--> --> Abstract: Reconstructing a Source Rock System - Silurian Onlap in Time and Space, by Pieter Spaak; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Reconstructing a Source Rock System - Silurian Onlap in Time and Space

Pieter Spaak1

(1) Shell International E&P, The Hague, Netherlands.

With the rising interest in unconventional oil and gas plays, the need for better understanding and detailed reconstructions of source rock systems becomes even more important than for conventional plays. In this presentation, the base Silurian source rock on the Gondwana margin will be reviewed.

The most prolific Palaeozoic source rocks in the Middle East and North Africa (MENA area), are associated with a major second order transgression, following a Late Ordovician glacial event. These Silurian source beds are responsible for numerous hydrocarbon accumulations, including the Arabian ‘super-giants’ (North Dome/South Pars and Ghawar) and major Algerian gas fields (Hassi R’Mel and Hassi Messaoud). Over 200 biostratigraphically constrained data points from this central Gondwana area, allow the reconstruction of the flood in time and space and show a ‘duration’ of onlap of typically 5-9Ma. Some areas only became flooded after 24Ma in the Late Silurian (Ludlow). Within this ‘base’ Silurian transgressive framework, source rocks are found at various levels.

The melting of an Ordovician polar ice cap will have caused a considerable sea level rise with associated run-off and nutrient input, facilitating source rock formation. However, analogous to more recent deglaciation events, such a ‘glacial’ sea level rise will probably have been relatively fast and consequently, the extended duration of onlap, more likely represents ongoing tectonic subsidence of the Silurian Gondwana margin and cratonic interior. In addition, the onlap ‘distribution’ reflects flooding of a structurally and glacially modified and varied landscape, and this topographic variability was also instrumental in the formation of the ‘source beds’.