Quartz Cementation in Sedimentary Basins

Abstract

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. However, sandstones buried to less than about 2.5-3.0 km have very little quartz cement. 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 cement, but the volume of silica released is difficult to quantify. 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 10[sup 8] Cm[sup 3]/cm[sup 2] would be required. Calculations indicate that for cooling (rising) pore water, the rate of calcite dissolutionmore » 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 several times higher than for quartz. We suggest 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. 60 refs., 5 figs., 1 tab.« less