Investigating the stratigraphic role of the Tirgan Formation on water wells dischargein the north and east of Bojnourd (NE Iran)

To supply water for drinking and industry, several water wells have been drilled in the Kopet-Dagh basin. These water wells have been drilled in the north and northeast of Bojnourd in the Tirgan Formation. There is a significant relationship between stratigraphy and water wells flow rates. Water wells which are drilled in the north-east of the area in comparison with those drilled in the east of the area, have higher flow rates. A complete stratigraphic section of Tirgan Formation (i.e. Baba Musa) with a thickness of 510 m was measured, sampled and studied as well as drilling log data of each of the water wells. The Baba Musa section can be divided into five rock units composed of limestone, marl and marly-limestone. Limestone rock units are considered as karst-aquifer and marly-limestone and marl are considered to be semi-permeable layers and impermeable, respectively. Water wells drilled in the upper limestone have the highest flow rates than other water wells. The wells that have not penetrated the entire upper limestone while penetrated the middle limestone and upper marly-limestone, although they are twice as deep, have a much lower flow rates. Therefore, the best water-bearing layer of the Tirgan Formation is the upper limestone.