Sedimentology and reservoir characteristics of Jurassic Samanasuk Formation of Salt Range and Hazara areas, Upper Indus Basin, Pakistan

The present study focuses on the Samanasuk Formation, which is exposed in Chichali Gorge (Surghar Range), Nammal Gorge (Western Salt Range), and Sangar Gali (Hazara area). The study aims to analyze the lithofacies, petrographic characteristics, diagenetic features, ooid types, and depositional settings of the formation. Outcrop data revealed four distinct lithofacies have been identified: limestone facies (SF1), dolomitic limestone facies (SF2), oolitic limestone facies (SF3), and sandy limestone facies (SF4). In the Chichali Gorge and Nammal Gorge sections, the petrographic study further classified the formation into three facies: mudstone, wackestone, and packstone. This classification reveals a progradational stacking pattern, where the coarse-grained facies predominantly overlie the fine-grained facies. Additionally, the petrographic analysis indicates various depositional environments ranging from the beach to the inner ramp. The Samanasuk Formation in the studied sections exhibits five major diagenetic features, representing five significant diagenetic processes: micritic envelopes, cementation, hard ground surfaces, physical and chemical compaction, and dolomitization. Furthermore, two types of ooids have been identified within the formation, namely Radial Ooids and Concentric Ooids. Radial Ooids are formed in the beach-to-inner ramp environment, while Concentric Ooids are formed in the inner ramp environment. To assess the petrophysical properties, wireline logs from the Chanda deep-01 and Isakhel-01 wells were interpreted. The analysis and log interpretation aided in characterizing lithologic units of reservoir zones, determining reservoir properties, differentiating shale-bearing zones from non-shale-bearing zones, and understanding the depositional settings. The petrophysical evaluation of the Samanasuk Formation, conducted through outcrop and thin section studies, reveals shoaling upward cycles, indicating that the formation belongs to the High Stand System Tract (HST). The depositional model suggests that the Samanasuk Formation was deposited during the Middle Jurassic in a shallow marine environment extending from the beach to the inner ramp.