ABSTRACT: Role of Compaction in Determining Sandstone Porosity
David W. Houseknecht
At the depositional surface, porosity of sand is controlled by sorting and packing arrangement, both of which reflect depositional processes. With burial, that original porosity can be reduced by compaction and cementation, and can be increased by dissolution of grains and/or cement. The relative importance of compaction and cementation to porosity reduction can be effectively evaluated by quantification of intergranular volume and cement volume. Intergranular volume (IGV) is irreversibly reduced by compaction and is occluded but not destroyed by cementation. Compaction and cementation fundamentally control the intergranular porosity of a sand because intergranular porosity = intergranular volume - intergranular cement.
During relatively shallow burial, intergranular volume is reduced by mechanical compaction, which includes rotation of brittle (nonductile) grains, fracture or cleavage of brittle grains, and plastic deformation of ductile grains. The extent to which mechanical compaction reduces IGV depends on original texture and framework grain composition. In well-sorted sands (original IGV ^sim40%) composed exclusively of brittle grains, mechanical compaction can reduce IGV to 20-30%, depending on quartz content and grain shape. In well-sorted sands composed of 25% or more ductile lithic fragments, mechanical compaction can totally eliminate IGV. In sandstones containing chemically unstable framework constituents, grain dissolution commonly results in additional mechanical compaction. Porosity ge erated by grain dissolution during relatively shallow burial is reduced significantly by mechanical compaction during deeper burial unless sufficient cement has been precipitated to act as stress bridges around grain dissolution pores.
During relatively deep burial, intergranular volume is reduced by chemical compaction, which involves intergranular pressure solution (and, locally, stylolitization). The extent to which chemical compaction reduces IGV depends on a number of interactive variables. Chemical compaction is promoted by fine grain size, illitic grain coatings, rapid burial, deep burial, high temperatures, and fluid flow. In contrast, chemical compaction is inhibited by the presence of cement, slow burial, and overpressured conditions. In quartzarenites, chemical compaction may reduce IGV to 15% or less.
Quantification of compaction and cementation in sandstones ranging in age from Miocene to Ordovician reveals that compaction (both mechanical and chemical) systematically reduces IGV with increased burial, whereas cementation is locally variable and reduces porosity by occluding the IGV established by compaction. Thus, an understanding of how compaction proceeds in sandstones of various compositions in a variety of basin types is fundamentally important to the predicting porosity.
AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990