Thermal maturity modelling and geochemical investigation of Kashafrud Formation in the Eastern Kopeh-Dagh, NE Iran

Hydrocarbon (HC) exploration in the Eastern Kopeh-Dagh basin (southwest of Amo-Darya Basin) have faced challenges due to the structural complexity and limited geochemical study. Kopeh-Dagh fold and thrust belt and Kopeh-Dagh foredeep were moved from each other by the Main Kopeh-Dagh Fault (Fig. 1). All drilled anticlines in Kopeh-Dagh fold and thrust belt are dry which are in contrast with the Kopeh-Dagh foredeep. Generation, migration, and accumulation of petroleum occured in the region during the Late Miocene and migration of gas started not earlier than 10 Ma years ago (Moussavi-Harami & Brenner, 1992, 1993). It is suggeseted that the hydrocarbon potential of Kashafrud Formation is by the dark shale and abundant vascular-plant fragments (Poursoltani et al., 2007; Taheri et al., 2009) with sufficient thermal maturity to generate hydrocarbons (Poursoltani & Gibling, 2011). Chaman Bid Formation with total organic carbon 0.8% introduced as gas prone (Ghasemi-Nejad et al., 2005). The main objectives of this study are to geochemically investigate the Kashafrud Formation as well as constructing burial history and one dimension (1D) thermal maturity models in the eastern Kopeh-dagh (wells B and E; Fig. 1).

A total of 56 samples (15 cuttings and 25 outcrops) were collected from Kashafrud Formation in the Eastern Kopeh-Dagh (Fig.1). Bulk geochemical parameters (such as TOC, S1, S2 …) of all samples were obtained by using Standard Rock-Eval 6 pyrolazer (Table 1) in Basic Method (Behar et al., 2001). Temis suite software (1D) developed by French Institute of Petroleum (IFP) along with procedure presented by Hantschel & Kauerauf (2009), were employed for reconstructing the thermal maturity and burial history of the studied wells. The heat flow is a critical input parameter in basin modelling. In the Eastern Kopeh-Dagh, the paleo-heat flow has been affected by the tectonic evolution and rifting phase. The basement of Eastern Kopeh-Dagh basin consists of Carboniferous basic volcanoes (Ulmishek, 2004) which corroborated with the heat flow values of 60 mW/m2 (Allen & Allen, 2013). With the occurrence of rifting from 166 to 173 Ma, the heat-flow values reached to the maximum of 80 and 105 mW/m2 for B and E wells, respectively (Fig. 3). The rift affected the heat flow model, a higher heat flow occurrence during rifting phase and an exponential reduction after the post-rift phase (McKenzie, 1978). For well B which is located in Kopeh-Dagh foredeep a cooling history with decreasing heat-flow values was modelled from 72 to 166 Ma and then remained constant about 30 Ma years. Finally due to strike-slip movement in the Eastern Kopeh-Dagh, an increase of 9 mW/m2 values of heat flow is suggested (Fig. 3). But for well E from Kopeh-Dagh fold and thrust belt, heat the flow values are higher than Kopeh-Dagh foredeep. On the other hand the same scenario is played by tectonic activity (Fig. 3). It can therefore be observed that there is a good consistency between the temperature gradient and the heat flow.

Geochemically, Kashafrud Formation (Aalenian-Bathonian) showed poor/fair hydrocarbon generative potential (McCarthy et al., 2011) (S2 versus TOC; Fig. 4), Type II, mixed II-III and IV kerogens (Dembicki Jr, 2009) (HI versus OI; Fig. 5) and thermal maturity from late oil window to condensate-wet gas (Hackley, 2012) (PI against Tmax; Fig. 6). For thermal maturity modelling, zones I2, I4 and I6 in Well B (Kopeh-Dagh foredeep) and I1, I3, and I5 in Well E (Kopeh-Dagh folded) were introduced as the source rock intervals of Kashafrud Formation (Fig. 7). The model indicates the onset of oil-generation in the Well B zones begining during late Middle Jurassic-Lower Cretaceous time (107-167 Ma). Peak oil generation occurred during Cretaceous time (72-115 Ma) and condensate-wet gas generation started during Late Cretaceous-Early Eocene time (45-90 Ma). I4 and I6 (deepest interval) zones experienced maximum 2-2.5% calculated vitrinite reflectance (VRo) and 192-205 °C (Fig.9) and I2 interval endured minimum 1.58% VRo and 171°C during Early Oligocene (30 Ma). High heat flow during rifting as well as the remained thickness of Kashafrud Formation caused early hydrocarbon generation in zone I6 (Figs. 7 and 9). But, due to uplift and temperature reduction as low as 130-175 °C (Fig. 9), the hydrocarbon generation has been stopped. The onset of oil-generation in the Well E zones began during late Early Cretaceous-Late Cretaceous time (83-110 Ma; Fig. 7). Peak oil generation occurred during Late Cretaceous-Early Oligocene time (30-90 Ma). I3 and I5 (deepest zone) intervals experienced maximum 1.1-1.4% VRo and 135-155 °C (Figs. 7 and 9) and I1 interval endured 0.95% VRo and 128°C during Early Oligocene. Here, due to temperature reduction (as low as 63-95 °C; Fig. 9) caused by upliftment hydrocarbon generation has been stopped.

Based on quantity and quality of geochemical parameters such as TOC, S2 and HI, Kashafrud Formation has potential to generate mostly gas with some condensate. However presence of bitumes in all cutting samples points to low permeability of Kashafrud Formation which refered to cracking of generated HC. Furthermore the effects of some low potential shale gas on gaseous HC of carbonate and clastic reservoirs in the Eastern Kopeh-Dagh (Saadati et al., 2016) which is in accordance with lower than 2% of TOC content of Kashafrud Formation. Finally, due to over pass of the main phase of hydrocarbon generation in Kopeh-Dagh foredeep and ceased hydrocarbon generation because of uplifting in Kopeh-Dagh folded and foredeep, the formation of anticlinal traps in the Lower Oligocene folding does not have any effect on HC accumulation in Eastern Kopeh Dagh Basin and paleo-geomorphological traps must be considered as the future exploration targets.

The authors would like to extend their thanks to the Exploration Directorate of the National Iranian Oil Company (NIOC) for providing the samples and the financial support of the project. Also support of Petroleum Geology and Geochemistry Research Centre (PGGRC) is highly appreciated. No doubt this manuscript could not come to this final stage without the valuable view points and suggestions of Professor Moussavi-Harammi for which the authors are very much grateful.