Carbonate facies, diagenesis and sequence stratigraphy of an eocene nummulitic seservoir interval (jdeir formation), offshore NW Libya

This study investigates the sedimentology, petrology and depositional environments of a major Early Eocene nummulitic reservoir unit: the Jdeir Formation, from offshore NW Libya in the Mediterranean Sea. This formation is a prolific hydrocarbon-producing unit that was deposited as part of the Mesozoic-Cenozoic stratigraphic fill of the Sabratah Basin. The Sabratah Basin is an elongate ESE/WNW trending fault-bounded basin that originated as a left lateral pull-apart basin during the Late Triassic-Early Jurassic. Presented in this thesis is a review of the tectonostratigraphic setting of the Jdeir Formation, an evaluation of the facies and an interpretation of the depositional environment of the platform. During this Master project, diagenesis was also evaluated with the aim of better understanding reservoir development of the Jdeir Formation. On the basis of detailed core description and petrographic study eight facies have been distinguished. These are: (1) Planktonic Foraminifera Fades, (2) Discocyclina-Nummulitc Facies, (3) Nummulite Facies, (4) Alveolina Facies, (5) Peloidal-Bioclastic Facies, (6) Mollusc Facies, (7) Echinoderm Facies and (8) Sandy- Bioclastic Facies. These are interpreted as having been deposited in open-marine, fore-bank, bank, lagoonal (back-bank) and restricted lagoonal environments. Nummulitic rudstones, dominated by B-forms with minor A-forms, comprise the upper part of bank, and float/rudstones form the lower part of the bank. Abrasion and fragmentation of bioclasts resulted from the transport of sediment from palaeohighs and their reaccumulation into intra-, or back-bank environments. Discocyclina and planktonic foraminifera-rich facies formed in open-marine environments, with the former accumulating towards the base of the photic zone. The back-bank or lagoonal deposits are highly variable, were sometimes affected by siliciclastic influx, and may be dominated by molluscs, echinoderm debris or imperforate foraminifera. Facies and thickness variations between the three wells were controlled by the palaeotopography and palaeoenvironments of the Sabratah Basin. It is appears that the depositional settings of the shallow platform deposits were more variable than has-been previously documented. The carbonates of the Jdeir Formation have been altered by a variety of diagenetic processes as inferred from petrography, cathodoluminescence, SEM and stable isotope analysis. The diagenetic sequence involved initial marine diagenesis including micritization, which has obliterated much of the original fabric of many skeletal grains, and the formation of micrite envelopes also occurred at this time. Leaching of bioclasts and the matrix has created vuggy and mouldic porosity, which is now commonly partially or completely filled by drusy and minor blocky calcite cements and rare kaolinite cement. Recrystallization of micrite to microspar and pseudospar is attributed to meteoric diagenesis, which has occluded some of the original porosity. The final diagenetic features include development of dissolution seams, stylolites and fractures, coarse calcite spar, dolomitization, silicification and pyrite. All these later diagenetic features are inferred to have occurred in a burial environment. As a result of these diagenetic processes, Nummulitic limestones of the Jdeir Formation have lost most of their primary porosity. Secondary porosity formed through the dissolution of aragonitle skeletons and micritic matrix during meteoric diagenesis, and possible also late burial diagenesis. The overall sequence stratigraphic interpretation of the Jdeir Formation would most likely be transgressive deposits (transgressive system tract) based on underlying early dolomite which is associated with tidal flat deposits and overlying deep marine deposits. Intra-formation smaller-scale trends most likely formed under transgressive, stillstand and regressive conditions. The incomplete well data through the Jdeir Formation and different numbers of transgressive and regressive cycles in each well makes formation-wide correlation problematic

Sedimentology, Diagenesis and Reservoir Characteristics of Eocene Carbonates Sirt Basin, Libya

ABSTRACT The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin (at the northern margin of the African continent) is strongly influenced by depositional facies and various diagenetic modifications. This thesis investigates the petrography, sedimentology, diagenetic evolution and hydrocarbon potential of the Middle Eocene Gialo Formation in the subsurface of the north-central Sirt Basin based on data from core samples and well logs from five boreholes in the Assumood and Sahl gas-fields. Reducing risk in exploration demands an understanding of reservoir facies development, which is governed by the type and distribution of depositional facies and their diagenetic history. Seven major carbonate facies (and 20 microfacies) have been identified in this study and are interpreted as predominantly deposited under shallow-marine conditions within the photic zone, as indicated from their richness in phototrophic fauna and flora. These include lagoon (back-bank), main bank, fore-bank and open-marine facies, all of which were deposited on a homoclinal ramp type of carbonate platform. The type and distribution of the Gialo depositional facies were influenced by basin-floor architecture and environmental controls. The basin floor was shaped through pre-Eocene structural development into a series of elevated platforms and deep troughs. Platform facies were deposited across three broad facies belts: (1) inner-ramp, dominated by dasycladacean molluscan wackestone/packstone, nummulitic-bryozoan packstone, bryozoan wackestone; (2) mid-ramp, dominated by nummulitic packstone and Discocyclina-nummulitic wackestone; and (3) outer-ramp, dominated by fragmented nummulitic packstone. Troughs were dominated by thick successions of lime mudstone containing rare fine skeletal fragments and nummulites, with deposition taking place in a deeper-marine environment, below the photic zone. Present-day reservoir characteristics of the Gialo Formation are the net result of modification to the original depositional characteristics caused by diagenesis. This diagenesis took place on the seafloor, under burial, and in the meteoric diagenetic environment. Early marine diagenetic processes affecting the Middle Eocene Gialo carbonates resulted in micritization of bioclasts. Later diagenesis in meteoric to burial environments resulted in dissolution of aragonitic bioclasts, cementation (syntaxial overgrowths on echinoid grains, and blocky to equant, non-ferroan cements), neomorphism, pressure dissolution, compaction and fracturing. δ18O and δ13C values in the Gialo Formation range between -1.06 and -4.16‰ PDB, and 0.76 and 1.89‰ PDB, respectively. These values are mostly marine values, although some alteration is likely. The more negative oxygen of the cements suggests precipitation within the shallow-burial environment under the influence of meteoric water and / or precipitation at higher temperatures during further burial. The carbon isotopic signatures are typical marine values. There is a strong relationship between porosity and the diagenetic processes that-affected the Gialo sediments. Generally the porosity in the Assumood and Sahl fields is either primary or secondary, enhanced by dissolution and fracturing of the sediments. Reduction in porosity in the investigated sediments is mainly due to cementation and compaction. The common pore-types in the Gialo Formation are intergranular, moldic, intragranular, vuggy and scattered fractures. Porosity ranges from poor to very good (<1% to ~37%) and permeability varies from low to high (<1mD to 100mD). These variations in porosity and permeability are strongly related to facies changes, which were influenced by depositional environment and diagenetic processes. Shallow-water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality. The Gialo Formation is an important gas producing reservoir in the Assumood, Sahl and other surrounding fields. The gas which is generated from the gas-prone Sirt Shale source rock of the northern Ajdabiya Trough possibly migrated onto the Assumood Ridge from the northeast through late Cretaceous, Paleocene and early Eocene carbonates, before being trapped beneath the Augila Shale (Upper Eocene) which is the principal regional seal in the area. This integrated study has helped to understand the reservoir heterogeneity and potential of the Gialo carbonates and based on this current wells are being completed appropriately, as, hopefully, will future wells too. The facies pattern is different from one well to another, which does suggest that there was a strong tectonic control, that is differential tectonic subsidence and/or fault control, or that deposition was controlled by autocyclic processes. The different vertical positions and numbers of transgressive-regressive cycles in each well make formation-wide correlation problematic. The lack of correlation in terms of cycle thickness, as well as facies, between wells, also suggests autocyclic processes. Third and fourth-order relative sea-level (RSL) changes do not appear to have been a major control on deposition during this Middle Eocene time.

Robust and flexible multi-scale medial axis computation

The principle of the multi-scale medial axis (MMA) is important in that any object is detected at a blurring scale proportional to the size of the object. Thus it provides a sound balance between noise removal and preserving detail. The robustness of the MMA has been reflected in many existing applications in object segmentation, recognition, description and registration. This thesis aims to improve the computational aspects of the MMA. The MMA is obtained by computing ridges in a “medialness” scale-space derived from an image. In computing the medialness scale-space, we propose an edge-free medialness algorithm, the Concordance-based Medial Axis Transform (CMAT). It not only depends on the symmetry of the positions of boundaries, but also is related to the symmetry of the intensity contrasts at boundaries. Therefore it excludes spurious MMA branches arising from isolated boundaries. In addition, the localisation accuracy for the position and width of an object, as well as the robustness under noisy conditions, is preserved in the CMAT. In computing ridges in the medialness space, we propose the sliding window algorithm for extracting locally optimal scale ridges. It is simple and efficient in that it can readily separate the scale dimension from the search space but avoids the difficult task of constructing surfaces of connected maxima. It can extract a complete set of MMA for interfering objects in scale-space, e.g. embedded or adjacent objects. These algorithms are evaluated using a quantitative study of their performance for 1-D signals and qualitative testing on 2-D images

Pre- and protopalatial Minoan larnax : individuals vs collective identity in pre- and protopalatial Crete

Prepalatial and Protopalatial larnakes offer a corpus of material with their own biography which has long been ignored, passed over, or forgotten. They represent the beginning of a mortuary tradition of burials in ceramic containers that spans a millennium and eventually crosses the Aegean, appearing at select locations on mainland Greece. Their appearance at the EM III -- MM IA transition -- a crucial moment in Minoan history -- has provided fodder for interpretations of nascent individualism on Crete, thus replacing the communal ideology so prevalent in the earlier Minoan mortuary landscapes and built urban environments. This study provides an overview of Prepalatial and Protopalatial larnakes, their contexts, the associated funerary assemblages, and the regional implications made apparent throughout the course of research. The results of this study conclude that the larnax operated within an interwoven communal tradition that embodied not a single or elite individual, but the collective as a whole

Discerning Migration In The Archaeological Record: A Case Study At Chichén Itzá

Migration, as a theory to explain aspects in the archaeological record, has fallen out of favor in Mesoamerican archaeology, possibly due to a lack of a standard definition or description of migration. Migration as an explanation of change in Maya civilizations has been around since the 1950\u27s and the culture-history era of American archaeology. Since the early 1990\u27s, migration has been treated as a process, one that can be discerned in pre-literate cultures as well as historical ones. Models of the migration process are being developed and tested. One type of migration, elite dominance migration, is a particularly suitable process to study in Mesoamerica. A model of elite dominance migration might include the following attributes: advance contact by the migrating culture, migration to a center, maintenance of contact with the sending population, spatial concentration of the incoming population, migration of a selected population of elites, and a cause or push factor. To find these attributes, the receiving population is studied to determine if there are multiple changes in the material record consistent with an intrusion of an outside group. And, there should be a rough chronological correlation between the sending and receiving populations. The Maya site of Chichén Itzá is a classic case study, and provides a starting point as a possible receiving population of an elite dominance migration. There is an abundance of scholarship devoted to the question of the relationship between Chichén Itzá and Tula, Hidalgo, in Central Mexico. The iconographic similarities between the two sites are numerous and have been thoroughly discussed in the literature. But, there is much more evidence that should be examined in applying a model of elite dominance, such as architecture, artifacts (including ceramics and obsidian), burial and caching practices, and site configuration. Comparing all of these categories at the two sites, one reaches two conclusions: there are multiple lines of evidence for change in the material record across the spectrum of categories at Chichén Itzá, and, to a lesser extent, at Tula, Hidalgo, indicating a population intrusion. And, secondly, there are abundant similarities in architecture, caching practices, ceramics, and other aspects of the material record that support the assertion of strong contacts between the two sites. Applying the model of elite dominance migration to Chichén Itzá, the majority of the markers for this type of migration can be seen in the material record of the site, as well as the site of Tula, Hidalgo. Chichén Itzá has the attributes of a receiving population, with an elite dominance migration of Central Mexican people taking place there, either from Tula, Hidalgo or from a third, as yet unspecified site that impacted both Chichén Itzá and Tula

Visual inspection : image sampling, algorithms and architectures

The thesis concerns the hexagonal sampling of images, the processing of industrially derived images, and the design of a novel processor element that can be assembled into pipelines to effect fast, economic and reliable processing. A hexagonally sampled two dimensional image can require 13.4% fewer sampling points than a square sampled equivalent. The grid symmetry results in simpler processing operators that compute more efficiently than square grid operators. Computation savings approaching 44% arc demonstrated. New hexagonal operators arc reported including a Gaussian smoothing filter, a binary thinner, and an edge detector with comparable accuracy to that of the Sobel detector. The design of hexagonal arrays of sensors is considered. Operators requiring small local areas of support are shown to be sufficient for processing controlled lighting and industrial images. Case studies show that small features in hexagonally processed images maintain their shape better, and that processes can tolerate a lower signal to noise ratio, than that for equivalent square processed images. The modelling of small defects in surfaces has been studied in depth. The flexible programmable processor element can perform the low level local operators required for industrial image processing on both square and hexagonal grids. The element has been specified and simulated by a high level computer program. A fast communication channel allows for dynamic reprogramming by a control computer, and the video rate element can be assembled into various pipeline architectures, that may eventually be adaptively controlled

Facies and sequence stratigraphy of the tamet formation (middle eocene), eastern sirte basin, Libya

The middle Eocene Tamet Formation on the eastern side of the Sirte Basin, is largely shallow-marine carbonate platform-to-basin transition formed along passive margin. The stratigraphy and deposition of this formation were largely controlled by eustatic processes of superimposed short-term and long-term of relative sea-level fluctuations. Tamet sediments were mostly deposited in subtidal environments, which ranged from above fairweather- to below storm wave-base. Intrinisic processes such as storm and wave redeposition and reworking may have acted to inhibit aggradation into the zone of peritidal sedimentation. Estimates of water depth during deposition range from a few metres to a few tens metres maximum. There are three major plat formal facies associations in the transition from deep subtidal to hypersaline deposits. These facies associations are defined and interpreted on the basis of their constituent microfacies and depend on their palaeogeographic setting on the platform. The spatial distribution of the complete spectrum of the facies associations suggest that deposition took place under low-energy conditions, as a stacked prograding homoclinal ramp. Ten microfacies types have been distinguished, and their vertical interrelationships reflect metre-scale, shallowing-upward subtidal cycles, which are considered as the basic building blocks of the Tamet ramp. Two different types of subtidal cycle have recognised in the study area. Open-marine subtidal cycles are present along the outer through inner ramp within the transgressive and most of the highstand deposits. They are characterised by relatively deep subtidal microfacies at the base, gradationally overlain by shallow subtidal microfacies. Hypersaline subtidal cycles are present only upon the Cyrenaica Platform and predominated during the late phase of highstand deposition; they are composed of dolomitised shallow subtidal microfacies, capped by anhydrite. The middle Eocene across this area is not a single carbonate ramp but rather an amalgamation of stacked ramps. Facies associations and cycles within the Tamet Formation have allowed the recognition of three depositional sequences separated by stratigraphic transitional zones. Each sequence represents a prograded ramp. The development of a sequence framework is based on the metre-scale cycle architecture and maintaining microfacies interpretations. Most of the sequences are interpreted as transgressive-highstand deposits. Each transgressive ramp is typically characterised by an aggradational patterns of relatively deep subtidal mud-rich carbonates deposited in a catch-up depositional system and episodically affected by storm events. Away from the ramp-margin, the transgressive facies change and stratigraphically thin into lagoonal facies deposited under keep-up conditions. The subsequent highstand ramp begins with an aggradational geometry but finally shifts into a distinct progradational pattern. The highstand cycles cover a broader area than that occupied by the transgressive sediments and are made up of mud-poor packstones reflecting a keep-up depositional system. The Cyrenaica Platform at this time was occupied by a very shallow and hypersaline sea. Carbonate sedimentation was shut off and replaced by precipitation of shallow-water evaporites; associated with this was dolomitisation, marking the end of sequence. Several lines of evidence suggest that the magnitude of the middle Eocene sea-level fluctuations on the eastern Sirte Basin were relatively low. First, if the magnitude of the oscillations had been greater, then the sea-level falls would potentially have lead to formation of major sequence boundaries on the Tamet sequence upper surfaces. Second, if the magnitude was larger, then the rapid sea-level rises would have caused drowning of the ramp or domination by "catch-up" style of subtidal deposits. However, the Haq et al. (1987) sea-level chart in some circumstances may require modification, at least in terms of magnitudes and eustatic sea-level rises