Genesis of large-amplitude tilting oil-water contact in Sarvak Formation in South Azadegan Oilfield, Iran

The Sarvak reservoir of the South Azadegan Oilfield in Iran is a thick, block-like limestone reservoir of slack shelf sea deposition. The oil-water contact (OWC) of the reservoir is characterized as a large-amplitude one tilting from north to south. Hydrodynamic force and reservoir heterogeneity were studied, indicating that neither of which were the causes for the tilted OWC of the Sarvak reservoir. By investigating the evolution of areal tectonic setting, utilizing a formation flattening technique to recover the paleo-structure of different geologic stages, and combining with accumulation factor analysis, it was determined that the adjustment of traps after the Zagros orogeny led to the tilted oil-water contact. A paleo-trap which decreased from north to south (reversed from the present situations) formed in the Sarvak Formation in the SA Oilfield after the late Paleocene. The Zagros orogeny during the Neogene caused the adjustment of the trap, and the former low point in the south uplifted significantly and gave birth to a new secondary trap. As a result, the present tectonic setting with two high points (higher in the north and lower in the south) was formed. The balance in the paleo-trap was broken, and hydrocarbon migrated to the secondary trap in the south and was still adjusting now. The large-amplitude tilting oil-water contact showed that the paleo-reservoir was still adjusting and unstable.