Reservoir characterization of the Fahliyan Formation in Dorood Oilfield, Persian Gulf

Abstract

The reservoir characteristics of giant oil fields in the Persian Gulf are poorly described in the geological literature due to their economic importance to host nations. The Berriasian–Early Valanginian, Lower Fahliyan Formation hosts several giant oil fields in the Persian Gulf, Iran, and in this study, sedimentary microfacies (MF), reservoir characteristics, petrophysics, rock types, and electrofacies of one of these fields are presented. Five MFs are defined and interpreted as representing deposition in lagoon, shoal, and mid shelf settings. MFs deposited in high-energy shoal settings have the best reservoir properties and mainly comprise grainstones and packstones; diagenetic alteration of these MFs reduces reservoir quality. MFs from lagoon and mid-outer shelf settings mainly consist of wackestone with poorer reservoir quality.Cluster analysis applied to well-log characteristics to define electrofacies. Electrofacies are then compared to effective porosity and integrated with the capillary pressure plots to define reservoir rock types. The resulting rock types were correlated to electrofacies and MFs. Here, in most cases, high reservoir quality rock types (RT3 and RT4) correlate well to high porosity and permeability electrofacies (EF1 and EF2) and grain-supported sedimentary microfacies (MF3 and MF4) and vice versa, but in some rare cases, the best reservoir rock types do not correlate to depositional facies due to the complex pattern of heterogeneity in pore types and spatial distribution that reveals considerable impact of diagenesis in the reservoir. This study reinforces the importance of accounting for diagenetic effects (particularly destructive digenetic phenomena which impact permeability) in reservoir rock typing.