Abstract

Approximately 200 m of middle Green River Formation (Eocene) strata crop out in Nine Mile Canyon, southwest Uinta Basin. This interval is characterized by predominantly mudstone with subordinate interbedded sandstone and minor carbonate units. The succession can be divided into cyclic ~10-m-thick shoreface-offshore lacustrine-dominated intervals that alternate with 20- to 30-m-thick fluvial-deltaic-floodplain terrestrially dominated intervals. This study focuses on the fine- to medium-grained sandbodies that are interpreted as fluvial-channels and which constitute the main reservoir rock in oil fields north of the study area. The sandbodies are assessed in terms of their geometry, internal heterogeneity, and overall connectivity, and controls on their variability are discussed.

Mostly laterally amalgamated channel sandbodies, on the order of 5 m thick and with variably effective internal permeability barriers, are characteristic of the lower part of two floodplain-dominated intervals. They are interpreted as lowstand deposits that followed forced regression from a deep-lake highstand (likely of a scale where Laramide lakes merged). More sporadic and isolated lenticular channel sandbodies, commonly with inclined heterolithic strata indicative of high sinuosity planforms but also of internal permeability barriers, occur elsewhere in the other floodplain-dominated intervals. They are interpreted as transgressive systems tract deposits and as lowstand deposits associated with lesser magnitude lake-level falls. Vertical connectivity between units is uncommon due to their low net:gross. Thus, although sheet-like amalgamated channel sandbodies may sit on a disconformity or angular unconformity, the underlying units are mostly mudstone with only sporadically distributed sandbodies present to form a sand-on-sand contact. Additionally, over large areas the amalgamated sandbodies commonly sit with a tabular nature on oil shale or micrite rather than truncating them, suggesting an early lithification control on vertical connectivity.