Cap Rock

Definition

The upper rock material that is more resistant to erosion than the underlying rock material; it also refers to a sedimentary unit of lower hydraulic conductivity than that of the underlying oil or gas reservoir rock that restricts upward migration of hydrocarbons, thus effectively capping the reservoir.

In geomorphology, the upper rock material that is more resistant to erosion than the underlying rock material is called cap rock. Cap rock typically forms a distinctive ledge at the crest of an escarpment (Fig. 1). An irregular escarpment that extends for more than 250 km in the northern part of western Texas in the American southwest marks the boundary between a gently undulating upland surface known as the High Plains of West Texas and New Mexico, with elevations ranging from 1,000 to 1,500 m, and the dissected rolling plains of Central Texas to the east, with elevations typically 300–500 m lower (Collins 1984). Approximately 120 km southeast of Amarillo, Texas, is Caprock...