Abstract
This study focuses on the thermal maturity of Permian deposits from the Zagros Basin, Southwest Iran, employing both optical methods (Thermal Alteration Index, Palynomorph Darkness Index, Vitrinite Reflectance, UV Fluorescence) and geochemical analyses of organic matter (Rock Eval Pyrolysis and MicroRaman spectroscopy) applied to the Faraghan Formation along two investigated Darreh Yas and Kuh e Faraghan surface sections. Furthermore, an integrated palynofacies and lithofacies analysis was carried out in order to integrate the few studies on the depositional environment. The Faraghan Formation, which is widely distributed in the Zagros area, generally consists of shale intercalated with sandstones and pebble conglomerates in the lower part, followed by a succession of sandstone, siltstone and shaly intercalations and with carbonate levels at the top. The integrated palynofacies and lithofacies data confirm a coastal depositional setting evolving upwards to a shallow marine carbonate environment upwards. Rock Eval Pyrolysis and Vitrinite Reflectance analysis showed that the organic matter from samples of the Darreh Yas and Kuh e Faraghan sections fall in the mature to postmature range with respect to the oil to gas generation window, restricting the thermal maturity range proposed by previous authors. Similar results were obtained with MicroRaman spectroscopy and optical analysis such as Thermal Alteration Index and UV Fluorescence. Palynomorph Darkness Index values were compared with Rock Eval Pyrolysis and vitrinite reflectance values and discussed for the first time in the late stage of oil generation.
Highlights
The Paleozoic and Mesozoic successions of the Middle East and North Africa are wellknown and have been extensively studied for the potential of their petroleum systems [1,2,3,4,5]
This study aims to determine the hydrocarbon source rock potential of the Faraghan Formation in terms of its thermal maturity combining different geochemical (Rock Eval Pyrolysis and MicroRaman spectroscopy) and optical methods (VR, Thermal Alteration Index (TAI), Palynomorph Darkness Index (PDI) and UV Fluorescence) applied to OM [42,61,62]
A multidisciplinary strategy based on geochemical (MicroRaman spectroscopy and Rock Eval Pyrolysis) and optical methods (VR, TAI, Spore Color Index (SCI), PDI and UV Fluorescence) has established that the Faraghan Formation spans the floor of the oil window, ranging from the ceiling of the oil window into the floor of the wet gas window in terms of thermal maturity, further constraining the conclusions of previous workers
Summary
The Paleozoic and Mesozoic successions of the Middle East and North Africa are wellknown and have been extensively studied for the potential of their petroleum systems [1,2,3,4,5]. Despite its increasing importance in hydrocarbon exploration, few studies have yielded data concerning the thermal maturity of organic matter (OM) in Iran’s Permian sequences [9,13]. We try to fill this gap by focusing on the Faraghan Formation which is widely distributed in the Zagros area (SW Iran; Figure 1 [14,15,16,17,18,19,20] and was recently attributed to the middle Permian (Guadalupian [21,22]). Thermal maturity studies need to be performed at local scale considering the peculiar variability in terms of intrabasinal sediment thickness and subsidence rates
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