We studied the distribution of organic carbon in rocks of the Bazhenov horizon, a unique object of predominantly biogenic sedimentation in the West Siberian sedimentary basin. The contents of organic carbon in the rocks were determined using the data from 4094 core analyses and core-log relationships derived from 48,500 radioactive- and electrical-log measurements. For the Bazhenov and Tutleima Formations, both approaches gave the same results. The average content of organic carbon in the rocks is 7.7%. These data were used to compile a detailed map of the distribution of organic carbon contents in sedimentary rocks of the basin. It was shown that the average organic carbon content in the rocks increases from 2–4% on the periphery of the basin to 10–12% in its central, deepest part. The distribution of Corg values in the basin is highly asymmetric. The highest Corg values are observed in the southwestern part of the basin interior, where beds with > 10% Corg range in thickness from 5 to 12–15 m. In sections, the highest Corg values are observed in their middle and upper parts, composed predominantly of silicites and mixtites enriched in biogenic silica.