Quartz Cementation in Sedimentary Basins

Exploration in deeper parts of sedimentary basins requires a better understanding of the rate and timing of quartz cementation, one of the main factors controlling reservoir quality in well-sorted sandstones. In the recent literature on sandstone diagenesis, quartz cementation is frequently interpreted as having occurred at relatively shallow depths and at low temperatures. In continually subsiding sedimentary basins like the North Sea and the Gulf Coast, however, sandstones buried to less than about 2.5-3.0 km have very little quartz cement. Fluid inclusion data from North Sea reservoirs indicate that most of the quartz cement forms at temperatures exceeding 90–100°C. Fluid inclusion temperatures in quartz overgrowths commonly approach bottom-hole temperatures, suggesting that quartz cementation may continue, probably at a reduced rate, after oil emplacement. Evidence of local dissolution of quartz by pressure solution is usually well developed in North Sea sandstones that contain significant quartz cement, but the volume of silica released is difficult to quantify. Amorphous silica and opal CT may be important sources of silica for quartz cementation at 70–80°C, and the existence of such metastable silica phases at lower temperatures is evidence that quartz does not then precipitate for kinetic reasons. Modeling shows that diffusion of silica is insignificant on a large scale (hundreds of meters), but is important on a smaller scale, particularly when amorphous silica and opal CT are present. At higher temperatures, the pore water will approach equilibrium with respect to quartz. Quartz precipitation will then result from upward (cooling) pore water flow. Calculations show that if enough silica to precipitate a significant volume of quartz (1%) from external sources is to be introduced into sandstones by pore water flow, a flux of about 108 cm3/cm2 would be required. Such fluxes could be obtained only on a local scale and by assuming an extreme degree of focusing of compactional water or by thermal convection. Modeling of coupled carbonate and silicate reactions can be useful in constraining our theories of solid transport in sedimentary basins. Calculations indicate that for cooling (rising) pore water, the rate of calcite dissolution exceeds the rate of quartz cementation by a factor of 30 to 300, depending on the pH of the pore water, which is assumed to be buffered by the silicate minerals. If the pore water is buffered by the carbonate system, the rate of calcite dissolution upon cooling is smaller, but still several times higher than for quartz. If significant volumes of quartz cement are introduced by advective flow of cooling pore water, all calcite present should, in most cases, have been dissolved, given the higher reverse solubility/temperature function of carbonates. We suggest that the source of quartz cement in sandstones is predominantly local (<10 m), sourced from within the sandstones, mainly by pressure solution. Therefore, one must predict the porosity loss in reservoirs from internal textural and mineralogical properties of the sandstones in addition to temperature and pressure.