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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 49 (1965)

Issue: 9. (September)

First Page: 1575

Last Page: 1575

Title: Stratigraphic Traps in a Valley Fill, Western Nebraska: ABSTRACT

Author(s): J. C. Harms

Article Type: Meeting abstract

Abstract:

Oil is trapped in a trend of valley-fill sandstones in the Cretaceous "J" formation in Cheyenne and Morrill Counties, Nebraska. The valley fill is composed chiefly of porous and permeable sandstone, strikes north-south, and is about 1,500 ft. wide and 50 ft. thick. Oil has accumulated in the valley fill trend where it crosses the axes of northwest-plunging anticlines. Updip (eastward) escape of oil is prevented by the discontinuous nature of sandstones with low oil-entry pressures in the enclosing marine sediments of the "J" formation. The traps therefore are a combination of stratigraphic and structural.

The "J" formation in this area is a sandy and silty unit 38-77 ft. thick deposited in predominantly marine environments. The "J" is overlain and underlain by dark gray marine shale. The formation can be divided into two members, each relatively thin and with distinctive mineralogy, sedimentary structures, fossil content, and electric-log character. These members can be traced over hundreds of square miles in western Nebraska. After the deposition of the younger member, emergence caused a narrow valley to be cut and filled by a stream. Within the area of stream erosion, most of the previously deposited sediments of the "J" were removed. The sandstones of the valley fill also have distinctive sedimentary structures, textures, and electric-log character. The trend of the valley fill is n arly straight, suggesting that erosion and deposition were the work of a meandering stream whose width was less than the width of the valley.

Seven fields have been discovered along the valley fill trend within the study area. One well out of every 1.8 wells drilled into the valley fill has been completed successfully. These wells are rated as good producers and have long productive lives by Denver basin standards. Some production has been developed in marine sandstones of the "J" formation near the area of the valley fill, but only one well out of approximately every 15 drilled is successfully completed, productivity is low, and total reserves are smaller. Therefore, stratigraphic study leading to an improved understanding of the genesis and form of the sandstone reservoirs is of considerable economic value.

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Copyright 1997 American Association of Petroleum Geologists