Pore-Size Distribution And NMR In Microporous Cherty Sandstones

Nuclear Magnetic Resonance (NMR) longitudinal relaxation curves measured in the laboratory on water-saturated samples of the Cut Bank formation are converted to sums of exponentials using a procedure introduced by Gallegos and Smith. Under the assumption that nuclear magnetization in each pore decays as a single exponential, the exponential decomposition thus obtained can be interpreted as a pore-size distribution. The exponential decomposition contains a good deal of detail, but is still quite repeatable, as shown by synthetic data and by duplicate samples. The exponential decomposition qualitatively agrees well with pore-size distributions seen in thin-section micrographs. In particular, the thin sections show large variations in the ratio of microporosity to open pores, and in the size of the open pores, which are also seen in the NMR exponential decomposition. Quantitative comparison of NMR with pore diameters determined from digital analysis of thin-section images is also very good. By matching the exponential decomposition of the NMR measurement with the image-analysis pore diameters, a value for the power of the rock surface to promote NMR decay, of 0.001 cm/set is determined, in line with values seen in the literature. Pores unresolved by optical microscopy are an important part of the porosity; a threshold of 85 milliseconds or less for the NMR longitudinal relaxation time constant (T1) was found to correspond to the optical resolution limit of 5 microns diameter. Comparison of pore sizes with mercury injection is made, but is somewhat unsatisfactory because the ratio of pore throat size to pore body size varies widely in these samples.