Hydrocarbon Exploration
With the investigation depth range from dozens of meters to dozens of kilometers, magnetotellurics (MT) is a highly appropriate 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. Since 2000 Nord-West Ltd has acquired MT data at more than 100 000 survey sites throughout the globe and most of them were obtained for hydrocarbon purposes.

The magnetotelluric method has been developed since 1960s mainly as a tool of sedimentary basin imaging [Berdichevsky, 1960, 1968; Keller, 1968; Vozoff, 1972]. At the same time, seismic methods are traditionally considered to be the main instrument in hydrocarbon prospecting and exploration, and in most cases advanced seismic technologies make it possible to solve tasks of defining prospective geological structures and/or formations. However, there are several geological settings, where seismic methods face serious difficulties, while the MT application can substantially increase credibility of geological interpretation of geophysical data.

Favorable for magnetotellurics geological settings are sedimentary basins covered with thick flood basalt cover or permafrost. The Siberian traps in Russia, Parana Basin in Brazil [Palshin et al., 2017], as well as sedimentary basins covered by Deccan basalts in India may serve as examples. Besides, MT proves to be an efficient complementary method of hydrocarbon prospecting in intracratonic sedimentary basins of the Northern Sea and in folded trust zones, such as Zagros in Iran, Subandean fold belt in Peru and Bolivia or Yenisei-Khatanga Depression in Siberia [Palshin et al. 2020; Palshin et al., 2021]. Another type of sedimentary basins where MT is efficient is associated with salt-dome tectonics [eg, Aleksanova et al., 2009]. Salt diapirs are found in many sedimentary basins, having salt deposits overlain by formations with sufficient thickness, as well as in fold belt areas. Classical salt diapirs (domes) are formed due to gravitational instability in regions that have not undergone significant tectonic stress; however some salt domes are found in tectonically active regions. The first type includes areas of salt-dome tectonics in the Gulf of Mexico, the north of continental Europe and the North Sea, the Caspian depression, while the Middle East salt diapirs found in Iraq, Iran and the Arabian Peninsula belong to the second one.

Magnetotellurics has certain advantages when applied to map the structure in folded regions, where the geological boundaries are characterized by large dip angles, which also makes interpretation of seismic data challenging. Subandian, Taimyr and Zagros fold belts, northwest part of Colombia are among the examples of promising oil-and-gas-bearing folded regions [Palshin et a., 2020; Palshin et al., 2021]. The electrical conductivity of sedimentary rocks depends on the clay content, pore space geometry, porosity, fluid saturation, and, finally, the electrical conductivity of the pore fluid. The first four factors characterize the petrophysical and hydrophysical properties of the rock. The latter one is the most important, since it’s the main factor controlling the electrical conductivity of the majority of non-clay sedimentary rocks. The resistivity of sedimentary rocks depends on several factors and, as a rule, an unambiguous interpretation of the observed anomalies in terms of geological structure is hardly possible without additional petrophysical and hydrophysical information. In many cases, when data or at least some estimates on lithology, petrophysical and hydrophysical properties of sediments (clay content, pore space geometry, fluid saturation, typical salinity of brines) are available, it becomes possible to estimate the effective porosity as reservoir quality index (or clay content as cap rock quality). Due to its ability of estimating the reservoir properties of sedimentary rock and/or cap rock properties based on electrical conductivity, the MT method could be especially useful in the hydrocarbon prospecting [Palshin et al., 2017].

Taymyr fold belt, Russia
Since 2005 Nord-West Ltd. has been participating in multi-method geophysical studies in the territory of Taymyr Peninsula (North of Siberia, Russia). The total length of the completed survey lines is about 20 000 lin. km. The main components of the geophysical technology applied for the studies of both the Mountain Taymyr and the Yenisei-Khatanga Trough, are the 2-D CMP seismic survey and the MT. Also, the previously collected data of gravity and magnetic prospecting have been utilized for the interpretation. MT acquisition was conducted along the seismic lines and a joint interpretation of resistivity and seismic images with the involvement of gravity and magnetic prospecting data made it possible to clarify the understanding of the geological structure of the region to identify a series of large previously unknown geological objects that aren’t exposed at the earth’s surface and identify hydrocarbon-prospective areas as well.

Fig. 1 shows the geophysical studies results along one of the survey lines. In the middle part of the line, a large Gydan-Taymyr Trough has been mapped, with the thickness of the sedimentary formation reaching 20 km (filled with about 10 km of Paleozoic sediments and the Upper Riphean sequence of comparable thickness). A joint analysis of the seismic and resistivity models has shown that the largest anomalies of the seismic wave field coincide with the resistivity anomalies; taking the available paleo reconstructions into account, it can be assumed that these anomalies correspond to reef structures and manifestations of salt-dome tectonics (Fig. 2). It was concluded that the distinct-boundary high-resistivity zones correspond to the regions of loss of correlation between the seismic reflectors and resistivity boundaries. In one case, those anomalies are accompanied by low gravity field, which suggests that they might be associated with low-density anhydrite bodies (salt domes), while other certain resistive regions fall within the elevated gravity field zones, which is the evidence for their carbonate (reef) origin.
Fig. 1. Overlayed seismic and resistivity images (top) and final model interpreted in terms of geological structure (bottom). The black lines indicate the major faults.
Fig. 2. An example of the identification of salt domes based on the joint analysis of resistivity and seismic images. The top surface of the salt domes is shown with a black line.
Magnetotellurics plays an important role as a part of the geophysical technology used to study the Mountain Taymyr. MT studies made it possible to predict a generalized lithology pattern of the Phanerozoic formations in the area, to clarify the location of the bottom of the traps and Jurassic-Cretaceous sediments, to identify the non-outcropped intrusive bodies in the upper part of the section, to make important conclusions about the oil and gas potential of the area, and to outline the strategy for further studies. Application of magnetotellurics confirmed the existence of the previously-unknown sufficiently large structures, assessed as prospective in terms of oil and gas potential.
Subandino Fold 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 details 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 characterized by a quasi-2D structure 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. Disharmonic folding is also typical for this region: folding in upper structural levels does not coincide with those in the lower levels.

The Subandean fold belt is characterized by very complex structures with steep (even vertical) dips in the anticline nuclei. Available seismic data have not provided enough information in the axis of the structures, which might lead to a false interpretation when planning exploration wells. Detailed MT studies were carried out in addition to seismic acquisition to understand the structural behavior of the study area.

Fig. 3. MT projects in Subandean Fold Belt, Bolivia:
1 – Subandino Norte, 2 – Subandino Sur, 3 – Itacaray.
Subandino Norte
Broadband MT data were acquired along 15 profiles following old seismic profiles with length from 30 to 50 km oriented across geological strike. Due to quasi-2D structure of the Subandean fold belt the main interpretation tool was a 2D inversion. A TM mode in our particular case is more informative, but a bimodal inversion with a downweighted TE mode at a limited period band clearly gave the best result. Theoretically TM mode has a better resolution to resistive targets, while TE is responsible for imaging conductive objects, but in practice for real geological settings even for quasi-2D fold belts where targets are geological structures, which could be both resistive (anticlines) and conductive (synclines) a bimodal inversion is preferable. There is also another reason for applying a bimodal inversion: a regularized 2D inversion of TM mode-only data in some cases can result in “overfitting”, when a number of artefacts appear in resistivity images. These artefacts are caused by deviation of a real structure from a 2D one. Application of a bimodal inversion even with a downweighted TE mode helps to avoid such situations and obtain reliable resistivity images for both resistive anticlines and conductive synclines. A case study showed that a 2D bimodal inversion with a reference background resistivity model as a starting one (soft constraint) is the most efficient approach. The procedure consists of three main stages: (1) unconstrained 2D and 3D MT data inversion, (2) construction of 2D reference background resistivity models using all available data: unconstrained MT data inversion, seismic and logging data and constrained inversion – in our case MT data inversion with a reference background model as starting one. An example of resistivity image is shown in Fig. 4.
Fig. 4. Resistivity model obtained by 2D bimodal inversion for exemplary profile at Subandino Norte with seismic data overlapped. Survey site locations are shown by triangles. Main structures: A1 – Lliquimuni anticline, A2 – Tacuaral anticline,
S1 – Mayaya syncline, S2 – Inicua syncline, R1 – resistive Paleozic sediments, R2 – crystalline basement [Palshin et al., 2020].
The results of MT data interpretation are presented as a scheme of the depth to Devonian sediments (second structural level) and a scheme of geological structure of upper structural level with faults zones outlined (see Fig. 5 and 6). Two structural resistivity levels were identified in resistivity images. Resistivity structural levels differ in resistivity and rheological properties: the lower one in more resistive and brittle, while the upper one is much less resistive and ductile. This fact determines disharmonic folding in the survey area. The boundary between levels corresponds approximately to the top of Gr. Retama (or Tomachi) Devonian formation, which shows how an important geological boundary (detachment) could be imaged by MT.

Depth to the top of Devonian formations differs in the northeastern and southwestern parts of the survey area: boundary between two regions could be defined (see Fig. 5 and 6). The structures (anticlines) in the upper level do not coincide with uplifts in the lower level. It's worth mentioning that Gr. Retama and lower Devonian formations are the main source rocks in the northern part of Subandean fold belt. At the NW part of the survey area the depressions with depth up to 8 km (a.s.l.) divided by elongated uplift 4–5 km was imaged, while at the south-eastern part a depression with depth about 5 km dividing two uplifts with depth up to 2 km was outlined. The topography of top of the of Gr. Retama formations correlates with structures outlined by surface geology only in a limited area (SE end of Lliquimuni anticline), while the rest part of the survey area spatial correlation of structures of in the upper and lower level is absent. Thus, the elongated depression is located in the NW part of the area and coincides with NW extension of Liquimuni anticline.

Fig. 5. Depth to the top of the second structural level. The boundary between two regions is shown by red dotted line. Black dashed lines show the location of Lliquimuni and Tacuaral anticlines axis. The location of observation sites is show as dots [Palshin et al., 2020].
Fig. 6. Geological structure of upper level. Resistivity at levels 0 m (a) and 1200 m (b).
1- backthrusts, 2- overthrusts, 3 – strike-slip [Palshin et al., 2020].
Another example is clearly seen in the SE part of the survey area where wide uplift the surface of Paleozoic formations coincides with two surface structures: Southern Inicua syncline and Sillar Anticline (see Fig. 5 and 6). The disconformity of structural levels could be explained by geological history and indicates only partial involvement of the lower structural level in the main phase of Andean folding. The lower structural level is characterized by resistivity increasing with depth and by duplex folding according to seismic data. Large numbers of fault zones were outlined in the upper structural level (see Fig. 6) by using resistivity models. Lliquimuni and Tacuaral anticlines are characterized by complicated structure (palm-tree structures). It could be explained by the peculiarity of stratigraphic sequences in the study area: interlayering of ductile clayish sediments and ductile sandstones and limestones. Clay searing is expected to be an important process responsible for fluid migration and accumulation in Subandean fold belt. Despite the spatial resolution of the resistivity images is less than that of 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 the possibility of correcting seismic results using modern approaches to joint inversions and vice versa – updating resistivity images using constraints from seismic data interpretation. Integrated approach, which includes joint application of seismic and MT method could open fresh opportunities of effective exploration of fold and thrust belts, which are commonly considered as ‘difficult’ places to explore for hydrocarbons [Palshin et al., 2020, 2021].
The MT method is increasingly used in hydrocarbon prospecting. It provides important information about the structure of buried folds, complex salt bodies and fault zones. The results of magnetotelluric studies make it possible to solve both structural and petrophysical problems in the search and exploration of hydrocarbons and deep aquifers.

The experience of studying areas of complex geological structure shows the feasibility of combining various geophysical methods to reduce the uncertainties of geological models. The use of MT data in combination with seismic survey and logging data besides other geological and geophysical studies can significantly increase the reliability of mapping prospective structures and determining the targets of exploratory drilling.
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