Logging response characteristics of karst formation in upper Ordovician assemblage of Daniudi gas field

Based on drilling core observation, thin section analysis and identification, and comprehensive analysis of conventional-imaging logging data, the logging response characteristics of karst formation in upper assemblage of lower Paleozoic Ordovician in Daniudi gas field were studied. The results show that from the top of weathering crust to bottom, the upper Ordovician assemblage can be divided into vertical seepage zone, horizontal undercurrent zone and deep slow current zone. The vertical seepage zone is mainly distributed in the Ma51+2 sub-member, with undulating gamma curve, significantly increased acoustic time difference and neutrons, and messy speckled distribution in electrical imaging. The horizontal undercurrent zone is mainly distributed in the Ma53+4 sub-member, with and the dentated gamma curve and a bright white block distribution of karst breccia in the electrical imaging. The deep slow current zone is mainly distributed in the Ma55 sub-member, with low and straight gamma curve value, dark irregular thin strip in the high conductance seam, and black cluster strip in the seam cavity.