Oil and Gas Migration

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.