Microfacies analysis and reservoir evaluation based on diagenetic features and log analysis of the Nammal Formation, Western and Central Salt Range, Upper Indus Basin, Pakistan

The Nammal Formation of Early Eocene age crops out at Nammal Gorge (NG) and Pail (PL) in the Salt Range, Upper Indus Basin, Pakistan. It comprises mainly medium to thick-bedded and rare thin-bedded limestone with mixed carbonate sequences of limestone, marl, and shale. Allochemical constituents and micrite estimated ratio leads in recognizing seven microfacies including Foraminifer wackestone, mudstone-wackestone, Foraminifer mudstone, and Assilina wackestone in the NG section and Foraminifer wackestone-packstone, Foraminifer wackestone, and Bioclastic packstone in PL section. Based on microfacies and fossil assemblages, the Nammal Formation is interpreted to have been deposited in outer to mid and inner ramp settings. The formation is largely transformed by diverse diagenetic episodes comprising micritization, cementation, dissolution, neomorphism, mechanical compaction, stylolitization, fractures, and vein formation. Fractures and dissolution are inferred to augment the prospect of hydrocarbon for reservoir of the Nammal Formation. ImageJ calculated porosities and log estimated average effective porosity range from 4 to 6% and 1 to 2%, respectively. The calculated log values in Dhermund-01 and Pindori-02 wells suggest that at certain limestone intervals of the encountered Nammal Formation, there is a compacted limestone that can be regarded as a tight reservoir for hydrocarbon accumulation.