ABSTRACT
Marine-bar and valley-fill stratigraphic traps in the Cretaceous “J” sandstone in Cheyenne and Banner Counties, Nebraska, illustrate control of reservoir shape, size, and characteristics by depositional environment.
Reservoirs deposited as shallow-marine bars are elliptical lenses 2-5 mi long, 0.5-1.5 mi wide, and less than 25 ft thick. Sandstone grades laterally into marine mudrock. There are two generations of bars in the area, closely spaced stratigraphically, but with different directions of elongation. These lenses now are tilted with a regional southwest dip. Entrapment is independent of structural closure. Most bar bodies are entirely oil filled.
Reservoirs deposited as a valley fill are within a prism of sandstone more than 20 mi long, 2,000 ft wide, and 50-80 ft thick. The boundaries of this body are erosional. Oil is trapped only where the valley-fill trend crosses plunging anticlines. The valley fill interconnects all pools as a single aquifer system.
Exploration and production efforts are guided by several considerations. Position of marine-bar reservoirs can be predicted by techniques which map gradients in sandstone-shale proportions, such as those based on mechanical logs. Bars in the study area are scattered and not in chains; orientation is varied. Structure is unimportant. In contrast, valley-fill reservoirs are separated by erosional boundaries from enclosing rocks; hence, they cannot be detected by examination of the enclosing facies. Where present, however, the valley fill has great continuity and persistence of trend. Structure is essential. Valley-fill reservoirs have water drive and high primary recovery, whereas marine-bar reservoirs have only solution-gas energy.
Environmental interpretation of these reservoirs is based on fossils, sedimentary structures, textures, facies relations, and geometry. A single core commonly allows correct interpretation. Exploration and production programs are guided profitably by use of environmental concepts at an early stage.
