ABSTRACT

Integrated petrographic and burial-history studies of Fall River sandstones from outcrop and the subsurface provide insight into the timing of compaction and quartz cementation, the two main porosity-reducing processes in quartzose sandstones. Petrographic study of 95 thin sections of Fall River fluvial valley-fill sandstones from outcrop, Donkey Creek field at 2 km burial depth, and Buck Draw field at 3.8 km indicates that reservoir quality differs significantly in these three areas. Fall River sandstones at the surface contain an average of 31% intergranular volume (IGV) and 2% quartz cement. In both Donkey Creek and Buck Draw fields, the sandstones average 22% IGV, but quartz-cement volume averages 8% in the shallower field and 12% in the deeper. Geometric mean permeability at the urface is 4700 md, compared with 42 md at 2 km and 2 md at 3.8 km.

Burial history of the Fall River sandstone differs greatly in the three areas. The outcropping sandstones were buried to 2 km and had reached 80°C by the end of the Cretaceous. They were then uplifted and have remained at near-surface temperatures since the Paleocene; the calculated time-temperature index (TTI) of these sandstones is 1. Fall River sandstones at Donkey Creek were also buried to 2 km and had reached 80°C by the end of the Cretaceous but remained at that depth during the Tertiary; TTI is 14. In Buck Draw field, Fall River sandstones were buried to 2.5 km during the Cretaceous and then continued to subside during the Tertiary, reaching depths of 4 km and temperatures of 140°C; TTI is 512.

Much of the compaction of Fall River sandstones had occurred by burial depths of 2 km, but additional time at that depth was required for completion of mechanical compaction and for minor chemical compaction. Although greater depth and higher temperatures did not increase chemical compaction, increasing thermal maturity resulted in more quartz cementation. Internal sources of silica from stylolitization and intergranular pressure solution account for only 10-25% of the volume of quartz cement in Fall River sandstones. Imported silica may have come from clay-mineral transformations in Cretaceous shales. Loss of porosity and permeability was greatest between TTI of 1 and 14 but continued, at a reduced rate, as TTI increased.