In 2010, Qatar Shell Upstream International B.V. (QSUI) re-entered Exploration in Qatar focusing on the relatively deep conventional Pre-Khuff gas plays with a view to discover additional hydrocarbons in the State of Qatar. The Pre-Khuff plays pose two main challenges; firstly the geophysical challenge of being able to image the deep Pre-Khuff structure and hence trapping configurations and secondly the geological challenge of being able to realistically predict reservoir and top seal quality. This paper highlights the approach to geological issues associated with Pre-Khuff exploration and how technologies were deployed to address pre drill exploration challenges in the venture.
Significant uncertainty exists in relation to the deep structure of the Qatar Arch particularly when in the region the Pre- Hercynian package has been shown to be structurally different to the overburden. 2D seismic reprocessing has helped to significantly improve seismic imaging encompassing clear improvements in seismic data quality with better reflector continuity and elucidation of structural and stratigraphic complexity. The overall structure of the Qatar Arch and the trap styles in the Pre-Khuff are assessed using a combination of gravity anomaly data and interpretation of 2D seismic profiles, while incremental restoration of 2D seismic lines provides insight on the timing of trap formation.
The Devonian Jauf and the Permo-Carboniferous Unayzah plays are primary targets in Block D. Key petroleum system risks are taken as top seal retention, reservoir quality and charge timing and migration. To address these geological factors, data has been re-assessed and new work undertaken on stratigraphic definition, depositional models, reservoir quality prediction, integrated charge evaluation and seal integrity of the Pre-Khuff. Pre-Khuff top seal effectiveness in the focus plays have been assessed through MICP analysis and demonstrates that the relevant seals can hold back significant gas columns. Revised depositional models, based on core observations, have been used to extend potentially successful play fairways into Qatar. New petrographic analysis programs address the diagenetic controls on reservoir quality and their relationship with depositional environments aiding the prediction of potential net viable reservoirs at depths greater than conventional cementation floors. Integrated charge evaluation using basin modeling and geochemical techniques comprising Compound Specific Isotope, TOC, visual kerogen and chitinozoan reflectance analysis suggests a working petroleum system for Pre-Khuff reservoirs. 1D basin models suggest the Pre-Khuff on the Qatar Arch has received gas charge over the last 100 Ma but despite these favorable conditions, a key risk remains the presence of effective vertical migration pathways (i.e. faults) from the source rock into the prospective reservoirs.