With increasing gas exploration activity in the Sultanate of Oman, a dedicated study of the lower Palaeozoic gas petroleum system of North Oman was undertaken in 2004. The study area covers over 50 thousand square kilometers and encompasses all major gas discoveries in the Sultanate. The study provides a 'fresh' look at the input parameters used to define both the thermal and migration models for gas expelled from source rocks of the upper Nafun Group (570–545Ma). The modelling results are calibrated against a recently analysed geochemical dataset describing the distribution and typing of North Oman gases in the lower Palaeozoic reservoirs.
A uniform basal heat flow of the range of 35–40 mW/m2 was preferred based on the calibration of available palaeothermal indicators including Vitirinite Reflectance measurements (VR); Apatite Fission Track Analysis (AFTA); and corrected present data bottom-hole formation temperature (BHT) data. No thermal evidence was found in support of major rifting events. This is also supported by the lack of growth faults on the seismic section across the whole region. Alternatively, accommodation space is possibly created by subsidence in a foreland basin setting as part of regional compressional setting (SRK, 2004).
The modelled maturity trend of Nafun source rocks across the North Oman Salt Basins (NOSB) shows two key expulsion peaks reflecting the basin's high subsidence during deposition of the Cambro-Ordovician Haima Supergroup and the subsequent Mesozoic carbonates. The modeling highlights areas with different expulsion histories and its impact on the risking of gas charge across the basin. The migration model highlights two key findings: Firstly, the presence of a narrow (~ 50 km wide) migration shadow zone, identified along the deepest part of the Ghaba Salt Basin (GSB) as a result of the presence of a thick section of Ara salt, supported by two dry deep wells previously drilled in this zone. Secondly, Haima-cutting faults and the presence of regional intra-Haima seals (Al Bashair and Mabrouk shale members) are identified as critical elements in providing access for a late (200-0 Ma) 'dry' gas expulsion that filled older traps.
The results of this study are used to construct charge risk segment maps for the North Oman, and provide a basis for dry versus wet gas type predictions to prospective traps.
