchuk and W.N. KrebsThe use of high-resolution bio- and chronostratigraphy to delineate and subdivide the strata of the Sirt Basin. Investigations utilizing varied microfossil data and unique biostratigraphic software techniques to construct chronostratigraphic frameworks for use in exploration and exploitation in the Sirt Basin. Identification of depositional sequences and correlation and calibration to other sedimentological and stratigraphic interpretations.
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OBC Systems for Imaging in Challanged Environments - Results and Remaining Challanges
Authors M.L. Johnson and M.W. NorrisA suite of tests were preformed in the Gulf of Mexico to evaluate the ability of conventional air gun arrays to generate low frequency energy. The tests utilized a MEMS OBC system to record the data generated by various air gun configurations. This presentation reports on the acoustic blanket configuration used with a deep air gun array.
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Azimuthal Seismic Fracture Detection in a Carbonate Reservoir - Integrated Seismic P-Wave Study
Authors J.M. DeGraff, M.K. Johns, D.Y. Wang, C.P. Lu, S.Z. Sun, S. Xu and D. ZhouUnderstanding the distribution of fractures in carbonate reservoirs can greatly improve modeling of production rate and flow anisotropy. Seismic anisotropy may be a cost-effective technique sensitive to open fractures affecting flow, and can provide volumetric information on fracture properties. This talk focuses on azimuthal AVO effects at interfaces between fractured and unfractured rocks. We calculate AzAVO attributes for a Cretaceous limestone reservoir in East Texas. The workflow integrates inversion results with rock physics, forward seismic modeling, dipole sonic and image logs, and core to interpret azimuthal attributes relative to geology and production data. AzAVO inversion results for the field show anomalies that broadly tie to fracture indicators and are geologically reasonable. Anisotropy magnitude aligns with many NE-trending faults, which is consistent with fault-related fracturing. AzAVO orientations generally parallel ENE-trending fractures in core, maximum horizontal stress from image logs, and fast-velocity direction from a dipole sonic log. However, moderate to poor seismic data quality and sparse geologic well control limit our confidence in quantitative fracture prediction. Overburden effects manifested by amplitude dimming degrade AzAVO inversion quality over the structural crest and highlight the sensitivity of the inversion algorithm to amplitude artifacts.
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Predicting Fractures Using Wide Azimuth Seismic Data in Tiguentourine Field, Algeria
Authors S.R. Tod, D. Sawyer, B. Taylor, R.G. Johnston, T.J. Allen, D. Buddery, A. Hutchison, A.M. Malik and D. AbdelouafiA small pilot area wide azimuth P-wave seismic survey was acquired over a part of the Tiguentourine gas condensate field in 2004 to test the ability of surface seismic data to accurately predict the P-wave anisotropy resulting from the preferential orientation of open fractures within the Cambro-Ordivician reservoir. Comparison of velocity anisotropy attribute estimation from the seismic data with image log data recorded in a number of wells shows an encouraging positive correlation that provides confidence in the use of the seismic attributes to assist in the locating of wells to target areas of high oriented fracturing in the believe that these can provide a considerable permeability assist and thus provide a significant boost to well productivity rates.
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Multi Azimuth 3D Bringing Clarity to Pre-Messinian Reservoir Imaging
Authors J. Keggin, M. Benson, W. Rietveld, T. Manning, B. Barley, P. Cook, E. Jones, C. Page and M. WidmaierA thin but complex layer of partially eroded anhydrite and other facies lie at a depth of around 3km across large areas of the Nile Delta, offshore Egypt. Wavefield distortion, attenuation and the generation of complex multiple diffraction noise cause the quality of the underlying seismic image to be highly variable. In this paper we describe the problem and then demonstrate how multi-azimuth seismic is able to improve the Pre-Messinian image
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Velocity Model Building for Pre-Stack Depth Migration - An Onshore Libya Case Study
Authors L. Auchterlonie, V. Vinje and R. TaylorTypically, velocity models for land datasets are built using a traditional “blocky” layer-based velocity model. Here we use a dataset from the Masrab field in the Sirte basin south-east of Tripoli, Libya, to compare this traditional way of building depth models with a “smooth”, volumetric velocity model building approach. We show that the volumetric approach provides improved image quality whilst decreasing the turnarund time.
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A Fast-Cycle Volumetric Approach for PSDM Velocity Model Building Applied to a Major Fault in Western Desert - Egypt
Authors M. Emile, A. Ramadan, S. Zimine, C. Pinson, P. Mitouard, V. Vinje, R. Saba, A. El-Fiki and G. BakryThe case history presented here is for a desert environment and contains a strong lateral velocity variation (up to 1500 m/s) across a major fault with a throw reaching 3 km. The results from this study show that the new velocity model buiding approach improved imaging in the shadow zone of the fault where traditional methods failed.
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Petrophysical Seismic Inversion Applied to the Troll Field
Authors A.J. van Wijngaarden, T. Couleou, A.N. Haaland, J.L. Formento and R. OnaWe present an application of petrophysical seismic inversion, a method driven by petro-elastic models, updating a fine-scale geological model in depth to make it fully compatible with pre-stack seismic measurements on a part of the Troll Field Central province, in the North Sea. The results are being evaluated for infill drilling and for a future 4D inversion to determine the remaining oil in the thin oil leg.
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EM Seabed Logging on the Troll Field
Authors S. Johnstad, B.A. Farrelly and C. RingstadA seabed logging R&D study was conducted on the Troll Field in December 2003 – January 2004. Different EM waveforms and frequencies were tested to optimize the hydrocarbon response from this shallow water gas and oil field. The survey consists of 41 receivers, deployed along a line crossing the Oil Province, the Western Gas Province and the Eastern Gas Province of the Troll Field. All receivers recorded two orthogonal components of the horizontal electric field, and 12 receivers measured in addition two orthogonal components of the horizontal magnetic field. The R&D study was carried out as an attempt to qualify seabed logging for shallow waters (less than 500 m) and optimize acquisition parameters for such cases. Up until now the marine controlled source EM method has been qualified for use in deep waters (more than 1000 m water depth) mainly because of the disturbing influence from the air wave in shallower water areas. The Troll Field seabed logging data is of excellent quality and helps in solving problems related to shallow water applications of this method.
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