Spatial and temporal changes in the organic matter supply during the Cenozoic basin evolution in the Danish North Sea.

The mudstones of the Upper Eocene - Middle Miocene Lark Formation in the Danish North Sea have not been the subject of a detailed organic petrographic and geochemical investigation despite its importance for paleoclimate studies and as a regional seal for potential CO2 storage sites. This study investigates the characteristics and quantity of organic matter in 391 samples of cuttings obtained from 7 wells from the Danish North Sea. The Hawk pyrolysis was conducted on all cuttings samples while reflected light and fluorescence microscopy was conducted on 38 pellets for determining the reflectance of primary huminite (HRo), the volume percentage of huminite and inertinite (H+I, vol.%) and the intensity ratio of red to green light of alginite (R/G quotient). The results show that the organic matter in the Lark Formation is generally immature (Tmax < 430°C, HRo mean value = 0.3%, R/G mean value = 0.51) and mainly consists of Type III kerogen which is dominated by huminite macerals, with the total organic carbon (TOC) content ranging from 0.33 wt.% to 7.34 wt.% (mean value = 2.54 wt.%). In a north distal site, we observed a long-term decrease in hydrogen index (HI) caused by the increase in TOC outpaced S2 during Oligocene to Early Miocene. In the south distal sites in the Danish Central Graben, we noticed the increase in HI resulted from the jump in TOC and S2 during Early Miocene to Middle Miocene. The increase in TOC was proved to be the result of the increased influx of terrestrial organic matter as we established the positive correlation (R²>0.95) between the volume percentage of total organic carbon (TOC, vol.%) and that of huminite and inertinite (H+I, vol.%). This increased influx proved further being the result of changes in organic matter supply from N to NE and then ENE direction during the Cenozoic Basin Evolution in the Danish North Sea. [ABSTRACT FROM AUTHOR]