Réservoirs silicoclastiques très enfouis : caractérisation diagénétique et modélisation appliquées aux champs pétroliers du Viking Graben (Mer du Nord)

Abstract : In siliciclastic reservoirs, porosity loss is mainly due to the mechanical compaction in the first 2-3 km of burial, the chemical compaction and mineral precipitation (quartz, clays, carbonates). Therefore, understanding the processes responsible of the inhibition of compaction and/or cementation permits to: i) contribute to the knowledge of the water-hydrocarbon-solid interaction mechanisms in deep diagenetic environment, ii) give new arguments for the reconstruction of fluid pathways at the basin scale iii) assist the oil industry for the intensive exploration. Analytical techniques such as quantitative petrography, fluid inclusion and basin/diagenesis modelling were coupled across 8 oil fields (and 11 wells) located in the Q3 block (Viking Graben, Northern North Sea) in order to understand the processes driving the variation of the Brent reservoir quality (Middle Jurassic). Quantitative petrographic study of Tarbert reservoir allowed to define similar depositional settings (mineralogy, porosity) among the 183 studied samples. The common diagenetic paragenesis is dominated by quartz cement, two generations of kaolinite (K1, associated with the destabilization of micas and K2, associated with the dissolution of feldspars), and precipitation of illite. The petrographic data do not explain the wide range of measured porosity and permeability on plugs from 8 to 27% and from 0.2 to 5000 mD respectively. In contrast with the conventional diagenetic model, the present study shows that inhibition of quartz cements did not play a major role in the preservation of porosity in the Q3 block. P-V-T-X-t estimates of fluid inclusion trapping coupled with basin modelling allowed reconstruction of three fluid migration pathways, associated with fluid overpressures in the Tarbert reservoir. 1) The northern part of the Viking Graben kitchen supplies Hild, Jura and Islay fields with an early migration of light oils (65-42 m.y.) and condensate from 35-15 m.y. Both migrations are associated with a great fluid overpressure from 100 to 200 bar. 2) The eastern part of the East Shetland kitchen supplies Alwyn, Dunbar, Grant fields, with heavy to light oils from 42-35 m.y., associated with a low fluid overpressure (30-40 bar). 3) The southern part of the East Shetland kitchen (long distance migration) supplies Forvie North and Central fields with a very late gas condensate migration (> 15 Ma). The relative timing of the fluid overpressure build-up with the degree of mechanical and chemical compaction appears to be the first order parameter governing the preservation of reservoir porosity across the Q3 block. The presence of unusual HT-LP (high temperature-low pressure) hydrocarbon inclusions in the fields near the graben could indicate an early heavy oil generation under hydrostatic pressure conditions. Although having no impact on the inhibition of effective stress, this high-temperature fluid migration could be attributed to thermal anomalies in the Northwest of Europe related to the North Atlantic opening. The combination of quantitative petrography, fluid inclusion and basin modelling allowed to point out the impact of regional fluid migrations on the well scale diagenesis and on the siliciclastic reservoir quality preservation
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Anthony Ong. Réservoirs silicoclastiques très enfouis : caractérisation diagénétique et modélisation appliquées aux champs pétroliers du Viking Graben (Mer du Nord). Sciences de la Terre. Université de Lorraine, 2013. Français. ⟨NNT : 2013LORR0065⟩. ⟨tel-01749803⟩

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