Source code for CONTRAfold is available at http://contra.stanford.edu/contrafold/.

This paper presents the results of the first application of high resolution magnetotellurics, a passive electromagnetic (EM) method, for hydrocarbon exploration in an onshore project in the North of Mexico. In general, controlled source electromagnetic (CSEM) methods are used in combination with magnetotelluric surveys for mapping and imaging subsurface structures for exploration purposes offshore. In this onshore study, high resolution magnetotelluric instrumentation and a novel methodology were developed to capture and analyze frequencies from the naturally occurring electromagnetic waves returning to the surface from the subsurface of the earth. Since natural electromagnetic energy is being captured, magnetotelluric surveying utilizing surface generated higher frequencies satisfies the level of resolution required for depth of investigation, depth accuracy, and thickness resolution in Eagle Ford shale play. The recorded passive electromagnetic data were analyzed, and the raw EM signal demodulated to an audio format for analysis. The resulting audio signal was then interpreted to obtain an estimate of porosity from signal strength and character and fluid contents characteristics from signal phase changes. The results were integrated and presented in electric log type format, base maps, and cross plots for interpretation, correlation, and identification of hydrocarbon trends along horizons. The main contributions of this work consist of the following. Firstly, our results will enormously contribute in reducing the uncertainty associated with seismic interpretation alone for petroleum exploration to generate new prospects and to validate existing ones. Secondly, as unconventional resources become increasingly important, identification of hydrocarbon bearing fracture and orientation using this technique will contribute to development plans for both: horizontal drilling planning and design and hydraulic fracturing.