Print this pageSyncementation Crack-Tip and Crack-Seal Microtextures and Their Implications for Fracture Connectivity and Porosity Interpretation
JOHNS, MARY K., STEPHEN E. LAUBACH, and KITTY L. MILLIKEN
In quartz-rich sandstones, temporal relations between fracturing and cementation can be deduced from textures visible using scanning electron microscope-based cathodoluminescence (scanned CL) imaging. Kinematic and cementation history recorded in Cretaceous sandstones from gas wells in the western United States reveal the changing character of connectivity and porosity in fractures.
A Dakota sandstone core shows multigenerational fracture growth near a step between two en echelon macrofractures. In plan view, early microfractures obliquely span the step between tips of large fractures, indicating low contrast between crack-normal and crack-parallel compressive stresses. These are crosscut by later microfractures aligned parallel to the macrofracture trend. Here microfractures record a growth history in the process zone surrounding the interacting crack tips. Their textures suggest increased far-field crack-parallel compressive stress and decreed fracture connectivity during fracture growth.
Several units, including Travis Peak and Frontier Formation, show crack-seal microtextures within quartz-lined macrofractures. Scanned CL reveals elongate shards that have been separated from wall rock by tens of episodes of cracking and sealing. Such textures are commonly associated with residual fracture porosity because cement precipitation rarely keeps up with fracture opening. Where not overprinted by postkinematic cement, these textures are indicators of potential fracture porosity.
Where microfractures are appropriate proxies for large fractures, such observations can be used to assess fracture connectivity and fracture openness even in the absence of fractures visible to the unaided eye.