Organic matter accumulation and carbon sequestration in the Paleocene-Eocene Thermal Maximum: Insights from the Kongdian Formation in Cangdong Sag, Bohai Bay Basin

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) has attracted much attention due to resembling the modern climatic change. This study focuses on the organic matter accumulation and carbon sequestration under the PETM by the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin using petrography, geochemistry, and X-ray diffraction (XRD). The PETM happened in the second member of the Kongdian Formation (Ek₂) which experienced a process of warm-dry to hot-humid to warm-dry variations. Hence the Ek₂^1-2 section has been divided into pre-, core-, and post-PETM. During the pre-PETM, a continuous hot climate strengthened the evaporation to induce the lake salinizing which aggravated water stratification and reducing condition. Intense hydrological cycle resulted in rising terrestrial flux and falling salinity, and more oxidic water column at the core-PETM stage. The salinity continuously falls from the highest point at the post-PETM stage, corresponding to the evaporation weakening and terrestrial input increasing. This caused gradually weakening water stratification and oxidic water column. More saline and reducing conditions promoted the boom of aquatic organism and favorable preservation, so more saturated and aromatic hydrocarbons have been formed at the pre- and post-PETM stages. By contrast, the core-PETM section contains more higher plants with unfavorable preservation, generating less asphaltene. Combined with carbonate and TOC contents, the interplay of organic–inorganic carbon sequestration occurred at the pre- and post-PETM stages, distinct from that at the core-PETM. Injection of light oil from the pre- and post-PETM sections leads to high OSI values of the core-PETM which has good shale oil potentials.