Interpretation of Compaction and Quartz Cementation Processes from Integrated Petrographic and Burial-History Analyses, Lower Cretaceous Fall River Formation, Wyoming and South Dakota

Shirley P. Dutton

The Fall River Formation, exposed in outcrop around the Black Hills Uplift, produces oil from reservoirs at depths of 1.8 to 4.0 km in the Powder River Basin. 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 these sandstones. Petrographic study of 85 thin sections of Fall River fluvial, incised-valley sandstones from outcrop, Donkey Creek field at 2 km, and Buck Draw field at 3.8 km, indicates that reservoir quality differs significantly in the three areas. Fall River sandstones at the surface contain an average of 30% intergranular volume (IGV) and 3% quartz cement. In both Donkey Creek and Buck Draw ields, the sandstones average 22% IGV, but quartz-cement volume averages 8% in the shallower field and 12% in the deeper. Permeability at the surface averages 4,800 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 but were then uplifted and have remained at near-surface temperatures since the Paleocene; 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 to occur. Greater depth and higher temperatures did not increase chemical compaction, but increasing thermal maturity resulted in more quartz cementation. Loss of reservoir quality was greatest at a TTI of 1 to 14 but continued, at a reduced rate, as TTI increased.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995