The Precambrian-Cambrian Athel Silicilyte is an enigmatic chert unit of up to 390 m thick found as slabs (each slab typically 2 × 6 km across) entrapped within salt domes at a depth of 4-5 km in the South Oman Salt Basin. This formation is a prolific self-charged reservoir with high porosity (up to 34 %) and high oil saturation (80 %). Despite its economic value, the origin and the variability of this formation are not fully understood. This study therefore aims to investigate the variability and the origin (silica source and precipitation mechanism) of the Athel Silicilyte. Data obtained from core, wireline log and petrographical analysis were employed to establish the vertical and the lateral variability and, with the assistance of geochemical data, the likely source and precipitation mechanism of silica was determined.The Athel Silicilyte is only present in the deepest parts of the South Oman Salt Basin within the fault-bounded Athel Basin, bounded by two silt-rich mudstone units. Six lithofacies were recognised in the Athel Silicilyte that reflect variability in detrital material contents (three silica-rich facies), sediment remobilisation (slumped and brecciated lithofacies) and diagenetic modification (carbonate-bearing lithofacies), with the silica-rich facies being the most abundant (> 97 % of the total thickness). The Athel Silicilyte exhibits wavy discontinuous lamination and it is predominately (silica-rich facies average = 80 wt. %) composed of connected-networks of microcrystalline quartz (1-5 µm). Other constituent components are detrital quartz (3 wt. %), illite (10 wt. %), pyrite (4 wt. %) and organic carbon (TOC = 3 wt. %). The detrital contents increase towards the Upper Athel Silicilyte and towards the basin margins (ranging from 3 to 30 wt. %). The wavy laminations are interpreted to have a microbial origin. The homogeneity, loose packing of detrital grains and preservation of 390 m thick laminated fabric suggest that the Athel Silicilyte precipitated syndepositionally in microbial layers during low detrital input. The intense detrital input during bounding mudstones sedimentation inhibited Athel Silicilyte precipitation as a result of the rapid burial of microbial layers. The Athel Silicilyte shows strong enrichments of redox-sensitive elements (U, V and Mo), Mn-depletion, positive Ce anomalies, and small framboidal pyrites (3.6-4.0 µm), suggesting that the water column was euxinic during precipitation.Based on the petrographical evidence for the Athel Silicilyte forming as a syndepositional precipitate alongside seawater-like rare earth element (REE) characteristics, silica is interpreted to have been sourced directly from seawater. Mass balance calculations support this interpretation, indicating that silica-rich Precambrian seawater provided the significant silica mass in the Athel Silicilyte. The ability of dissolved silica to form hydrogen bonds with the functional groups (e.g. carboxyl and hydroxyl) in microbial layers was the key for Athel Silicilyte precipitation. The formation of hydrogen bonds was made possible under euxinic conditions, where the pH values were probably lower (< 7) than for the normal seawater as a result of HS- and H+ production by sulphate reducing bacteria and HS- oxidation at the redox boundary by sulphur oxidising bacteria. Consequently, dissolved silica was concentrated in microbial layer microenvironemnts, resulting in silica nucleation and polymerisation.

Made available in DSpace on 2014-12-17T14:06:55Z (GMT). No. of bitstreams: 1 CarinaGMM_DISSERT.pdf: 2881294 bytes, checksum: 3b5fb2cb077e76e3477ec04b95724ee5 (MD5) Previous issue date: 2009-12-15 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / One of the major current challenges for oilwell companies is the extraction of oil from evaporitic zones, also known as pre-salt basins. Deep reservoirs are found under thick salt layers formed from the evaporation of sea water. Salt layers seal the flow of oil from underneath rock formations, which store hydrocarbons and increase the probability of success in oil and gas exploration. Oilwells are cemented using Portland-based slurries to promote mechanical stability and zonal isolation. For pre-salt oilwells, NaCl must be added to saturate the cement slurries, however, the presence of salt in the composition of slurries affects their overall behavior. Therefore, the objective of the present study was to evaluate the effect of the addition of 5 to 25% NaCl on selected properties of Portland-based slurries. A series of tests were carried out to assess the rheological behavior, thickening time, free water and ultrassonic compressive strength. In addition, the slurries were also characterized by thermal analysis, X ray diffraction and scanning electron microscopy. The results showed that the addition of NaCl affected the thickening time of the slurries. NaCl contents up to 10% shortened the thickening time of the slurries. On the other hand, concentrations in excess of 20% not only extended the thickening time, but also reduced the strength of hardened slurries. The addition of NaCl resulted in the formation of a different crystalline phase called Friedel?s salt, where free chlorine is bonded to tricalcium aluminate / Atualmente, um dos grandes desafios para as empresas petrol?feras ? a explora??o de petr?leo em bacias dominadas por estruturas salinas, mais conhecidas por zonas evapor?ticas. A camada de sal ? formada pela evapora??o da ?gua do mar, cuja composi??o qu?mica inclui altas concentra??es de cloreto de s?dio (NaCl). Reservat?rios profundos s?o encontrados abaixo de espessas camadas de sal que vedam o fluxo de ?leo para fora da rocha, aprisionando hidrocarbonetos e aumentando, assim, a probabilidade de sucesso na prospec??o de ?leo e g?s. Atendendo ?s condi??es de po?o, s?o utilizadas pastas de cimento do tipo Portland para isolar, promover resist?ncia mec?nica ao po?o e impedir a intercomunica??o zonal das forma??es rochosas produtoras. Deste modo, as pastas de cimento devem atender ?s necessidades do sal, pois este se encontra na natureza com distintas caracter?sticas. Assim, este trabalho teve como objetivo avaliar o comportamento de sistemas de pastas saturadas com NaCl, em concentra??es de 5% a 25%, frente as propriedades f?sicas e qu?micas das mesmas. Tais propriedades foram analisadas atrav?s de ensaios laboratoriais de reologia, tempo de espessamento, ?gua livre e resist?ncia ? compress?o (UCA), bem como com o uso de t?cnicas de caracteriza??o t?rmica, difra??o de raios X e microscopia eletr?nica de varredura. Os resultados mostraram que a presen?a do NaCl, quando em pequenas concentra??es (at? 10%) atua como acelerador de pega. J? quando em altas concentra??es (a partir de 20%), n?o s? apresenta efeito oposto em rela??o ? pega, retardando-a, bem como atua deleteriamente na resist?ncia ? compress?o. Foi observada, tamb?m, a forma??o de uma nova fase denominada sal de Friedel, resultado da liga??o dos cloretos livres ao aluminato tric?lcico, incorporado ?s fases s?lidas do cimento hidratado

The Onion Creek Diapir is one of many salt domes proximal to the Uncompahgre thrust front of the ancestral rockies in the Paradox Basin. It is comprised of Paradox Formation evaporites and large blocks of Honaker Trail Formation carbonates that were deformed by loading of Permian Cutler Formation progradational alluvial to fluvial fans. The history of salt movement in the Onion Creek Diapir is recorded in the near-salt strata. Large salt bodies and their adjacent mini-basins evolve conforming to a complex relationship between salt withdrawal, creating localized accommodation, and sediment deposition. Migrating mini-basin depo-centers, thinned and folded strata, and spatial facies trends reveal the relative rates of diapirism and sedimentation. The study area outcrop, north of the diapir, is divided by significant stratigraphic horizons that help define depositional periods. Six measured sections in the study area reveal higher preservation rates of fine grained floodplain deposits, typically destroyed in alluvial environments, than at locations correlating to stratigraphic levels high in the outcrop suggesting a low accommodation environment evolving into higher accommodation where stacked channel complexes are preserved. Preserved slump folding at the base of the outcrop reveals that although some salt emergence occurred in the earliest depositional period it was not significant enough to preclude sediment deposition or to divert the Cutler fluvial network and destroy floodplain facies. A 3-D digital outcrop, modeled from photogrammetric data, illustrates the development of localized accommodation, attracting fluvial channel in a near-salt, tight axial syncline during the later depositional period. These evidences suggest a greater emergence of the diapir and likely diversion of the Cutler channel complexes.