Abstract: Inferring Fracture Permeability from Rock Microstructure

LAUBACH, STEPHEN E., and RANDALL MARRETT

Although the degree to which fractures are open and interconnected governs their ability to transmit fluid, information on fracture apertures and connectivity is usually incomplete. Hydraulic apertures depend on fracture growth, diagenesis, and modern state of stress. Many rocks contain fractures that are lined--but not locally bridged--by delicate acicular crystals that demonstrate mechanical closure has not occurred since mineral growth (in some cases, greater than 100 m.y.) even where fractures are not aligned with SHmax and pressures are near hydrostatic. Elsewhere fractures are occluded by diagenetic minerals. Using temporal relations of mineral precipitates and microstructural movement indicators, cemented rocks can be subdivided into those dominated, volumetrically, by pre-, syn-, and postkinematic cements. SEM-based cathodoluminescence observations reveal that synkinematic cements are marked by competition between fracturing and cement precipitation that tends to preserve fracture porosity. The principal cause of fracture closure in these rocks is occlusion by postkinematic cement. Core, well log, and production data from oil and gas reservoirs show that the volume of postkinematic cement is an indicator of fracture occlusion.

Mechanical apertures, connectivity, and scaling patterns of these properties reflect fracture propagation and linkage. Scaling relations provide the basis for predicting spatial frequencies of opening-mode macrofractures from observed frequencies of microfractures. Fractures having mechanical apertures smaller than 1 micrometer and larger than 10 cm follow power-law scaling, although individual data sets typically span only 1-2 orders of magnitude in this scale range. Study of coupled diagenesis and fracture processes--and resulting microstructures--provides new types of evidence for inferring fracture permeability.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria