Compaction of Lithic Sands: Experimental Results and Applications (1)

Abstract

Compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present. These experiments produced a product that is texturally indistinguishable from natural lithic sandstones. Generally, sedimentary lithic fragments are more ductile than metamorphic lithic fragments. Volcanic lithic fragments, when altered to phyllosilicate minerals by weathering or diagenesis, are extremely ductile. Mineralogy and intragranular microporosity influence ductility. Ratios based on weight percent of soft, ductile minerals (clay and micas) to hard, brittle minerals (quartz and feldspar) within the lithic fragments provides a good indication of ductility. A ratio greater than 1 indicates the lithic material will be h ghly ductile. Microporous lithic fragments are more ductile than dense lithic fragments. Physical compaction models were developed based on experimental data from well-sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress, the type of lithic material, and the quantity of lithic material. These models are derived from laboratory sandstones with at least 25% lithic grains in the following categories: moderately ductile metamorphic lithic fragments, highly ductile shale lithic fragments, and extremely ductile altered volcanic lithic fragments. Application of the compaction models of the experimental compaction techniques to modern subsurface sand samples (or outcrop equivalents) can provide valuable estimates of the preserved porosities (i.e., reservoir potential) for lithic sandstones that may be exploratory objecti es in frontier basins. Examples using the models and experimental compaction techniques for exploration are given. Experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation. An early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack. A later cement does not have this beneficial effect. Overpressure, when developed early, retards compaction by reducing the effective stress. Of course, a late-developing overpressure is not effective at preserving porosity because compaction is an irreversible process.