Basement reservoirs are a special type of important oil reservoirs that until recently have generally been neglected as targets for exploration.

Most basement reservoirs occur either on platforms or in intermontane basins. They are rare in foredeep basins.

The basement reservoirs always occur on highs or uplifts within the basin, and have been subjected to long periods of weathering and erosion. Younger sediments (presumably source beds) directly overlie basement, providing opportunity for entrapment of oil in the basement rock.

Almost all basement reservoirs occur below a regional unconformity. All source beds of basement reservoirs lie above the reservoir rock, but all source beds lying above the reservoir rock do not necessarily form basement reservoirs.

Although most source beds lie above an unconformity, some lie below it and form a so-called second crop. Unconformities play an important role in basement reservoirs, as they are the passageway for oil migration, provide conditions for long-distance migration, and connect source and reservoir rocks which may lie on opposite sides of the unconformity and at great distances from each other.

The unconformity surface provides evidence that basement rocks have undergone weathering, erosion, solution, and leaching for so long a time that porosity and permeability have increased greatly, and the accumulation of petroleum was facilitated.

Oil may accumulate in any igneous, metamorphic, or sedimentary rocks with secondary fissures, dissolved interstices, and caverns, or in sandstone and carbonate rocks with primary porosity. Carbonates form the best basement reservoirs because, being hard and brittle, they not only develop secondary fissures, but are readily dissolved by ground water so that original pores are enlarged and new porosity produced.

Basement reservoirs are characterized by thick reservoir rocks. Porosity and permeability are irregular. Production from basement reservoirs is usually high and reserves are large.

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