Insight into Water Occurrence and Pore Size Distribution by Nuclear Magnetic Resonance in Marine Shale Reservoirs, Southern China

The exploitation of shale gas has been a hot topic, and the understanding of water occurrence and pore size distribution in shale reservoirs plays a crucial role in efficiently developing this type of resource. In this study, we target moderate-shallow marine shales from the Lower Cambrian Niutitang Formation and deep marine shales from the Lower Silurian Longmaxi Formation. Low-field nuclear magnetic resonance is applied to investigate the water occurrence and pore size distribution together with complementary experiments including mineral component analysis, scanning electron microscopy (SEM), contact angle measurement, and high-speed centrifugation. The SEM analysis shows that the moderate-shallow shales have some inorganic nanopores and cracks but almost no organic pores, while the deep shales have abundant organic nanopores. The average movable water saturation of moderate-shallow and deep shales is only 15.5% and 15.2%, respectively. It implies that the high irreducible water saturation is a typical characteristic for both marine shales, which is ascribed to the strong capillary force of the nanopore inhibiting water flowing out of the pore space. By means of high-speed centrifugation and contact angle measurement, the converting factor of the T2 value into pore size was obtained. The average pore size in moderate-shallow shales is 19.4, 21.8, and 26.8 nm, respectively, while the average pore size in deep shales is 109, 57, and 68.4 nm, respectively, for the tested samples. This indicates that organic pores in deep shales contribute to the development of abundant nanopores. This study brings insight into characterizing the water occurrence and pore size distribution of moderate-shallow and deep marine shales.