ABSTRACT

We measured the NMR T2 distribution of four samples composed of natural carbonate skeletal grains and micrite sediments, two pure endmembers and two mixtures. Surface-area measurements and optical and scanning electron microscopy were used to characterize the pore geometry. The T2 distributions of the skeletal grains and micrite were not well separated despite large differences in pore size. This was due to large differences in the shape factor (α) which relates the surface-area-to-volume ratio of the pore space (S/V) to the radius (r) of the pores: S/V =α/r. The value of α for the intergranular porosity in the micrite ranged from 2 to 7, while the value of α for the intergranular porosity in the skeletal grains ranged from 42 to 119. The value of α for the intragranular porosity in the skeletal grains ranged from 10 to 51. With these observed differences in α we conclude that there is no simple transform, using a single value of α, that could derive an accurate pore-size distribution from the NMR T2 distribution for carbonates that are a mixture of skeletal grains and micrite. In the samples composed of a mixture of skeletal grains and 20 and 50% micrite, micrite filled in the intragranular and intergranular pores in the skeletal grains. The peak relaxation time of the skeletal grains was shifted to earlier time when micrite was present due to the reduction in pore size. The results suggest that the value of α decreases as the micrite-to-skeletal-grain ratio increases until micrite makes up the total porosity, at which point α of the mixture approaches that of pure micrite.

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