Fabrics of Tertiary shales from the Rhinegraben (southwestern Germany) were studied by methods for fine-particle measurement including grain-size distribution, porosity, pore-size distribution, and specific surface area.

The most frequent pore radii (rm) are found to decrease with increasing overburden, this decrease in relation to porosity being more rapid in shales, which are cemented by carbonate. The specific surface area (Sg) is not affected by pressure down to a depth of approx. 1,000 m. With increasing compaction pressure the specific surface areas are found to decrease indicating a fusion of clay-mineral particles, which corresponds to the alteration of smectite minerals into 10-Å mica.

The sorting index of pore-size distribution (Sop) remains constant throughout the shallow-burial range. Below 1,000 m the sorting index increases considerably. This effect is caused by the coarse non-clay components of the shales, since on both sides of a nearly isometric quartz grain, for instance, large pores may be kept open even under high compaction pressure. This effect is also indicated by the skewness of the pore-size distribution, which shifts from negative to positive values near 1,000 m of overburden. Above this level fine pores predominate while deeper the relatively coarse pores are more abundant.

This study shows that the conventional mechanical model of a clay-mineral fabric reacting on overburden pressure is applicable to the shallow-burial range only. After mineral transformations have commenced, the fabrics loose their original grain-size distribution, so that their porosities are no longer controlled solely by mechanical effects. With the Rhinegraben shales the shallow-burial range is limited to a depth of about 1,000 m.