The influences of thermal alteration, evaporation fractionation and water washing on the Carboniferous oils were evaluated by using chemical and isotopic compositions of light hydrocarbons (C6—C8) in Carboniferous crude oils from the Tz4 Oilfield, NW China. The results suggest that hydrocarbons in the CI Fm and CII and CIII Fms have distinct accumulation processes. Oil cracking probably occurred in the deeply-buried Lower Ordovician and Cambrian paleo-reservoirs through geological time scale, these thermally altered oils migrated upward along faults and finally accumulated in the Carboniferous. Natural gases from hydrocarbon secondary cracking should have a critical implication in forming gas accumulations in this region recently.