Litho- and biostratigraphy and hydrocarbon source-rock potential of the Jurassic–Paleogene strata in the Kala Chitta Range, northwestern Himalayas, Pakistan

Abstract

In this study Jurassic–Paleogene strata were investigated to understand the litho- and biostratigraphic framework and hydrocarbon source-rock potential of various stratal packages. Biostratigraphic controls were used to establish the chronostratigraphic framework of Jurassic–Paleogene strata in the area. The Lower Jurassic (Hettangian) clastics saw an unconformity during the Sinemurian–Pliensbachian, while the Lower Jurassic (Toarcian)–Middle Jurassic (Bajocian) clastic–carbonate mixed strata is also separated by a Bathonian unconformity from the Middle Jurassic (Callovian) to the Upper Jurassic (Tithonian) carbonate sequence. The Upper Jurassic Oxfordian strata are missing, while the Upper Jurassic (Kimmeridgian)–Lower Cretaceous (Valanginian) glauconitic sandstone and clays are the conformable sequences. The Lower Cretaceous (Hauterivian)–Upper Cretaceous (Turonian) clastics is a conformable sequence that is separated by a Coniacian–Santonian unconformity from the Upper Cretaceous (Campanian) pelagic carbonates. The Cretaceous–Tertiary boundary is marked by laterites, while the Paleocene (Thanetian) sequence is represented by a shale- and sandstone-dominated sequence. The Paleocene (Thanetian)–Early Eocene (Ilerdian) siliciclastic–carbonate mixed sequence marks the last episode of Tethyan sedimentation. Total organic content (TOC), organic petrography and Rock-Eval pyrolysis (REP) techniques were used to evaluate the hydrocarbon source-rock potential, kerogen type and level of maturity of the hydrocarbons. The majority of studied samples show the occurrence of type IV kerogen. However, the Middle Jurassic (Callovian)–Upper Jurassic (Tithonian) carbonate sequence of the Samana Suk Formation, the Kimmeridgian–Valanginian Chichali Formation, the Paleocene (Thanetian) sequence of the Hangu Formation and the Paleocene (Thanetian)–Early Eocene (Ilerdian) Patala Formation confirms the Type III kerogen, poor–fair source-rock quality, immature–mature, gas- and oil-prone indigenous hydrocarbon occurrence in the region.