Abstract:

Resistivity of basalts is commonly lower than 50 Ω·m in the deep part of oil and gas bearing basins represented by the Paleogene basalts in the Liaohe depression. The low resistance characteristics may lead to multiple interpretations of lithology identification and fluid property analysis in reservoir logging interpretation. Through the combination of drilling core description, thin section analysis, scanning electron microscope, mineral X-ray diffraction analysis, and resistivity logging data of 25 wells, the mineralogical characteristics and conductive mechanism of low resistivity basalts are summarized, and the petroleum geological  significance of low resistivity basalt is discussed. The research shows thatprimary minerals in basalt, including plagioclase, olivine and pyroxene, are high resistivity non-conductive minerals, while a small amount of accessory minerals, such as magnetite and other conductive secondary minerals formed by burial diagenesis, are the key internal factors causing low resistivity of basalts. According to different conductive mechanisms, there are three main types of conductive minerals including metal minerals, zeolites, and clay minerals.  The resistivity of basalts is mainly determined by magnetite and saponite due to their strong conductivity. The low resistivity of basalt in the deep part of basin indicates that it has undergone a certain degree of alteration. Intercrystalline microporosity and microfractures formed by clay mineralization and the matrix dissolution porosity formed by superimposed dissolution are beneficial to the improvement of pore connectivity and reservoir permeability in basaltic reservoirs. 

Key words: volcanic reservoir, basalt, resistivity logging, clay mineralization, volcanic alteration, Liaohe depression