UNDRILLED LARAMIDE ANTICLINES IN THE CORDILLERAN FORELAND OF WYOMING REQUIRE EVALUATION: NOT ALL WILL CONTAIN COMMERCIAL HYDROCARBONS. WHICH ONES WILL? THE PAST IS THE KEY TO THE PRESENT

Seminal geologic studies of the structural entrapment of oil and gas in the Rockies reveal key parameters as to why some Laramide anticlines are productive while others are not. These papers, many written long ago, are worth reevaluation for those interested in per well production that exceeds the resource plays now in vogue. In the Green River Basin, Lost Soldier-Wertz fields (the most prolific per/surface acre accumulations in the Rockies) produce from every porous rock under closure, including Precambrian granite. Lost Soldier, the anticline containing mostly oil, is structurally higher than its twin anticline, Wertz, containing abundant gas. Their relative positions during initial hydrocarbon accumulation have been reversed during the latter, wrench-fault phase of Laramide tectonics. Some large, adjacent anticlines are not productive, while other smaller ones are: The timing of the structural development versus hydrocarbon migration is critical. Nearby, Lost Soldier vintage structures, primarily hidden under late Tertiary alluvium, remain undrilled.

In the Wind River Basin, Winkleman Dome anticline is a prolific producer but an attached anticline, one order of magnitude larger, and structurally higher, is barren. Circle Ridge anticline produces, in part, from Phosphoria Formation, yet the same formation outcrops on the anticlinal crest via an unbroken lithologic continuum. Northwest Sheldon Dome Field produces more oil from the single, farthest down dip Tensleep producer than the combined production of all the Tensleep wells on the crest of the anticline. Emigrant Gap anticline has been drilled on today’s structural crest (good porosity and live oil shows were found) but despite good structural closure it was not productive. Isopach studies indicate the old, pre-Laramide anticlinal closure may today be at a lower, undrilled structural position similar to the productive but structurally lower Winkleman Oil Field-structurally higher Sage Creek Anticline complex. Perhaps ideas from earlier publications which indicated pre-Laramide hydrocarbon migration and downdip entrapment can be applied to Emigrant Gap Anticline.