Early Palaeozoic-age non-associated gas fields operated by Petroleum Development Oman (PDO) in the Sultanate of Oman have traditionally comprised good reservoir quality sandstones located on three- or four-way dip-closed structural highs. While gas exploration success has continued over the last five years, this has been restricted to discoveries in much poorer quality ("tight") sandstone reservoirs. Significant challenges exist: target reservoirs are deep - over 4500 m (18,000 ft) with high reservoir temperatures (> 170°C). Porosities range from less than 3 to 10% with (ambient) permeabilities ranging from 0.001 to 1 mD.
These tight reservoirs have elevated pressures (above hydrostatic) and many wells record GDT (Gas-Down-To) situations (i.e. no GWC recorded). Furthermore, basin modelling indicates that peak hydrocarbon generation occurred during the Palaeozoic and Mesozoic and may have continued until Early Tertiary times in some areas.
A study was started in 2008 to analyse the above data applying a range of techniques including basin modelling, geochemistry, regional well results evaluation together with pressure data analysis and comparison with global analogues. This resulted in approval for a four well exploration campaign to evaluate diverse locations across north Oman addressing the quest for tight gas in a basin-centre setting. Drilling of the first exploration well started in late 2009 with the aim of proving the presence of deep gas accumulations and ultimately gain an indication of commercial attractiveness.
This paper presents the key criteria expected to influence the deep gas play prospectivity (i.e. presence of favourable reservoir, hydrocarbon charge and retention) and the steps to mature this opportunity. We also highlight an approach to progressing an unconventional gas opportunity in a challenging geological environment, in the Middle East, where the maturation of this resource type is currently in its infancy.