Self Sourcing of Ara Group Carbonate Stringer Oils in the Neoproterozoic-Cambrian South Oman Salt Basin from indigenous kerogen and bitumen

Love, Gordon D.¹, Emmanuelle Grosjean², Charlotte Stalvies³, Colin E. Snape⁴, Will Meredith⁴, David A. Fike⁵, John P. Grotzinger⁶, Paul N. Taylor⁷, Mark J. Newall⁸, Roger E. Summons⁵ (1) UC Riverside, Riverside, CA (2) Geoscience Australia, Canberra, Australia (3) University of Newcastle, Newcastle upon Tyne, United Kingdom (4) University of Nottingham, Nottingham, United Kingdom (5) Massachusetts Institute of Technology, Cambridge, MA (6) California Institute of Technology, Pasadena, CA (7) Petroleum Development Oman, Muscat, Oman (8) Shell Egypt N.V, Heliopolis Cairo, Egypt

The South Oman Salt Basin (SOSB) is a prolific petroleum-producing basin and host to the world's oldest known commercial deposits. Most of the SOSB oils have been associated with the source rocks located within the Neoproterozoic to Cambrian Huqf Supergroup but the assignment of oils to specific Huqf intervals or facies has been hampered by the geochemical similarity of the organic matter across the whole Huqf sequence.

We sought to apply a range of newer techniques that would allow us to detect drilling and handling contaminants that might confound correlations, distinguish between migrated and in-situ bitumens and open the door to identification of ‘novel' biomarkers that were reflective of the distinctive nature of Neoproterozoic biota. Thus we employed catalytic hydropyrolysis to examine the hydrocarbon products generated directly from fragmentation of kerogens (Love et al. 1995). Oils, rocks and kerogen pyrolysates were analysed by GC, GC-MS in SIM and scan modes and, most usefully, in GC-MS in Multiple Reaction Monitoring (MRM) mode.

The effective source rocks that are positively identified as contributing to hydrocarbons in SOSB comprise the Ara carbonate stringers, the Athel basinal shales and the Athel Silicilyte. No unambiguous molecular evidence was found for contributions of liquid hydrocarbons from Nafun Group sediments (Buah, Shuram, Khufai and Masirah Bay formations) although there remains the possibility that these rocks have contributed gas.

Hierarchical cluster and principal component analysis based on MRM-GC-MS biomarker data revealed that the Ara carbonate stringer oils formed two geographically and stratigraphically resolved groupings. However, it was the hydropyrolysis approach that proved Ara carbonate stringer oils were predominantly self-sourced and genetically related to Ara carbonate stringer kerogens and heavy bitumens which are generated as early catagenetic intermediates.