FLUVIAL SANDSTONE DISTRIBUTION IN THE JURASSIC SALT WASH MEMBER OF THE MORRISON FORMATION AROUND THE GYPSUM VALLEY SALT DIAPIR

This study focuses on documenting and interpreting the controls on the fluvial architecture and reservoir facies distribution of the Jurassic Salt Wash Member of the Morrison Formation that was deposited over and around the Gypsum Valley Salt wall in the Paradox Basin of Colorado. The Salt Wash Member is an eastward prograding distributive fluvial system deposited in the Sevier foreland basin and, is generally thought to post-date diapirism in the Paradox Basin. However, our work shows that the Salt Wash Member was influenced by varying degrees of syndepositional salt tectonism ranging from minibasin and top salt subsidence to locally rising topographic highs over the Gypsum Valley Salt Diapir. The local salt tectonism overprinted the more regional trends in the medial fluvial distributive architecture and influenced the reservoir quality of the Salt Wash Member around the salt wall.

Twenty-four composite stratigraphic sections were measured at different positions around the salt wall including diapir margin, top salt Hat minibasin, and distally away from the diapir margin. The Salt Wash Member at Gypsum Valley can be broken into four stratigraphic units in ascending order: 1) Interbedded sandstones and mudstones 2) amalgamated sandstone channels 3) isolated sandstone channels, and 4) laterally stacked sandstone channels. Units 2 and 4 have thick sandstone reservoir packages with thin mudstones. The amalgamated sandstone channel unit is the thickest (28.8 m) distal to the Gypsum Valley Salt Diapir to the northeast diapir margin. This unit has the highest concentration of sandy channel fills throughout Gypsum Valley with the highest at 82% of the section on the eastern margin. This unit thins to 10 m and pinches out to the south on the eastern margin of the Gypsum Valley Salt Diapir. The laterally stacked sandstone channel unit contains the most laterally continuous channel fills and is thickest (105 m) in the northeastern part of the diapir. It thins (10 m) to the southern part of the diapir on both the eastern and western margins. This unit has the highest sandstone percentages distal from the diapir on the north western margin at 84% and on the southeastern margin of the diapir at 98%.