Romain Debarre¹, Remi Eschard², François Guillocheau³

(1) Institut Français du Pétrole, Rueil-Malmaison, France
(2) Institute Francais du Petrole, Rueil-Malmaison Cedex, France
(3) Géosciences Rennes, Université de Rennes-1, Rennes, France

ABSTRACT: Fluvio-Lacustrine Genetic Sequences of the Jurassic Morrison Formation in Southeastern Utah (U.S.A.)

Sediments of the Jurassic Morrison Formation outcropping in Southeastern Utah have classically been interpreted as deposited in a large fluvial system. However, detailed facies analysis performed on five sites along a 200 Km long profile have highlighted the importance of fluvio-lacustrine facies. Their sequential organization was controlled by lake-level variations.

Fifteen architectural elements were determined from sedimentological analysis, photomosaic interpretation and paleocurrent data. They were regrouped into four major environments: (A) "deep" lacustrine, (B) fluvio-lacustrine deltas, (C) ephemeral sheet-floods and (D) humid alluvial fan which characterize lacustrine depositional environments connected to ephemeral fluvial systems.

The existence of a lacustrine geomorphic base-level allows a relatively traditional approach of the stratigraphy, where stratigraphic cycles and facies preservation are controlled by the accommodation potential and the sediment supply variations. Two 4th order stratigraphic cycles (about 60m thick each) limited by lacustrine MFS have been correlated at a regional scale. The cycle first records the progradation of fluvio-lacustrine deltas and fluvial systems followed by a lacustrine transgression. Each 4th order cycle results of stacked 5th order cycles (about 20m thick each) characterized by aggradational or progradational stratigraphic pattern. These patterns are characterized by different association and preservation of architectural elements. Laterally continuous sandstone layers of fluvio-lacustrine deltas and fluvial deposits occur during progradation, whereas aggradation produces disconnected reservoir units of fluvio-lacustrine deltas.

This outcrop study highlights the importance of fluvio-lacustrine sediments in continental strata. The model of sequence presented here can be easily applied in subsurface to help reservoir characterization and prediction.

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