Final answer:
Sandstone is preferred as a reservoir rock over shale for conventional petroleum extraction due to its higher permeability, allowing for easier flow of hydrocarbons, and its layered characteristics that are more accessible through drilling.
Step-by-step explanation:
Sandstone is a much more common reservoir rock for conventionally extracted petroleum than shale because sandstone's permeability allows petroleum to flow more freely. Sandstone's composition, typically consisting of sand-sized grains, particularly quartz, provides the necessary pore space for oil and gas to accumulate and flow through. In contrast, shale has extremely low permeability, which hampers the flow of oil and gas, thus making it necessary to employ unconventional extraction techniques like hydraulic fracturing (fracking) to release the trapped hydrocarbons. Additionally, as a clastic sedimentary rock, sandstone often occurs in layers, which can be more easily identified and tapped by drilling, as opposed to the more consolidated and less porous shale deposits.