6.3 The global picture
The global occurrence of petroleum is very patchy and there are sound geological reasons for this. The most significant is the distribution of continental and oceanic crust, because source rocks, the prerequisite for any petroleum system, are confined to continental crust, including continental shelves. Elsewhere, and mainly concealed beneath the world's great oceans, vast areas of oceanic crust have no source rocks and therefore no petroleum potential. Similarly, igneous and most metamorphic rocks cannot source and rarely host petroleum, so areas where they predominate, such as Scandinavia and the Canadian Shield, are poor in petroleum resources.
In contrast, petroleum-rich countries generally have one of the following two features:
Particularly prolific petroleum basins within their borders. The top five countries in terms of their share of proved world oil reserves (as at end 2004) are: Saudi Arabia 22.1%, Iran 11.1%, Iraq 9.7%, Kuwait 8.3% and the United Arab Emirates 8.2%.
Large continental or continental shelf areas, which are statistically more likely to contain sedimentary basins with the key ingredients for petroleum. For example, the five largest countries in the world (by area) contain the following share of total proven world oil reserves (as at end 2004): Russian Federation 6.1%, Canada 1.4%, China 1.4%, United States 2.5% and Brazil 0.9%.
There are specific features of the geology of the Middle East that make it so richly endowed with petroleum. The region contains several world-class source rocks ranging in age from Palaeozoic to Tertiary, with very thick reservoirs and seals above them, in enormous, low-relief anticlines. In addition, most of its reserves were easily discovered because of the simplicity and sheer size of the traps.