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When a reservoir fills with petroleum, the process commonly takes about 10 Ma. Cementation of a sandstone reservoir can occur in a similarly short time. There is much direct evidence that these two processes can occur at about the same time. In petroleum-bearing sandstone reservoirs it is common to find inclusions of petroleum trapped in diagenetic minerals. The effect on reservoir porosity and permeability caused by the interaction of these two processes is dramatic. Fields with a relatively early petroleum charge, earlier that is than cementation, can be saved from the ravages of reservoir quality destruction. Fields in which the two processes were acting at the same time commonly display porositydepth gradients which are twice the regional average. If cementation beats petroleum in this 'race for space' there may be no field to produce, as the petroleum may not be recoverable from the low quality reservoir.We believe that the processes of petroleum generation/migration and cementation may share a common cause. Both processes are commonly associated with major changes within a basin: rapid burial/heating of the sediment pile and/or major faulting which changes the basin 'plumbing'.

The Vøring Basin forms an integral part of the passive margin off central Norway. Three phases of extension and three phases of compression record the transition from rifting to sea-floor spreading between Norway and Greenland. The first regional extensional event is of Early Cenomanian age, causing relative uplift of the Gjallar Ridge and development of a regional depocentre in the Rås Basin. A second extensional event is represented by Late Campanian faulting on the Nyk High and along the Fles Fault Complex. A compressional event in the Late Maastrichtian led to inversion of the Râs Basin. This was followed by a regional extensional event in Late Palaeocene, leading to the continental break-up between Norway and Greenland in Earliest Eocene. After the onset of sea-floor spreading, the Vøring Basin has remained in overall compression, with two major compressional events identified in Early Eocene and Mid-Oligocene.

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Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south-west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association 11) comes from meta-eclogitic pods with foliated amphibolite rims. In each case the sapphirine-bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite f plagioclase and records pressures of around 14-16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.The host gneisses equilibrated at approximately 8 to 10kbar and 700-750"C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16-1.0Ga. It is unlikely that the higher pressures recorded in the metaeclogitic pods represent an earlier high-pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association I1 this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750" C. These probably represent the peak conditions for granulite facies metamorphism in the south-west Grenville Province.