Paleoenvironmental reconstruction of a heterogeneous, tidally influenced reservoir analogue : the Neslen Formation near Harley Dome, Book Cliffs, Utah

A sedimentologic, ichnologic, and architectural analysis of the upper Sego and Neslen fms (Campanian) along the Book Cliffs near Harley Dome, UT document a paleoenvironmental evolution from wave and tidally-modified deltaic deposits of the upper Sego Fm to tidally influenced estuarine deposits of the Neslen Fm. Paleoenvironments of the upper Sego include tidal barforms and interdistributary bays while those of the Neslen include distal to proximal tidal-fluvial channels, sandy and muddy tidal flats, and floodplains. A total of 28 trace fossil forms were identified in the upper Sego and Neslen fms including 14 fully marine traces, 11 facies breaking traces, (those traces that can exist in fresh, brackish, or marine salinities), 1 freshwater trace, and 2 continental traces. Ichnology was critical for refining paleoenvironmental interpretations and was used to distinguish between deposits of distal tidal-fluvial channels and proximal tidal-fluvial channels, and tidal flats and floodplains. The Teredolites ichnofacies was used as an indicator of marine flooding events. Neslen sandbodies preserve inclined heterolithic stratification in point bars of estuarine tidal-fluvial channels. In contrast to recent studies, bayhead deltas and deltaic deposits were not identified in the Neslen near Harley Dome. Carbonates that include stromatolites and oolites are identified for the first time in the Neslen. The predominance of distal to proximal tidal-fluvial channels and tidal flats, abundant marine ichnology, the recurrence of the Teredolites ichnofacies and carbonates, and the lack of bayhead delta or wave-modified deposits suggests deposition within a distal, yet protected estuarine environment. The Neslen Fm near Harley Dome provides an analogue for a heterolithic, paralic reservoir system. Due to the heterogeneity that is evident at multiple scales within tidally influenced systems, the outcrops of the Neslen Fm in the Harley Dome, UT area provide an analogue to examine the distribution of various heterogeneities and facies in estuarine deposits that could ultimately affect reservoir performance.Geological Science