Geological and Geochemical Attributes of Paleozoic Source Rocks and their Remaining Potential for Unconventional Resources in Erg Oriental Algerian Sahara
The Erg Oriental in Algeria refers to the western part of the Ghadames basin that extends across Algeria, Tunisia, and Libya. There is significant oil and gas production from Berkine and Illizi of south-eastern Algeria. The Paleozoic sedimentary section of these basins reveals significant petroleum source rocks within early Silurian (Llandovery) and upper Devonian (Frasnian) that were deposited along the Northern shelf of the African Craton. These source facies are primarily black shale, extending across North Africa to the Middle East and correlating to major worldwide anoxia events. Analogues are reported all around the world including North and South America, Russia, Pakistan, China. It is estimated that more that 10% of the word's petroleum reserves are derived from Paleozoic source rocks, primarily Silurian and Devonian in age.
Although the source rock potential of these hot shales was identified in some of the earliest exploration reports in North Africa, their potential for shale oil/gas resources only emerged recently. On-the-ground assessments for their unconventional resource potential have yet to be conducted.
Mineral and organic attributes of the Silurian and Devonian source rocks in the Berkine/ Illizi basin are analogous to the Silurian-aged source rocks in the Permian, Anadarko, and Michigan basins, and the Devonian source rocks to the Marcellus, Woodford, and Bakken in the US.
Silurian and Devonian organofacies are mostly sapropelic (type II kerogen). The Silurian has an abundance of graptolites and the Devonian contains Acritarchs and spores. Both contain organofacies with predominantly spores and pollens with frequent thin layers of Tasmanaceae algae (type I), deposited in the shallow marine environment of Gondwana's epicontinental seas. Younger Paleozoic source rocks (Carboniferous) are mainly ligno-humic (type III kerogen) that originated from terrestrial land plants.
The organic content of Silurian and Devonian black shales of the Erg Oriental is appropriately assessed through conventional techniques (% TOC); however, maturity measures (Vitrinite Ro%) show inconsistencies with depth of burial. Vitrinite Ro% is particularly unrealistic when compared to the thermal stress of the recovered hydrocarbon fluids. This is due to an absence of vitrinite macerals for measurement. Most commonly exinite is misidentified as vitrinite, leading to higher-than-normal reflectance readings. RockEval pyrolysis displays reliable results in the oil window but fails to differentiate the condensate or dry gas generation windows.
This presentation highlights the importance of source rock maturity in the context of unconventional resource assessment, discusses the limitations of commonly used maturity markers (Ro, Tmax) and examines alternatives to mitigate some pitfalls. The study shows organic matter has undergone a complex geological history, with differences occurring due to local burial depth variations during the Carboniferous. It also points out the good remaining oil potential for the Upper Devonian on three quarters of the studied area, and the fact that the Silurian hot shale (up to 10% TOC) is mostly in the gas window. Silurian and Devonian hot shales have provided for huge oil and gas resources in Illizi and Berkine's conventional reservoirs but more importantly hold significant unconventional resources yet to be developed.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013