Evaluating Seal Potential: Reducing Uncertainties in Determining Hydrocarbon Retention of Caprocks

Abstract

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. 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 petroleum systems and field scale prospects in the petroleum industry. Reducing the uncertainties associated with the evaluation of seals can be done by understanding the seal potential of the seal as well as of any faults or fractures which pass through it. Seal potential is defined as the 1) seal capacity, 2) seal geometry and 3) 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 possibly 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 any faults that displace it. Seal integrity refers to geomechanical properties of the caprock. These properties are controlled by lithology, thickness, ductility and fracture density, and are determined by microscopic and macroscopic analyses of the caprock. In addition, regional and local stress fields as well as any stress changes induced by tectonic activity, or the injection or withdrawal of water or CO2.influence seal integrity and must be taken into account. 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. 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.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019