Opposite Contributions of Types I and II Organic Matter to Nanoporosity in Lacustrine Shale

Nanoporosity is a key factor for evaluating shale oil/gas potential and recovery. Organic matter (OM) can be a primary host of nanopores in shale. The Triassic Yanchang Formation in the Ordos Basin is a promising target for shale oil/gas recovery in China. However, the relationship between OM and nanoporosity in the Yanchang shale remains controversial. In this study, 19 Yanchang shale cores were collected and analyzed. A twofold effect of OM on nanoporosity was observed, even though the total pore volume and specific surface area correlated negatively with the OM content. Nanoporosity in samples with total organic carbon (TOC) content of < 7% increased with increasing maturity, while in the other samples, it showed a decreasing trend. This phenomenon is attributed to the two different OM inputs in the studied shale samples. In shale samples with TOC content of < 7%, OM was mainly type II and a new generation of nanopores contributed more to nanoporosity than nanopore occupation by newly generated hydrocarbons with increasing maturity. The situation was opposite in the other shale samples, in which OM was mainly type I. Clay-related pores were an important contributor to nanoporosity. Nanoporosity correlated positively with quartz content but negatively to feldspar content, and these are attributed to pore protection from compaction of quartz and dissolution and conversion of feldspar, respectively. These findings provide a reasonable explanation for the diverse observations of relationships between OM and nanoporosity in the Yanchang shale and enhance the understanding of pore evolution in organic-rich shale.