Diffusive coupling in heptane-saturated kerogen isolates evidenced by NMR [formula omitted]-[formula omitted] and [formula omitted]-[formula omitted] maps

Abstract

It is well known that in unconventional shale, the 1H NMR T1/T2 ratio for light producible hydrocarbons in the kerogen (T1/T2≃3↔6) is larger than water in the clays (T1/T2≃2). While such a contrast in T1/T2 is potentially useful for fluid typing and saturation in shale, the underlying mechanism remains elusive. Our previous NMR reports at f0 = 2.3 MHz of heptane-saturated kerogen isolates distinguished two signals (a) inter-granular heptane between the kerogen grains with small T1/T2≃2.4, and (b) dissolved heptane in the kerogen grain with large T1/T2≃70. In this report we present new evidence of diffusive coupling effects between the inter-granular heptane and the dissolved heptane using T1-T2 correlation maps as a function of saturation and T2-T2 exchange maps. Our results indicate a phenomenon not previously reported; namely, different diffusive-coupling regimes between T1 (with intermediate coupling) and T2 (with weak coupling). We find that T1/T2 of the inter-granular heptane increases from T1/T2≃2.1→6.5 with decreasing saturation of the inter-granular pore-space, which we attribute to increasing confinement and the difference in diffusive coupling between T1 and T2. Our findings imply that the large range T1/T2≃3↔6 of the inter-granular (i.e. the more producible) hydrocarbons in shale is due to confinement together with diffusive-coupling effects with the dissolved (i.e. the less producible) hydrocarbons in kerogen.