SEDIMENTARY ROCKS/Sandstones, Diagenesis and Porosity Evolution 149 Figure 14 Porosity/depth relationships for fields within the Ula Trend (Norwegian Central Graben) Intrafield porosity gradients are about twice those observed for water bearing sandstones (regional gradient) in the same area It is possible that oil em placement limited cementation within oil bearing reservoir inter vals From Kupecz JA, Gluyas JG, and Bloch S (1997) Reservoir Quality Prediction in Sandstones and Carbonates, American Association of Petroleum Geologists’ Memoir 69 Tulsa: American Association of Petroleum Geologists 8% porosity loss per extra kilometre of burial depth, whilst that seen in the Ula and Gyda Fields is 16% km (Figure 14) This same pattern of porosity loss also occurs within individual coarsening-up sequences within the reservoir interval, particularly in the direction of known mature oil source Detailed observations on the distribution of petroleum-filled fluid inclusions indicate an exponential decline in such inclusions from field crests to field flanks, with the same sort of distribution occurring in the individual retrogradational cycles The sympathetic patterns of porosity and fluid inclusion distribution are most easily explained by considering that diagenesis was progressively retarded as the fields filled with oil The coarse, permeable tops of the retrogradational cycles formed the natural migration pathways of oil into the structures and these, too, had retarded diagenesis Impact of Diagenesis on Porosity and Permeability From a physical perspective, sands and sandstones comprise two basic components: solid and void In the preceding sections, the intrinsic properties of the solid component, its grain size and sorting, and its mineralogy have been examined The void space is now examined The void in a sand or sandstone is porosity, an intricate network of pores connected by pore throats (Figure 1) At the Earth’s surface, the void space is commonly filled by a combination of water and air (depending on the elevation of the sandstone relative to the local water table) In the subsurface, the void space can, in addition to water, contain petroleum (oil and/or hydrocarbon gas) and possibly non-petroleum gas (CO2, H2S, N2, and He2) Figure 15 Measured relationship between porosity and per meability for the Fontainbleau Sandstone, and modelled relation ship for a monodisperse sphere pack with a grain size of 0.2 mm Reproduced from Cade CA, Evans IJ, and Bryant SL (1994) Analysis of permeability controls: a new approach Clay Minerals 29: 491 501 Porosity is measured as a percentage (or fraction) of the rock plus void For sands, porosity commonly lies in the range 35–50% Well-sorted sands are more porous than poorly sorted sands, and loosely packed sands are more porous than tightly packed sands Sandstones commonly have a lower porosity than sands This is because compaction and mineral precipitation (diagenesis) reduce the pore space between grains In extreme instances, the porosity of sandstone can be close to 0% The permeability is a measure of the rate at which fluid can be transmitted through a porous medium It’s unit is the Darcy (D), such that a rock has a permeability of D if a potential gradient of 1atmÂ10 m induces a flow rate of 10 m3 s 10 4m and a liquid viscosity of 1cP For loose sands, the unit of permeability is the darcy, whereas, for sandstones, a more convenient unit is the millidarcy There is no particular reason why porosity and permeability should be related, other than that, for a rock to have non-zero permeability, it must also have non-zero porosity However, for individual sands and sandstones, porosity and permeability are commonly positively correlated (more porous sandstones tend to be more permeable than less porous sandstones) Where it does exist in granular porous media, the correlation between porosity and permeability commonly reflects the variation in one or possibly more of the components For example, the Fontainbleau Sandstone of the Paris Basin is essentially monodisperse (perfectly sorted) and uncompacted However, the quantity of cement varies between 0% and about 40% For this