Origin of chemical structure and pore heterogeneity of over-matured organic matter in the Paleozoic marine shales, China

Abstract

Organic matter (OM) pores play a leading role in gas storage for many shale formations, and understanding their development mechanism is critical to evaluating the mechanisms of shale gas enrichment. However, an insufficient knowledge of OM compositional and structural complexity makes the origin of pore heterogeneity still unclear. This study correlates high-resolution scanning electron microscopy (SEM) and Raman microscopy to investigate the association of OM molecular structure and pore development in the Cambrian Niutitang shales and Silurian Longmaxi shales, south China. Results show that pyrobitumen, the predominant OM maceral, can be divided into two types (PB1 and PB2) which have differences in petrographic characteristics, Raman parameters, and pore distribution. In the shales with good gas preservation, the pore-filling PB2 shows a lower degree of aromaticity, a higher apparent transformation ratio (15 %–40 %), and larger pore size (peak of 80–90 nm) than that of the less porous PB1. In the poorly-preserved shales, different pyrobitumen exhibit homogeneous molecular structure and smaller pore size (peak of 30–40 nm). The poor development of OM-hosted pores in the Niutitang shale is related to the increased OM graphitization at a Ro > 3.5 %. The heterogeneity of OM structure and porosity in different Longmaxi shales would be originally impacted by a distinction in the degree of oil expulsion under different preservation condition. In the well-preserved shales, a higher retention of aliphatic-rich oil may lead to the more porous and less aromatic pyrobitumen, and thus a higher current gas content. The study provides new insights into the OM chemical controls on nanoscale pore heterogeneity, which is significant for shale gas exploration.