Sequential extraction was performed on two oil sandstones from the Upper Carboniferous oil columns of TZ401 well. The free oils of these two oil sandstones and a crude oil from the Lower Carboniferous oil column of this well have low ratios of C28/（C27 ＋ C28＋ C29） steranes and gammacerane/C31 hopanes, ranging of 0.11-0.16 and 0.09-0.15, respectively, similar to those from the Middle-Upper Ordovician source rock. However, these two ratios for the adsorbed and inclusion oils of these two oil sandstones are relatively high, ranging of 0.29-0.31 and 0.26-0.40, respectively, similar to those of the Cambrian-Lower Ordovician source rock. This result demonstrates that the initial oil charging the reservoirs was derived from the Cambrian-Lower Ordovician source rock, whereas the later charging oil was derived from the Middle-Upper Ordovician source rock.
