E. Diaz, M. Prasad, M. A. Gutierrez, J. Dvorkin, and G. Mavko

Rock Physics Laboratory, Geophysics Department, Stanford University, Stanford, CA

ABSTRACT: Rock Physics of Glauconite and Glauconitic Sandstone Reservoirs

We report measurements on glauconitic sandstones as a function of pressure. Glauconite is an iron rich variety of clay that can be found as individual pellets, composite grains, and intergranular cement. Identifying glauconite in the subsurface is important for depositional environment interpretation, stratigraphic correlation, dating, tracing of unconformities, geochemical exploration in marine environments, and reservoir quality prediction in glauconitic sandstones.

In order to identify, track, and characterize glauconitic sandstones from seismic data, it is important to relate the elastic properties of glauconitic sandstone to their physical properties, most important among which are mineralogy, porosity, and rock texture. To this end, this paper presents laboratory measurements of ultrasonic velocities, porosity and permeability of glauconitic sandstones from the Caballos Formation, Putumayo Basin (Colombia). To differentiate between glauconite bearing and non-glauconite bearing rocks, we compare their physical and acoustic properties along with those of a pure glauconite sample. Based on laboratory data, we have established that rocks containing glauconite have at the same porosity smaller permeability and smaller velocity than sands without glauconite.

Probably the most exciting and practically meaningful result is that the quality of reservoir rock (specifically, permeability) can be discriminated not only by porosity but also by Poisson’s ratio (or Vp/Vs ratio). Specifically, at the same porosity, the samples with lower permeability have higher Poisson’s ratio. In other words, our rock-physics diagnostic of Caballos Formation shows that we can determine porosity and permeability from a combination of P- and S-wave data.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado