Occurrence of organic matter in different mudstone lithofacies and its influence on specific surface area:A case study of the Paleogene in the Zhanhua Sag, Bohai Bay Basin

Some core and cast thin section samples of shales were collected from different depths of the lower section of the third member of the Paleogene Shahejie Formation in well L in the Zhanhua Sag. X-ray diffraction (XRD), pyrolysis and specific surface area (SSA) measurements were applied to examinethe characteristics of organic matter (OM) occurrence and analyze the effect of OM on the SSA of different mudstone lithofacies. Results showed that the shales can be categorized into lithofacies A (lack of organic laminae) and lithofacies B (rich in organic laminae). The former has a low TOC content with high SSA, and the latter has a high TOC content with low SSA. The relationship between TOC and mineral SSA is convergent in the lithofacies A and divergent in the lithofacies B, which indicates that OM in the lithofacies A is mainly adsorbed on mineral surfaces while OM in lithofacies B is chiefly accumulated as organic laminae. The variation of OM occurrence clearly affected shale SSA. The SSA of lithofacies A is lower than that of lithofacies B, and the contribution of minerals to SSA is similar in both lithofacies, which indicates that the variation of SSA in different lithofacies (A and B) is correlated with the abundance of organic laminae.