Research deal with the diagenetic grade of a 2.7 km thick sequence of lower Palaeozoic marine sediments in the Tobolka 1 deep exploration borehole and these was investigated by analysing organic and mineral components and fluid inclusions embedded in vein minerals. The combined evidence from organic matter, clay minerals and fluid inclusions points to maximum paleotemperatures of ~ 100–230 °C in the Silurian–Devonian strata, and up to ~ 250 °C or even higher in the Ordovician strata. Apatite fission track analysis (AFTA) data suggests that maximum heating occurred between 340 and 380 Ma, i.e. during Variscan orogenic deformations. These paleotemperatures and the degree of diagenesis may have been produced by a post-Middle Devonian burial or tectonic load that was subsequently eroded, or by higher paleogeothermal gradients, or by a combination of both.