The PESGB thank ENI for sponsoring this London Evening wine reception

Global Neoproterozoic Petroleum Systems:
Snowballs, Source Rocks and the search for "Old Oil"

Jonathan Craig, ENI
(jonathan.craig@agip.it)

Tuesday 12th September 2006, doors open 5.45 pm for 6 pm
NB: Change of Venue:
 Royal Over-Seas League, Park Place, St James's Street, London SW1A 1LR

To view Fig 1, Fig 2

The Neoproterozoic eon, stretching from 1000 Ma to the base of the Cambrian at 542 Ma, is relatively poorly known from a petroleum perspective despite the existence of proven plays in many parts of the world including Oman, Mauritania, Siberia, India, Pakistan, Australia and the United States. In geological terms, the Neoproterozoic was a period of massive atmospheric, climatic and tectonic change. It was dominated by the “Freeze-Fry” cycles of the Cryogenian (850-650 Ma) “Snowball Earth” glaciations (Fig. 1) when global mean surface temperature oscillated between -50°C and +50°C over periods of ±5-10 Ma and atmospheric CO₂ levels may have periodically reached up to 350 times those of today. Perceived wisdom suggests that these extreme climatic variations were ultimately responsible for the emergence of the first recognisable animal life around 600 Ma during the Ediacaran (630 Ma – 542 Ma) and, in turn, for the eventual explosion of life forms in the latest Neoproterozoic and Early Cambrian.

Many of the proven “Infracambrian” (Neoproterozoic to Early Cambrian) hydrocarbon plays around the world depend on the presence of prolific “Infracambrian” source rocks. There is increasing evidence that deposition of many of these organic-rich units was triggered by strong post-glacial sea level rises on a global scale following “Snowball Earth” type glaciations, coupled, in some areas at least, with basin development and rifting on a more local scale.

One region of the Neoproterozoic world which is exciting interest in the search for stratigraphically deeper “Infracambrian” plays and associated “Old Oil” (or even, gas) is the so-called “North Gondwana Margin”, which extends from northern South America, across North Africa and the Middle East and into Pakistan and N.W. India (Fig. 2). The interest is rooted in the greatly improved understanding of the prolific Neoproterozoic to Early Cambrian (Huqf Supergroup), intra-salt Ara “Stringer” carbonate and Athel silicilyte plays, and the increasingly important older “pre-salt” plays, in the South Oman Salt Basin. Similar plays are already being actively parsued within the age-equivalent Marwar and Machh supergroups to the east, in the Sindh and Punjab regions of Pakistan and in Rajasthan in Western India. However, attention is increasingly turning to the potential for age-equivalent and possibly older Neoproterozoic plays across the rest of the Middle East and North Africa.

The equivalent “Infracambrian” succession in North Africa is now widely accessible for research and is already emerging as a hydrocarbon exploration target with considerable potential and with proven petroleum systems in several different areas. Large basins with excellent surface outcrops and thick sedimentary fills of Neoproterozoic and Early Palaeozoic age are widespread throughout North Africa and the Middle East (Fig. 2).

New biostratigraphic analysis has recently provided the first definitive Late Riphean (Tonian – Cryogenian) age dates for reservoir sequences containing gas in the Taoudenni Basin in Mauritania (to add to similar dates obtained for subsurface sequences in the Cyrenaica Platform bordering the East Sirte Basin of Libya in the 1970’s). New fieldwork in the Taoudenni Basin, in the Anti-Atlas region of Morocco and in the Kufra Basin in Libya has also added to our understanding of Infracambrian reservoir, source and seal relationships and confirmed the widespread presence of stromatolitic carbonate units of potential reservoir facies across many parts of North Africa.

Neoproterozoic to Early Cambrian organic-rich strata were deposited in both high latitudes (e.g. Mauritania) and low latitudes (e.g. Oman) along the North Gondwana Margin. Some of the black shales encountered on the West African Craton may be as old as 1000 Ma and clearly predate the Pan-African orogenic event. These are substantially older than the majority of the Infracambrian organic-rich units that occur across much of North Africa and the Middle East (including those in Oman) which predominantly range from c. 850 to c. 540 Ma in age. Infracambrian black shales and organic-rich dolomites, which are broadly age equivalent to the prolific Infracambrian source rocks in Oman, are known to generate light oil and heavy oil respectively, further east in Pakistan and India.

Post-glacial black shale deposition is also a well-documented phenomenon in northern Gondwana during subsequent Palaeozoic times. A post-glacial sea level rise of some 50-100 m, following the Late Ordovician Saharan glaciation, flooded large areas of northern Africa and Arabia during the earliest Silurian. The associated transgression was global and pushed the coastline of northern Gondwana southwards by many hundreds of kilometres. As appears to be the case during the Infracambrian, Silurian black shale deposition was restricted to the anoxic parts of the infrashelf basins. The Late Ordovician-Early Silurian systems may form a good analogue for post-glacial source rock depositional systems, both spatially and temporally.

Further investigations of the Infracambrian black shales globally are required in order for us to better understand their relationship with the “Snowball Earth” glaciations. In particular, high resolution biostratigraphic, isotope and other palaeotemperature proxy records need to be documented for suitable type sections of the organic-rich facies and constrained with absolute radiometric dates, so that the onset and termination of black shale deposition can be compiled with precision and placed within the Neoproterozoic to Early Cambrian climatic context.

Our knowledge of the Neoproterozoic and its petroleum systems is improving rapidly. There is already a widespread and growing perception that the “Infracambrian” succession will continue to form an important target for future exploration, not only in North Africa, but elsewhere along the North Gondwana Margin and globally.

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