NMR measurement of oil shale magnetic relaxation at high magnetic field

Nuclear magnetic resonance (NMR) at low field is used extensively to provide porosity and
pore-size distributions in reservoir rocks. For unconventional resources, due to low porosity and
permeability of the samples, much of the signal exists at very short T₂ relaxation times. In
addition, the organic content of many shales will also produce signal at short relaxation times.
Despite recent improvements in low-field technology, limitations still exist that make it difficult
to account for all hydrogen-rich constituents in very tight rocks, such as shales. The short pulses
and dead times along with stronger gradients available when using high-field NMR equipment
provides a more complete measurement of hydrogen-bearing phases due to the ability to probe
shorter T₂ relaxation times (<10^-5
sec) than can be examined using low-field equipment. Access
to these shorter T₂ times allows for confirmation of partially resolved peaks observed in low-field
NMR data that have been attributed to solid organic phases in oil shales. High-field (300 MHz or
7 T) NMR measurements of spin-spin T₂ and spin-lattice T₁ magnetic relaxation of raw and
artificially matured oil shales have potential to provide data complementary to low field (2 MHz
or 0.05T) measurements. Measurements of high-field T₂ and T₁-T₂ correlations are presented.
These data can be interpreted in terms of organic matter phases and mineral-bound water known
to be present in the shale samples, as confirmed by Fourier transform infrared spectroscopy, and
show distributions of hydrogen-bearing phases present in the shales that are similar to those
observed in low field measurements.