Effect of saturating fluid composition on the rock microstructure, porosity, permeability and Vp under high pressure and temperature

The experiments on amphibolite and sandstone samples pressurized by gas ('d ry conditions'), pure water and water solutions of different composition un der temperatures up to 850 degrees C at equal confining and pore pressure o f 300 MPa revealed the different Vp behavior. On heating under gas pressure Vp decreases within all the temperature range, on heating under pressure o f water and water solutions Vp at first decreases, but reaching a minimum v alue at temperature of about 650 degrees C increases. In order to interpret these data Vp, porosity, permeability and pore size d istributions were measured on quenched samples. Microstructure and micropro be investigations also were carried out. The results obtained revealed poro sity and permeability changes due to the rock microstructure changes with V p trends synchronously. These microstructure changes initiating Vp changes were associated with rock thermal decompaction and geochemical transformati on caused by the effects of water fluids of different composition: metasoma tic and hydration-dehydration reactions, silicification and phase transitio ns (alpha-beta quartz transition and partial melting). The experimental data demonstrated that the presence of fluids with differe nt composition might lead to occurrence of different Vp values in the rock of the same type. Hence, seismic boundaries may occur in the rocks of the s ame chemical composition. (C) 2000 Elsevier Science Ltd. All rights reserve d.