EXPERIMENTAL CHARACTERIZATION OF THE HYDROCARBON SEALING EFFICIENCY OF CAP ROCKS

Jurassic shares and mudrocks from the Haltenbanken area offshore Norwa y and red claystones from Carboniferous and Permian intervals of North ern Germany were used in a study of the hydrocarbon sealing efficiency of elastic sediments. The investigations comprised geochemical and mi neralogical analysis of the pelitic rocks, petrophysical characterisat ion by mercury porosimetry and specific surface area measurements, and laboratory experiments to assess the transport properties with respec t to both molecular transport (diffusion) and volume flow (Darcy flow) . Effective diffusion coefficients of methane in the water-saturated r ock samples at 150 degrees C lay between 1.4 x 10(-11) and 4.5 x 10-(1 0)m(2)/s and showed a distinct correlation with TOC content. Permeabil ity coefficients, measured by means of a steady-state method, ranged f rom <1 nDarcy (<10(-21)m(2)) for Permian (Rotliegend) and Carboniferou s red claystones up to 4.3 mu Darcy (4.3 x 10(-18) m(2)) for a bioturb ated Jurassic siltstone. The experimental data were used to calculate maximum sustainable gas and petroleum column heights, hydrocarbon leak age rates by pressure-driven volume flow (Darcy flow), and diffusive g as losses for simple, hypothetical scenarios. Computed maximum gas col umn heights range from 20 m up to >2000 m. Hydrocarbon column heights calculated on the basis of a rich condensate lay between 3 and 340 m. Depending on temperature, pressure, reservoir geometry and seal thickn ess, diffusive losses can be expected to require tens of millions of y ears to significantly affect the contents of commercial size natural g as reservoirs. (C) 1997 Elsevier Science Ltd.