78 Geophysics in Geothermal Exploration inversion, and the bottom image (Figure 2.17c, top) shows the results of the constrained inversion previously shown. Both models are equivalent mCSEM wise (same mCSEM data fit) as they have the same transverse resistance (integration of the resistivity with respect to depth). The constrained results are preferred given the a priori information available. A qualitative interpretation of the CSEM inversion results supports the outcome of the Alternative, Central and Bjaaland wells. A prominent resistivity anomaly is recovered at Central, in which there was a significant oil discovery, which agrees with the high resistivity values measured at the reservoir location. On the other hand, the Realgrunnen structures penetrated at Alternative and Bjaaland, two dry wells, are related to low resistivity values that support the petrophysical outcome. Such analysis of the CSEM data in isolation does not allow to go beyond the previous qualitative conclusion. Only a quantitative approach that integrates the resistivity measurements with the seismic analysis can lead to reservoir properties Figure 2.18 is an MT example (Avram, 2017). Uzbekistan, along with Russia and other FSU countries have long histories deploying MT measurements on their oil and gas fields especially for those one that are poorly covered by seismic. In the example, Uzbekgeofizyka partnered with Phoenix Geophysics deployed a very large, dense MT survey over East Buzakhur. The results shown here concern the East Buzakhur – Karabay contact (Figure 2.18a). MT was tasked with the following: 1. bring additional information regarding the extent of the hydrocarbon system West of the main fault, 2. confirm and map the faults that control the East extent of the hydrocarbon system, 3. bring additional information that better characterize the source of these systems, their origin, and their relations from one basin to the next one, if any. After data analysis and preprocessing, the MT resistivity pseudo sections are converted in resistivity by inversion (Figure 2.18b). The data analysis has put into evidence the following: MT responses have different behavior whether they are collected over the known deposit or away from it. The main structural N-S fault is visible and seems to control the Eastern trap of the reservoir. mCSEM is best suited for deep waters acquisition layout. Shallow waters could lead to absence of sensitivity to resistive targets. Resolution of mCSEM is lower than for reflected amplitude seismic methods, but better than for potential fields methods. Transmitter frequencies must be chosen regarding target depth and host rock resistivity, keeping in mind skin depth concept. MT data could help to invert mCSEM data to image subsurface resistivities distribution. Combination of MT and gravity methods can be used in a Multiphysics approach to reduce the uncertainties and enhance resolution (Ceci et al., 2024a,b). For more information, we recommend the reading of the e-book devoted to electromagnetic methods in geophysics (http://books.ifpenergiesnouvelles.fr/ebooks/ ifpen-electro/).
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