Seals are defined as generally ductile rocks with a very high capillary entry pressure which 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 trap 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° API gravity oil as a standard, an arbitrary scale of seal types is defined:

SEAL TYPES
35° API Oil Column Held

Comparators of known seal types composed of samples of the rock type, SEM photographs and capillary pressure curve are used to estimate seal type under a binocular microscope at 50X magnification. A catalogue of petrophysical properties and photomicrographs allows one to make reliable estimates of seal capacity of unknown samples in cores and cuttings with a binocular microscope.

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