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Ten J Sandstone oil fields form a long, narrow, northeast-southwest trend in western Nebraska. Except for these fields, this area is nonproductive
of oil and gas. It is proposed that this group of related fields be termed the Scotts Bluff trend. Subsurface mapping indicates that recurrent movement along Precambrian basement faults has enhanced reservoir quality and localized oil migration, favoring oil accumulation along the trend.
The J Sandstone dips gently southwest across the northeastern Denver basin. Low-relief closures and structural noses are critical elements in several structural-stratigraphic traps. However, most of the traps are controlled primarily by an updip facies change from porous, permeable sandstone to siltstone and shale.
Most oil production is from the J1 member, whose commercial production limits coincide with porous sandstone bodies at least 5 ft (1.5 m) thick. Three fields each have produced over 1,000,000 bbl of oil. The J1 was deposited in elongate, elliptical, northwest-trending marine bars that are rhythmically separated by laterally equivalent shales. The central-bar facies, which includes most of the reservoir rock, grades into bioturbated bar-margin siltstones which, in turn, grade into interbar shales.
Long-distance lateral migration into the shallow reservoirs of the trend from thermally mature shales of the Dakota and Benton Groups near the basin's axis is indicated. Recognition of the trend's characteristics will reduce exploration risk and help realize the area's considerable economic potential.
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