Abstract: The current OWC (COWC) and paleo-OWC (POWC) in CIHI reservoir in Tazhong-4 oilfield are found in buried depth of 2 5 10m and 2 610m, respectively, and the latter is horizontal in occurrence. The late faulting activity resulted in upward migration of crude oil in CIII reservoir,accumulation in CI and CII reservoirs,causing the POWC in CIII reservoir raising of 100m. If CIHI reservoir was formed in Late Permian and modified after Permian,a tilted or irregular POWC should exist due to tectonic movement and varied trap shapes. Therefore, when using oil-gas-water contact variation to determine petroleum accumulation period, the assumption that according to back-stripping analysis, make inverse calculation the paleo-burial depth of COWC in each well, and take the time of the paleo-burial depth with re-exhibited horizontal form of OWC as the petroleum accumulation period is unrealistic or wrong. It is impossibly so coincident that original OWC will re-exhibit a horizontal form in late period after long-term geological evolution, especially under the condition of trap shape changes. From the ge-ologic records, POWC is generally in tilted and even irregular forms. The corect method should be to identify POWC firstly, and then make determination of the related petroleum accumulation period, based on the time of earlier horizontal form of POWC.

Key words: Tarim basin, Tazhong-4 field, OWC, petroleum migration, petroleum accumulation, reservoir