Diagenetic characteristics and their implications on the reservoir potential of Bajocian Sandstone, Jaisalmer Basin, western Rajasthan, India

Abstract

Petrography and scanning electron microscopy (SEM) investigations aided by energy dispersive X-ray (EDX) analysis and quantitative measurement of reservoir properties were used to extensively examine the physical and diagenetic characteristics of the Joyan Member Sandstone of Jaisalmer Formation. The Joyan Member Sandstone is fine- to medium-grained, moderate to well sorted, sublitharenite to litharenite. Mechanical compaction, precipitation of calcareous, ferruginous and silica cements, clay minerals, dissolution and alteration of unstable clastic grains such as feldspar and rock fragments, and grain fracturing are the identified diagenetic features. Feldspar and rock fragments underwent significant changes to kaolinite and chlorite while silica cement primarily originated from the dissolution and alteration of these grains and pressure solution. Mechanical compaction and the authigenic cements like calcareous, ferruginous, and silica reduced primary porosity, while secondary porosity was created by dissolution of clastic grains and cements. Compaction reduced porosity from an anticipated original 40% to around 13.4%. Porosity was reduced by cementation to 20.8%. Cementation reduced the porosity of the Joyan Member Sandstone somewhat more than compaction. Calcareous cementation played a major role in the porosity evolution of Joyan Member Sandstone. During early burial, the early calcareous cement occupied most of the pore spaces, leading to a significant reduction in porosity. However, incomplete filling or scattered patches of calcareous cement helped to preserve some primary porosity. In addition to calcareous cement, clay minerals like kaolinite and chlorite also acted as pore-filling and pore-lining cements. Kaolinite had a booklet-like or lamellar pattern contributing to minor porosity loss through pore-occlusion, while pore lining chlorite helped to retain porosity by preventing syntaxial silica overgrowth. Extensive dissolution of calcareous cement significantly increased the secondary porosity. Diagenesis affects reservoir quality by reducing initial porosity through cementation and compaction, and then increasing it through dissolution of early calcareous cement and unstable grains. The diagenesis of the studied sandstone is closely linked to its potential as a reservoir.