Quartz Cementation in Oil Field Sandstones: A Review of the Key Controversies

Despite quartz cement being the most important pore-occluding mineral in deeply buried (> 2500 m) sandstones, its origin and the controls on its distribution are still subject to disagreement and debate. Depending on the primary mineralogy and conditions of pressure and temperature, internal SiO₂ sources include pressure dissolution of quartz grains, feldspar-alteration reactions, clay-mineral transformations and dissolution of amorphous silica. However, sources external to a given sandbody are still proposed, despite the lack of direct evidence and problems of the low solubility of silica and the vast quantities of water required to accomplish quartz cementation. Redistribution of silica between facies in sandstone sequences and a systematic bias of sampling only reservoir facies may be partially responsible for apparent advective import. Quartz cementation is strongly affected by temperature, quartz only being a major cement in rocks that have been heated to above about 80°C. However, quartz cementation is likely to be a kinetically controlled process such that small amounts may develop slowly even at low temperatures. It is not yet proven whether quartz cement forms continuously at slow rates or rapidly over short periods. Pressure (effective stress) is potentially an important control on quartz cementation, since quartz cement tends to be less abundant in overpressured sandstones than in normally pressured sandstones. Primary lithofacies exerts a strong control on quartz cementation because quartz needs clean substrates to form: quartz cementation is most likely to be inhibited by the presence of grain coating clay, infiltrated clays and microquartz. The emplacement of petroleum in a sandstone probably inhibits quartz cementation although the degree of inhibition would depend on the remaining water saturation, wettability and fluid pressure in the sandstone. Import of silica through advection will be entirely halted in oil fields because the two-phase relative permeability of a reservoir to water becomes extremely low at high oil saturations. Direct empirical evidence of the effects of petroleum on quartz cementation are as yet lacking due to incomplete data collection. Fundamental rate data for many of the component processes involved in quartz cementation are known to different degrees. Although no predictive models yet incorporate all the sources and controls discussed in this paper, it should be possible, at some time in the future, to bring all the controls into a unified model. Realistic quantitative modelling of quartz cementation based upon all the potential sources of silica may ultimately be possible for a sandstone unit if silica sources, thermal history, transport mechanism, pore pressure history, local variations in lithology and petroleum-filling history for the reservoir are known or predictable.