Abstract

These stratigraphic traps in the Cretaceous 'J' sandstone illustrate control of reservoir characteristics by depositional environment. The sandstone reservoirs deposited as shallow-marine bars are elliptical lenses; the sandstone grades laterally into marine mudrock. The bars are scattered and their orientation is varied. Entrapment is independent of structural closure and most bar bodies are oil filled. The position of marine-bar reservoirs can be predicted by techniques which map gradients in sandstone-shale proportions. The valley-fill reservoirs are within a long narrow prism of sandstone; the boundaries of the reservoirs are erosional. Oil is trapped only where the valley-fill trend crosses plunging anticlines. These reservoirs cannot be detected by examination of the enclosing facies. Environmental interpretation of these reservoirs is based on fossils, sedimentary structures, textures, facies relations, and geometry.

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