Final answer:
Oil naturally gushes to the surface in primary recovery due to the pressure in underground reservoirs. Early 'gushers' were caused when drilling penetrated confining layers above the pressurized porous rock containing oil. Over time, advanced techniques like hydraulic fracturing and horizontal drilling are required as natural pressure declines.
Step-by-step explanation:
The natural gushing of oil to the surface during primary recovery is mainly due to the built-up pressure of the underground reservoirs where the oil is found. In geological formations, oil is trapped in porous rock which is confined under a layer of non-porous rock, trapping the oil under pressure. When the confining layer is penetrated, the pressure difference causes the oil to rush to the surface, sometimes resulting in the dramatic 'gushers' that characterized early oil exploration.
The natural pressure could be due to several factors, including the natural gas that is often found along with the oil exerting pressure from above, the expansion of gas as it rises, or the displacement of oil by water entering from the edges of the reservoir. However, as easily accessible reserves have been depleted over time, modern extraction techniques must compensate for reduced natural pressure. These include hydraulic fracturing to create pathways in the rock, drilling horizontally to access more of the oil-bearing formation, and using pumps to artificially lift the oil.