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

Abstract

Abstract 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. Keywords: Permeable, Impermeable, Semi-permeable layer, Drilling Log, Tirgan Formation Introduction Various consumers in Iran are dependent on groundwater resources of alluvial aquifers. In the last few decades, due to water shortage in alluvial aquifers, karst aquifers have been widely considered. The high costs of drilling wells in karstic Formations lead to more precise studies to determine appropriate drilling locations. Tirgan Formation is one of the most important karstic formations in the Kopet-Dagh area in northeast of Iran. Several wells in this formation have been drilled for drinking water and industrial consumptions. Some of the wells drilled in the Tirgan Formation, although not much depth (137_140 m), have very high flow rates, while some wells with high depth (250 m) have a lower flow rates. Several factors play a role in the development of karst which one of the most important is stratigraphy. There is a relationship between karst development with lithology and limestone thickness. Generally, thicker limestone rock units contain more karst development. Based on hydrogeological characteristics, in some cases, two or more formations can be considered as a hydrogeological unit. However, sometimes it is necessary to split a formation into separate hydrogeological units. Due to the large development of the Tirgan Formation in the studied area as well as its high discharge potential, this formation has been widely studied. Simple bedding and the sequence of thick limestone with marl or marl-limestone rock units have played a significant role in the development of karst in the Tirgan formation. Accurate understanding of the location and depth of the water bearing rock units in this formation depends on detailed stratigraphic studies. Material and methods In this research, in the north and eastern part of Bojnourd, a stratigraphic section of Tirgan, in Baba Musa mountain, was measured and sampled (85 samples for thin sections). In order to nomenclature and interpretation of the microfacies of the Tirgan Formation, procedures of Flügel (2004) and Dunham (1962) have been used. Like systematic studies of stratigraphy, the physical properties of the layers including thickness, layering, color, hardness, erosion, slope and topography, dip and thickness of layers around water wells were also evaluated. Water wells information including, lithology logs and geophysical logs (gamma, self-potential, electrical resistance) were also used. Water level data-sets during drilling, pumping, and subsequent years were evaluated. The geological profiles of all water wells were mapped and the depths of well penetration were determined. By analyzing the above-mentioned data-sets, the Tirgan Formation is divided into several layers with deferent permeabilities. Discussion of Results and Conclusions In this research, the information extracted from the Baba Musa stratigraphic column and data-sets obtained from the water wells in the studied area were combined. According to the information integration, the Tirgan Formation is divided into five rock units which are three limestones and two marly limestone rock units. The arrangement of these rock units from base to top are as follows: basal limestone, basal marly limestone, middle limestone, upper marly limestone and upper limestone. The stratigraphic position of limestone and marly- limestone units results in the recharged water, mainly penetrates into the upper limestone layer. Therefore, due to the presence of limestone, marl and marly limestone rock units, the Tirgan Formation is divided into permeable, impermeable or semi-impermeable units. In addition, the degree of purity of the upper limestone layer is much greater than that of the other two limestone layers. These two factors cause the degree of karstification in upper limestone is much greater than that of the other two limestone layers. Information obtained from the water wells show that they have penetrated the upper limestone rock unit or up to the middle limestone rock unit. Given the above-mentioned characteristics, those water wells drilled in the upper limestone unit have the highest flow rate in comparison with other ones.

Keywords

• permeable
• impermeable
• semi-permeable layer
• drilling log
• tirgan formation