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Abstract: Comparative Study of the Sealing Properties of Cap Rocks from Northern Germany and Haltenbanken Area, Norway

Stefan Schlomer, Wolfgang Eckert, Bernhard M. Krooss

A comprehensive study was conducted to determine the sealing properties of 12 Permian and Upper Carboniferous cap rock samples from North German gas reservoirs and 15 Jurassic shales from the Haltenbanken area offshore Norway. The samples were characterised by standard mineralogical (XRD, SEM, optical microscopy) and geochemical (XRF, TC, TOC, Rock-Eval) methods. Samples from the North German Permian basin were red beds with essentially no organic matter (OM) comprising shales, siltstones and brecciae. The shale cap rocks from Haltenbanken had TOC contents between 0.8 and 5.5%.

Permeability measurements with fresh water yielded values ranging from below the detection limit of < 0.1 nanodarcy (10-22 m2) up to 100 nanodarcy for the red shales and siltstones. The brecciae which are envisaged as regional lateral seals had permeabilities around 1300 nanodarcy. Permeabilities of the Jurassic shales from Norway ranged between 2 and 400 nanodarcy.

Diffusion experiments were carried out on the water-saturated cap rocks with methane at 150° C. For the M-containing shales from Haltenbanken diffusion coefficients ranged from 1.4-11.4 × 10-11 m2/s whereas the values for the red bed samples from Northern Germany were generally higher (10-45 × 10-11 m2/s). The effect of O on the diffusion coefficient is attributed to sorption/desorption processes.

Mercury porosimetry yielded displacement pressures between 16 and 800 bar for the Hal\tenbanken samples corresponding to equivalent pore radii of 400-10 nm. For the North German shale cap rocks displacement pressures were usually higher and the pore radius distributions showed most prominent pore radii close to or below the experimental limit of 2.1 nm.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France