No description available.

A detailed 3D seismic investigation coupled with key well data of the Cantarell Field located in the southern Gulf of Mexico was undertaken with the view to developing a seismic-structural framework in order to address the geological features observed in the Cantarell Field. The Cantarell Field hosts a world class petroleum system. The most productive reservoir of the giant Cantarell Field is a complex brecciated and fractured Upper Cretaceous carbonate system that has been interpreted as a response to subaerial exposure and karstification. The 3D seismic interpretation shows the Cantarell area to be intensely deformed by a complex array of thrusts, normal, reverse and strike-slip fault systems that uplifted the Mesozoic carbonate sequences. This work details the structural evolution and shows how the broader understanding of the structural history allows a re-examination of the reservoir model and in particular the development of key fracture sets, that are so critical for the production strategy deployed on the field. From the regional interpretation of seismic lines coupled with key well data a depositional setting is interpreted to have evolved from a shallow ramp during the Jurassic to a scarp dominated platform margin at the end to the Cretaceous. During the Cenozoic the basin underwent significant subsidence and the sedimentation style switched from carbonate dominated to a siliciclastic dominated. The structural interpretation allowed suggesting three main phases of structural deformation, which exhibit evidence of reactivation: (1) Jurassic to Early Cretaceous Early extension, (2) Mid Miocene to Pliocene compression, and (3) Pleistocene to recent mild extension. From the spatial distribution of seismic facies assemblages, this work proposes a polyphase model for the origin of the Upper Cretaceous calcareous breccias: (1) Late Cretaceous sedimentary breccias occurring in a basinal setting as cyclic talus deposits in a carbonate-slope-apron system. (2) During the Neogene structural evolution the area resulted in a complex array of thrusts and fractures which resulted in the development of tectonic breccias occurring at various scales. (3) Subaerial to medium depth karstification related to the Neogene structural evolution of the area which resulted in a thrust controlled uplift of 1000 metres, affecting the breccias situated at the crest of the Akal block. This model has major implications for future field development and the understanding of the fractured reservoir system in this basin.

This thesis uses a combination of industry seismic (2D and 3D) and well data to investigate the typologies, genetics and mechanisms of soft-sediment deformational processes on the continental margin of Israel and their impact on the exploration and production of hydrocarbons. Research has been focused on the two major types of soft-sediment deformation in the region: clastic diapirism and submarine slope instability (i.e. submarine slumping). Such processes have occurred almost continuously throughout the post-Messinian history of the Israeli margin, and have played a critical role in its overall evolution and construction. Detailed analysis of the timing of occurrence, areal distribution and 3D appearance of the resultant structures has enabled an enhanced understanding of the causes, processes and results of soft- sediment deformational events to be obtained. Clastic diapirism occurred during the first stages of refilling of the Mediterranean Sea after the Messinian Salinity Crisis, and was restricted to an area underlain by the Afiq Submarine Canyon (Oligocene in origin). The resultant bodies correspond to a series of four-way dip mounded features, and ridge-like structures that are mainly distributed along the axis and one of the flanks of the canyon, respectively. Seismicity and hydrocarbon generation have been proposed here as the main triggering mechanisms. Clastic diapirism plays a decisive role in the hydrocarbon prospectivity of the region as it largely modifies the reservoir properties and architectures of the largest accumulations of hydrocarbons discovered to date in Israel. Submarine slope instability (i.e. submarine slumping) is the second dominant typology of soft-sediment deformation in the continental margin of Israel. Submarine slumping initiated during the Late Pliocene with the Israel Slump Complex (ISC), one of the biggest submarine slump deposits in the world described to date. Since then, slope failure has occurred almost continually up to the present day. Submarine failure in the area is linked to the dynamics of subsidence and deformation of the transform margin of the eastern Mediterranean. Seismicity and presence of gas in the sediments, together with localised oversteepening, have been proposed as the main triggering mechanisms. The high spatial resolution provided by the 3D seismic data has enabled two principal types of submarine landslides to be distinguished according to their mechanism of frontal emplacement: frontally confined and emergent. In the first, the landslide undergoes a restricted downslope displacement and does not overrun the undeformed downslope strata. In the second significant downslope translation occurs since the landslide is able to abandon its original basal shear surface and translate freely over the seafloor. Such division is of critical importance as the formational mechanisms, and processes of translation and cessation are fundamentally different.

No description available.

The geology of the Rambler area, Newfoundland, is described. The rocks of the area consist mainly of ultramafic and tholeiitic pillow lavas and associated pyroclastic rocks. Clastic sedimentary rocks, acidic lavas and pyroclastics, and thin units of an acidic ‘chert-like’ rock are interbanded with the lavas and pyroclastics. Metadolerite dikes intrude all other rocks in the area. The field and petrographic relationships suggest that the area has undergone at least three deformational and two meta- morphic events. The first deformational event was mainly constrictive and produced rock fabrics which locally approach those of L tectonites; mineral and particle lineations are abundant. This deformational event was accompanied by meta- morphism to the quartz-albite-epidote-almandine subfacies of the greenschist fades. Structures produced by the first deformational event were folded by at least two later deformations into earlier tight, northeast plunging, isoclinal folds which were followed by development of open recumbent folds. A second metamorphic event which appears to have post dated the folding, attained conditions of biotite grade green- schist metamorphism. Major and trace element data are presented for the country rocks. The tholeiitic lavas are chemically similar to present day ocean floor basalts and the ultramafics to rocks described from the Komatii formation in the Barberton Mountain Land, South Africa, by R.P. Viljoen and M.J. Viljoen in 1970. The association of ultramafic lavas, ocean floor type lavas basic pyroclastics, ' chert - like' sediments, acidic lavas and low potassium basic intrusives (metadolerites) suggests that the Rambler area represents an ancient ocean floor complex - probably part of an ophiolite complex. The geology and mineralogy of several of the sulfide deposits are described, their textures being explainable in terms of the deformational and metamorphic events affecting the area. The sulfide deposits have been deformed and metamorphosed during the first constrictive deformation and their textures are the result of recrystallization under conditions of the biotite grade greenschist facies of metamorphism. Fluidinclusion filling temperatures on the quartz gangue in remobilised sulfide veins gave temperatures (uncorrected for pressure) of crystallization of 145~330 C. The geochemistry of the East Mine ore body is described and the genesis of the sulfide deposit s is discussed in the light of the geochemical and petrological data. Although it is difficult to establish clearly the mode of formation of the highly metamorphosed and deformed sulfide deposits the bulk of available in formation favours a 'syngenetic’ rather than an ' epigenetic replacement’ mode of formation.

Gold occurrences in the Lupa goldfield, SW Tanzania, are typical of the orogenic Au deposit type. Auriferous quartz veins and mylonitic shear zones cross cut Archean granitoids (ca. 2740 Ma) and Paleoproterozoic felsic-mafic intrusions (1960–1880 Ma) comprising a Paleoproterozoic magmatic arc at the Tanzanian cratonic margin. Lu-Hf zircon results provide evidence for ca. 3.1 Ga basement beneath the Lupa goldfield and imply significant portions of the Ubendian Belt represent re-worked Archean crust. A greenschist facies metamorphic mineral assemblage overprints all of the magmatic phases and U-Pb dating of syn-metamorphic titanite hosted by a foliated Archean granitoid at ca. 1950 Ma suggests the onset of this tectonothermal episode occurred during the Paleoproterozoic. The majority of mineralization is hosted by a network of brittle-ductile and mylonitic shear zones, which record evidence for transpressional deformation of triclinic, or lower order, symmetry. Relacement of igneous feldspar (plagioclase and K feldspar) with intrinsically weaker phyllosilicates, during sericitization of the granitic wall rock, created the ideal conditions for strain localization and locally may have led to the onset of crystal plastic deformation processes. Continued feedback between fluid, rock and deformation generated interconnected networks of weak mylonitic shear zones that are subject to reactivation. Quartz veins are the other significant host for Au and possess geometries that imply mineralization occurred concurrently with episodic fluid pressure fluctuations. Re-Os molybdenite, pyrite and chalcopyrite geochronology ages record a protracted metallogenic history and provide evidence for at least two mineralizing events at ca. 1940 and 1885 Ma. Each metallogenic event is represented in detail by a hydrothermal history that occurred at a time scale less than the resolution of the Re-Os method. High precision U-Pb zircon ages for the Saza Granodiorite overlap with Re-Os ages and provide unequivocal evidence for magmatism concomitant with sulphidation, however the wide range of Re-Os sulphide ages precludes a genetic relationship between any individual intrusion and Au. The goldfield-wide metallogenic event at ca. 1885 Ma occurred concurrently with eclogite facies metamorphism elsewhere in the Ubendian Belt and provides one of Earth’s earliest examples of subduction processes temporally linked to orogenic Au deposit formation. Mesoproterozoic Re-Os ages (ca. 1125 Ma) correlated to the Kibaran/Irumide Orogenies and compatible with inferred Pb loss events provide evidence for sulphidation during at least two discrete orogenic cycles and suggest mid-crustal mylonitic shear zones represent long-lived zones of structural weakness.

No description available.

Examination of the opaque minerals in an intrusive sheet of analcite dolerite shows that they consist partly of a cubic solid solution, referred to for brevity as 'magnetite', and partly of a rhombohedral solid solution referred to as 'ilmenite'. Systematic variation of the properties of the 'magnetites' indicates an increase in titanium dontent towards the more slowly cooled central part of the sheet. 'Ilmenite' usually occurs as oriented intergrowths in 'magnetite’ crystals and contains 10-20% ferric oxide in solid solution. Less commonly, ‘ilmenite' also occurs as a discrete phase believed to have crystallised before the formation of the 'magnetite'-'ilmenite' intergrowths, and this seems not to contain ferric oxide in solid solution. 'Magnetite’ -'ilmenite' relationships are considered in terms of an Fe(_3)O9_4)-FeTiO(_3) phase diagram constructed from the experimental data of previous workers. The content of opaque minerals decreases regularly from 9% in the marginal rocks to less than 1% at the centre of the intrusion. The intensity of natural remanent magnetization decreases systematically away from the margins of the sheet and is clearly dependent on the content of 'magnetite' in the intrusion. The intrusion is reversely magnetized and has a mean direction of magnetization that confirms its age as Tertiary in agreement with geological data. However, there is considerable variation in the palaeomagnetic directions within the sheet due to the multiple natures of the intrusive processes. This is especially clear in the inclinations. The contact relations of the intrusion suggest that the reverse magnetization is a primary feature although the possibility of self-reversal cannot be ruled out entirely. Constructional details are given of instruments designed for the accurate automatic measurement of the Curie temperature of a minute ferromagnetic sample, and for the determination of the saturation magnetization of such samples at room temperature.

Understanding source-deposit relationships in VMS systems is important for mineral exploration and to increase knowledge of seafloor hydrothermal processes and ocean–crust fluxes. Although it is known that metals are stripped from oceanic crust by hydrothermal fluids and redeposited in orebodies, some aspects are poorly understood. It has been proposed that metal depleted epidosites (epidote–quartz–chlorite–Fe-oxide–titanite units within sheeted dyke complexes) are source rocks for Cyprus-type VMS deposits. To test this hypothesis, the Spilia-Kannavia epidosite zone in the Troodos ophiolite, Cyprus, was investigated. This zone (≥1.9 km3), at the base of the Sheeted Dyke Complex, extends ~5 km parallel and ~2 km across dyke strike and vertically ≥400 m. Differentiation trends defined by LA-ICP-MS analysis of volcanic glass and SIMS analysis of minerals in fresh basalts indicate that olivine and pyroxene were important hosts of Co, Ni and Mn but not Cu or significant Zn. Protoliths for the Spilia-Kannavia epidosite zone were Zn rich, with moderate Mn and Co and low Cu and Ni relative to average Troodos basalts. SIMS analysis indicates that, during epidositisation, chlorite and amphibole are the only silicate minerals that retain Co, Ni and Zn, whilst epidote hosts Mn. Concentrations of Cu are low in all silicates. Base metal mobilisation was controlled by chlorite and amphibole dissolution although concentrations in chlorite and epidote vary with alteration. During alteration, the Spilia-Kannavia epidosite zone released ~0.4 Mt Zn, ~0.06 Mt Ni, ~5.2 Mt MnO, and 0.14 Mt Cu. This exceeds the Zn within any Cyprus-type VMS deposit, and provides enough Cu for a medium-large deposit. In comparison, published data suggest a similar volume of non-epidosite zone diabase could mobilise similar amounts of Zn, less Cu and Ni, and no Co or Mn, but it is not clear if all of these base metals are removed from the sheeted dyke complex. It is concluded that the epidosite zone formation is the only way to mobilise significant amounts of base metals within Cyprus-type VMS systems.

More than 20 bentonite deposits crop out on the islands of Milos, Kimolos and Chios, Aegean Sea, Greece many of which are currently under exploitation. The bentonite deposits have been formed at the expense of volcaniclastic rocks, probably pyroclastic flows in the majority of the deposits, under subaqueous conditions. The presence of abundant authigenic K-feldspar in most deposits suggests that alteration was diagenetic and took place at very low temperatures. Alteration of the glass involves mobilization of alkalis, and uptake of Mg and S. Al and Ti are essentially immobile while the behaviour of Si and Ca is controlled by both the nature of the parent rock and the composition of smectite. Fe displays small scale migration controlled by the prevailing redox conditions. Zr, Nb, Cr, Ni, V and the LREE are essentially immobile, while Ba, Sr, Rb, Zn, Y and the HREE are mobile. The behaviour of Th is controlled by the phase which hosts the LREE. The mobility of chemical elements has caused variation in the chemistry of the microenvironment in which smectites formed, resulting in large variations in the chemistry of smectites. Smectites might have been formed through an Ostwald Ripening-like process affecting the pore fluid chemistry and thus the chemistry of other phases like zeolites. Beidellites coexist with Cheto- but not with Wyoming-type montmorillonites. The crystal chemistry of smectites is affected by the nature of the parent rock, but the conditions prevailing during alteration might modify this "inherited" factor, as indicated by 1000 EPMA microanalyses which complement XRF, XRD, IR, DTA/TG, SEM, TEM and HRTEM data. Almost all deposits have been affected by a later hydrothermal alteration which has effectively "diluted" the original smectite content either by conversion of smectite to illite/smectite, kaolinite/halloysite and/or alunite or by precipitation of new phases (carbonates, phosphates, sulphides). Illitization of smectite probably proceeds by an Ostwald ripening-like mechanism characterized by low supersaturation conditions. The Greek bentonites have good swelling properties after Na-activation, and cation exchange capacity which ranges from 40 to 105meq/100gr. These properties are closely related in bentonites containing smectites with similar crystal chemistry. The bentonites with low CEC contain abundant opal-CT. The rheological properties vary and are probably related with the degree of disaggregation of the smectite tactoids. Low grade bentonites might be suitable for the drilling industry. Acid activation increased the surface area of the bentonites up to 5 times and rendered them suitable for decolourization of crude edible oils. However, the maximum bleaching efficiency is not associated with maximum surface area. Mg-smectites are activated easier, but their decolourization properties deteriorate faster than Al-smectites. High grade Greek bentonites have foundry properties comparable to those of commercial products after Naactivation. This treatment increases the wet tensile strength but its effect on the other foundry properties is unpredictable. The properties of the bentonites have been degraded by the superimposed hydrothermal alteration, in general, although in some cases limited kaolinization of smectite seems to have a positive effect on the rheological properties.