Publications

The accumulation of hydrocarbons requires a porous reservoir rock (such as a sandstone or limestone) overlain by an impermeable caprock or seal. The importance of the caprock is that it provides containment of buoyant hydrocarbon Any lithology can theoretically act as a caprock, however, shales, evaporates (halite and anhydrite) are the most common seals and are responsible for the majority of all trapping of oil and gas in hydrocarbon reservoirs as well as other gasses such as CO2, N2 and He. Factors such as lithology, thickness, ductility and fracture density influence the seal properties, and are determined by microscopic and macroscopic analyses of the caprock. Determining which seals have the potential to trap economically viable hydrocarbon accumulations, versus those that hold sub-economic volumes, has become an important aspect of evaluating both basin-wide hydrocarbon systems and field scale prospects in the petroleum industry. Similarly, determining the viability of caprocks for the retention of economic volumes of CO2 is a critical element in the selection of sites for safe CO2 injection and secure storage in commercial scale carbon capture and storage (CCS) projects. The successful evaluation of petroleum systems includes a thorough understanding of the seal potential of the top seal or caprock as well as of any faults or fractures which pass through the caprock. Seal potential is defined as the seal capacity, seal geometry and seal integrity of the caprock. Seal capacity refers to the hydrocarbon column height that the caprock can retain before capillary forces allow the migration of the hydrocarbon into and through the caprock. Seal geometry refers to the thickness and lateral extent of the caprock. The caprock must have sufficient lateral extent to cover whatever structural, stratigraphic or hydrodynamic trap is trapping the hydrocarbon accumulation. In addition, it must be thick enough to maintain an effective seal across faults that displace it. Seal integrity refers to geomechanical properties of the caprock. These properties are controlled by caprock mineralogy, regional and local stress fields as well as any stress changes induced by tectonic activity, or the injection or withdrawal of water or CO2. The modification of the effective stress state can lead to a number of deleterious effects ranging from fault reactivation to alterations of the actual in situ stress field. Faults and fractures may either enhance or retard the rates of fluid migration, and it is therefore crucial to fully understand the locations, geometries and permeabilities of such features. The presence of faults and their extent within caprock formations can be determined by seismic reflection techniques and analysis of well core or well bore imaging.