INTERACTION BETWEEN DIAGENETIC REACTIONS AND MECHANICS OF OPENING-MODE FRACTURES IN SANDSTONES: THE MESSAGE FROM CRACK-SEAL TEXTURE

Fracture systems reflect interactions among mechanical and chemical processes integrated over geologic timescales. In the deep subsurface (>1000 m), where fractures are new rock surfaces created in the presence of high temperatures and reactive fluids, the effects of diagenesis on fracture attributes and mechanics can be significant. Profound effects on fracture mechanics arise from cement precipitation within the growing fracture system. Porosity, roughness, sensitivity to effective stress changes, and aspect ratio are among the fracture attributes modified by diagenetic reactions in the growing fracture system. One manifestation of diagenetic processes within growing fractures is crack-seal texture and associated fracture porosity. Although crack-seal texture is documented primarily in veins in low-temperature metamorphic rocks and faults, it is a widespread attribute of opening-mode fractures in flat-lying and nearly flat-lying sedimentary rocks in basinal and platform settings as well as fractures associated with faulting and folding. Where regional opening-mode fractures are unrelated to faults and folds, crack-seal texture reflects episodic fracture growth driven by interplay of cementation, compaction, stylolitization, pore pressure changes, burial-related temperature changes, and overburden and remote tectonic loads. In moderately to deeply buried sandstones, crack-seal texture is most frequently associated with quartz cement precipitation, possibly because throughout much of a sandstone's burial history, rock-dominated fluid and substrate chemistry dictates that this cement is most likely to precipitate. Use of SEM-based cathodoluminescence detectors reveals the complex nature of the crack-seal texture within quartz-lined macrofractures. Macrofractures in sandstones show crack-seal texture that is less regular and more variable along fracture length than the crack-seal texture typically shown in higher temperature veins.

The message from widespread linked crack-seal textures and fracture porosity and other evidence of interacting mechanical and chemical processes is that successful comprehensive models for subsurface fracture systems will have equal mechanical and diagenetic components.