Sonoro Energy Ltd (SNV)

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To answer your question: Different Reservoirs

We are not talking about light oil lying beneath heavier oil within the same reservoir; rather light oil reservoirs lying below heavier oil reservoirs. Wikipedia talks about how reservoirs are formed:

http://en.wikipedia.org/wiki/Petroleum


Crude oil reservoirs

Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs.

Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.



So to answer your question as to why the light oil reservoirs are found below heavier ones, it has to do with the Geothermal Gradient. For instance in Western Canada, the increase in temperature with depth (Geothermal Gradient) is 1.8 degrees C per 100 meters. Without going into all the chemistry, light oil reservoirs need a higher temperature to be formed, which is usually deeper.