Abstract: Evaluation of Seals and Flow Barriers

Robert M. Sneider

Seals and flow barriers are generally ductile rocks with a very high capillary entry pressure that can trap hydrocarbons. Seals are rocks or immobile fluids that lie above, lateral to, and below reservoirs; flow barriers are within reservoirs and impede or prevent fluid flow.

Petrophysical and petrographic studies of conventional and sidewall cores from known seal-reservoir couplets within hydrocarbon-producing reservoirs provide a database for quantifying the capacity of a rock to trap a hydrocarbon column. The most important property of a seal or flow barrier is its pore-size distribution as measured in thin section, scanning electron microscope images, and very high air-mercury capillary pressure curves measured across bedding.

Pore-size distribution and interconnection, and rock ductility determine the seal or the barrier capacity. Using the density difference between marine water and 35° API oil as a standard, an arbitrary scale of capacity is defined: type A, >300 m; type B, >=150 to <300 m; type C, >=30 to <150 m; type D, >=15 to <30 m; type E, <15; type F, waste zone rocks.

Catalogs of geological and petrophysical properties, and comparators of known seal/barrier types allow accurate estimation of seal and flow barrier capacity from cuttings and sidewall core samples examined with a binocular microscope. Key information in the seal/flow barrier catalog includes petrographic descriptions, porosity/permeability data measured under confining stress, SEM and thin section photographs, capillary pressure tests on horizontal and vertical cores and simulated cuttings, and acoustic properties. Empirical adjustment factors derived from catalog measurements allow one to estimate capacity from cuttings alone.

Oil and gas field examples show the influence of seal capacity on trapped accumulations and demonstrate how flow barriers influence primary production and supplemental recovery processes. Production buildup tests can determine if a fault is sealing or nonsealing. Seal/flow barrier capacity of some formations can be estimated from well logs when calibrated with rock information.

AAPG Search and Discovery Article #90949©1995-1996 AAPG International Distinguished Lecturers