The eastern Junggar Basin, controlled by continental extension (rift), was deposited by lacustrine dominated sediments during middle Permian Lucaogou period. An unusual porphyritic-like texture was observed in Lucaogou/Pingdiquan dark fine-grained organic-rich sediments in two sub-tectonic units in the basin. The “phenocrysts” are composed of two types of mineral assemblages. The first is a coarse euhedral calcite assemblage in the Jimusar Sag, and the second consists of dolomite, analcime, and pyrite in the Shishugou Sag. The lithological and mineralogical features indicate a hydrothermal origin for these phenocryst-like minerals. The chondrite-normalized rare earth element patterns show flat or positive Ce anomalies and negative Eu anomalies, which reflect a suboxic to anoxic, off-axis site from the center of the fault system, where the temperature of the hydrothermal fluid might be less than 250 °C. The high ratios of Ba_N/La_Nat 1.6–65.5, strongly positive Sr anomalies at Sr/Sr* = 5.54–39.9, and relatively low⁸⁷Sr/⁸⁶Sr isotopes at 0.705002–0.705776 in the coarse calcite suggest an origin of mixed sources of lake water, underlying biogenetic sediments, and deep magmatic water. However, the low⁸⁷Sr/⁸⁶Sr ratios of 0.705321–0.705968 in the dolomite and δ³⁴S_V-CDTof 10.8‰–12.3‰ in the pyrite indicate that water–underlying-rock interaction and the abiotic thermochemical sulfate reduction of lake water or organic matter might have participated together resulting in the precipitation of the dolomite–analcime–pyrite assemblages in the Shishugou Sag.

The biostratigraphic information from the last remnants of the Tethys Seaway in northern Pakistan was used to date the cessation of marine sedimentation as a result of the closure of the eastern Tethys Seaway and to give a minimum time for the India–Asia collision. The current detailed biostratigraphic investigations of the Eocene Kohat Formation in the Kohat Basin divulged that the rock unit in the study area contains age-diagnostic larger benthic foraminiferal (LBF) species of middle Eocene age. Based on the association of identified LBF species, two local biozones, i.e. PKBZ 1 and PKBZ 2, are documented and are comparable with published SBZ 13 and SBZ 14 zones, suggesting an early to middle Lutetian age of the rock unit. The integration of current biostratigraphic information of the Kohat Basin with detailed literature on other basins suggests that the closure of the eastern Tethys Seaway in Pakistan occurred in four stages: (i) closure of the northwestern Kohat region during the early Ypresian (55.9–55 Ma), (ii) followed by the closure in the Potwar region, apart from its northwestern domain, during the middle to late Ypresian (54–49.9 Ma), (iii) closure of the Tethys Seaway in the Hazara–Kashmir and northwestern Potwar regions during the early Lutetian (49–45.8 Ma) and (iv) the final stage of Tethys closure has occurred in the Kohat region during the middle Lutetian 1 (45.8–43.6 Ma). The biostratigraphic information in the current study suggests that the final stage of eastern Tethys closure in northern Pakistan occurred at 45.8–43.6 Ma, and thus it gives a minimum age for the India–Asia collision. Such diachroneity can be observed elsewhere in the eastern Tethys, including Iran, Nepal, Bangladesh, Turkey and Tibet. The closure started first in eastern Tethys, continued in both eastern and western Tethys, and was finally completed in eastern Tethys.

The exploration of hydrocarbon resources plays a critical role in fulfilling the world’s increasing demand for energy. In this regard, the distribution and source rock characteristics of coal measure stratum in the whole basin are important and must be studied. However, systematic research has not been conducted yet. In this study, organic geochemical data, drilling data, and fieldwork were used to examine the source rock distribution of the Upper Paleozoic stratum. The results revealed that Upper Paleozoic coal-bearing rock series are mostly present in the Benxi, Taiyuan, and Shanxi formations, and mudstones and coals are formed frequently in tidal flat deposits. The layers of the coal and mudstone are primarily thick on the western sides, eastern sides, and northern sides, thin in the middle region, and gradually thinner from north to south. The source rocks are mostly enriched in the east and west of the basin. The results of the Rock-Eval pyrolysis analysis indicated that the overwhelming majority of the coal comprises excellent source rocks, all limestones are poor source rocks, and most mudstones are good–excellent source rocks. The kerogen type of most of the rock samples is type Ⅲ, suggesting typical “gas source” kerogen. Humus is derived from terrestrial organism and aquatic algae remnants, indicating a diversified parent material input. These results evidence that studied source rocks are at the postmature-dry gas stage.