Petrophysical Properties of Seals

Abstract

Seals are defined as generally ductile rocks with a very high capillary entry pressure that can dam up hydrocarbons. Petrophysical and petrographic studies of conventional and sidewall cores from known seal-reservoir couplets of hydrocarbon-producing reservoirs provide a basis to quantify the capacity of a rock to seal a hydrocarbon column. The most important property of a seal is its pore-size distribution as measured in thin section, scanning electron microscope, and very high pressure (up to 50,000 psi) air-mercury capillary pressure curves determined across bedding surfaces in vertical plugs. Seal quality or capacity is determined by pore-size distribution and interconnection and ductility. Using the density difference of normal saline water and 35{degree} API oil as a standard, an arbitrary scale of seal types has been developed. Comparators of known seal types composed of samples of the rock type, SEM photographs, and capillary pressure curves are used to estimate seal type under a binocular microscope at 50X magnification. A catalog of seal petrophysical properties and microphotographs allows one to make reliable estimates of seal capacity of unknown samples in cores and cuttings with a binocular microscope.