Effects of diagenetic processes on seismic velocity anisotropy in near-surface sandstone and carbonate rocks

Abstract

The effects of diagenetic processes on velocity anisotropy in Ferron sandstone and Ellenburger carbonate core samples are investigated using petrophysical/petrological and P-wave velocity ( V p) data at room pressure and ambient (air dry) saturation. The V p data are measured in both vertical ( V p v) and horizontal ( V p h) directions (parallel and perpendicular to the axes of the cores, respectively), using a 125-kHz sonic probe. The petrophysical/petrological properties for the Ferron sandstone are bulk porosity, fluid permeability and volume fractions of quartz, clay, feldspar and calcite; for the Ellenburger carbonate, they are bulk porosity, vertical permeability, horizontal permeability, maximum permeability, grain density and bulk density. In the Ferron sandstone samples, the V p h values are higher (∼8.5%) than the V p v values, which is attributed to laminated bedding or alignment of grain orientation. Correlations between V p anisotropy and minerals that represent diagenetic processes are clearly demonstrated in the Ferron sandstone. Anisotropy decreases with decreasing feldspar, increasing calcite and increasing clay. This correlation suggests that calcite cementation and pore-filling kaolinite reduce velocity anisotropy of the Ferron sandstone by reduction of the net grain alignment or preferred pore orientations. The heterogeneity of the Ellenburger carbonate caused by multiple karsting and burials does not provide observable correlations between the velocity anisotropy and petrophysical/petrographic properties. In the Ellenburger carbonate, V p v values are mostly higher (∼7.9%) than the V p h values and are likely to be related to near-vertically aligned regional fractures. The orientation and amount of velocity anisotropy and of permeability is scale dependent when fractures are present. Seismic anisotropy is closely related to subsurface lithology, rock fabric and texture, and to its diagenetic history. Consequently, analysis of seismic anisotropy provides an important tool for reservoir characterization and environmental and engineering applications.