English abstract
In North Oman, 20 fields, with a combined oil-in-place of 500 million cubic meters (3,145 million barrels), have reserves booked in the Permian-Carboniferous Haushi (Gharif and Al-Khlata formations) reservoirs. In addition, some 150 prospects are recognized with a combined ultimate recoverable reserves expectation of 39 million cubic meters (245 million barrels). Following a successful exploration campaign, the success ratio decreased progressively. An integrated charge and retention study was therefore carried out to revitalize the play. The aim of this study was to establish a framework for predicting fluid content (gas, oil, and water). Components of this framework were a charge study to explain the hydrocarbon distribution on a basin scale and a retention study addressing top and fault seal quality. The oil in the Haushi has been generated by three different source rocks: 'Q', Huqf Group (Shuram Formation) and Haima Group (Safiq Formation). However, oil-typing shows that over 90% of the oil-in-place is derived from the 'Q' source rock. This seems to indicate a charge problem for prospects outside the 'Q' migration fairways. The 'Q' kitchen was defined with the help of chemical odometers (nitrogen compounds whose relative concentrations provide a measure of migration distance). Measurements on 18 'Q' oils suggest a source area on the western margin of the Ghaba Salt Basin, immediately to the east of the Ramlat Rawl and Saih Rawl. Seismic lines across this area show salt-filled rim synclines, which possibly contain the 'Q' source beds. The two main sealing lithologies are carbonate (Khuff and Haushi limestone) and shale (intra-Haushi shales), which can act as both top seal and/or fault seal. Based on log response, seal lithologies for gas-only, gas-and-oil and oil-only could not be discriminated. Recent mud logs over the Khuff Formation (top seal of the Gharif reservoirs) showed no evidence for a gas fraction, suggesting that the Khuff is an efficient gas seal. However, it has been observed that gas fields tend to have a thicker Khuff seal and smaller fault throw than the oil fields. This trend is related to the regional westward tilting of the basin during Khuff deposition. Fault seal has been analyzed on across-fault juxtaposition diagrams. Highly-faulted prospects, above salt domes, are water-bearing. However, apart from these extreme cases, seal breaching is not seen as a general problem. Sand-sand juxtaposition areas along faults were calculated in several Gharif accumulations. Although cross-fault juxtaposition was identified as the critical factor determining the hydrocarbon contacts in some moderately faulted fields (e.g. Burhaan), there seems to be no general relation between hydrocarbon fill and Khuff thickness. An opportunity map was generated for North Oman. A strategy for exploring for stratigraphic traps in the 'Q' migration fairways is believed to have the greatest chance of success.
