Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Synkinematic Carbonate Fracture-Sealing Cements in Opening-Mode Fractures: Characteristics and Models
Opening-mode fractures in carbonate and siliciclastic rocks commonly contain carbonate cements. These cements can precipitate during (synkinematic) or after (postkinematic) fracture opening. In fractured dolostones, synkinematic cement is typically dolomite, whereas in limestones the synkinematic cement is normally calcite. Quartz is the dominant synkinematic cement in sandstones, although synkinematic carbonate cements have also been observed. In this contribution, we examine the morphology and internal structure of synkinematic carbonate cements in a variety of fractured reservoir rocks using SEM-based cathodoluminescence (SEM-CL), secondary- and backscattered-electron images, and element mapping. We also present image captures of geometric crystal growth models for dolomite fracture fill in fractured dolostones, and compare them with natural dolomite cement structures.
A systematic, size-dependent relationship governs the amount and internal structure of synkinematic cement. Narrow fractures seal completely, in some cases showing crack-seal texture that indicates multiple incremental openings. We have observed crack-seal texture in calcite, dolomite, and ferroan dolomite cements. Wider fractures are bridged and lined with cement and can preserve considerable effective porosity. Dolomite bridges commonly have rhombic shapes that our models have been able to replicate. In addition, the crystal growth model predicts morphologies that are analogous to features in SEM-CL images that superficially appear to be caused by crystal dissolution.