Abstract

The hydrocarbon potential of the Crawford thrust plate of the Wyoming-Idaho-Utah thrust belt is evaluated using geochemical, seismic and well data. Generation of hydrocarbons in footwall Cretaceous rocks and movement of Cretaceous-sourced hydrocarbons into rocks of the southern Crawford thrust plate is indicated.

Exploration history and geochemical data indicate that the Crawford thrust plate lacks a significant viable sourcerock. However, mapping footwall cutoffs of major stratigraphic units beneath the Crawford thrust plane delineates a zone of Cretaceous age source rocks which are in contact with the overlying thrust plate. Thermal alteration index (TAI) values of 2.5-3.0 place these rocks within the oil-generative window. Microscopic organic analysis (MOA) indicates that footwall Cretaceous rocks contain predominately Type II (oil-prone) kerogen. A Lopatin reconstruction of burial history, supported by measured TAI values, shows the entry of footwall source rocks into the oil-generative window to occur after the thrusting that provided traps in the hanging wall. Finally, chromatographic analyses of extracts from oil-stained cuttings of potential reservoir rocks correlate with known Cretaceous oil analyses.

Possible explanations for the lack of significant hydrocarbon accumulations in this seemingly favorable area to date include: 1) lack of hanging wall traps; 2) insufficient quantity or low organic richness of footwall source material; 3) inadequate reservoir rock; and 4) lack of migration pathways. Perhaps the generated hydrocarbons remain dispersed along migration pathways or were dissipated into the atmosphere via updip migration and leakage. Remaining potential appears limited to subthrust structures, truncation traps at the updip edge of Cretaceous source beds. and obscure hanging wall traps.