Reservoir investigation of shale gas using the magnetotelluric method in Nandan, Guangxi, China

The rugged terrain, complex near-surface geology and harsh deep subsurface conditions in Nandan, Guangxi, restrict shale gas reservoir research. This paper adopts magnetotelluric (MT) surveys down to 4 km to analyse shale gas enrichment rules, controlling factors and reservoir properties. The results show that regional strata are stable with dip angles below 10° and weak tectonic deformation. Shale gas occurs in Lower Carboniferous Luzhai Formation (C1lz) mud shale, with thickness between 200–500 m and burial depth of 300–3000 m. Its resistivity varies from 1 to 7420 Ω m, averaging 164 Ω m. This shale contains abundant organic matter: 85.96% of samples have TOC above 3%, with a maximum value near 12%. The organic matter has reached the over-mature thermal evolution stage, showing strong hydrocarbon generation capacity. Sufficient shale thickness and moderate burial depth create favourable accumulation conditions. The upper mudstone of the Luzhai third member and tight limestone of the Baping Formation form intact cap rocks with little denudation, offering good sealing for gas preservation. The study area underwent slow tectonic uplift (15.77 m/Ma), avoiding intense fracturing and helping retain shale gas. Without man-made seismic sources, MT surveys fit the mountainous topography well and accurately map deep structures and shale reservoirs. This study verifies the reliability of MT prospecting in Nandan and supplies key geophysical support for shale gas evaluation across South China.