3D numerical modelling of marine organic matter distribution: example of the early Jurassic sequences of the Lusitanian Basin (Portugal)
Résumé
Due to the multiple controlling factors involved, it is a challenging task to identify and quantify the
processes influencing the distribution and heterogeneity of marine organic-rich rocks. To improve
our understanding of these deposits, we model their burial history and stratigraphic evolution as well
as processes linked to marine organic matter history throughout the Lower Jurassic in of the Northern
Lusitanian Basin (Western Iberian Margin). This 15-Ma-long interval is modelled using 100-kyr
time steps to simulate lithologies and organic matter heterogeneity as layers with a thickness of 2–
5 m, depending on the sedimentation rate in the basin. The model is calibrated by well and outcrop
data which provide structural and biostratigraphic constraints, as well as information on the depositional
facies and geochemistry of the sediments. The results show that the presence of organic-rich
intervals is linked to first-order variations in the basin geometry and sedimentation rates. Without
considering any variation of primary productivity or oxygen content in surface sea waters, the
parameters of basin geometry and sedimentation rate are sufficient to predict the main characteristics
of source rocks, i.e. their occurrence, thickness and mineralogy at the basin scale. However, to fit the
measured organic carbon contents, we need to take account of other parameters such as variations of
primary productivity or changes in dissolved oxygen concentration.