Authigenic Silica in Shales - From Important Factor in Early Pore Occlusion and Fracturing Behavior?

Juergen Schieber
Geological Sciences, Indiana University, Bloomington, IN

The Late Devonian Chattanooga and New Albany Shales (eastern US), and the Campanian Kenilworth Member (Blackhawk Formation, Book Cliffs of Utah) have been studied by SEM, Scanned-CL, and TEM for microfabrics and mineralogical features.

In Kenilworth shales, silt and sand sized grains of volcanic origin are common, altered to clays (smectite), and squeezed and deformed during compaction. In the shale matrix abundant submicron size, rounded quartz grains appear to be of non-detrital origin. Differential compaction of clay minerals around these grains suggests an early diagenetic, pre-compaction origin. This quartz, constituting 20-30% of the background clay matrix, adds to the overall quartz content of the shale. Early diagenetic silica deposition occluded pore space and led to faster reduction of porosity and permeability during burial.

In Devonian black shales of the eastern US, organic matter-rich shales are associated with low fracture density. Interbedded gray mudstones show abundant fractures with limited continuity due to more plastic behavior. Early diagenetic cements of dolomite, quartz, phosphate, and pyrite formed close to the sediment-water interface and reduced early burial porosity. Laminae of wave and current reworked silt and sand improve lateral permeability, further improved by bioturbation that connects permeable layers vertically.

The sealing capacity of shale successions results from a combination of oceanographic (organic matter, cement minerals), sedimentologic (winnowing), and biological (bioturbation) factors, and by their history along the continuum of deposition, exposure, erosion, weathering, and final burial. These parameters need to be examined comprehensively for the prediction of sealing capacity in a sequence stratigraphic context.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas