Oil wells have been dated as being drilled as far back as
the third century AD. In China, bamboo poles were drilled to depths of around
240 metres to extract oil. Until the 1970s, most oil wells were drilled
vertically, however, modern technologies have enabled horizontal drilling to be
conducted. This allows for the collection of oil from locations which may be
difficult to reach from directly above. The “deepest” drill hole in the world
is often reported as the O-14 in Russia, which has an overall length of 15 km,
however, less than 1 km of this is vertical and the rest is horizontal.
Burial of organic
matter in oceanic basins can form oil and gas over millions of years. (https://www.flickr.com/photos/121935927@N06/13598599604,
accessed 27/5/2020)
To form oil and gas we need four constituents: an abundance
of dead organisms, heat, pressure, and time. The creation story of oil and gas
begins in the ocean. Organic matter (like dead plankton) sinks to the bottom of
the ocean. Where it is calm and poorly oxygenated the organic material mixes
with fine sediment and is converted into a foul-smelling mud (kerogen). Over
time, more sediment accumulates on top and the mud becomes buried. It takes
millions of years for oil and gas to form from this kerogen. Typically, kerogen
buried at a depth of around 2 to 4 kilometres will form oil and at 4 to 6
kilometres it will form gas.
Oil and gas
migrate upwards from the source rock through permeable rocks until they are
trapped or reach the surface. (https://commons.wikimedia.org/wiki/File:Anticline_trap.svg,
accessed 27/5/2020)
As the solid organic material is converted into fluid
hydrocarbons (oil and gas) its volume increases. This causes an increase in
pressure which leads to mobilisation of the oil and gas from the source rock. Oil and gas will migrate upwards through
porous and permeable rocks like sandstone, or heavily fractured rocks. If the
fluid reaches an impermeable barrier, such as a layer of clay, it becomes
trapped. An accumulation of oil and gas under this impermeable barrier is known as a reservoir.
To release tight gas
reserves hydraulic fracturing (fracking) is used. (https://www.flickr.com/photos/121935927@N06/13598599604,
accessed 27/5/2020)
The first oil well drilled in America was only around 30 metres
deep. However, the easily accessible oil which could be found close to the
surface was consumed quickly. Over the past sixty years, the average vertical
depth of an oil well has increased from around 1 kilometre to nearly 2 kilometres.
This increase in depth leads to more technical challenges and adds a lot more
time to a project, all of this comes at a cost (drilling costs). Many oil and gas companies are now finding it more profitable to
collect unconventional, “tight” (stuck in low permeability rock such as shale) oil
and gas through fracking methods.This is occurring at Chevron’s shale
operation in Texas.
To investigate oil migration try the experiment Oil and Gas Migration to Traps. You might like to check out the video on it first.