April 18. Saudi Arabian Oil

April 18, 1906. At 5:12 in the morning San Francisco was rocked by a powerful earthquake. The magnitude was 7.8 or 7.9 and the death toll is estimated today at about 3,000 people. 

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Now back to the Silurian. Oil is another resource associated with Silurian rocks.  Worldwide, something like 9% or so of all the oil comes from Silurian source rocks – and most of that is in Saudi Arabia. The Silurian Qalibah Formation is a thick shale and sandstone package. The lowest part is a black shale rich in organic material. It seems to have been deposited relatively early in the Silurian, when the land was being flooded as the glacial period at the end of the Ordovician Period ended. It appears that this shale was deposited during a sea-level drop, an anomaly during a time of generally rising sea levels as the ice melted. The receding sea might have stranded piles of plants and other shallow-sea life in shallow pools on land, where enough organic material was caught in the mud to make these black shales. It may be that there was a proliferation of life associated with the end of the glaciation – there was such a proliferation, but how fast and extensive it was can be debated. But with the glaciers’ withdrawal, there was more land to erode and to provide nutrients into the sea for life to take advantage of in the relatively suddenly warmer waters. The exact origin of these rocks remains somewhat contentious.

They call these source beds “hot shales” because they also tend to have concentrations of radioactive elements, which make them easy to recognize on well logs, which are basically measurements of information taken down a drill hole. One tool measures gamma-ray intensity, which is greater in radioactive materials. Similar rocks of similar age are found in North Africa as well – Algeria, Libya, and Egypt have such shales and oil explorationists are really only just beginning to understand them.

The known oil from Silurian source rocks in Arabia and Algeria amounts to probably about 95% of the Silurian-sourced oil in the world, but there’s some oil in Silurian rocks in the U.S. as well. We’ll get to that in a few days. 

—Richard I. Gibson

References:
Hot Shale 
Role of glaciation
North Africa