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Assessing HPHT Sandstone Reservoir Quality: Identifying the Reality


Quartz cementation is one of the most important cements governing reservoir quality in sandstone reservoirs. The presence of clay mineral coatings plays a crucial role in preserving anomalous high porosity in deeply buried sandstones by inhibiting porosity-occluding macroquartz cementation. Previous laboratory experiments and modelling have identified the role played by high temperatures (>100°C) in controlling authigenic clay coatings on detrital quartz grains. It is evident that for higher temperature reservoirs, more robust and greater grain coating coverage is required to preserve significant amounts of porosity. However, the role played by pore fluid pressure in overpressured reservoirs has been identified as significant for preserving anomalously high porosities. Overpressures (low vertical effective stress) can limit and/or prevent quartz cementation by forestalling the onset of intergranular pressure solution. In this study, a series of hydrothermal reactor experiments have been undertaken to simulate quartz cementation and grain coatings, particularly chlorite, with different temperature, pore fluid chemistry and pore fluid pressures to mimic the conditions of deeply buried reservoirs. The experiments were performed in a Parker Autoclave Engineers self-sealing reactor vessel rated to 350 MPa and operating temperatures up to 350 °C. Clean quartz sand with no clay coatings or detrital grain components and naturally-occurring sandstone from the Lower Jurassic Cook Formation of the Oseberg Field (Norway), were used as starting material. Pre-existing berthierine, an aluminous Fe2+ rich clay which can act as a precursor for chlorite has been identified in the Cook formation and is of particular importance for the experimental procedure of this study. A variety of artificial solutions and a source of silica gel maintaining silica supersaturation during all experimental runs were used to elucidate this problem. Both starting material and end-products were inspected using, SEM/SEM-EDS, Micro-CT and automated mineralogical imaging and petrography of new mineral precipitates. The hydrothermal reactor experiments have revealed that quartz cementation can be achieved in the laboratory under controlled physico-chemical conditions. Preliminary results showed that the patchy amorphous berthierine clays transformed to crystalline robust grain coating Fe-chlorite cements facilitating better grain coat coverage. The applied methods have allowed for the first-time quantification of the grain coating volume increase at specific temperature steps. With increasing pressures, changes in thickness and morphology of authigenic chlorite are also observed. However, any breaks in chlorite coat coverage can be detrimental to reservoir quality resulting in macroquartz growth.