caprock

Caprock or cap rock is a hard, resistant, and impermeable layer of rock that overlies and protects a layer of softer material. In petroleum geology the softer lower layer may be a reservoir of oil or natural gas. The caprock acts similarly to the crust on a pie where the crust (caprock) prevents leakage of the soft filling (softer material). In landforms like mesas and buttes the caprock consists of sturdy erosion-resistant rock like granite or basalt that resists weathering more strongly than its supporting rock and results in differential erosion. It also influences hydrology by creating waterfalls and aquifers where the movement of water is restricted and channeled by the harder layers. As petroleum reservoirs caprocks are often buried under other layers of rock and protected from weathering, they can consist of softer materials like sandstone, limestone, and evaporites, as long as they are highly impermeable and resist the flow of hydrocarbons through them.

== Geological characteristics == Caprock is typically composed of erosion-resistant materials. Common caprock materials include strongly cemented sandstone, limestone, basalt, and evaporites like anhydrite, gypsum, or halite, which form over salt domes. The formation of caprock occurs through processes such as differential erosion, where resistant rocks remain as elevated features while softer rocks erode away; depositional processes, including chemical precipitation of volcanic activity; and diagenesis, where sediments transform into hard rock over geological time. These processes collectively create durable layers that shape landscapes and preserve subsurface resources.