Abstract

Nearshore marine sandstone is an important oil and gas reservoir rock. The seaward pinch-out of these sandstone bodies into marine shales is a well-known exploration target. However, the landward pinch-out of these same sandstone bodies is more complex stratigraphically and relatively under-explored as a potential stratigraphic play. The landward pinch-out of nearshore marine sandstones have proved their potential for hydrocarbon production.

Two Upper Cretaceous deltaic shoreline deposits of east-central Utah are compared. Wave-dominated shoreline sandstones of the Blackhawk Formation in the Book Cliffs, and river-dominated shoreline sandstones of the Ferron Sandstone Member of the Mancos Shale along the Coal Cliffs of Utah, were examined to better understand the nature of the landward pinch-out trap. Drill-hole data and measured sections from the two study areas indicate the river-dominated shoreline deposits (Ferron Sandstone) abruptly pinch-out into coastal-plain deposits. The wave-dominated (Blackhawk Formation) pinch-out of shoreline sandstone occurs within 0.25–0.5 miles (402–805 m). The pinch-out is into sandy facies of marginal marine origin and eventually into finer-grained coastal-plain rocks.

The river-dominated landward pinch-out play is potentially smaller in size due to the discontinuity and heterogeneity of the shoreline deposits. More homogeneous and continuous wave-dominated shorelines provide potential for large-size reservoirs associated with a landward pinch-out trap. Down-cutting channels may breach the trap in either delta system, although more likely in the river-dominated system. Fine-grained coastal-plain deposits have low permeability and provide excellent trapping characteristics in both depositional systems.