Organic petrography of Ordovician - Silurian rocks and their implications to hydrocarbon generation in the Akkas-1 well, western Iraq: New evidence for the Ordovician Gloeocapsomorpha and glaciation events

Abstract

The Ordovician-Silurian hot shales are proved to be source for hydrocarbon in Akkas Field. New set of core samples of these important strata from Khabour and Akkas formations were examined from Akkas-1 well by organic petrography. The core samples of the Ordovician were found to be enriched in the liptinitic macerals that are mostly telalginite and lamalginite with lower abundances of vitrinite-like, inertinite and non-fluorescing network-forming matrix bituminite macerals. The studied Ordovician samples showed organic matter composition with large liquid hydrocarbon generation potentiality. The telalginite grains belong to the mat-forming algae that are found mostly in clusters and colonies. The early colonies are assigned to the Ordovician cosmopolitan Gloeocapsomorpha cyanobacterium. Several morphological variations in the telalginite particles are noticed through time that verifies their evolution. This is first record of this important oil prone organic matter in this region. In addition, unique micro-laminae are identified and attributed to the record of seasonal changes or varves. These varves indicate the summer/winter cyclic changes accompanied the Late Ordovician glaciation at western part of Arabian Peninsula and its influence on sedimentation in nearby basins. This cyclic pattern is also recorded for the first time as glaciation evidence in the Akkas-1 well. The seasonal changes of the summer/winter cycle are accompanied with some distinctions in the quantity and quality of the telalginite particles. The summer micro-laminae have large abundance of both organic matter and liptinite maceral. The Silurian hot shale has abundant liptinite maceral that are mostly bituminite and lamalginite. In addition, they are rich in redox sensitive trace elements that reveal anoxic depositional conditions. Ultralaminar structures are documented for the time in the bituminite of Silurian shale, which suggest a micro-algal origin. The source rock in the Silurian shale has a large potentiality to generate liquid hydrocarbon.