Considering oil forms under compression of trees and the like, doesn’t that mean there must have been a lot of life and vegetation there a long time ago? Why did all of that dissappear and only leave mostly barren wasteland?
In: 4590
Geologists talk about a ‘source rock’ for oil and gas reserves – this is the rock that contains the organic material that was converted into hydrocarbons which then move to other rocks where they accumulate in reservoirs. The giant Saudi oil fields are believed to have formed from shales such as the Qusaiba formed in the Silurian between 440 and 400 million years ago and the younger Hanifa. Shales are rocks made of fine clay minerals laid down in quiet, still seas. The seas where the shale was forming were teaming with microscopic life, especially algae. When these died, they sank to the bottom of the ocean along with the mud. The water immediately above the seafloor contained very little dissolved oxygen, so the organic remains of the algae didn’t decay. Instead, they accumulated to make up a considerable volume of the shale.
The ocean basin was gradually deepening, so more and more mud and organic ooze continued to accumulate. The pressure of the newer sediments piled pressure on the stuff below and as it got deeper it got warmer until the dead algae began to convert into hydrocarbons.
Over time, the oil and gas in the shale began to migrate upwards into younger reservoir rocks. In Saudi Arabia, there are huge thicknesses and extents of sandstones and carbonates containing pores and cracks where oil and gas can accumulate overlaid in turn by rocks that prevent them escaping. The reservoir for the gigantic Ghawar field is a limestone which is more than 1/3 empty space, leaving lots of space for oil.
There are even bigger fields in Venezuela, but the oil there is dominated by extremely sticky, high-sulfur crudes which are expensive to lift and transport and then need sophisticated refining to turn them into reasonably non-polluting fuels, so they have been much less preferable to Saudi crude. The best oils in the World for uses such as petrol are the very light, low-sulfur oils found in Libya and the North Sea which means they are sold at a premium over Middle Eastern oil.
Latest Answers