Abstract: The Making of a Quartzite: Where is the Quartzose Sandstone Porosity Basement?

W. Naylor Stone, Raymond Siever

Studies of deeply buried quartzose sandstones and low-grade metamorphic quartzites from the Greater Green River and Anadarko basins, the Sabine Uplift (East Texas), and the Pennsylvania coal fields have led to a general model for the quartzose sandstone lithification process. Compactional processes (mechanical and pressure solution) result in intergranular volume (= intergranular porosity + cement + matrix) decline to an average of 24% within the first 2 kilometers of burial. With additional burial, the dominant porosity decline process becomes quartz cementation. Quartz cementation requires the importation of large volumes of silica-bearing fluids; therefore, two of the rate limiting steps in the porosity decline process are the aqueous fluid flow rate and the rate of supply of silic to the fluids from sources external to the sandstone.

Empirical porosity prediction models which relate observed porosity and quartz cementation to thermal maturity allow an estimate of the mean and range in porosity possible at a given thermal maturity. Our current model (and data from the literature) suggests that average porosities are less than 5% at thermal maturities shortly beyond the end of the oil window. However, since small amounts of pressure solution or small amounts of quartz cement can result in high porosity packing frameworks that are stable to great lithostatic pressures, high porosity sandstones can exist at great depth. To explore for anomalously high porosity at high thermal maturities one must identify units which (1) have had low aqueous fluid flow rates throughout their burial history; (2) are distant from potenti l silica sources; or, (3) have grain surface contamination which prevents quartz cement nucleation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994