AAPG Middle East Region Geoscience Technology Workshop

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New Play and Petroleum System in Cretaceous Lower Aruma Group, Masirah Graben, Oman.


A new play and petroleum system has been uncovered in the Masirah Graben, offshore Oman. The petroleum system hosts the recently discovered GA South field, which is slated for development in late 2019. The GA South-1 well was drilled by Masirah Oil Limited in 2014, testing a Mesozoic fault block, and TD’ing in the Pre-Cambrian Abu Mahara Formation. The main target was a carbonate reservoir in the Natih Formation. However, a 20m thick sandstone section (16m net) in the Fayah Member of the Lower Aruma Group was found to be oil bearing in the upper 8 meters at well location, with an estimated oil column of 38.5 m. The sandstone is well developed and interpreted to be a near-shore marine deposit. A tight marl representing the Arada Member of the Upper Aruma Group, provides the top seal. Underlying the Fayah Member sandstone is a 10m thick marl layer; Lower Shargi Member. The basal layer of the Lower Aruma section is a coaly sandstone interpreted to be a paralic deposit representing the Qitqwat Member of the Lower Aruma Group. 3 other wells have been drilled by Masirah Oil Limited in the past 5 years, in addition to three wells drilled by Sun Oil and Amoco in the 1970s and 1980s. The Manarah-1 well was drilled in 2015, and the upper part of the Qitqat member was cored. Sedimentological analysis of the core indicates a fluvial component in addition to near-shore marine sands, suggesting a more proximal location. By integrating the core with 3D seismic from 2015, the Masirah Oil Limited team has put together a comprehensive depositional model for the Lower Aruma clastic section. Overall, the Aruma clastics developed as a band of proximal marine deposition between the uplifted area in the north, and the deeper basin in the south. The upper part was subsequently selectively eroded on fault blocks developed during transpression. Chemical analysis of the oil in the GA South 1 well shows an elevated pristane/phytane ratio, which to our knowledge has not before been documented in Oman. This high ratio suggests a terrestrial source rock, with the coal in the Lower Aruma sands being the only plausible candidate. Extracts from cuttings from the same interval, but deeper, in the Karamah-1 well also display elevated pristane/phytane ratio. RockEval analysis of cuttings from the Lower Aruma coaly sand section suggests it is immature for oil expulsion in the GA South area. 3D modeling of source rock maturation suggests that the Lower Aruma coaly section is oil-mature deeper in the basin due south of GA South 1, from where the oil migrated northwards up along a fault block ridge into the GA South structure. Thus, we have proven the presence of a new play and petroleum system in the Middle Cretaceous Lower Aruma Group offshore Eastern Oman, including a new coaly source rock, a new clastic reservoir target, a migration pathway into the structure, and with the Upper Aruma marl providing seal. Further drilling of development and exploration wells will shed additional light on the petroleum system.