K. Tawengi

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Case Study on Field Scale Seismic Definition of the Upper Ordovician Reservoir Configuration, Murzuq Basin, South-West Libya

Proceedings (2012)

Robert Gruenwald A. Ponte Neil McDougall K. Tawengi

As result of combined exploration efforts by REMSA S.A. as operator and OMV as partner several fields were discovered in the Murzuq Basin. 3D Q Seismic reflection datasets are used to unravel the complexity of reservoir-seal contacts of the Late Ordovician glacial deposits on a field scale. Three wells, apparently drilled within a four-way dip closure, targeting the Mamuniyat sandstone reservoir were studied to explain the unexpected lateral disconnection. As a result of detailed well and 3D seismic data integration, three hypotheses were evaluated to explain the limits of the structural closure. These include; the presence of a non reservoir barrier laterally separating the Upper Mamuniyat reservoir in wells A&C from well B, analyzed on a variance time slice close to the OWC, a low VRMS due to the presence of Bir Tlacsin, and detection of different generation Mamuniyat sandstone bodies based on VoxelGeo amplitude analysis. Geo-bodies of the Upper Ordovician depositional system were imaged on seismic on a regional scale, explaining the presence of different generation Upper Mamuniyat on a field scale. The resulting reservoir separation is complimentary to VRMS and coherency cube observations and in agreement with regional FMI and core facies interpretation.

Tarim basin

Reflection

10.3997/2214-4609.20148235

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Citations (1)

Trazas fósiles e Icnofacies y su relación con el ambiente sedimentario, Formación Hawaz, Ordovícico, Libia

Geotemas ( Madrid ) (2005)

Jordi M. de Gibert Emilio Ramos M. Marzo K. Tawengi Néstor D. Bolatti

The ichnological study of the Hawaz Formation evidence the existence of an important correlation between lithofacies associations and trace fossil assemblages. Recognized ichnoassemblages can be linked to two archetypal Ichnofacies: Skolithos and Cruziana. The first is represented by strogly bioturbated sandstones with Skolithos, a vertical dwelling burrow od a suspension-feeder. These piperocks characterize persistent high-energy settings, such as those found in shoreface or inner platform setting. In contrast, the Cruziana ichnofacies is constituted by horizontal locomotion, feeding or resting trace, mostly produced by deposit-feeders. The most abundant trace fossils are Cruziana, Rusophycus and Artrophycus. This Ichnofacies is found in hetherolitic facies corresponding to lower-energy shallow subtidal setting. The incorporation of ichnological data to sedimentological analysis demonstrates to be a powerful tool in paleoenvironmental interpretation of depositional settings.

Ichnofacies

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Burrow

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Stratigraphy and sedimentology of the Middle Ordovician Hawaz Formation (Murzuq Basin, Libya)

AAPG Bulletin (2006)

Emilio Ramos Mariano Marzo Jordi M. de Gibert K. Tawengi Abdalla A. Khoja

The Middle Ordovician Hawaz Formation is a 200-m (660-ft)-thick succession made up of fine-grained quartzarenites displaying a variable degree of bioturbation. It records the deposition in a large-scale, low-gradient estuary, which was partially controlled by tectonic extension. The upper boundary of the formation is marked by two erosion surfaces (unconformities U1 and U2), related to the Late Ordovician glaciation. The U1 and U2 erosion surfaces generated a pronounced paleotopography that controlled the deposition of the Upper Ordovician sequences. Tectonism influenced the paleogeography, although faults were unimportant from the point of view of sedimentary thickness. Tectonic subsidence was moderate, and accumulation rates were low. Physiography favored tidal power, especially during transgressive episodes, when the coastal embayment was flooded. We defined 11 lithofacies, forming 6 facies associations. These associations are subtidal sandstones; storm-reworked, shoreface sandstones; shoreface-to-beach sandstones; channel-sandstone bodies; nearshore to inner-platform sandstones; and K-bentonites. Trace-fossil assemblages match Skolithos and Cruziana archetypal ichnofacies. On the basis of the dominant facies associations and ichnofacies, we divided the formation into three informal units, from base to top: HW.1, HW.2 and HW.3. Periodically, volcanic ash was supplied to the basin from distal eruptive centers and was preserved as thin beds of K-bentonite interstratified with the shoreface sandstones, but not with the tidal-dominated sandstones. We divided the Hawaz Formation into five third-order depositional sequences. Lowstand deposits were not identified. The lower boundaries of transgressive systems tracts are tidal ravinement surfaces or sequence boundaries, whereas the upper boundaries are flooding surfaces. The transgressive systems tracts are constituted by early transgressive tidal deposits separated by a wave ravinement surface from the late transgressive storm-dominated deposits. Highstand systems tracts consist of bioturbated shoreface-to-beach sandstones, which record seaward, shoreline progradation. Emilio obtained his Ph.D. in geology from the Universitat de Barcelona in 1988. Since then, he has been a lecturer in basin analysis and petroleum geology. He has been involved in several research projects on sedimentology and basin analysis in Spain, northern Africa, Antarctica and South America. His present-day research interests include three-dimensional modeling of sedimentary bodies and reservoirs. A professor of stratigraphy at the Universitat de Barcelona, M. Marzo's research interest focuses on the application of clastic sedimentology, sequence stratigraphy, reservoir modeling, and basin analysis to the exploration and production of hydrocarbons. He has been involved in several research projects funded by oil companies in southern Europe, the North Sea, South America, and northern Africa. Jordi M. de Gibert received his Ph.D. from the Universitat de Barcelona in 1996. After a period at the University of Utah, he returned to Barcelona in 1999, where he currently holds a position as a tenure-track lecturer. His interests and areas of expertise include trace fossils, their paleobiological significance, and their implications for understanding ancient depositional environments. K. Tawengi received a B.Sc. degree in geology from Alfateh University, Libya, in 1984, and an M.Sc. degree in sedimentology and stratigraphy from Durham University, England, in 1996. He worked as an explorationist with Agip Oil Company in Libya from 1985 to 2000 and since then has worked as a senior exploration member with REPSOL Exploration in Murzuq S.A. His main fields of interest are sedimentology, stratigraphy, and subsurface geology. A. Khoja graduated in 1972 from the University of Libya. He received a diploma in petroleum geosciences from Oxford Polytechnic (1991) and an M.Sc. degree from Oxford Brookes University (1993). He joined the National Oil Corporation of Libya in 1972 and is presently the regional studies superintendent in the National Oil Exploration Department. Nestor obtained his degree in geology from Cordoba University (Argentina) in 1982 and his postgraduate in petroleum geology in the University of Cuyo, Argentina. He joined YPF in 1984 and worked in their exploratory department in Mendoza, Plaza Huincul, and Neuquen until the year 2000. Subsequently, he became Libya team leader for REPSOL- YPF in Madrid. He is currently director of exploration and production in Brazil.

10.1306/03090605075

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Facies and sequence stratigraphic implications of the Statfjord formation (upper triassic-lower Jurassic), northern north sea, UK

K. Tawengi

Detailed sedimentological logging of core, and interpretation of wireline logs from the Brent Oil Field, northern North Sea, have allowed for the identification of sedimentary facies and environments of the Upper Triassic-Lower Jurassic Statfjord Formation. The Statfjord Formation is interpreted to have been deposited within a diversity of sedimentary environments. These include delta plain, meandering rivers, braided rivers, tidally influenced fluvial systems and a shallow, high energy transgressive marine environment. Within the fluvially-dominated part, which is by far the most important volumetrically, two distinct facies associations were recognized: channel and overbank. The channel facies association is in turn divided into three discrete end members: low sinuosity, high sinuosity and minor/crevasse channel facies. Similarly, the overbank facies association comprises a variety of sedimentary facies including levee, crevasse splay, swamp, flood plain and rare lacustrine and lacustrine deltas. Low sinuosity channel facies are divided on the basis of their architectural style, into three different types. Type 1 low sinuosity channel facies dominates the northern part of the study area and resembles the sandy Platte River model of Miall (1977). Type 2 low sinuosity channel facies dominates the southern part of the study area and is characterized by numerous internal scour surfaces. This channel type is interpreted to have been deposited within a relatively proximal braided river system. Type 3 low sinuosity charmel facies is a transitional facies between the proceeding two low sinuosity channel types and is similar in character to the South Saskatchewan and the Battery Point River models of Cant and Walker (1978). Documented point bar sediments within the high sinuosity channel facies association indicate that channel bankfull depths may have reached as much as 4.5 m. The lack of evidence of exposure of the in-channel sediments in addition to the thick, consistent bedforms suggest that the rivers were probably of a perennial nature. Palaeoflow indicators obtained form HDT log analysis reveal that the Statfjord Formation m the Brent Field has been deposited by a north to northwest flowing drainage systems. Thorium/potassium cross-plots from NGS logs indicate an upward reduction in sandstone mineralogical maturity which may imply an upward reduction in the degree of reworking of sediments and a least sediment transport. The overall spatial and temporal consistency of the palaeocurrent indicators coupled with the upward decrease of sandstone maturity further indicate that no flow reversal occurred throughout the geological evolution of the Statfjord Formation. Sequence stratigraphic concepts applied in conjunction with detailed facies analysis indicate that sedimentary facies of the Statfjord Formation are arranged within two successive depositional sequences. Although the sequences are incomplete they possess all the attributes of the depositional sequences described by Posamentier and Vail (1988) and Van Wagoner et al. (1990). The lower sequence comprises sediments deposited within a highstand systems tract. The lowermost part is characterised by low net/gross ratio and is interpreted to have been deposited within early highstand systems tract most likely in an upper delta plain setting. As the rate of additional subaerial accommodation space approaches zero during a stillstand and eventually reverses, high sinuosity rivers are thought to have migrated laterally during a late highstand combing the previously deposited fine-grained sediments. The upper sequence overlies a regional sequence boundary unconformity and comprises an early lowstand wedge systems tract which is characterised by braided river sediments deposited during a slightiy rising base level. As the rate of additional subaerial accommodation space increases isolated high sinuosity river sediments are interpreted to have been deposited during a late lowstand wedge systems tract. Cyclic rhythmites reflecting neap-spring-neap tidal cyclicity of a diurnal palaeotidal regime represent the first 'local' major marine flooding. These are probably equivalent to a condensed section in the coeval marine realm and mark the onset of a transgressive systems tract: Continued rnarine transgression is documented by the deposition of the overlying high-energy, transgressive marine sandstone which is probably separated from the underlying tidally influenced strata by the occurrence of a ravinement surface indicating a progressively landward migrating shore line. In. considering the relative tectonic quiescence of the area during the deposition of the formation which occurred during a post-rift thermal subsidence phase, the close link between variations m the stacking patterns of facies tracts and the eustatic curve reveal that the stratigraphy and deposition of the Statfjord Formation were largely controlled by processes of sea level fluctuations in a near-shore setting.

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Overbank

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