Organoclay interaction response to sedimentary environment and sources in Paleogene shales from the Nanpu Sag: Insights for organic matter accumulation mechanism

Abstract

Shales play a crucial role in the global carbon cycle through organic matter accumulation and hydrocarbon generation. However, the influence of organoclay interaction on organic matter enrichment and hydrocarbon generation remains unclear. To address this, PY-GC/MS, along with a series of experiments, was employed to investigate the Ed₃-Es₁ Formation in the Nanpu Sag. Organic matter in the shales occurs in two forms: mineral-bound organic matter and free organic matter. Mineral-bound organic matter, chemically bound to clay minerals, is primarily derived from aquatic organic matter. Free organic matter, physically associated with minerals, mainly originates from terrestrial plants. The organic matter in the source rocks from the Nanpu Sag varies in occurrence between units. In Es₁, it is primarily composed of mineral-bound organic matter, whereas in Ed₃, it consists of both mineral-bound organic matter and free organic matter. The diversity was the result of the combined influences of sedimentary environment, mineral input, and organic matter source. The deep palaeo-lake during Es₁ deposition, characterised by moderately high salinity and low-energy hydrodynamic conditions, promoted the development of aquatic organic matter and the input of fine-grained minerals, thereby facilitating the formation of mineral-bound organic matter. The shallow palaeo-lake during Ed₃ deposition, characterised by relatively low salinity and strong hydrodynamic conditions, experienced a substantial input of terrestrial detrital minerals and plant fragments, resulting in the abundant presence of free organic matter in the shales. Furthermore, the hydrocarbons produced from the different units exhibit significant variation in yield and composition. Above all, source rock assessment should take organoclay interactions into account. This research can also enhance the understanding of the influence of lacustrine sediment on the global carbon cycle.