Precambrian Hydrocarbon Systems of Oman
Joachim E. Amthor
Petroleum Development Oman, Muscat, Oman
In the Sultanate of Oman, the Huqf Supergroup comprises the oldest sedimentary sequences. Lying above Pan-African basement and below the Angudan unconformity, it spans over 200 My of earth history from the Late Neoproterozoic (Cryogenian and Ediacaran) to the earliest Cambrian. The Terminal Neoproterozoic to early Cambrian Ara Group of the South Oman Salt Basin comprises some of the oldest known commercial hydrocarbon reservoirs: the Ara ‘Stringer’ carbonates (dolomitized ‘microbial’carbonates) and the Athel Silicilyte (laminated organic-rich cherts). These are source-reservoir plays with source, reservoir and seal rocks deposited during a single depositional sequence (i.e. the Ara Group) in latest Neoproterozoic.
The interplay of tectonic, climatic and depositional processes was fundamental to the uniqueness of Oman’s old Huqf petroleum systems.
Proven ‘Post-glacial’ Neoproterozoic–Early Cambrian petroleum systems in Oman are mainly associated with fault-bounded salt basins which are filled with mixed carbonate, evaporite and shale/chert successions of latest Neoproterozoic and earliest Cambrian age. The Ara Group giant salt basins formed (and were later deformed) during transtensional and transpressional local tectonics within a compressional plate setting. The presence of thick salt sequences is the key for 1) The deposition and preservation of prolific source rocks in close proximity to reservoirs (most of the South Oman oils have been proven to be associated with the source rocks of the Ara Group), 2) The early in-situ generation of hydrocarbons and overpressure development which arrests diagenesis and preserves matrix porosity, and 3) The absence of oil expulsion, resulting in present-day high oil saturations.
Conceptual plays are related to the widespread presence of carbonate and clastic units of potential reservoir facies and of black shales with potential source rock characteristics in the pre-salt sequences in Oman. However, these Neoproterozoic petroleum systems and plays not associated with salt sequences have significant reservoir and hydrocarbon retention risks. Hydrocarbon source rock deposition is intimately linked to climate and, in some cases, specifically to periods of post-glacial marine transgression. There is already a widespread perception that the unconventional Neoproterozoic shale plays will form an important target for future exploration not only in Oman.
In support of the ongoing exploration and development activities, Petroleum Development Oman (PDO) has sponsored numerous research projects. These studies have addressed the Huqf Supergroup in outcrops and in the subsurface and have produced a wealth of new data and interpretations. Oman has now emerged as one of the localities in the world that provides direct constraints on both the Sturtian (740-700Ma) and Marinoan (665-635Ma) glacial episodes, as well as very precise dating of the Late Precambrian - Cambrian faunal turnover in Ara Group carbonates. The combination of precise radiometric ages and detailed carbon isotopic data in the terminal Neoproterozoic of Oman make its record an important component of the global composite carbon isotopic record.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013