Oil production from the Cretaceous 'J' interval in western Nebraska is mainly from lenticular bodies of crossbedded, fluvial sandstone in a north-trending zone of valley fill about 1,500 feet wide, 50 feet thick, and at least 20 miles long; seven fields have been discovered along this valley fill trend. Oil accumulated where valley fill crosses the axes of northwest plunging anticlines and updip escape was prevented by higher oil entry pressures in enclosing marine sediments. Some production comes from nearby marine Cretaceous sandstone but the success ratio of completion in the valley-fill is nearly 6 times better.

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