DEPOSITIONAL HISTORY OF GACHSARAN FORMATION (CAP ROCK OF ASMARI RESERVOIR) AT AB TEYMUR OIL FIELD

Gachsaran Formation with 700 meters thickness is a cap rock for Asmari reservoir at Ab Teymur Oil Field in Dezful Embayment. It is mainly composed of anhydrite, halite and grey to red marl with carbonate intebeds. This research has been done based on gamma-ray logs, graphic well logs, petrography of 200 thin sections of cuttings as well as Scanning Electron Microscope (SEM) analysis with EDX attachment. The Gachsaran Formation consists of many small cycles with high frequency that may have formed during fluctuation of shallow water resulted from glacial processes, tectonic activities as well as deposition of plastic sediments and subsidence in the Zagros foreland basin. Each cycle usually begin with lagoonal limestone and marl during trangressive phase and continue with evaporites that formed during regressive phase. At this stage, sabkha is formed and deposition of anhydrite started. Salt pans are formed during the last period of regression in central part of basin and affected by flooding, concentration, and desiccation stages. Sedimentary cycles of Gachsaran Formation are very similar to many ancient and recent evaporites such as Arab Formation and recent sabkha environment in Persian Gulf.