Mapping electrical structures in the southern Great Khingan Range, north-east China, through two-dimensional magnetotelluric sounding

Abstract

Magnetotelluric (MT) measurements were performed in the southern Great Khingan Range from 2012 to 2014 to reveal the tectonic framework of the Jarud rift basin, and to identify the strata distribution of the Permian Linxi Formation. The dark mudstones in the Linxi Formation have shown the potential of shale gas. The study area is geographically situated in the Jarud Banner and Ar Horqin Banner, Inner Mongolia, China. It is covered predominantly with Cretaceous–Jurassic igneous rocks except for the small south-eastern part. It is tectonically located in the southern Great Khingan Range on the western periphery of the Songliao Basin, north of the Xar Moron Fault.MT data were acquired on six north–west and five north–east profiles in the study area. After geoelectric dimensionality analysis, strike analysis and 2D inversion, the resulting resistivity models revealed numerous large faults, part of which formed the borders of the Jarud Basin. In contrast to the original extent of the Jurassic Jarud Basin, the new borders should extend further to the north. According to well logging data and geological information, two Paleozoic depressions inside the Jarud Basin, Hunnitu depression and Gadasu depression were discovered and inferred to possibly contain Linxi strata. Meanwhile, the Mesozoic Xiretu depression outside of the Jarud Basin was discovered and inferred to possibly contain Cretaceous hydrocarbon-bearing strata. In particular, the Gadasu depression, with an area of ~500 km2, contains reasonably thick conductive sediments exceeding 4 km in depth, which were inferred to be dark mudstones possibly pertaining to shale gas. Overall, the MT work will be helpful for the assessment of potential shale gas.