A regional sequence stratigraphic model is proposed for the Oligo-Miocene Asmari and Pabdeh Formations in the Dezful Embayment of SW Iran. The model is based on both new detailed sedimentological observations in outcrops, core and well logs, and an improved high-resolution chronostratigraphic framework constrained by Sr isotope stratigraphy and biostratigraphy. A better understanding of the stratigraphic architecture distinguishes four, geographically separated types of Asmari reservoirs.Three Oligocene sequences (of Rupelian, early Chattian and late Chattian age) and three Miocene sequences (of early Aquitanian, late Aquitanian and early Burdigalian age) have been distinguished, representing a period of 15.4 Ma. The stratigraphic architecture of these sequences is primarily controlled by glacio-eustatic sea-level fluctuations, which determined the distribution of carbonates, sandstones and anhydrites in this sedimentary system. Tectonic control became important in the Burdigalian with a regional tilt down towards the NE. The lithological heterogeneity, the complex geometries, and both early and late diagenetic alterations are the basis for a classification of four main stratigraphic reference types for the Asmari Reservoirs: Type 1, sandstone dominated; Type 2, mixed carbonate-siliciclastic; Type 3, mixed carbonate-anhydrite; and Type 4, carbonate dominated. The sequence stratigraphic model predicts how and when these types change laterally from one to another.

Many factors control accurate determination of water saturation (Sw). Cementation exponent (m) and tortuosity factor (a) are from those that have been focus of many studies. Log-log plot of porosity (φ) versus formation factor (F) is used to determine m and a. The cementation exponent is determined from the negative slope of the least square fit straight line of the plotted points, while the tortuosity factor is the intercept of the line where φ = 1. In heterogeneous carbonate reservoirs where pores and pore throat networks are complex due to various diagenetic processes, F and φ scatter significantly on the φ-F plot. This will cause a small coefficient of determination between F and φ and thus less reliable m and a. Although classification of data based on petrofacies and/or permeability may improve the correlation to some extent, but data still show significant scatter. Introducing current zone indicator (CZI) and electrical flow unit (EFU), this study has established a new approach to classify φ and F data. The approach enables one to obtain more accurate m and a and thus more realistic calculation of Sw. This study also shows forcing a to any fixed value, will lead to both optimistic and pessimistic estimation of Sw within a reservoir.

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