The origin of separate oil and gas accumulations in adjacent anticlines in Central Iran

• In adjacent anticlines one (Alborz) has trapped oil, the other (Sarajeh) gas and condensate.
• Sarajeh is south verging, Alborz north verging above a SW-dipping detachment.
• Downbuilding of the Upper Red Formation above salt caused the south vergence.
• Vergence direction caused relatively late closure of Sarajeh, when source basin was mature for gas.

In Central Iran two adjacent anticlines contain hydrocarbons within the Late Oligocene–Early Miocene Qom Formation. The anticlines formed at approximately similar times (Late Miocene–Pliocene), have similar overall geometries, but the Alborz Anticline is an oil field, while the Sarajeh Anticline is a gas field, and both are underfilled structures. Basin modeling and structural restorations identified why the two different accumulations developed. The Sarajeh and Alborz anticlines are modified detachment anticlines. Asymmetric development of growth strata on the SW margin of the folds was caused by salt withdrawal and normal fault activity on the basin margin, which modified the detachment fold style and tilted the basal detachment to the SW. The detachment dies out to the NE as the basal (Lower Red Formation) evaporites pinch out in the same direction, yet the Alborz Anticline is north vergent, while the Sarajeh Anticline is south vergent. The mini salt-withdrawal basin is best developed on the southern limb of the Sarajeh Anticline and is responsible for the SW vergence. This difference in vergence permitted the Alborz Anticline to attain 4-way dip closure earlier in its structural history than the Sarajeh Anticline, and consequently the trap was at least partially developed as a four-way closure during oil generation and expulsion. For the Sarajeh Anticline oil migrated across the fold when it was still unclosed, continued subsidence of the SW fold limb resulted in gas generation and expulsion during the time when the fold attained closure.