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The ion content and mineralogy of a North Sea Cretaceous shale formation

  • T. G. J. Jones (a1), T. L. Hughes (a1) and P. Tomkins (a1)

Abstract

The shale from the Witch Ground Graben in the North Sea consists mostly of clay and calcite with relatively small amounts of quartz and plagioclase. The clay mineralogy is dominated by smectite and illite with varying amounts of chlorite and kaolinite. The ion composition was determined by ion chromatography, the porewater anions and cations being removed by leaching the shale with water, while the exchange cations were removed by reacting the shale with a large excess of tetramethylammonium ions. The clay mineralogy from XRD is consistent with the measured values of CEC. The water content and wireline log conductivity of the shale is controlled largely by the CEC (i.e. clay mineralogy). The cation content of the shale section is dominated by Na, with only small concentrations of K, Mg and Ca ions. The total anion concentration, which is dominated by chloride, shows an inverse relation to the concentration of exchange sites in the shale, suggesting that the compaction of the shale is to some extent controlled by a Donnan or salt-exclusion mechanism. Application of the Donnan equilibrium led to the reasonable conclusion that the shale is in compactional equilibrium with an external reservoir of about four times the anion content of seawater. The extent of salt exclusion in the shale is relatively low—a consequence of the high salt concentration in the external formation and the relatively low concentration of exchange sites in the shale.

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