USING DIAGENESIS TO UNRAVEL FRACTURE HISTORY AND SELECT SUITABLE OUTCROP ANALOGS FOR BURIED FRACTURE SYSTEMS

Basinal fracture systems reflect interactions among mechanical and chemical processes integrated over geologic timescales. In the subsurface (>1000 m), where fractures are new rock surfaces created in the presence of hot, reactive fluids, fracture size distribution, aspect ratio and porosity distribution, spatial arrangement, and sensitivity to effective stress changes are among the fracture attributes modified by diagenetic reactions. One manifestation of diagenetic processes within growing fractures is crack-seal texture and associated fracture porosity. Core studies show that isolated mm-scale bridges of cement having crack-seal texture are widespread in opening-mode fractures in flat-lying and nearly flat-lying sedimentary rocks in basinal and platform settings as well as in 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. The message from widespread linked crack-seal textures and fracture porosity, and other evidence of interacting mechanical and chemical processes, is that models for subsurface fracture systems must have equal mechanical and diagenetic components. Here we report exceptionally well preserved basin fractures in outcrops from Mexico, Scotland and Wyoming where fractures possess characteristic crack-seal bridges. Associated patterns of power-law size distributions and clustering are accounted for with mechanical models that incorporate diagenesis. Dough requires cooking to be a cake, and outcrop fractures must have appropriate diagenetic attributes to be suitable guides to those in the subsurface.