Genetic mechanism and development of the unsteady Sarvak play of the Azadegan oil field, southwest of Iran

The upper Cretaceous Sarvak reservoir in the Azadegan oil field of southwest Iran has its oil–water contact nearly horizontal from the north to the center and dips steeply from the center to the south. The purpose of this paper is to interpret this abnormal reservoir feature by examining the accumulation elements, characteristics, and evolution based on the 3D seismic, coring, and well logging data. Generally, in the field, the Sarvak reservoir is massive and vertically heterogeneous, and impermeable interlayers are rare. The distribution of petrophysical properties is mainly dominated by the depositional paleo-geomorphology and degrades from north to south laterally. The source is the lower Cretaceous Kazhdumi Formation of the eastern Dezful sag, and the seal is the muddy dense limestone of the Cenozoic Gurpi and Pebdeh Formations. Combined with the trap evolution, the accumulation evolution can be summarized as follows: the Sarvak play became a paleo-anticlinal trap in the Alpine tectonic activity after the late Cretaceous (96 Ma) and then was relatively peaceful in the later long geologic period. The Kazhdumi Formation entered in the oil window at the early Miocene (12–10 Ma) and charged the Sarvak bed, thus forming the paleo-reservoir. Impacted by the Zagros Orogeny, the paleo-reservoir trap experienced a strong secondary deformation in the late Pliocene (4 Ma), which shows as the paleo-trap shrank dramatically and the pre-low southern area uplifted and formed a new secondary anticline trap, hence evolving to the current two structural highs with the south point (secondary trap) higher than the north (paleo-trap). The trap deformation broke the paleo-reservoir kinetic equilibrium and caused the secondary reservoir adjustment. The upper seal prevented vertical oil dissipation, and thus, the migration is mainly in interior Sarvak bed from northern paleo-reservoir to the southern secondary trap. The strong reservoir heterogeneity and the degradation trend of reservoir properties along migration path (north to south) made the reservoir readjustment extremely slow, plus the short and insufficient re-balance time, making the Sarvak form an “unsteady reservoir” which is still in the readjustment process and has not reached a new balance state. The current abnormal oil–water contact versus the trap evolutionary trend indicates the secondary readjustment is still in its early stage and has only impacted part of paleo-reservoir. Consequently, not all of the reservoir is dominated by the current structure, and some parts still stay at the paleo-reservoir form. From the overview above, we suggest the following for the future development: In the northern structural high, the field development should be focused on the original paleo-reservoir zone. In the southern structural high, compared with the secondary reservoir of the Sarvak with the tilted oil–water contact and huge geologic uncertainty, the lower sandstone reservoirs are more reliable and could be developed first, and then the deployment optimized of the upper Sarvak after obtaining sufficient geological data. By the hints of the similar reservoir characteristics and tectonic inheritance with Sarvak, the lower Cretaceous Fahliyan carbonate reservoir is also proved to be an unsteady reservoir with a tilted oil–water contact.