Sequence Stratigraphy and Depositional Controls of Nahr Umr Iron-Rich Sheet Sands in a Unique Mid Cretaceous Reservoir (Offshore Qatar)

Abstract

The Nahr Umr Formation forms a laterally extensive but heterogeneous sand payzone in the Al Shaheen Field. An improved understanding of this Cretaceous sandstone reservoir has been obtained through an integrated seismic core and palynological data. A high resolution sequence stratigraphic model has been established for this thin, iron-rich siliciclastic unit which is overlying the Shuaiba Formation in offshore Qatar.Two 3rd order sequences are defined and subdivided into eight 4th order sequences. The base of the first 3rd order sequence, which overlies the Shuaiba Formation, is associated with a major incised valley system backfilled with clean sandstones during transgression. During highstand a condensed facies of glauconitic and iron-oolitic facies, locally siderite cemented, were deposited along with patches of clean partially cemented sandstones and shales. The second 3rd order sequence starts at the base with laterally extensive sheet sand, locally rich in glauconite, calcite and siderite cements. Subsequent 4th order sequences demonstrate subtle tectonic uplift in the north, and are characterized by significant lateral facies changes. Palynological analyses were essential to constrain the correlations, and provided additional environmental information.The proposed sequence stratigraphic model is used in 3D reservoir modeling and in placement and drilling horizontal Nahr Umr wells.