Determination of Key Beds from the Cap Rocks of Oil Reservoirs Using a Novel Method, Case Study: The Gachsaran Formation, Southwest Iran

The use of key beds in the cap rocks of the oil reservoirs is crucial. Lack of awareness of these key beds will have serious risks and damages. The Gachsaran oil field is located 220 km southwest of Ahwaz-Iran. The former caprock consists of six key beds (A, B, C, D, E, and F). At the time of writing this paper these key beds are being used during excavations and drilling to determine the site of casing points. What has made utilizing these key beds during excavations difficult however, is use of diamond drill bits which results in the shattering of excavated samples. As such, it has become challenging to learn more about these key beds through studying petrographic thin sections in microscopy. These key beds were observed during the investigation at the drill sites. Later, the excavated samples were studied in the Energy-Dispersive X-ray Spectroscopy (EDS) machine, using a semi-quantitative method to analyze the elemental differences in those key beds. In addition to the identification of various lithological and diagenetic properties of the caprock, this study leads to the introduction of four new key beds based on geochemical properties. Some of the most important differences within the formation include those between the caprock anhydrite and the non-caprock anhydrite, namely the presence of potassium and titanium and the absence of chlorine and sodium in the caprock section. The four new key beds introduced from this study using elemental differences were named as key beds-1, 2, 3, and 4.