ABSTRACT

Previous studies have shown that carbonate rock with rounded moldic macropores has a higher compressional velocity at a given porosity than rock with interparticle or intercrystalline porosity and high-aspect-ratio macropores. Our study of Permian dolograinstone challenges this commonly accepted result. Petrophysical and acoustic velocities were measured on 55 mini core plugs collected along an approximately 1 km long horizontal transect within a single grainstone unit having lateral changes from grainstone with dominantly interparticle porosity to grainstone with mostly moldic porosity. The studied dolograinstone samples have a large scattering of acoustic velocity (up to 1500  m/s) at equal porosity. Some oomoldic grainstone samples have either higher velocity or significantly lower velocity than that of the intercrystalline grainstone samples. Petrographic analysis under plain light and UV light combined with mercury injection capillary pressure data and scanning electron microscopy analysis indicates that moldic samples with a lower than average velocity for a given porosity have a matrix with between 50% and 75% of the total porosity as intercrystalline microporosity between large moldic pores. Conversely, rocks with a higher than average velocity for a given porosity find a lack of micropores in the microcrystalline part of the matrix. The amount of micro- and mesoporosity in the matrix is the primary control on acoustic properties of those moldic dolograinstone samples. Therefore, the matrix microcrystalline structure can exert a stronger control on acoustic property than do the macropore types and shapes. These results indicate that using acoustic data from the subsurface to identify pore shape and reservoir properties still remains a challenge.

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