Idaho Falls

idaho falls prospect

The following is a compilation of the geologic history and a discussion of the risks for the Idaho Falls Prospect as a means of justifying the drilling of a frontier wildcat in the Snake River Plain (SNP). The prospect potential is extremely high, based on the unusually strong results of the various exploration techniques employed.

• The prospect lies on the traces of the Williard and Bannock branches of the Idaho­Wyoming Overthrust, and given the thicker Mesozoic and Paleozoic section present, more than one thrust-fold sheet may be prospective.
• The SNP lies directly on the NE-SW trace of the Brockton-Froid fault system, one of the major basement fault systems in the center of the continent, and an important element for production in the Williston Basin.
• Reactivation Brockton-Froid basement fault system is evident from the early Tertiary Structural Zone, and is parallel with the faults which define and are responsible for the Humboldt Structure Zone and the Snake River Graben.
• Activity along the reactivated basement fault system is responsible for the migrating mantle hotspot, the resultant basalt, rhyolite and ash flow tuffs. The mantle hotspot resides today beneath the Yellowstone volcano, and is responsible for the extremely high heat flow values beneath the SRP.
• The high heat flow, associated volcanism and intrusion, and depth of burial are likely responsible for a higher organic maturity for the sediments. The Paleozoic sediments may be over-mature and strictly gas productive, whereas the Mesozoic sediments are likely gas and condensate.
• The equivalent producing zones in the Wyoming thrust belt fields are likely the main objectives: 1) Jurassic Nugget sandstone, and 2) Jurassic Twin Creek Limestone formations. Welded ash flow tuffs (ignimbrites), shallow sandstones and dolomites within the Plio-Pleistocene Snake River Group are potential reservoirs sourced by lacustrine oil shales or biogenic gas from lignite beds.
• The gravity and magnetic maps show the orthogonal fault system where the older faults segment the linear pattern of the Snake River Plain. The Idaho Falls prospect appears to be underpinned by a structure(s) that likely served as a structural obstacle over which the stratigraphic successions and the individual thrust members of the Idaho-Wyoming thrust belt system have been ramped up.
• The most likely leading edges of the thrusts across the prospect are the NNW -SSE oriented Williard, Bannock Meade and Absaroka thrust plates south of the contact with the Snake River Plane.
• Though down-dropped into a Plio-Pleistocene aged graben with bounding normal faults, evidence from the gravity and magnetic data, as well as the hydrocarbon detecting techniques, indicate that the earlier fold-thrusts are intact, and probably segmented by oblique faults parallel to the SRP.
• The down-dropped thrust-folded traps were covered with a relatively thin layer of alternating high-silica rhyolite flows and volcanic ash, alternating with repetitive basalt lava flows. During quiescent periods of igneous activity, the SRP formed lakes (most well known is Lake Boise in the western part of the SRP), that are intermontane and similar to Lake Bonneville and the Great Salt Lake of Utah. However, the basement and structural configuration beneath the prospect was a topographic high around which had a thin veneer of just several thousand feet compared to the much thicker volcanic flows and sediment deposited to the west and east.
• The prospect area was a major depocenter for extra-thick accumulations of Paleozoic and Mesozoic sediment, providing an abundance of potential reservoirs and thick accumulations of source rocks in the Mississippian, Permian and Cretaceous.
• Radiometric pattern, as well as gravity and magnetic data indicate side bounding faults parallel to SRP graben faults in NE-SW. Potential segmentation of the prospect by NE-SW Plio-Pleistocene trending "graben" parallel faults is likely.
• The hydrocarbon detecting methods indicate a very large accumulation, which would indicate that deep, large structures with substantial thicknesses of reservoir are the likely origin of the anomalies; lenticular traps at shallow depths are not responsible for the size of these anomalies, but may provide serendipitous secondary reservoirs on the way to the "sub-basalt" depths.