We present organic geochemical data (vanadyl and Ni-porphyrin contents, pristane/phytane ratios and n-alkane compositions) relating to oils from Palaeozoic reservoirs in the SE West Siberian Basin. Analogous data front organic material in Palaeozoic and overlying Jurassic source rocks is also presented. The Jurassic succession here consists of siliciclastic units deposited during alternating marine transgressions and regressions. Our results indicate that vanadyl and Ni-porphyrins occur predominantly in transgressive units, and that pristanelphytane ratios increase progressively lip through the Jurassic section. However for Jurassic sediments in which vanadyl porphyrins are,present, the Pr/ Ph ratio is strongly facies dependent and is relatively high in transgressive units and lower in regressive units. By contrast, (C-21 + C-23 +C-25)/3C(20) n-alkane ratios are relatively low in transgressive deposits and higher in regressive deposits. There is a close relationship between this parameter and PrIPh ratio. Oils from weathered Palaeozoic reservoirs have variable compositions. In most cases, these compositions appear to be influenced by the depositional characteristics of the overlying Jurassic source intervals. Our data point to a mainly Jurassic origin for the oils in SE Western Siberia. However there is some evidence that a Palaeozoic source may be significant in the south of the studied area.