The physical, mechanical, and structural effects of diagenesis in siliciclastic rocks

摘要

Diagenesis results in systematic and predictable changes to sediment framework in siliciclastic sedimentary rocks. These diagenetic changes impact hydrologic, mechanical, and structural properties of the sediments. In well-constrained systems such as the St. Peter Sandstone, these changes can be quantified and used to calibrate physical models, improving predictive capabilities. As diagenesis progresses and porosity decreases, grain contacts increase in both number and length. Pores become smaller, more uniform in size, and more circular---changes that result in decreased permeability. The consistent, progressive nature of these changes allows us to calibrate the proportionality constant of the Kozeny-Carman relationship, improving permeability prediction.;These diagenetic changes progressively strengthen and stiffen the rock; increasing elastic moduli, confined compressive strength, and ultrasonic velocity. The number of grain contacts, a measure of the connectivity of the solid framework, is a good predictor of mechanical behavior. Prediction of mechanical behavior can be improved by considering the length of grain contacts as well. This is especially true for inelastic failure where a rock is subjected to stress states outside its range of elastic behavior.;In syntectonic siliciclastic rocks adjacent to the San Gregorio fault, preserved structures record transitions in mechanical behavior with progressive diagenesis. Progressive localization of structures, from distributed fabric development, to localized discrete deformation bands, and finally brittle deformation in outcrop-scale faults, is recorded through crosscutting relationships. Therefore, diagenetic changes that increase strength fundamentally affect the way sediment deforms. Deformation transitions from inter- to trans-granular as increasing intergrain strength results in progressive localization and dilatancy of deformation structures.