Modeling overthrust structures for hydrocarbon exploration by 3D magnetotellurics

Abstract

The seismic reflection method is widely used in hydrocarbon exploration due to its ability to provide detailed imaging of sedimentary layers. However, in overthrust terrains where high velocity rocks (e.g., volcanic cover) overlie low velocity sediments, seismic methods often struggle to distinguish hydrocarbon-bearing zones from other sedimentary layers. These challenges arise due to seismic velocity reduction at the base of volcanic layers and the dispersive nature of volcanic rocks. In such complex geological settings, magnetotellurics (MT) offers an alternative approach, as it can penetrate deeper and resolve subsurface resistivity contrasts more effectively. This study evaluates three-dimensional (3D) MT for hydrocarbon exploration in overthrust settings with high resistivity cover layers. Synthetic models incorporating overthrust structures were developed, and their MT responses were analyzed through 3D inversion. The results demonstrate that 3D MT inversions adequately recover hydrocarbon-bearing structures but may underestimate their resistivity due to the influence of overlying layers. Constrained inversions, incorporating additional constraints such as basement resistivity values, improve overall model resolution, yet do not significantly enhance hydrocarbon trap definition. These findings suggest that incorporating additional constraints can enhance subsurface imaging. Ultimately, this study suggests that 3D MT should be more widely utilized in contractional tectonic settings, particularly in fold-and-thrust belts, to enhance hydrocarbon exploration strategies. While MT alone provides valuable imaging capabilities, its integration with other geophysical methods could further improves inversion accuracy and hydrocarbon detection in complex geological environments.