Sylvie Bourquin¹, Christophe Rigollet², Philippe Bourges³

(1) CNRS, Rennes, France
(2) ENSMP, Fontainebleau, France
(3) ELF

ABSTRACT: High-resolution sequence stratigraphy of an alluvial fan environment: fluvial reservoir implications - Example of the Chaunoy Sandstones (Triassic, France)

The Chaunoy Sandstones Formation (Norian) is one of most famous reservoir of the Paris Basin. It is composed of small ribbon channel deposits, interbedded with flood-plain and lacustrine environments. We performed a high-resolution sequence stratigraphy analysis of these continental deposits, based on analysis of high-frequency fluctuations of baselevel. This is defined from sedimentological studies and calibrated on well-log signatures. The baselevel corresponds here to lake-level. Well log correlations show a direct relationship between lake-level variations and channel amalgamation. Periods of high facies preservation, i.e. baselevel rise, correspond to well-developed channel facies, a progressively increase of lacustrine or flood-plain deposits : the channels become isolated. Periods of low facies preservation, i.e. baselevel fall, correspond to paleosols development and/or amalgamated channels.

The Chaunoy Formation is much thicker in the Northwest of the basin (130m north of the Bray Fault compared with less than 60m to the south). Based on baselevel variations, the formation can be subdivided into three stratigraphic units from base to top: Chaunoy I, II and III. The unit II and III are only observed north of the Bray Fault and are more lacustrine. This can be explained by climatic environment variations, from arid to more humid. Moreover, the increasing thickness of sandstone deposits northward can be related to both fault activity and a general tilting of the basin. These different paleogeographic changes, of tectonic and climatic origin, and the baselevel variations strongly controlled the reservoir quality and the geometry of the seals. These primary heterogeneities will be discussed.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado