148 SEDIMENTARY ROCKS/Sandstones, Diagenesis and Porosity Evolution Figure 12 Global secular curve for sulphur and oxygen isotope covariance in marine derived sulphate minerals The Permian interval is highlighted, as is the distribution of data obtained from anhydrite and barite cements in the Rotliegend Sandstones of the Amethyst Field (North Sea) The sulphur isotope data clearly indicate derivation from the overlying Zechstein Modified from Gluyas JG, Jolley EJ, and Primmer TP (1997) Element mobility during diagenesis: sulphate cementation of Rotliegend sandstones, Southern North Sea Marine and Petroleum Geology 14: 1001 1012 Diagenesis and Petroleum Emplacement Figure 13 Pore water evolution for the Permian Rotliegend Sandstone of the Village Fields Area (North Sea) deduced from analysis of stable isotope ratios, fluid inclusion homogenization temperatures, and radiometric dating in and of mineral cements concretions, which precipitated soon after deposition, with surrounding sandstones It is tempting to deduce from such studies that (some) sandstones import silica and export potash during diagenesis Critics of such studies point to the data obtained from formation water isotopic analysis, which have been used to suggest that the water budget is severely limited in the deep subsurface and there is insufficient water to transport the observed cement volumes to the site of precipitation Others researchers invoke local sources of silica from pressure dissolution along stylolite seams, although this too is not a panacea, as many sandstones are without such pressure dissolution phenomena A possible relationship between diagenesis and petroleum emplacement has already been touched upon in the section on ‘Mineral Dissolution’ Here, the likelihood of significant porosity creation by organic acids was questioned Much more controversial is the effect on diagenesis caused by oil emplacement There are two extreme viewpoints: oil emplacement halts diagenesis by displacing the formation water, and diagenesis continues unaffected by oil emplacement It is probable that the truth lies somewhere between these two extremes Ample evidence exists of continued diagenesis in the presence of (possibly) low oil saturations Oilfilled fluid inclusions occur in many mineral cements (Figure 10) However, quantitative analysis of these same inclusion distributions often indicates that the presence of petroleum inhibits mineral precipitation Studies on several sandstones, including those from the Upper Jurassic of the North Sea, have shown that cementation and petroleum migration commonly occur at the same time In some papers, this has been referred to by the acronym SMAC (synchronous migration and cementation) and in others as the ‘Race for Space’ Oilfields so affected have highly porous sandstone at their crest and low-porosity sandstone at the oil–water contact The rate of porosity decline as a function of depth is perhaps twice that of the regional porosity gradient determined from waterbearing sandstones In the instance of the North Sea sandstones mentioned above, the regional gradient is