150 SEDIMENTARY ROCKS/Sandstones, Diagenesis and Porosity Evolution Figure 16 (A) Modelled relationship between porosity and permeability for monodisperse (perfectly sorted) sands of different grain size (B) Modelled relationship between porosity and permeability for a medium grained sand with different sorting characteristics (xw, vw, w, m, p, and vp srted denote extra well, very well, well, medium, poorly, and very poorly sorted, respectively) Reproduced from Cade CA, Evans IJ, and Bryant SL (1994) Analysis of permeability controls: a new approach Clay Minerals 29: 491 501 sandstone, there is a non-linear correlation between porosity and permeability on a semi-logarithmic plot (Figure 15) The increasing rate of decline in permeability at low porosity is due to progressive closure of the pore throats between pores The relatively simple relationship between porosity and permeability for the Fontainbleau Sandstone has been used as the foundation for a comprehensive predictive model for permeability based on a real physical model of a porous medium The model combines data derived from the perfectly sorted porous medium with empirical curves linking porosity and permeability for less well-sorted sands (Figure 16) Cements are then modelled as grain rimming or pore filling, and solid (such as quartz or carbonate) or microporous (clays) Controls on Diagenetic Processes In broad terms, near-surface diagenetic processes are much better understood than those occurring at depth Geochemical and isotopic studies have revealed the importance of bacterial reactions in modifying pore water and inducing the precipitation of carbonates, oxides of iron and manganese, and sulphides For the deep subsurface, we know less about what triggers diagenesis, although we can, as shown above, determine when and under what conditions diagenetic reactions occurred A recurrent observation is that major diagenetic events commonly accompany or follow immediately after significant geological events This is almost self-evident in the case of mineral dissolution beneath unconformities, but in other situations it is a little more subtle For example, most of the clay and carbonate minerals in the Permian Rotliegend Sandstone of the southern North Sea precipitated towards the end of the Jurassic, a time of major rifting in the area At the same time, there was a fundamental change in the connate water from Zechstein (evaporated seawater) derived to meteoric, yet saline, water There was also a dramatic loss of overpressure from the reservoir system (the reservoirs are normally pressured today) It is tempting to conclude that the rifting led to failure of the salt seals above the Rotliegend, and massive pore water revolution, so causing cementation In contrast, the Middle Jurassic Brent Sandstone over much of the northern North Sea was cemented at around the Paleocene to Eocene boundary This, too, may have been associated with the ingress of meteoric water as the rift shoulder became elevated Although it is possible that such external factors were the cause of cementation in these two sequences, it seems probable that the degree and style of cementation was controlled by the conditions within any particular sandstone (temperature, pressure, mineralogical composition) See Also Analytical Methods: Fission Track Analysis; Geochemical Analysis (Including X-Ray); Geochronological Techniques Diagenesis, Overview Fluid Inclusions Minerals: Feldspars; Quartz Petroleum Geology: The Petroleum System Sedimentary Rocks: Mineralogy and Classification Sedimentary Processes: Fluxes and Budgets Weathering Further Reading Burley SD and Worden RH (2003) Sandstone Diagenesis Recent and Ancient, Reprints Series, International Asso ciation of Sedimentologists, vol Oxford: Blackwell Science