Experimental characterisation of the hydrocarbon sealing efficiency of cap rocks

Abstract

Jurassic shales and mudrocks from the Haltenbanken area offshore Norway and red claystones from Carboniferous and Permian intervals of Northern Germany were used in a study of the hydrocarbon sealing efficiency of clastic sediments. The investigations comprised geochemical and mineralogical analysis of the pelitic rocks, petrophysical characterisation by mercury porosimetry and specific surface area measurements, and laboratory experiments to assess the transport properties with respect to both molecular transport (diffusion) and volume flow (Darcy flow). Effective diffusion coefficients of methane in the water-saturated rock samples at 150°C lay between 1.4 × 10 −11 and 4.5 × 10 −10m 2/s and showed a distinct correlation with TOC content. Permeability coefficients, measured by means of a steady-state method, ranged from <1 nDarcy (<10 −21m 2) for Permian (Rotliegend) and Carboniferous red claystones up to 4.3 μDarcy (4.3 × 10 −18m 2) for a bioturbated Jurassic siltstone. The experimental data were used to calculate maximum sustainable gas and petroleum column heights, hydrocarbon leakage rates by pressure-driven volume flow (Darcy flow), and diffusive gas losses for simple, hypothetical scenarios. Computed maximum gas column 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 thickness, diffusive losses can be expected to require tens of millions of years to significantly affect the contents of commercial size natural gas reservoirs.