Fracking drill rig. Fracking is a drilling technique is used to extract gas and oil from deep rock formations such as shale deposits. A mixture of sand, chemicals and water is pumped under high pressure from the surface to underground horizontal wells. The pressure forms fissures (fractures) in the shale. The sand keeps the fissures open, allowing gas and fluids to flow into the well and rise to the surface. The main drill rig is used to drill a well prior to the hydraulic fracturing process. Fracking can cause small earthquakes (microseismic events), and there are concerns about effects on water tables. This rig is at Belfield, North Dakota, USA, on the Bakken shale formation.