Reservoir rocks

An oil and gas reservoir may be defined as a body of porous and permeable rock containing oil and gas through which fluids may move towards openings under the pressure that exist or that can be applied.

The lithologic properties of the reservoir rock are important in determining storage capacity resistance of flow and the rate at which fluids may enter the wells. The size and shape of the pore spaces, their continuity and the percentage of the total volume of the rock that they represent are factors of great importance. Reservoir rocks are usually sand, sandstones, unconsolidated sands, conglomerates or limestones: that is rocks affording sufficient space to permit storage of fluids to the extent of 10 per cent or more of the volume of the rock and having pore spaces sufficiently large and continuous to allow movement of fluids through them.

The term “oil sand” has often been loosely applied to all kinds and types of rock which serve as underground reservoirs for petroleum. The term is known to have originated in Pennsylvania, where the oil reservoir rocks in the first wells drilled were sandstones. These sandstones, after being finally pulverized by the drill bit and hoisted to the surface in a bailer, appeared to the drillers as sands and since they contained oil, the name “oil sand” was readily and conveniently adopted.

Sedimentary rocks, or rocks which have been eroded and transported by wind and stream action and finally deposited from suspension in water, often contain sufficient pore space after compaction and consolidation to permit accumulation of petroleum in commercial quantities. Oil being accumulated in sedimentary makes them one of the most important objectives for oil geologists. Common rocks of this type are fissured and sandy shales, porous or channeled limestones, and various gradations of sandstones.

Igneous rocks, or those that have been produced by the cooling of molten rock masses /magma/, on account of their being usually dense when in place and unaltered, seldom serve as reservoirs for oil and gas. Typical rocks of this class are granites and basalts /trap rocks/. However, sands and gravels composed largely of ingenious rock material which has been removed from the original ingenious rock mass by weathering and erosion sometimes serve as oil reservoir rocks.