In a conventional oil or gas field, where the oil or gas is in relatively porous and permeable rock (i.e. the pores are connected), the oil or gas can usually flow naturally from the reservoir rock to the wellbore. Nonetheless, a variety of techniques are often used to improve the flow of oil or gas, including hydraulic fracturing.
Rock formations that are very impermeable require artificially fracturing the rock using hydraulic fracturing in order to produce significant amounts of oil or gas.
Learn more: Hydraulic Fracturing