Final answer:
A sedimentary rock must possess high porosity and permeability to serve as a potential reservoir rock for oil or gas.
Step-by-step explanation:
The characteristic that is absolutely necessary for a sedimentary rock to have potential as a possible reservoir rock for oil or gas is porosity. Porosity refers to the empty spaces between grains in the rock, which allows for the storage of hydrocarbons.
In addition to porosity, permeability is essential, which is the ability of the rock to transmit fluids through it. Therefore, a good reservoir rock must have both high porosity to store oil or gas and high permeability to allow the movement of these fluids towards the production wells.