Geochemical Assessment of Early Cretaceous Garau Formation in Anjir and Kabir Kuh Anticlines of Lurestan Zone, Northwest of Zagros, Iran

Deposition of organic rich black shales and argillaceous limestones in the Berriasian-Turonian interval have been documented from different parts of the world. The Early Cretaceous Garau Formation, based on geological and geochemical reports is well exposed in northwest of Zagros, Iran (Lurestan Zone) as a hydrocarbon source rock. However, these reports lack comprehensive results of experimental analysis and studies. In this study, samples from Garau Formation obtained from Anjir anticline (n=12 cuttings from well ANR-1) and Kabir Kuh anticline (n=5 cuttings from well KAK-1 and 11 outcrop samples from Tange Garau) from Lurestan Zone were subjected to Rock–Eval pyrolysis, Gas Chromatography(GC), Gas Chromatography-Mass-Spectrometry (GC-MS) and vitrinite reflectance measurement (Ro%) techniques. Analytical results show that average values for TOC (1.75 and 2.55%), Tmax (445 and 475оC) and Ro (0.895 and 0.974%) in these two anticlines respectively, suggesting good to very good hydrocarbon source rock with late oil window and early gas generation maturity. Most samples have a mixture of kerogen types II/III and type III. Biomarker characteristics including normal alkanes, distribution of the hopanes and steranes indicated the presence of dominant marine organic matter with a minor terrestrial organic input that were deposited in a marine environment under reducing conditions. The results obtained from characteristic biomarkers also suggest that the Garau Formation is thermally mature in the above mentioned two anticlines which are in agreement with the results of Rock-Eval pyrolysis and vitrinite reflectance measurements.