Audio magnetotelluric detection of hidden karst in deep carbonate coverage area: A case study of Daguan area in Yunnan province, China
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摘要:
碳酸盐岩受水溶蚀易于发育岩溶，形成溶洞、地下河等，常导致油气钻探出现掉钻、卡钻等情况。调查和研究显示，云南大关地区奥陶系五峰—志留系龙马溪组富有机质页岩具有高含气性，是勘探开发的重要领域。然而，该地区分布的二叠系茅口组和栖霞组碳酸盐地层广泛发育岩溶构造，这在很大程度上制约了页岩气的进一步勘探。同时，这两套地层的埋深可能超过1 km，对地球物理方法的探测精度也提出了挑战。本文通过理论模型试算和已有钻孔（D0）的方法试验，明确了音频大地电磁方法识别深部隐伏岩溶的可行性。在预选井位区（D1）开展音频大地电磁探测，并根据电性模型重新部署了调查井（D2），实际钻井检验了方法的可靠性，并认为不连续分布的高电阻率异常区往往指示岩溶构造的存在，井位布设的有利区可能是高电阻率异常连续分布的块状或带状异常区。因此，音频大地电磁探测不仅对岩溶覆盖区的油气钻井选址具有先导作用，同时也拓宽了该方法的应用范围。
Abstract: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.
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Key words:
- Carbonate /
- deep buried karst /
- AMT
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