The
Mudstone-Dominated Caprock System of the CO2-Storage
Site at Ketzin, Germany
Förster, Andrea1, Niels Springer2, Gerhard Beutler3,
Joachim Luckert4, Ben Norden5, Holger
Lindgren2 (1) GFZ Potsdam, Potsdam, Germany (2) GEUS, Denmark (3) Martin Luther University Halle-Wittenberg, Germany (4) LBGR, Germany (5) GFZ Potsdam, Germany
Critical evaluation of the top seal
properties constitutes an integral part in Europe's first onshore pilot
storage of CO2 to be performed in 2007 at Ketzin,
in the Northeast German Basin. We report facies and physical and mineralogical properties of the
Upper Triassic (Middle Keuper) caprock
above the Stuttgart Formation serving as a CO2 reservoir. The Ketzin caprock sediments are of
continental playa type, consisting mainly of fine-grained clastics
(clayey and sandy siltstone) that alternate with thin-bedded lacustrine sediments (carbonates), and evaporates. The
playa-type succession is of basin-wide uniformity permitting marker horizons to
be traced over great distances. The caprock
characterization involved the study of outcrop analogues and analysis of well
logs and cores. Surface exposures offered a unique perspective on the stratigraphic relationships and are crucial in gaining a
better appreciation of factors that affect the sealing behavior. Analysis by
XRD, XRF, EMPA, SEM, AFM, MICP, DTA, and TG of a selection of core samples from
the Ktzi 38 borehole provided a first insight into
the expected sealing properties. The samples, mostly classified as clayey
siltstone cemented by dolomite, exhibit an average porosity of 10% and a
gas-permeability in the nanoDarcy range or below. The
rocks contain illite (42-74 wt%),
quartz (4-20 wt%), and dolomite (4-35 wt%), in addition to small fractions of
plagioclase (2-8 wt%), chlorite (1-3 wt%), amorphous phases, K-feldspar,
halite, and hematite. Pore bodies and pore throats are small (<500 nm and
10-36 nm, respectively), and the pore space is usually unconnected. The high
clay-mineral content and the observed pore-space geometry attest good sealing
properties.