Caprocks

Caprocks are geological formations that consist primarily of carbonate minerals, most commonly calcite or dolomite, which cap or seal subsurface hydrocarbon reservoirs such as oil and gas fields. These layers act as impermeable barriers, preventing the upward migration of fluids trapped beneath them. Caprocks are typically formed through diagenetic processes, including precipitation from groundwater, chemical alteration of pre-existing sediments, or the dissolution and reprecipitation of minerals due to pressure and temperature changes within the subsurface.

The effectiveness of a caprock as a seal depends on its thickness, lateral continuity, and the presence of microstructural features such as stylolites or clay minerals that reduce permeability. Common caprock types include chert, anhydrite, and evaporitic carbonates, though limestone and dolomite are the most widespread. These formations often develop in arid or restricted marine environments where evaporation rates exceed water input, promoting mineral precipitation.

In petroleum geology, caprocks are critical for trapping hydrocarbons in structural or stratigraphic traps. The integrity of a caprock can be compromised by fracturing, dissolution, or mechanical deformation, leading to leakage and potential hydrocarbon seepage. Geologists assess caprock quality through core analysis, seismic reflection studies, and well-log interpretations to evaluate their sealing potential. Understanding caprock properties is essential for exploration, reservoir modeling, and predicting the long-term stability of subsurface storage systems, including carbon capture and storage (CCS) sites.