Combined use of palynology and organic geochemistry in petroleum potential evaluation and paleoenvironmental interpretation of the Kazhdumi Formation (Aptian-Cenomanian) in the southwestern Zagros Bas

The Kazhdumi Formation of the Bangestan Group is a well-known and important source rock in most oil-fields in the Zagros Basin. In order to examine productivity of this formation in southwestern Iran, and to correlate petrographical and palynological data with geochemical properties, one of the best outcrops of this formation, located in Tange-Maghar 45 km northwest of Behbahan city, was sampled. Fifty-two rock samples were collected from the 270 m thick section which is made up of dark shales with intercalations of marl and limestones, and treated palynologically. Three palynofacies were differentiated based on statistical studies of palynological slides and the rock samples representing these palynofacies were geochemically analyzed. Palynofacies results were then correlated against the geochemical analysis. Palynofacies I (PF-1) with 90 to 100% Amorphous Organic Matter (AOM) presents high hydrogen index (HI) and total organic carbon (TOC) values. PF-1 contains kerogen type I/II and could potentially produce oil. The second palynofacies (PF-2) contains reduced amounts of AOM, HI and TOC and the kerogen is of type II/III, potentially producing oil and gas. Palynofacies III (PF-3) was recognized in a few samples and is characterized by low amounts of AOM, HI and TOC potentially producing meager amount of gas and oil. Plotting palynological data on Tyson ternary diagrams and Van Krevelen geochemical diagrams confirm that the kerogen is of type II/III, III and is originated mainly from algal organic matter. Potential for hydrocarbon generation is high for the samples falling in palynofacies I which contains high amount of kerogen type II while it is relatively good for samples representing palynofacies II with high amounts of kerogen type II, III. Tmax indicates that the samples are mainly mature or have entered the mature phase and are potentially able to produce oil and gas. The high amounts of clear AOM in most samples indicate dominance of redox environmental condition with low oxygen content. The HI/OI ratios curve indicates B and C restricts which confirms reducing environmental condition