Influence of pressure, salinity, temperature and grain size on silica diagenesis in quartzose sandstones

Abstract

Based on calculated quartz solubilities for near-neutral pH conditions in the 1–6-km depth range, there is a suggested predictable affect of pressure, temperature and salinity on pressure solution in quartzose sandstones. The following results were obtained: 1. (1) Quartz solubility related to increasing grain-contact pressure with depth is ∼ 7 times greater at 4 km than at 1 km ( m s = 1.5 · 10 −3 or 90 ppm vs. 2.0 · 10 −4 or 12 ppm, respectively). 2. (2) The difference between solubilities at grain contact and hydrostatic pressures increases with depth, excluding overpressuring (at 1 km Δm s = 1.0 · 10 −6 or 0.06 ppm; at 4 km Δm s = 4.7 · 10 −5 or 2.8 ppm; and at 6 km Δm s = 2.2 · 10 −4 or 13.0 ppm). 3. (3) Increasing pore-fluid salinity decreases solubility of quartz regardless of depth (e.g., at 3 km and m NaCl = 3, m s = 8.9 · 10 −4 or 54 ppm; at 3 km and m NaCl = 6, m s = 7.5 · 10 −4 or 45 ppm). 4. (4) Temperature has the greatest influence on quartz solubility of any parameter studied. Both high heat flow and high geothermal gradient enhance quartz solubility (e.g., at 1 km and 50°C, m s = 2.0 · 10 −4 or 12 ppm; at 4 km and 140°C, m s = 1.5 · 10 −3 or 90 ppm). These results indicate that pressure solution, as well as local and non-local migration and reprecipitation of silica, and thus reservoir quality destruction in quartzose sandstones, should be enhanced under higher temperature conditions, where grain-contact pressure is near lithostatic and pore-fluid salinity is low. Under conditions of low temperature, high pore-fluid salinity and grain-contact pressure close to hydrostatic, pressure solution is discouraged and higher reservoir quality more likely preserved. Grain-size differences in quartzose sandstones also influence pressure solution. For very fine-, fine- and medium-grained sand laminae, the average pressure solution index is about 3.3, 2.5 and 2 times that of coarse sand.