The impact of Milankovitch-scale climatic forcing on sediment supply

Abstract

Abstract Stratigraphy is the main technique to predict the occurrence of potential reservoirs and seals in the oil industry. Stratigraphic tools used are stratigraphic observations from drilled wells (rock samples and wireline logs) and seismic interpretation. Nowadays, computer modelling allows scenario forecasting of the sequence to be expected. Application of these tools indicates that the key input parameters are initial basin topography, subsidence, eustasy and sediment supply. The first three parameters are usually well constrained or can readily be deduced from well and seismic data. However, the fourth, sediment supply, is loosely defined, and because of that, it is used to calibrate the model. In the present article, the controls on sediment supply are reviewed and the impact of, in particular, Milankovitch-scale climatic forcing on sediment supply is tested on the Neogene of the Niger Drainage and Delta system. Sediment supply is controlled by a complex interplay of geographic and climatic factors, such as: (1) relief or mountain height; (2) the size and temporal storage capacity of the drainage area; (3) overall climate; (4) amount and seasonality of precipitation; and (5) vegetation cover. Studies in geomorphology have indicated that although complex, computer models can be made to predict erosional product for the present. A large-scale sediment supply prediction should be based on the combination of palaeogeographical, palaeoclimatological and erosion potential maps. More detailed sediment supply predictions should include Milankovitch scale variations. The climatic impact of these variations is mainly felt when we are in a transition between two climates, then the area will experience regular climatic changes, from e.g., wet (rainforest) to dry monsoonal climate, or low to high sediment supply, respectively. This concept was tested in the forward modelling package STRATAGEM on the Neogene of the Niger Delta. Four scenarios were evaluated, using various combinations of large and Milankovitch-scale sediment supply curves combined with global or Milankovitch-scale eustatic sea-level curves. The results confirm the impact of both large-scale and Milankovitch-scale climatic forcing on sediment supply. However, in an icehouse world, the various scenarios also suggest that the effect of eustatic sea-level fluctuations overshadows the impact of Milankovitch-scale sediment supply. By contrast, in a greenhouse world, the impact of Milankovitch-scale sediment supply is expected to be more significant in the absence of significant sea-level fluctuations.