Cementation advective

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

Although the presence of cements along fractures is indicative of water flow, precipitation does not necessarily occur by advection, but rather by ionic diffusion from the host sediments. Advective cementation requires the circulation of huge water volumes. For each pore volume of cement, 10 to 10 water volumes are required (Bathurst, 1975 Wood, 1986 Sharp et al., 1988). The distinction between cements formed by diffusive and advective... 

When the rate of quartz cementation becomes significant at temperatures exceeding 80-100°C, the porosity loss is mainly due to quartz cementation. Calculations show that the advective transport of silica in the pore water can be ignored as the low flow rates due to compaction are so low and because of the low solubility and solubility gradients of silica. The rate of import or export of silica is a function of the water flux (F), the solubility of quartz as a function of temperature (at) and the geothermal gradient (VT)... 

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