Stratigraphic Traps in a Valley Fill, Western Nebraska

Abstract

ABSTRACT Oil is trapped in a trend of valley-fill sandstone bodies in the Cretaceous “J” interval in Cheyenne and Morrill Counties, Nebraska. The valley fill is composed chiefly of porous and permeable sandstone; it trends north-south and is about 1,500 ft. wide and 50 ft. thick. Oil has accumulated in the valley fill where it crosses the axes of northwest-plunging anticlines. Updip (eastward) escape of oil is prevented by the enclosing marine sediments of the “J” interval, in which sandstone beds with low oil-entry pressures are discontinuous and separated by sandstone or shale beds with higher oil-entry pressures. The traps therefore involve a combination of stratigraphic and structural conditions. The “J” in this area is a sandstone and siltstone unit, 38–77 ft. thick, deposited in predominantly marine environments. The “J” is overlain and underlain by dark gray marine shale. The interval can be divided into two marine members, an upper and a lower, 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 deposition of the upper member, the area emerged and a stream cut a narrow valley which was filled mainly with sandstone of distinctive character. The stream in the valley cut through most or all of the two previously deposited marine members. The trend of the valley fill is nearly straight, suggesting that erosion and deposition were the work of a meandering stream whose width was much less than that of the valley. Seven fields have been discovered along the valley-fill trend within the study area. One well of every 1.9 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 also has been developed in marine sandstone beds of the “J” near the area of the valley fill, but only one well of approximately every 10.9 drilled is completed successfully; moreover, productivity commonly is low and total reserves are small. Therefore, stratigraphic study leading to an improved understanding of the genesis and form of the sandstone reservoirs is of considerable economic value.