Sequence Stratigraphy and Depositional Controls of Nahr Umr Iron-Rich Sheet Sands in a Unique Mid Cretaceous Reservoir (Offshore Qatar)
Authors: Buchem Frans S.P. , Elshayeb Tarek A. , Nygaard Jeppe , Gjerdingen Tore , Wendorff Maj , Hoch Esbern , Habib Kassim Hassan 
Published: May 2016

This reference is for an abstract only. A full paper was not submitted for this conference.

Summary
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.

Introduction
The lower part of the Nahr Umr Formation in offshore Qatar represents a unique, iron-rich siliciclastic reservoir within the stack of carbonate dominated reservoirs that make up the Al Shaheen Field. These siliciclastics vary in thickness from 6 to 12 metres, and are overlying the Aptian Shuaiba carbonate platform. The sand pay zones herein are thin, and the sedimentary system is characterized by significant lateral and vertical facies changes, creating a complex heterogeneous reservoir. The main challenge of the current study was to establish a high resolution sequence stratigraphic model at the scale of the block, which would provide the geometrical framework for the reservoir model, and increase our understanding of the lateral facies changes to support well planning and field development. Geologically Qatar takes an intermediate position between the northwest of the Arabian Gulf where Mid Cretaceous siliciclastics are well-known from the Burgan and neighbouring fields, and the south where the siliciclastic are absent.

Results
This study is based on an integrated dataset covering 2,500 km2 of Qatar Block 5, including ultra high-resolution 3D seismic, core of 28 vertical wells and palynological analyses.
Two 3rd order depositional sequences are defined (Figure 1). The basal sequence boundary of sequence 1 is documented in seismic and formed by a large incised channel system up to 8 km wide, cutting into the top of the Shuaiba Formation to a maximum depth in the order of 40 meters (Raven et al., 2008, 2009; Pink et al, 2008). During subsequent rise the incised valleys were filled with clean, tidally influenced sandstones, interpreted as transgressive, estuarine backfill deposits. This facies has been cored in the channel axis. Palynofacies analysis showed a gradual upward increase of the marine influence in these channel fills. The highstand deposits consist of oolitic ironstones interbedded in slightly sandy and silty claystones, locally rich in siderite and glauconite. This facies association is interpreted as a condensed shallow marine environment. Based on the palynological assemblage, the age of this sequence is Late Aptian to Early Albian. It documents a Late Aptian C-Early Albian sea level fall and rise that has also been inferred from other locations in the region.
Sequence 2 starts with a block-wide sheet sand, that is rich in glauconite, and has been dated as Middle Albian in age (Figure 1). Mapping of the subsequent 4th order sequences show a short period of uplift and erosion in the north of the block. This structural control influenced facies distribution, with iron-oolitic facies dominating in the northern area, whereas in the southern area, an alternation of glauconitic sandstones and carbonate beds is observed. With ongoing transgression, the environment deepened and facies changed to marls and limestones, typical of the Nahr Umr Formation proper.
The distribution of the different iron-rich facies has a major impact on the reservoir characteristics and distribution. The presented model allowed constructing facies maps for the different 4th order sequences and documenting their evolution through time. This information provided the input for the deterministic modeling of this reservoir.

Conclusion
The combination of detailed facies description, sequence definition, 3D seismic analysis and palynological dating allowed constructing a high resolution sequence stratigraphic model for the Nahr Umr sand unit in Block 5. It provides a model that is predictive with respect to lateral and vertical facies variations, and which is the basis for the reservoir evaluation and field development.

References
Pink, A., C. Sembritzky and M. Zahran, 2008. 3D Seismic data helps to lift the fog - structural and stratigraphic architectural observations from a recent 3D seismic survey, Block 4, Qatar. International Petroleum Technology Conference, Paper 12498.
Raven, M., P.-H. Larsen, H. Steinhardt, M. Emang, K. Habib and F.S.P. van Buchem 2008 Evidence for top Shu'aiba exposure and incision in Block 5, offshore Qatar: Regional consequences. GEO 2008 abstract, Geo Arabia, vol. 13, no. 1, p. 226.
Raven, M., F. van Buchem, P.-H. Larsen, F. Surlyk, H. Steinhart, D. Cross, N. Klem and M. Emang (in press) Late Aptian incised valleys and siliciclastic infill at the top of the Shu'aiba Formation (Block 5, offshore Qatar). In: F. van Buchem, M. Al-Husseini, F. Maurer and H. Droste (eds.): Aptian Stratigraphy of the Arabian Plate, GeoArabia Special Publication 4, Gulf Petrolink, Bahrain.