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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.

This reference is for an abstract only. A full paper was not submitted for this conference. Exploring for gas and gas condensate reservoirs in Oman has in recent years been focussing on deep reservoirs in the Haima Group. Formation evaluation is becoming increasingly challenging with increasing depth. Abnormal pressure regimes and high temperatures go hand in hand with severe borehole breakouts. These factors result in a difficult logging environment making it hard to acquire a good quality data set for formation evaluation leading to efficient completion decisions. Reservoir compaction and cementation increase with depth and typically the reservoirs that are targeted are characterised by low porosity and low permeability. The calculation of water saturation at low porosity is very sensitive to errors. One porosity unit error in this environment will result in a 30 % – 40 % error in the calculated water saturation. In addition, the porosity system in the Haima Group changes with depth. Micro porosity becomes the dominant porosity system at larger depths and this has significant consequences for formation evaluation. In particular with reductions in the resistivity index, the electrical behaviour changes and residual gas saturation increases. Production testing in PDO's exploration setting invariably involves running a liner, completion and some form of fracture stimulation. Therefore, production testing is expensive and time consuming, putting more emphasis on an appropriate "test or abandon" decision. A good understanding of the range of uncertainty on all available data types and their associated interpretation are essential ingredients for the completion decision. A number of examples from recent exploration wells are shown were production test results are compared with pre-test well interpretations with the aim of capturing the lessons learned and highlighting the progress made in recent years in PDO as well as demonstrating the remaining uncertainties.

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As gas operator on behalf of the Oman Government PDO has been active in a mature basin for ca 18 years. The traditional focus for gas exploration in the north has been on deep Haima reservoirs. These have been successfully targeted in a proven Haima silicilastic "play fairway" where all the classic play elements have been are recognised and areally defi ned in a target window of ca 4 -5km. The approach has led to the discovery and development of a number of signifi cant deep gas/condensate fi elds including: Barik, Saih Rawl, Saih Nihayda and Kauther.Periodic attempts (ca 5 year cycle) to move out of the fairway area based on a challenge of prior geological models, developed with new insights, have often led to disappointing results. Outside the mature fairway all the play elements (reservoir, charge, structure, seal) are still present but an additional overriding "reservoir recovery" play risk becomes signifi cant. Reservoir recovery captures (at a play level) subtleties in reservoir composition/facies, burial history and charge timing that have a signifi cant overprint on the overall play risk.To manage the "operational" factors impacting reservoir recovery in the exploration arena ongoing work pursues optimising drilling, completion and testing procedures. The presence of an open fracture network in a recent discovery has triggered a possible geological "game changer" with the potential to signifi cantly reduce recovery effectiveness risk and unlock signifi cant additional gas volumes across a large area.With the aid of a number of key indicators (rock mechanical properties, recent fl exure, fault orientation and late charge) a working model for the deliberate pursuit of fracture systems in an exploration setting (a "fracture play") is being developed. This workfl ow in conjunction with optimised operations may allow a redefi nition of recovery effectiveness risk.

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