Relationship between Acoustic and Petrophysical Properties of Permian Grainstones, Lawyer Canyon, Algerita Escarpment, West Texas

Janson, Xavier; Lucia, F. Jerry

This research investigates the relationship between acoustic properties and petrophysical properties within one outcropping grainstone cycle in the Permian San Andres Formation at Lawyer Canyon, Wets Texas.

The commonly accepted assumption for moldic carbonate rock is that rocks with rounded pores have a stiffer matrix and a higher Vp than rock with an high aspect ratio for a given porosity. One recent study has shown that the nature of the crystalline matrix between spherical pores is actually the main control on acoustic properties.

Acoustic properties were measured directly on the outcrop and on mini-core plug collected along a 900 meter transect within a single grainstone body that shows lateral changes from interparticle porosity dominated to oomoldic porosity dominated. The studied grainstones are dominated by small (150-µm) peloids, ooids, fusulinids, and mollusk fragments. Porosity varies from 11 to 29%, and permeability ranges from 0 to 60 md. Vp and Vs show variation up to 1500 m/s for a given porosity. The distinction in the velocity-porosity cross-plot between grainstone with interparticle porosity and moldic grainstone is not clear. If we plot acoustic properties against distance along the outcrop face, a separation in acoustic properties between the zone with moldic pores and the interparticle-porosity-dominated zone cannot be seen. Petrographic analysis under plain light microscope, shows no apparent difference between fast and slow samples for a given porosity. However, under UV light, samples with a lower-than-average velocity for a given porosity have a matrix between pores with significant amounts of intercrystalline microporosity. Conversely, rocks with a higher-than-average velocity for a given porosity all show a lack of microporosity in the matrix connecting the pores. The amount of intercrystalline microporosity in the matrix seems to be the primary control on stiffness of the rock framework and not the pore type, as previously assumed.

This study has direct implications for interpreting sonic log and calibrating seismic inversion techniques in reservoirs that have oomoldic pores.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013