The magnitude of undiscovered onshore oil and gas resources on Federal lands in the United States is an important resource appraisal question that remains unanswered. A new U.S. Geological Survey project to assess the resource potential of these lands relies on play analysis as its method of assessment.

The first result of this project is the delineation of two plays in the Wyoming-Utah-Idaho thrust belt province: the Absaroka Paleozoic (western line of folding) and Mesozoic (eastern line of folding) plays. The two plays are aligned northeast-southwest along two subparallel anticlinal trends formed during emplacement of the Absaroka thrust. Nine fields discovered in the Absaroka Paleozoic play produce sour gas and condensate from Paleozoic carbonate reservoirs in thrust folds in the hanging wall of the Absaroka thrust. Hydrocarbons in this play are believed to be derived from Cretaceous source rocks in the footwall of the thrust. In contrast, 14 fields discovered in the Absaroka Mesozoic play produce mainly oil and sweet gas from Mesozoic clastic reservoirs in thrust folds in the hanging wall of the same thrust. Hydrocarbons in the Mesozoic play are also believed to have been generated from the same Cretaceous source rocks in the Absaroka footwall. The boundary separating the two plays is drawn on the basis of distinct differences of types of hydrocarbons produced, and lithology and age of the producing reservoir.

Preliminary geologic estimates were made of potentially recoverable hydrocarbons from fields in both plays and were based on available field and well data. The estimates were fitted to graphs that show field sizes in millions of barrels of oil equivalent (BOE) by month and year of discovery. The resulting field-size distributions indicate that the largest fields were discovered early in the 10-year exploration period (1975-84) of both plays: Whitney-Carter Creek in the Paleozoic play (1,300 million BOE), and Anschutz Ranch East in the Mesozoic play (1,200 million BOE). It seems likely that future exploratory drilling may discover additional fields that fit historical field-size patterns similar to those established in both plays during the past 10 years.

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