بررسی نقش چینه‌‏شناسی سازند تیرگان در آب‌دهی چاه‌های آب در شمال و شرق بجنورد

به‌منظور تامین آب آشامیدنی و صنعت، چاه‌های متعددی در حوضۀ کپه‏‌داغ حفر و بهره‏‌برداری شده‌اند. هشت حلقه از این چاه‌ها برای صنایع و دو حلقه برای آب آشامیدنی در شرق و شمال‌شرق بجنورد و در سازند آهکی تیرگان حفر شده‏اند. بررسی آب‌دهی این چاه‌ها نشان می‌دهد ارتباط معناداری بین چینه‏شناسی و آب‌دهی چا‌ه‌ها برقرار است. چاه‌هایی که در سمت شمال‌‌شرق منطقۀ مدنظر حفر شده‌اند، در مقایسه با چاه‌های شرق منطقه آب‌دهی بیشتری دارند. برای بررسی دقیق‏تر وضعیت چینه‏شناسی منطقه، برش چینه‌شناسی کاملی از سازند تیرگان به ضخامت 510 متر در کوه باباموسی اندازه‏گیری، نمونه‏برداری و مطالعه شد. نتایج حاصل از این برش زمین‌شناسی با ستون چینه‏شناسی حاصل از لاگ‌های حفاری هر‌یک از چاه‌هایی که در سازند تیرگان حفاری شده‌اند، مقایسه شدند. برش چینه‌شناسی بابا‌موسی به پنج واحد سنگی آهکی، مارنی و آهکی مارنی ‌تقسیم می‌شود. واحد‏های سنگی آهکی لایۀ تراوا یا آبخوان کارستی و واحد‌های سنگی مارنی و آهکی– مارنی به‌ترتیب لایه‌‏های ناتراوا و نیمه‌تراوا محسوب می‌شوند. چاه‌های حفرشده در واحد سنگی آهک بالایی بیشترین آب‌دهی را نسبت به سایر چاه‌ها دارند؛ در مقایسه، چاه‌هایی که کل واحد سنگی آهک بالایی را قطع نکرده‌اند و به واحد‏های سنگی مارنی، آهک میانی و آهکی مارنی پایین‏تر نفوذ کرده‌اند، اگرچه تا دو برابر عمق دارند، آب‌دهی کمتری دارند؛ بنابراین، واحد سنگی آهک بالایی بهترین واحد سنگی دارای آب است. در این منطقه می‌توان سازند تیرگان را باتوجه‌به ویژگی‌های چینه‏شناسی به چند واحد هیدرواستراتیگرافی تقسیم کرد.

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

Abstract To supply water for drinking and industry, several water wells have been drilled in the KopetDagh 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 northeast 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 marlylimestone. Limestone rock units are considered as karstaquifer and marlylimestone and marl are considered to be semipermeable 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 marlylimestone, although they are twice as deep, have a much lower flow rates. Therefore, the best waterbearing layer of the Tirgan Formation is the upper limestone. Keywords: Permeable, Impermeable, Semipermeable 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 KopetDagh 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 marllimestone 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, selfpotential, electrical resistance) were also used. Water level datasets 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 abovementioned datasets, 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 datasets 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 semiimpermeable 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 abovementioned characteristics, those water wells drilled in the upper limestone unit have the highest flow rate in comparison with other ones.