ABSTRACT: Microbiostratigraphy, microfacies and depositional environments of the Sarvak and Ilam formations have been studied in the Gachsaran Oilfield (well no. 335). The thickness of the Sarvak and Ilam formations are 95.5 and 66.5 meters respectively and they consist of limestone with minor shale. Biostratigraphic analysis of these formations has resulted in the recognition of two biozones in the Sarvak Formation: 1) Nezzazata–alveolinid assemblage zone, 2) Rudist debris; and one biozone in the Ilam Formation: the Rotalia skourensis–algae assemblage zone. The age of the Sarvak Formation is considered as late Cenomanian and the Ilam Formation as Santonian. The zones match with the previous biozonation scheme of Wynd (1965) in the Zagros area. The intervals are subdivided into 11 microfacies that were deposited in four major depositional environments. These include shelf lagoon, platform margin, slope, and basin depositional settings. A carbonate shelf without an effective marginal barrier is considered as a depositional model for both formations.