Source Rock Occurrence in a Sequence Stratigraphic Framework<subtitle>The Example of the Lias of the Paris Basin</subtitle>

Abstract

The appraisal of the petroleum potential of a sedimentary basin requires a good evaluation of its source rocks. Sequence stratigraphy appears as a powerful tool for the study of basin-fill histories and is, at present, used for reservoir characterization purposes. Here, we demonstrate that this approach is also a powerful tool for predicting the organic matter distribution by providing a chronostratigraphic framework in which the role of the main parameters controlling its accumulation can be approached. The study was performed at the basin scale and covers a period of 25 m.y. where different orders of superimposed sequences were identified. It investigated the Lias (Lower Jurassic) of the Paris basin, an interval which is known as the bulk source rock for the oil pools in this basin. It used two methods, both applied on wireline logs: (1) the Carbolog method, which estimates the in-situ organic carbon content of the series, showed that the Liassic series was characterized by strong vertical and lateral variations of total organic carbon (TOC), and by the occurrence of several organic-rich intervals besides the well-known Schistes Carton; and (2) the stacking pattern method, which produced a consistent framework of three superimposed sequences which are in keeping with the global transgressive-regressive (T-R) Lias cycle. These are the genetic units (0.1 to 0.4 m.y.) of possible climatic origin, the genetic unit sets (0.6 to 1 m.y.) which might be of eustatic origin, and four minor T-R cycles (5 to 8 m.y.; i.e., the stage scale) of clearly tectonic origin. The study showed a correlation between the distribution of the organic matter and the sequence stratigraphic framework, at the different sequence orders evidenced