Anhydrite fabrics as an indicator for relative sea-level signatures in the sequence stratigraphic framework of a carbonate ramp

Abstract

Evaporite deposits are increasingly recognized for their role in developing oil reservoirs. In the Shadegan Oilfield, anhydrite is an evaporite mineral identified in the Asmari Formation (Oligocene-Miocene) as clastic- and carbonate-evaporite assemblages. Our study found that diagenesis plays a critical role in controlling the distribution of different fabrics (texture and structure) in the area. The anhydrites resulted from two progressive processes: early diagenesis and formation of anhydrites related to supratidal to shallow settings, and late diagenesis and crystallization of anhydrites in deeper environments. The evaporite-bearing intervals provide an opportunity to investigate the interactions between sea-level fluctuations and the growth of anhydrites during the development of the carbonate ramp of the Asmari Formation. Despite previous studies on the Asmari Formation, little is known about the relationships between the occurrence of its anhydrite fabrics and sequence stratigraphic framework. In our study, the continuous shallowing-upward trends, exposure of sediments, and subsequent diagenesis were responsible for the deposition of anhydrite. Semi-arid to arid climates followed, resulting in stacking of anhydrite and dolomites, particularly in the upper parts of the Asmari, corresponding to the Burdigalian.