Sedimentological and Geochemical Analysis of the Middle Jurassic Shinawari Formation, Upper Indus Basin, Pakistan: Implications for Palaeoenvironmental and Hydrocarbon Assessment

Abstract

Microfacies, palynofacies, and hydrocarbon source rock potential of Toarcian–Bathonian sediments of the Shinawari Formation are investigated in the Chichali Nala Section, Baroch Nala, Gulla Khel Nala, Surghar Range, and Askari Cement Factory Section, Nizampur Kala Chitta Range, Pakistan. The Shinawari Formation is dominated by a mix of limestone, sandstone, shale, marls, siltstone, and mudstone units with the association of laterite, hard grounds, coal layers, and coal disseminations. Microfacies analysis suggests that deposition of the Shinawari Formation occurred in peritidal lagoon, beach shoal to distal shelf setting. This interpretation is supported by sub-types of standard Tyson (in: Sedimentary organic matter, Springer, Berlin, 1995), palynofacies, which we have defined as palynofacies SFPF-A, attributed to a marginal dysoxic to an anoxic basinal setting, and palynofacies SFPF-B, deposited in heterolithic proximal platform settings. Some of the intervals have low ( $$\le 0.5\%$$ ) total organic carbon (TOC) values except for intervals with fair values (0.5–2.09%) in the Shinawari Formation in different stratigraphic sections. Despite some lower TOC values, and the presence of kerogen types III and II, spore colouration index of 5.0–8.0, thermal alteration index of 2–3+, and vitrinite reflectance (VR) of 1.16–1.35 indicate possible hydrocarbon source rock potential. The various geochemical plots obtained from TOC (%), VR and Rock-Eval pyrolysis data indicate that the Shinawari Formation specifies kerogen type III having Tmax ( $${}^{\circ }$$ C) range of 430–450 $${}^{\circ }$$ C and thermal maturity lies in the oil window.