(1) Indiana University, Bloomington, IN
ABSTRACT: SEM and TEM Study of Silica Diagenesis in Shales from the Cretaceous Blackhawk Formation, Book Cliffs, Utah: Implications for Sealing Capacity
Microfabrics and mineralogy of shales from the Kenilworth Member of the Blackhawk Formation (Campanian, Book Cliffs of Utah) were investigated by SEM and TEM. SEM examination shows that overall clay alignment and fabric anisotropy varies as a function of silt content and bioturbation, and also varies considerably over short distances (mm-scale). Silt and sand sized grains of volcanic origin are common. Their primary minerals are mostly altered to clays (smectite) and they frequently show deformation during compaction. Terrestrial organic particles with various degrees of silicification are conspicuous. The silica was probably derived from alteration of volcanic detritus, and possibly also from dissolution of biogenic opal. Under the TEM, submicron size rounded quartz grains are common. Their lack of sharp edges indicates a non-detrital origin. Set in a matrix of compacted clays, differential compaction of clay minerals around these grains indicates an early diagenetic, pre-compaction origin of the quartz. Compression of clay minerals around these quartz grains gives the shale a characteristic “augen”-like texture. The submicron “augen” quartz constitutes between 20-30% of the background clay matrix, and adds considerably to the overall quartz content of the shale. Similarly high proportions of “in situ” precipitated – rather than detrital – quartz have also been reported from other mudstone successions. These results indicate that early diagenetic silica deposition in pore spaces of compacting muds may be a significant factor that determines and influences porosity evolution and sealing capacity of mudstone successions.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.