Audio magnetotelluric detection of hidden karst in deep carbonate coverage area: A case study of Daguan area in Yunnan province, China

Karst is easy to develop in carbonate rocks through water dissolution, forming caves and underground rivers, which often causes issues like drill bit loss and sticking during oil and gas drilling. Previous research has shown that the organic-matter-rich shale of the Ordovician Wufeng to Silurian Longmaxi Formations in the Daguan area, Yunnan province, has a high gas content and is an important area for exploration and development. However, the Permian Maokou Formation and Qixia Formation carbonates in this area widely developed karst structures, which restricts the further exploration of shale gas. At the same time, the maximum burial depth of the two strata may be more than 1 km, which also poses a challenge to the detection accuracy of geophysical methods. In this paper, the feasibility of using audio magnetotelluric methods to identify deep concealed karst is confirmed by theoretical model calculation and pre-existing borehole (D0) test. Audio magnetotelluric profiles are implemented in the pre-selected borehole D1. According to the electrical model, the borehole D1 was redeployed to borehole D2. Subsequent drilling confirmed our reasoning and further suggested that the areas of discontinuous high resistivity often indicate the existence of karst structures, and favorable borehole locations may be the areas of massive or banded high resistivity. Therefore, audio magnetotelluric sounding not only plays a leading role in selecting oil and gas drilling sites in areas with karst covered, but also broadens the application scope of the method.