در مطالعۀ حاضر به بررسی ویژگیهای سنگهای منشأ احتمالی در شرق حوضۀ خزر جنوبی از پهنۀ ساختاری البرز پرداخته شد؛ به این منظور، برشهای سوچلما و پوروا در جنوب نکا و برشی در جنوب ساری انتخاب شدند. نمونههای مطالعهشده از رسوبات سنگ منشأ احتمالی به سن پالئوسن بر اساس تجزیهوتحلیل راک- اول بررسی شدند. بهمنظور مطالعههای تکمیلی، نتایج پیرولیز راک- اول و پتروگرافی آلی نمونههای رسوبات سنگ منشأ احتمالی در یکی از چاههای اکتشافی دشت گرگان نیز استفاده شدند. باتوجهبه نتایج مطالعۀ حاضر، بیشتر نمونههای رخنمون پتانسیل هیدروکربنزایی ندارند؛ همچنین نمونههای چاه اکتشافی بررسیشده نیز کربن آلی کل حدود 1 و کروژن تیپIII با بلوغ ناکافی دارند. در بررسی توالیهای رسوبی مشخص شد بیشتر نمونهها در حوضۀ رسوبی نزدیک به محیط قارهای و در شرایط پسروندۀ سطح نسبی آب دریا نهشته شدهاند.
عنوان مقاله [English]
Investigation of hydrocarbon generation potential of Paleocene sediments in the southern Caspian Basin
The present study investigates the geochemical properties of potential source rocks in the eastern Caspian Basin from Alborz structural zone. For this purpose, sections of Suchelma and Purva in south of Neka and an outcrop in south of Sari were selected. The studied samples of sedimentary source rocks of the Paleocene age were analyzed based on Rock-Eval analysis method. The results of Rock- Eval pyrolysis and organic petrography results of probable source rock sediments in one of the exploration wells of Gorgan plain were also studied. According to the results, most of the outcrops samples are immature and were deposited in a regressive continental environment. The samples of exploration borehole studied, are good in terms of organic matter richness but in terms of the amount of kerogen maturation are immature. According to the results of this study, these samples have weak hydrocarbon generating power and total organic carbon content of about 1% as well as kerogen type III. It was also specified in the determination of organic facies that most of the mentioned samples show deep environmental conditions adjacent to orogenic and continental sites and are deposited in a regressive environment.
Keywords: Source rock, Rock- Eval experiment, Maturity, Southern Caspian Basin, Hydrocarbon generation potential.
The Caspian Sea region is well known for its abundant oil reserves and oil and gas production has played an important commercial role in the region for more than 150 years especially in Azerbaijan and to a lesser extent in Turkmenistan (Smith- Rouch 2006). Identification of hydrocarbon systems in the Caspian sedimentary basin is of great importance for exploratory studies and to achieve the production of hydrocarbon resources. One of the main pillars of any oil system is the source rock. Therefore, it is necessary to identify the source rock and determine its location in any system to avoid imposing exorbitant costs and unsuccessful excavations. The relatively low geothermal gradients (around 1.5°C 100m-1) from rapid burial provide favorable thermal conditions for the preservation of hydrocarbons at relatively great depths (up to10+ km) (Nadirov 1990). A petroleum source rock is defined as any rock that has the capability to generate and expel enough hydrocarbons to form an accumulation of oil or gas. Definitions that do not include migration and accumulation are too general, because in a sense practically all rocks contain organic matter (OM) form some hydrocarbons. A potential source rock is one that is too immature to generate petroleum in its natural setting but will form significant quantities of petroleum when heated in the laboratory or during deep burial (Hunt 1996). The main source- rocks in the south Caspian basin are considered to be Paleogene- Miocene deposits (Maykop/ Diatom) (Guliyev et al. 2001). Paleocene deposits in Iran are important for hydrocarbon reserves due to their organic matter as a source rock. Therefore, this study investigates the hydrocarbon generation potential of Paleocene sediments in the east of southern Caspian Basin.
Materias and Methods
Three outcrops in Mazandaran province were selected for this study. From each of Suchelma and Purva outcrops, six and seven samples, respectively, and from south of Sari two samples of potential source rock sediments of the Paleocene age were analyzed for Rock- Eval pyrolysis. The Rock- Eval pyrolysis and Organic Petrography results of eight samples of the Paleocene deposits in one of the exploration wells of Gorgan plain were also studied. The Rock- Eval pyrolysis method has been extensively used for oil and gas exploration in sedimentary basins over the world (Behar et al. 2001). Vitrinite reflectance is the most reliable maturity parameter for measuring thermal maturity of sedimentary organic matter (Tokatli et al. 2006).
Discussion of Results and Conclusions
According to the results, most of the outcrops samples harvested were poor in terms of total organic carbon content and in terms of kerogen maturity were before the oil window. Therefore, they are weak in terms of hydrocarbon generation potential. These samples contain kerogen type IV. Also in determining the organic facies, it was found that most of the sediments were deposited in a continental regressive environment. The well samples are good in terms of organic matter richness, immature in terms of the amount of kerogen maturation and weak in terms of hydrocarbon generation potential. The measurement results of vitrinite reflectance also confirmed that the borehole samples were immature. These samples contain kerogen type III. It was also specified in the determination of organic facies that most of the well samples show deep environmental conditions adjacent to orogenic sites and are deposited in a regressive environment