ABSTRACT: Mass Transfer in Sandstones as Indicated from Bulk Chemical Analysis
James R. Boles, Patrick F. Dobson
Conventional bulk chemical analyses are useful for monitoring mass transfer in sandstones. Hypothetical chemical reactions can be compared to observed mineralogical and chemical changes in the rock. Porosity and permeability also can be correlated to chemical parameters. The main limitation of interpreting chemical changes in sandstone composition is seeing through detrital variations.
Bulk chemical analyses of arkosic sandstones from the San Joaquin basin of California show trends not easily resolved by other techniques. Detrital components in the sandstones are quartz, plagioclase, and K-feldspar, in proportions of 2/1/1, and variable amounts of smectite clay. Calcite cement and plagioclase porosity associated with kaolinite are the main diagenetic phases. Covariations in molar Si-Al-Ca and Ti-Al distinguish calcite-cemented (low porosity-permeability), clayey (high porosity-low permeability), and leached sandstones (high porosity-high permeability). Calcite-cemented sandstones have relatively high Si/Al ratios, implying that cementation occurred in sands that initially had high permeability (due to lack of clay).
Al mobility in these rocks is difficult to assess due to variations in initial clay content, subtle variations in quartz-feldspar contents, and diagenetic changes in rock volume. Plots of Si vs. Al ratioed against relatively immobile components not in plagioclase, such as Ti or Zr, show that plagioclase dissolution does not result in loss of Si and Al on a centimeter scale in these sandstones. Kaolinite forms as a sink for Al and some of the released silica. Surprisingly, the additional silica released appears to be conserved within the sandstones, yet it has not been recognized in our petrographic analyses.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990