Great Basin structure and potential prospects revealed by a structural contour map on top of the Mississippian Joana Limestone

Abstract

One of the objectives of 3-D seismic surveys is to create structural maps that reveal potential prospects. However, the Great Basin is too vast and too rugged to practically acquire a 3-D survey of the region. Another way to create a structural map is calibrate gravity surveys with stratigraphic and mapping data to create structural control points to constrain a structural contour map. The Mississippian Joana Limestone lies below thick Mississippian oil shales and above the Mississippian-Devonian Pilot oil shale that is age equivalent to the Bakken oil shale and provides an ideal datum on which to make a structural contour map because: 1) it is resistant and forms prominent cliffs in contrast with the non-resistant shales above or below; 2) it is easily seen on surface and subsurface gamma ray logs as it is less radioactive than the shales above or below; and 3) it forms a major seismic reflector as the acoustic impedance of the limestone contrast significantly with the shales. Few wells penetrate the Joana but by applying a regional stratigraphic database the top of the Joana can be projected in many wells. Another way to create control points is to apply geologic mapping and topography for the top of the exposed Joana. Also by applying both geologic mapping and topography the top of the Joana can be projected from the top of other formations. All these control points can be used to calibrate gravity surveys and multiply the number of control points. The new structural contour map reveals where potential oil shales are buried at optimum depths and it reveals structural highs where Paleozoic rocks are stacked up in thrust duplexes. The map was created six months before Noble Energy made public wells they located by acquiring a 3-D seismic survey in the Elko Basin, a Tertiary sub-basin within the Great Basin, for approximately $20,000,000. Their well locations fit well within a structural low on the structural contour map and confirm the utility of the map to reveal potential prospects.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016