Garau Formation as an unconventional hydrocarbon resource in southwestern Iran: a geochemical investigation

Abstract

Due to the growing need for hydrocarbons and the limited conventional resources, paying attention to unconventional oil and gas resources is essential. The upper Jurassic-Lower Cretaceous in western and southwestern Iran contains the most crucial source rocks of the Zagros sedimentary basin. One of the richest source rocks is the shaly Garau Formation. Owing to geochemical and geological characteristics, the Garau Formation, with Neocomian to Coniacian age and periodicity of radiolore black limestones with black pyrite bituminous shales, ammonite, and chertite shale, is the suitable potential sweet spot as oil shale and gas shale. In this study, a comprehensive regional study was conducted on the entire Garau Formation basin using Rock–Eval parameters and organic matter maturity data. In addition to investigating the hydrocarbon generation potential of the Garau Formation as a source rock, the prospect of this Formation as oil and gas shales in the entire Garau basin was investigated. For this purpose, 31 fields in 5 important sub-basins of Zagros, including East Lorestan, West Lorestan, Izeh zone, Dezful Embayment, and Abadan plain were investigated. The study showed that total organic carbon varies between 0.17 and 24.6, and more than 75% of total organic carbon had values greater than 1. By examining the quantity, quality, and maturity parameters and considering the necessary thresholds, it was found that East Lorestan and West Lorestan have the conditions for oil and gas shales. The highest quantity and best quality of organic matter are related to Eastern Lorestan. By moving west and south of Garau Basin, the quantity and quality of organic matter decrease, and the hydrogen index proves the results. Vitrinite reflection studies indicated that the lowest and highest levels of organic matter maturity belong to East Lorestan and West Lorestan, respectively. Therefore, the northeast of Garau Basin (Eastern Lorestan) has the potential for oil shale, and the northwest of the Basin (Western Lorestan) has the potential for gas shale.