Sequence Stratigraphy and Organic Matter Distribution of the Lias of the Paris Basin

Abstract

Sequence stratigraphy is a powerful tool for the study of basin fill histories. Here, it was used for a better understanding of the relation between basin evolution and organic matter distribution in the Lias of the Paris Basin. The study consisted of a three-dimensional investigation and the main results are synthesized on a SWNE cross section. It required the combination of two methods — the stacking pattern method and the Carbolog method — both applied to wells using wireline logs. The stacking pattern method, which necessitated the logs being calibrated on outcrops and on core material, and being biostratigraphically controlled, led to an accurate subdivision of the Liassic series into three superimposed sequences, traceable throughout the basin, inside the major transgressive-regressive Lias cycle. These three sequences are: the genetic units, the groups of genetic units, and four transgressive-regressive cycles with a duration of 5 to 10 Ma, and clearly of tectonic origin. The study pointed out the role played by the variations in subsidence regimes and by the reactivation of major basement faults for controlling the basin evolution at this “stage scale”. The combination of these results to the characteristic pattern of organic matter distribution, as issued from Carbolog analysis, demonstrated that this distribution is in keeping with the general sequence stratigraphic framework, at a basin scale; especially noteworthy is the general good correlation which exists, for the lower order sequences, between organic-rich horizons and maximum flooding surfaces, when deposition occurs below the storm wave base. The study also brought out the role of local conditions (dilution effect due to high sedimentation rate, for instance) which act within the sequence stratigraphic framework but which may also impose their own dominant influence.