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Sandstones may be viewed as chemical reaction vessels. The physical parameters of fluid-pressure and temperature are measurable and three dimensional observation of diagenetic minerals in the open pore spaces of sandstones permits correlation of mineral stability with these. Also, the chemical composition of formation waters in deeply buried sandstones from actively compacting basins appears predictable with respect to those ions that govern the stability of common authigenic minerals. The degree of compaction of argillaceous rocks and maturation of organic material largely determine the input of Ca⁺⁺, K⁺, CO₂ and H₂S into the sandstones and the semipermeability of the argillaceous caprocks determines the relative increase in concentration of ionized and non-ionized compounds at the sites of outflow of the waters from the sandstones.

These factors not only explain the formation of such common late diagenetic minerals as calcite, iron-rich carbonates, pyrite, kaolinite and chlorite but also lead to the concept of indicator minerals for hydrocarbon migration and the trapping potential of sandstones in compacting sedimentary sequences.

A first test of the applicability of this diagenetic exploration concept in a section of boreholes of different trapping potential on the Scotia Shelf showed very encouraging results.

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Supervisor: D. J. W. Piper

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