Evaluation of Reservoir Properties of Sylhet Limestone of Jaintia Group, North-Eastern Sylhet, Bangladesh

Abstract

The Sylhet Limestone in the Bengal Basin, formed in the Eocene Epoch and known for its fossil content, is significant in understanding the reservoir characteristics. Despite exposure in Jaflong and Takerghat of the Bengal Basin, little research has been conducted on reservoir characteristics. This study aimed to comprehensively examine the Sylhet Limestone Formation, encompassing its crystalline upper and fossiliferous lower sections. It utilized field investigations and laboratory analyses to address the gaps of sporadic or insufficient earlier studies. A thorough examination of thin sections from the Sylhet Limestone, exposed in the Dauki River area, provides insights into the textural and mineralogical attributes and the presence of skeletal fossils within the limestone. Based on the analysis of thin sections, the limestones are categorized as Rudestones and Packstones. The porosity observed in the exposed rocks ranges from 5% to 12%, with most pores associated with interconnected fractures and joints. However, thin-section studies also indicate evidence of diagenetic recrystallization and calcite cementation. Hence, closely spaced, interconnected joints and fractures filled with diagenetic calcite might deteriorate the reservoir quality. Notably, this limestone exhibits fossilized specimens such as Nummulite, Discocyclina, Alveolina, Assilina, and Ostracoda, among others. The combination of the fossil assemblage, limestone texture, and composition strongly suggests that this limestone formation was deposited in a shallow marine environment with minimal sediment input under a warm and humid climate. The petrographic analysis of the limestones concludes that the upper portion of the formation is fine-grained while the lower part is coarse-grained. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(2), 2023, P 119-137