2013: Advent Oil & Gas: Reserves and reservoirs

by on December 3, 2013

Having looked at how oil and gas are formed, it makes sense to look at where it all is now.  Everyone knows that you oil and gas are found underground, but you can’t just drill anywhere and find the stuff.

Oil and gas reservoirs need three things to form: a source rock, a reservoir rock, and some sort of seal to stop the gas escaping.

The source rock is formed from organic-rich sediment that has been compressed and heated over millions of years, as described in the last article.  This tends to be a soft ‘sedimentary’ rock like shale, mudstone, and siltstone.

The reservoir rock has a different set of requirements: it must be porous (so it contains gaps in between the mineral particles) and permeable (so these gaps are connected.)  To explain this difference, consider a kitchen sponge and a block of expanded polystyrene foam.  Both contain gaps, so they are porous; but in the polystyrene these gaps are unconnected and it is impermeable, whereas the gaps in the texture of the sponge are all connected, meaning that it’s permeable.  Obviously only a porous rock will have any space for oil and gas, but only a permeable one will allow it to flow and accumulate.  This makes sense when you consider that polystyrene foam will keep water out, while a sponge certainly won’t!

Also like sponges, permeable rocks tend to be saturated with water rather than oil or gas.  Since water is denser, oil and gas will end up rising above it and eventually floating or bubbling to the top – this is how tar pits form on the Earth’s surface.  For the reserves to stay trapped underground, there generally has to be an impermeable seal or cap rock above it to stop it rising.  A simple sort of trap is formed when impermeable rock is distorted into an upturned bowl shape, trapping oil and gas above water-saturated rock; you can try this by trapping air (or even olive oil!) with an actual upturned bowl and a basin of water.  More complex traps form when faults partly break up a cap rock formation, leading the reservoir to form in the highest point of the fault.  Another sort of reservoir has no separate cap rock at all, and is called tight oil: if the reservoir rock isn’t very permeable, the oil and gas simply won’t flow upwards and will stay in place.  Shale oil and shale gas, which I’ll discuss in another article, are ‘tight’ reserves.

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