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Modeling helps us to better understand the EM responses of different geological conditions and provides a pathway to the inversion and interpretation of EM data. 10.3390/min12060682 3 Li G., Gu X., Ren Z., Wu Q., Liu X., Zhang L., Xiao D., Zhou C. Deep Learning Optimized Dictionary Learning and Its Application in Eliminating Strong Magnetotelluric Noise. Electromagnetic (EM) methods, both airborne and ground, are some of the most widely used geophysical techniques in mineral exploration, in which natural or controlled sources are used to transmit EM waves to the Earth and measure the reflected EM signal. EM methods include the transient electromagnetic (TEM) method, radio imaging method (RIM), ground-penetrating radars (GPR), borehole radars (BHR), magnetotelluric (MT) method, audio-magnetotelluric (AMT) and controlled-source audio-magnetotelluric (CSAMT) method. [Extracted from the article]

The Lower Silurian Longmaxi Formation in the south of Sichuan is a key player in the exploration and development of shale gas in China. Due to a highly complex topographic area, electromagnetic methods (EM) become important exploration means in this area. Many studies have been conducted on the shale mineral composition and electrical properties of shale, however, the correlation between sedimentary environments and the electrical properties of shale remain poorly understood. The electrical properties and sedimentary environment of the organic‐rich shale of the Longmaxi Formation have been studied by means of X‐ray diffraction, organic geochemistry, scanning electron microscopy and complex resistivity measurements. The discovered high quartz content of the Longmaxi Formation shale results in low resistivity. Deep‐water shelf biogenic quartz contributes lower resistivity more than that of shallow‐water terrigenous quartz. The deep‐water anoxic and organic sedimentary environment led to major enrichment of pyrite, leading to a high polarization effect in shale. We present the correlation between the lithofacies types and electrical properties of Longmaxi Formation. The mixed siliceous shale lithofacies is the most favorable among the three lithofacies, which is characterized by high total organic carbon (TOC) content, high brittleness mineral content, high polarization and low resistivity ("three high and one low"). This feature is an effective identification of shale gas reservoirs by electromagnetic prospecting. Our study can provide constraints on electrical parameters of rocks for electromagnetic "sweet spot" exploration of shale gas, and so this has important geological significance to shale gas exploration and development. [ABSTRACT FROM AUTHOR]

The marine controlled-source electromagnetic (MCSEM) method is widely used in gas hydrate exploration. For the deployed transmission system of MCSEM, it is equipped with two groups of orthogonal transmission antennas to obtain more comprehensive information. The mainstream processing method involves using a single rotated field component for conventional 2D inversion to obtain the resistivity model. However, data preprocessing and inversion speed are relatively slow, making it difficult to quickly or nearly real-time identify gas hydrate anomalous areas. In order to leverage the benefits of multi-component data and obtain a potential real-time imaging method for the future, we have developed a rapid imaging technique for the deployed transmission system to obtain apparent resistivity pseudosections. The current and electromagnetic field data in different directions are replaced by rotational invariants, and an approximate formula is derived based on the half-space formula and test corrections. This formula maps the apparent resistivity and field data. Results from model tests demonstrate that the apparent resistivity pseudosections can qualitatively identify reservoirs, particularly in the lateral range. In 2015, the South China Sea gas hydrate exploration project acquired data using the deployed transmission system. The pseudosections reveal that the rapid imaging method can directly detect resistivity anomalies and serve as an approximate and effective display method for deployed transmission system of MCSEM, thanks to its simplicity, efficiency, and low computational cost. • A deployed transmission system with two groups of orthogonal antennas • Rapid imaging approach using approximate formula with rotational invariants • An effective and low-cost imaging method for marine controlled-source electromagnetic • Helpful to quickly and qualitatively discover the location of gas hydrate reservoirs [ABSTRACT FROM AUTHOR]

This paper mainly discussed the experimental relationships between electrical properties and geological parameters such as brittle mineral content, total organic carbon (TOC) content and microscopic pore structure, with examples from Longmaxi organic shale in Sichuan province of China. With results derived from X-ray diffraction, organic geochemical measurements, nitrogen adsorption method and tests of rock electrical parameters, we came to the conclusions as follows: (1) Shale samples dominated by organic pores have the lowest resistivity, medium resistivity was found in samples mainly containing intraparticle pores and intergranular pores, and intercrystal pore dominated samples have the highest resistivity. (2) Brittle mineral content has positive correlation with rock's resistivity, as well as porosity and connectivity of intergranular pores. Low brittle mineral content suppresses electric double layers where ions exchange with clay minerals, ultimately resulting in increasing rock resistivity and decreasing rock polarizability. (3) High TOC content (>3%) is the most critical factor contributing to low organic shale resistivity, as increasing TOC content improves pore structure and reduces resistivity. The isomorphic substitution of the organic matter also serves to enhance shale conductivity and polarization. (4) Pyrite content has negative correlation with resistivity, while positive correlation with polarizability. Our study thus provides a series of basic parameters for shale gas exploration and exploitation, and discusses the geological features of significance for shale gas development in the Longmaxi Formation of this area. This is achieved by the determination of rock electrical parameters, which can be used to judge organic carbon and brittle mineral content to delineate shale areas as ‘brittle shale sweet spots’. In addition, in the process of monitoring hydrofracturing, the electrical characteristics of the low resistivity anomaly correspond to the possible existence of a fracture or fracture zone, which could provide more reliable geological structure information for the later exploitation of shale gas. [ABSTRACT FROM AUTHOR]