| Summary: | Reservoir quality relates to the presence of porosity and the connectivity of the pores of the reservoir rock which controls permeability. Deeply buried sandstones in sedimentary basins lose their reservoir quality due to compaction and cementation. Quartz cement is the most volumetrically important porosity occluding cement in sandstones buried to depths greater than about 3000m. Precipitation of quartz cement requires a source of silica, transportation of the silica in solution from source to the point of precipitation and clean grain surfaces to grow on. For these processes to take place water is required to dissolve the mineral grains that provide the source of silica, to provide aqueous fluid pathway from the site of mineral dissolution to the site of precipitation and water is required at the site of mineral precipitation to enable mineral growth. In oil or gas fields, displacing the aqueous pore fluid by petroleum disrupts the pathway between the reactants and points of precipitation. If the oil saturation becomes high; (i) the residual (irreducible) water becomes isolated within a continuous hydrocarbon phase or (ii) the aqueous pathway becomes tortuous and diffusion becomes slow or (iii) grain surfaces become coated by oil if the sandstone is oil wet. However, over the years a controversy has developed as to whether oil emplacement into reservoir stops quartz cementation and preserves porosity at depth. The research presented in this thesis used core samples, wireline logs and well reports collected from the oil and water legs and the transition zone of Upper Jurassic marine sandstones of the Ula and Tambar fields from the Norwegian North Sea. They were studied using a range of techniques: core analysis, core logging, downhole wireline analysis, optical microscopy, scanning electron microscope (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), fluid inclusion UV-petrology and thermometry. The distributions of all potential controls on porosity and permeability have been quantified so that it has been possible to assess the influence of all possible controls on quartz cement as well as fluid type. Thus the roles of depositional facies, grain size, sorting, chlorite coats and microcrystalline quartz coats have all been assessed. The main diagenetic cements in both Ula and Tambar fields are quartz overgrowths, grain coating microcrystalline quartz, K-feldspar cement, illite, dolomite and minor amounts of calcite and chlorite. Fluid inclusion evidence shows that quartz cementation was probably a continuous process and is still taking place in both fields. Quartz cementation occurred in the presence of some oil, as shown by the presence of oil inclusions within quartz cement in both Ula and Tambar. However, there are far fewer oil inclusions in quartz cement in Tambar than Ula suggesting that oil emplacement occurred later in Tambar than Ula. There is less quartz cement in the coarser grained sandstones in the oil legs than the water legs of Ula and Tambar suggesting that quartz cementation has been inhibited, in these facies, by the addition of oil. Finer-grained facies in both fields have more grain-coating microcrystalline quartz that has effectively inhibited quartz cementation in both oil and water legs. Stable isotope data show that the carbonate cements in the oil leg grew at relatively lower temperatures than those in the water legs. Precipitation temperature for carbonate cement in the oil leg stopped at the time of oil emplacement but carbonate cements in the water leg carried on growing or recrystallising at higher temperatures and show progressive input of source-rock derived CO2 (as the carbon isotopes get progressively lighter) in both Ula and Tambar fields. Reservoir quality in the Ula field is primarily controlled by a combination of depositional facies, mechanical compaction and early oil emplacement and locally by facies-controlled microcrystalline quartz. In the Tambar field early-formed grain-coating microcrystalline quartz mainly controls the reservoir quality and effects of oil emplacement are not as significant as in Ula due to the later oil charge. The results of this work have academic and economic significance. Understanding the controls on reservoir quality and the effect of oil emplacement on quartz cementation may be used (1) as analogues in other basins of known petroleum charge history, (2) to improve appropriate reserve calculation and well planning during the appraisal stage and (3) to assist in reliable prediction of aquifer performance during production, and lead to proper decision on number and positions of injection wells during the later life of the field.
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