Hydrocarbon Exploration
With the investigation depth range from dozens of meters to dozens of kilometers, magnetotellurics (MT) is the best electromagnetic method for hydrocarbon prospecting, which is especially effecient when used together with other techniques of deep geophysical exploration, such as seismic, potential field and TDEM surveying. Nord-West Ltd. has successfully carried out a number of large MT projects aimed for hydrocarbon exploration all over the world, a couple of which are given below.

Subandino Foled Belt, Bolivia
This unique MT & TDEM project (by the time of completion - the largest in Latin America) has been carried out in 2017 by Nord-West Ltd. in cooperation with Bolpegas SRL under a contract with the Bolivian national oil & gas company YPFB. The main goal was to understand better the geological structure of the sedimentary basin and petroleum system in the two study areas: Subandino Sur and Subandino Norte.

In order to acquire 3628 MT and 1130 TDEM records in less than 1 year Nord-West has brought to Bolivia more than 50 experienced field data operators and used the unprecedented number of 54 sets of MT instrumentation at once. The field logistics included off-road, ferry, helicopter and boat transportation, as well as long-distance foot marching and setting remote camps in the jungle. The employed technology incuded broadband MT data acquisition in the period range from 0.0001 to 1000 s. The data were acquired about 14 –20 h (overnight) for better quality. More detailes on data acquisition, analysis and interpretation could be found in [Palshin et al., 2020].

The Subandean fold belt is a thin-skinned in-sequence system with
several detachment levels. The geological structure of the Subandean
fold belt is typical for many fold belts. It is characterized by quasi-2D structures with wide relatively low resistivity synclines with horizontal layering and narrow complicated anticlines often fragmented by a large number of fault zones and subvertical layering forming mountain ridges with steep slopes. Also, the folding is often disharmonic between the upper and lower structural levels.

As a result, the quality of archive seismic data obtained within the Subandean fold belt ranges from bad to null. Only in the northwestern sector of the area, where the structural complexity diminishes, the seismic response improves. The first wells were proposed in embedded anticline structures, according to the original interpretation based on the surface geology. Unfortunately, after drilling the results showed that the wells did not reach the crest but ended up in the flanks of the interpreted structures. None of these wells reached the proposed objectives in a proper structural position and even no significant hydrocarbon shows were reported during drilling.

Such misplacements of the first deep (3-4 km) wells drilled in the area were caused by the presence of low-angle subsurface overthrust system. Consequently, one of the main tasks of MT exploration was to map the top of the gas bearing Devonian rock formations. These maps show the locations of its main 'crests' and 'troughs' thus helping to predict the hydrocarbon migration paths and choose better places for deep drilling.

Advanced in MT technology made it possible to get new geological information about the structure of the sedimentary cover in the study area. Despite the spatial resolution of resistivity images obtained is less to compare with seismic stacks, they give valuable additional independent information. In particular, for the northern Subandean fold belt geological settings MT gives the true position of the buried axes of anticlinal folds in lower structural level corresponding to Carboniferous-Devonian formations. The use of MT data opens up the possibility of correcting seismic results using modern approaches to joint inversions and vice versa – updating resistivity images using constraints from seismic data interpretation.

Taymyr, Russia
Detailed geological and geophysical study of the region of Taymyr Orogen and Yenisey-Khatanga Trough in the Northern Siberia shows that it has large Paleozoic-Triassic anticlinal structures and thus is very interesting from the standpoint of hydrocarbon exploration. The direct evidences of favorable conditions of oil & gas generation in this region are numerous surface hydrocarbon manifestations. Due to the high geological complexity of the area under study, the reliable geophysical results could be achieved only by making use of joint interpretation of seismic, MT and potential field data. In the below example one may see how the inclusion of the MT data collected by Nord-West Ltd. into the interpretation scheme helps to find and delineate salt diapirs and prospective reef structures, base of the trappean complex, etc.
More details on this and other MT surveys for evaluating oil & gas occurences in Siberia could be found in [Afanasenkov, Yakovlev, 2018].