Origin and sweet spots of typical low-resistivity oil reservoirs of fine-grained sedimentary rocks

Fine-grained sedimentary rocks, in which low-resistivity oil and gas reservoirs are well developed, have become a significant target in unconventional hydrocarbon exploration. The study serves to reveal the genetic mechanism of typical lowresistivity oil reservoirs of fine-grained sedimentary rocks in applying the data from logging, seismic and thin-section electron microscopy (EM), as well as well facies-controlled reservoir parameter distribution, while proposing three sweet-spot distribution patterns under low resistivity. The research results are shown as follows. First, the origin of low-resistivity reservoirs mainly includes the large specific surface area, high content of mixed layer illites/smectites, well-developed micropore network and high bound water content, high-salinity water and high content of conductive minerals. Second, we propose three typical types of fine-grained sedimentary rocks with low resistivity, including tight low-resistivity oil reservoir with high mud content (i. e., tight reservoir of low resistivity), low-resistivity oil reservoir with high content of conductive minerals (i. e., conducive mineral-dominant reservoir of low resistivity) and water network low-resistivity oil reservoir with well-connected loose sandstones (i. e., well-connected water network conductive reservoir of low resistivity). Meanwhile, their genetic mechanisms are analyzed built on this. Third, the sweet-spot distribution patterns in subtle low-resistivity reservoirs are clarified. The tight reservoir of low resistivity has the sweet spots mainly developed in those of relatively higher resistivity; the conductive mineral-dominant type has the sweet spots mainly grown in the dissolved section of the dolomitic siltstone matrix and the section with well-developed dolomitic mud/shale fractures; meanwhile, the well-connected water network conductive reservoir of low resistivity has the sweet spots mainly developed in the ultra-fine-grained rocks with dual-mode pore structure. The results are of great significance to guiding the exploration of fine-grained sedimentary oil/gas reservoirs.