Shuaiba Formation, Depositional Facies Distribution Revealed From Extended Reach Horizontal Wells in Al Shaheen Field, Offshore, Qatar
Authors: Van Buchem Frans , Maurer Florian , 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
Evaluation of logs, cuttings and biostratigraphy data from closely spaced, extended reach horizontal wells has been used to obtain sub-seismic detailed insight into the facies distribution of the Lower Cretaceous Shuaiba Formation. The horizontal development provides a unique opportunity to map lateral facies change in the complex depositional facies systems related to platform-to-basin transition and the heterogeneous platform-top environment. This facies mapping is of critical importance for reservoir modelling, well planning, geosteering and well completion design in the Shuaiba Formation of the Al Shaheen Field, Offshore Qatar.

Introduction
The Aptian Shuaiba Formation of Al Shaheen Filed forms an important oil reservoir of the offshore Qatar. Well logs and cores from vertical appraisal wells were used to map small and medium-scale sequences across the reservoir. The limited facies information from vertical wells was then extended by information from extended-reach horizontal wells. A limited number of horizontal cores and several hundred thin sections from cuttings of horizontal wells were used to improve the facies model in between widely spaced vertical wells.
The process is used dynamically by continuously feeding detailed facies evaluations from long reach horizontal wells into the reservoir model and thus refining the three-dimensional understanding of the Shuaiba reservoir.

Results
A total of 18 depositional facies were recognised from vertical and horizontal cores, and they have been grouped into five facies associations. These facies also can be identified in thin sections from cuttings and which resemble the five main depositional environments:1) Microbial-dominated inner platform: This environment comprises Lithocodium and sponge/coral boundstone, oncoidal/Lithocodium floatstone and algal lump/ orbitolinid wackestone to packstone. These microbially-dominated carbonates were deposited on the platform top in a low-energy setting.2) Muddy open lagoon: Sediments from this environment are made up of wackestone with orbitolinids, echinoids and gastropods and were deposited in a low-energy, deeper water lagoon and to a minor degree on the platform slope.3) Grainy barrier: This facies belt, separating lagoon and foreslope, consists of skeletal packstone to grainstone with common rudists, echinoid debris, orbitolinid and miliolid foraminifera.4) Distal ramp: The facies of this low-energy ramp setting comprise argillaceous orbitolinid mudstone to packstone and fossil-poor, laminated dolomitic mudstone.5) Intrashelf basin: Mixed pelagic-benthic mudstone and wackestone were deposited in deeper water settings of the foreslope and the Bab intrashelf basin.
A sequence-stratigraphic framework was defined based on facies interpretation, special surfaces and biostratigraphic dating. The Shuaiba-Hawar 3rd order depositional sequence has been subdivided into five medium-scale sequences. These sequences show variations in facies composition and architecture, reflecting changes in sea level and depositional topography. Sequences 1 to 3 show an aggradation with a change in topography from ramp to rimmed carbonate platform. This phase is dated as Early Aptian in age based on the occurrence of the foraminifer Palorbitolina lenticularis. Sequences 4 and 5 show progradation towards south into the Bab Basin and are dated as early Late Aptian based on the occurrence of Mesorbitolina parva.
Along with the identification of different facies, the precise position of facies belts can be mapped once a sufficient number of horizontal wells has been drilled. As shown in Figure 1, the location of the grainy barrier was mapped using horizontal wells drilled from the same platform location. This facies distribution reveals a roughly west-east trending platform margin with a lagoon attached in the north and the Bab intrashelf Basin located further south.
Timing and depositional patterns of the Shuaiba formation in Offshore Qatar resemble the evolution of the Shuaiba platform on the southern margin of the Bab Basin in the United Arab Emirates and Oman. There the Shuaiba platform also shows an initial phase of aggradation which is later changing to progradation into the Bab Basin (van Buchem et al, in press). Microbially-dominated platform top carbonates are equally limited to the aggradation phase, however, in Abu Dhabi extensive rudist biostromes only develop in the platform progradation phase.

Implications and Conclusions
The field-wide facies mapping is of fundamental importance in the ongoing Al Shaheen field development, offshore, Qatar. Depending on the orientation of wells relative to the facies belts, well spacing has to be adjusted to compensate for higher or lower permeability in order to reach optimum sweep of the reservoir. Likewise, along-hole facies and permeability changes are acknowledged in the design of well completions. While drilling, the established facies model supports geosteering to place the trajectory in the optimum reservoir zone inclusive steering through several sequences in the direction of progradation. Lastly, facies mapping from cuttings is an inexpensive way to constrain deterministic reservoir models without the need of drilling vertical appraisal wells between distally located platforms. Due to the similarity of the Shuaiba carbonates around the Bab Basin, the here presented facies model and mapping technique are applicable to Shuaiba reservoirs throughout the southern Gulf region.

References:
van Buchem, F., Al-Husseini, M., Maurer, F. and Droste, H. (in press). Aptian Stratigraphy of the Arabian Plate. GeoArabia Special Publication nr. 4, Gulf Petrolink, Bahrain.