Steam-assisted gravity drainage (SAGD) is an oil extraction method used primarily for heavy oil and bitumen. In this process, steam is injected into underground reservoirs to heat the thick oil, reducing its viscosity. This allows the oil to flow more easily to the surface. Once the oil is heated, it moves through the reservoir and is collected through production wells. The steam is generated using natural gas, making SAGD an efficient way to extract oil from deposits that are otherwise difficult to access. This technique is commonly used in regions like Canada's oil sands.