Examples of sandstone reservoirs are found in Western Canada Sedimentary Basin, coastal basins of Equatorial Africa, United States’ Gulf Coast, Rocky Mountains and Southern North Sea (Stoneley 39). 2. Carbonate Rock Reservoirs Carbonate reservoirs comprise of geologic formation that are naturally fractured which are characterized by heterogeneous permeability and porosity distributions. These are the predominant types of reservoirs all over the world accounting for about 50% of all reservoirs. The distinctive aspect of carbonate reservoirs is their intrabasinal origin. Their mode of formation was primarily dependent on organic activities. They were formed through biochemical processes in special environments. Organisms involved in their formation also contributed in determination of their qualities. Carbonate reservoirs are highly susceptible to processes of modification as a result of post-depositional mechanisms. The variations observed among various carbonate reservoirs result from processes such as lithification and compaction (Ahr 50). A notable feature of these reservoirs is their high sensitivity to environmental changes. Changes in temperatures affect biogenic activities thus affecting sediment production. This aspect is what makes carbonate reservoir development depth dependent. Favorable conditions enhance organic productivity while in unfavorable conditions productivity ceases. Carbonate reservoirs are considered autochthonous since they develop close to their final depositional sites. Water energy and basin configuration influences deposition which leads to formation of carbonate reservoirs. Examples of carbonate reservoirs are found in Big Horn Basin in Wyoming and south Caspian basin which encompasses...
They develop better vugs and fractures that are relatively important for flow and storage of fluids. Carbonate reservoirs are also harder and tighter than sandstone reservoirs (Renpu 9). Primary porosity of sandstone reservoirs is exclusively interparticle while that of carbonate reservoirs can either be intraparticle, interparticle, intercrystalline, vuggy, fenestral or cavernous. Porosity-permeability relationships of sandstones are relatively consistent and dependent on particle texture while that of carbonate reservoirs are highly varied and independent of particle size (Ahr 53).
Shale reservoirs differ from sandstone reservoirs in that shale reservoirs can contain organic mudstones which are not present in sandstone reservoirs. They can also have open fractures which are not common in sandstone reservoirs. Conglomerate reservoirs differ from sandstone reservoirs in that conglomerate reservoirs exhibit an extra-high heterogeneity as a result of a complex pore structure. They also display rapid changes in permeability and porosity between their layers which is not the case with sandstone reservoirs (Zou 298).
Advantages of Sandstone Reservoirs
Porosity of sandstone reservoirs is uniform over time since it does not diminish with increasing saturation of the reservoir. Chemical processes do not affect the permeability and porosity of sandstone reservoirs. Effects and of micro and macro organisms do not impede fluid transfer through sandstone reservoirs as can be the case with carbonate reservoirs (Renpu 11).