BESSEREAU, G., A. Y. HUC, and B. CARPENTIER, Institut Francais du Petrole, Rueil-Malmaison, France; and F. GUILLOCHEAU, Centre de Geochimie de la Surface, Strasbourg, France

ABSTRACT: Source-Rock in a Sequence Stratigraphic Framework, An Example: The Lias of the Paris Basin

Understanding of source-rock distribution is of primary importance in the appraisal of the petroleum potential of a sedimentary basin. Sequence stratigraphy, which is a powerful tool for study of basin fill histories, can be successfully used to better understand the relations between basin evolution, sedimentary environment succession and organic matter distribution.

Such an approach has been applied to the Lias of the Paris Basin, along a SW-NE transect chosen to cover the major paleogeographic and tectonic features of the Paris Basin. In this interval known as the source-rock for the oil pools in the basin, profiles of organic matter, which has been determined using the Carbolog method (based on sonic and resistivity logs), exhibit strong vertical and lateral variations; striking is the presence of several organic rich horizons, beside the well-known Schistes Carbon, and their lateral continuity over the basin.

The sequence stratigraphic analysis has been based on well log correlations of characteristic stacking patterns of genetic units. This subdivision has been correlated with outcrop sections and core material, and biostratigraphically controlled on the ammonite zone level. Four orders of sequences have been distinguished: (1) the overall transgressive-regressive cycle covering the whole Lias, is a result of an acceleration of subsidence during the opening of the Neo-Tethys; (2) four smaller scale transgressive-regressive cycles have been found with a duration of 5-10 Ma. They are related to block reactivation as a result of tectonic stress regimes within the Paris Basin; (3) groups of genetic unit (0.6-1 Ma) which could be of eustatic origin; and (4) genetic units (100 to 400 ka), proba ly of high frequency eustatic origin.

The comparison of the organic content distribution to this sequence stratigraphic framework, showed: (1) a general good correlation exists between organic rich horizons and Maximum Flooding Surfaces when deposition occurred below the storm-wavebase; in a wider sense organic matter concentration tends to be high in the late stage of Landward Stepping (i.e. TST) and early highstand Vertical Stacking (i.e. HST). (2) The sequence hierarchy also seems to be reflected in the organic matter distribution, i.e. the amount of organic carbon stored is determined by the position in the lower order sequences.

But the basin scale analysis also showed the influence of tectonics and paleoceanographic conditions on the final organic matter distribution and composition (dilution effect due to subsidence increase...), which do act within the sequence stratigraphic framework but may impose their own dominant influence from time to time.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.