Application of gravity and magnetic techniques to model the geometry of the northern margin of the Onion Creek salt diapir, Paradox Basin, Utah

Abstract

The Onion Creek salt diapir lies within the Paradox Basin of southeast Utah where it forms part of a group of salt structures that separate the Paradox Basin into smaller sub-basins. A series of anomalous, tight folds occur on the northern side of the Onion Creek diapir within the Permian Cutler Group. These folds are thought to be associated with a shallow detachment horizon with three possible origins: 1) a weak shale layer within the Cutler Group; 2) a salt namakier; or 3) a salt shoulder. We collected and analyzed gravity and magnetics data across a portion of the concealed Onion Creek salt body. Since the salt is less dense and less magnetic than the Cutler Group siliciclastics, these geophysical data aid in defining the extent of subsurface salt. Our gravity data show a free-air anomaly low over the diapir with a gradual increase in values as more of the Cutler Group covers the subsurface salt. Magnetic data display a similar trend, but also suggest more complicated 3-D structure exists beneath the study area. Forward and inverse modeling indicated a salt shoulder model best fit the geophysical data. These results suggest gravity and magnetic methods are a low-cost method to evaluate plausible subsurface salt structure for oil and gas exploration studies.