Author: Rajesh Pandey , Dinesh Kumar and A.S. Maurya

Abstract - Systematic geological investigation has been carried out in Tertiary sequences of Jaisalmer Basin, which includes understanding of widespread heterogeneity within lithofacies of Paleocene and Eocene sequences deposited under various depositional environments. These sequences are gas bearing in Manhera Tibba, Ghotaru and Kharatar gas fields in India and Sui and Mari gas fields of Pakistan. On surface exposures base of Tertiary is demarcated by Habur Formation, which is a fossiliferous sequence of calcareous sandstone, sandy limestone and coquinoidal limestone, while in subsurface, Marly Parh Formation. Overlying, Sanu Formation is represented by an arenaceous sequence of fine to medium grained highly current bedded. Sanu Formation has been divided in three main gas bearing reservoirs D2, D4 and D6. D2 and D6 units are represented by fine grained, subangular to subrounded detrital quartz grain with calcareous and ferruginous matrix while D4 unit is carbonate mudstone and wackestone. Petrographic and clay mineralogy suggest that Sanu Formation is deposited in warm, arid or semiarid alkaline continental environment (deltic) with marine influences at the base and top. Khuiala and Bandah formations are represented by fossiliferous limestone, shales, marls, bentonitic clays and fullers' earth of 25m-30m thickness. Khuiala Formation has been subdivided into two main reservoirs B4 and C2-C4. B4 unit is carbonate mudstone while C2-C4 limestone is mainly packestone and algal wackestone with good porosity, resulted due to dolomitization and solution activity. Contact between Khuiala and Bandah are gradational. Bandah Formation has two established reservoirs A4 and B2. A4 unit has carbonate mudstone with lower porosity, B2 has nummulitic wackestone and packestone. Khuiala and Bandah formations represent inner to middle shelf environment. Recent Shumar Formation unconformably overlies the Bandah Formation.

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