Understanding organic matter supply in the Eocene to Miocene Lark Formation, Danish North Sea

Abstract

The significance of the Lark Formation is underscored by the variations in regional depositional environments and climatic conditions that characterized the late Cenozoic sedimentary flux into the Danish North Sea basin. This study marks the first systematic investigation of sedimentary organic matter in the Lark Formation. A total of 391 drill cuttings from 7 wells in the Danish North Sea were collected and analyzed. All 391 samples were analyzed by pyrolysis geochemistry, and thirty-eight were examined petrographically using reflected light to document maceral composition. This allowed the investigation into spatial and temporal variations in the distribution and properties of organic matter within the Danish North Sea Basin from the latest Eocene to the Middle Miocene. The results reveal that the organic matter comprises primarily Type III kerogen and is thermally immature, as indicated by thermal indicators (T_max < 430 °C, mean HRo = 0.3%, mean R/G = 0.51). The kerogen is predominantly composed of huminite (abundant), liptinite (less abundant), and inertinite (trace quantity) macerals.

The increasing trend in total organic carbon (TOC) commenced mainly in the northeastern part of the basin (F-1 well) in the late Oligocene. From the late Oligocene to the Early and Middle Miocene, there was a progressive clockwise shift in this increasing trend of sedimentary organic carbon content towards the southernmost part of the Danish North Sea basin (Adda-3, E-1, Dany-1X and Jens-1 wells). The results of the analysis further demonstrate that this rise in sedimentary organic carbon is primarily driven by the increased content of huminite and inertinite.

Given the immaturity of the organic matter and its consistent preservation conditions, the spatial and temporal increase in the abundance of huminite and inertinite is attributed to the influx of allochthonous organic matter from terrestrial sources. This influx was primarily influenced by climatic changes and subsequent alterations in runoff. Lower runoff during cooler and drier climates from the latest Eocene to the late Oligocene resulted in a low influx of allochthonous organic matter. Conversely, higher runoff during warmer and more humid conditions in the Early Miocene and earliest Middle Miocene markedly increased its influx. The distribution of allochthonous organic matter in the study area depended on the positioning of basin entrances and depocenters, which respectively determined the supply routes and accumulation sites. This further contributed to the relatively higher abundance of allochthonous organic matter in the areas near the corresponding stratum depocenters.