Tidal1y deposited valley-fill sandstones, when sourced from the sea, as is the case in Sun Ranch field, Wyoming, generally form sand bodies which contain smaller pores, (finer grained) and contain significantly more pore throat plugging by detrital (and diagenetic) clay than most superficially similar fluvial or distributary channel sandstones. The detrital clay occurs as thin beds, drapes, clasts and "grains." Calculated log porosities are significantly lower where clay occurs in otherwise high quality reservoir sandstones. The quality of the reservoir is commonly masked on logs by the effect of the clay clasts.

Individual tidal valley-fill deposits are demonstrably separable laterally and generally have several locally continuous horizontal shaley beds which form baffles or barriers that slow or prevent vertical flow. These very thin horizontal beds are: (1) dominantly clay and fine sand and silts deposited during marine to brackish flooding events; (2) are easily recognizable in core but not on most logs; (3) have very low porosity and permeability; (4) prevent or deter vertical flow between reservoir compartments and act as baffles to flow; and (5) are the focus of diagenetic clay formation. Because of the significant amounts of clay present in some tidal and tidally-reworked fluvial valley fills, they have significantly lower porosity and permeability and smaller "port sizes" than most fluvial and deltaic deposits.

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