## 2 Geological Background  
The West Siberia Basin (WSB) comprises three total petroleum systems (TPS), Bazhenov-Neocomian (upper Jurassic-lower Cretaceous), Togur-Tyumen (early to middle Jurassic),  
Figure 1: Location map (**left**) and lithost stratigraphic log of Mesozoic strata (**right**) in northern West Siberia Basin. On the right, black bold dashes in the Middle Jurassic and Cretaceous correspond coal occurrence. Black triangles in Bazhenov Formation correspond organic matter. and Northern West Siberian Mesozoic Composite TPS. The first two are oil-prone whereas the third one is mainly gas-prone. The ST gas field is situated in the northern part of WSB, more precisely in a peninsula adjacent to the Kara Sea (Figure 1). This region encompasses a complete sedimentary cover above the pre-Mesozoic basement (Figure 1), the environment of which transitioned from continental to transitional and to deep sea. Sedimentary thickness varies from 2 km to a maximum of 9 km [10]. Continental clastics of the Triassic Tampey series uncomformably overlie the basement [19]. During the early to middle Jurassic, this study area underwent eustatic events to deposit lacustrine and estuarine siltstone and organic-rich shales of Togur and Radom members within the dominantly continental coal-bearing Tyumen Formation. These shale beds serve as hydrocarbon source rocks as well as seals between sandstone reservoirs within the Togur-Tyumen TPS [11; 19]. Upper Jurassic evolved into marine shales of Abalak Formation which separates the Bazhenov-Neocomian and Togur-Tyumen TPS. Overlying is the upper Jurassic Bazhenov Formation which is organic-rich siliceous shale deposited widely in the whole basin under a deep-water anoxic environment. Bazhenov Formation is regarded as a principal source rock as it has generated more than 80% of the oil and gas for the WSB. Then, the basin was filled by the upper Jurassic Achimov Formation and Neocomian programing clastic clinoforms, which form good stratigraphic traps above the Bazhenov source [5]. Although most of the discovered reserves are in structural traps, the stratigraphic traps in Jurassic-Neocomian are also promising and undiscovered [2]. Jurassic strata in most of the fields in northern WSB lie within the oil-condensate window, but the low hydrogen index, due to the increased thermal maturity calculated in the ST gas field, imparts gas-prone intervals in Jurassic strata [5; 10]. Refs. [1; 9] suggested that there may exist dry gas in Aptian-Cenomanian and wet gas in Jurassic-Neocomian sequences. The West Siberia Basin is not only physically the world's largest petroliferous basin, but also produces more than 70% of Russia's oil [20; 21]. Particularly, its northern part hosts Russia's huge gas reserves. Triassic to Paleogene sedimentary sequences are established on the Paleozoic and Precambrian basement [9]. Natural gas, condensate gas, and condensate oil fields were found in Jurassic-Cretaceous strata. Pre-Jurassic deposits and basement complexes are also promising [5]. In the south of the peninsula, the Lower-Middle Jurassic is alluvial lake facies, while shallow to deep marine deposits are available in the north and middle of the peninsula. The Late Jurassic organic-rich siliceous shales of the Bazhenov Formation were deposited in the whole study area under deep-sea conditions [10]. The structural shielding traps are developed in the Jurassic strata in the northwest active fault zone as a result of the local uplift. During the Late Jurassic (Callovian to Volgian), the basin experienced a big transgression. Therefore, the upper Jurassic sediments derived from high terrains of the southern WSB are the coarsest and most continental near the basement, while the Bazhenov Formation is the global maximum flooding surface, and the sediments show a fining upward feature (Figure 1). Sediments mainly come from the marine transgression period of the Upper Jurassic in the southeast, in a large-scale reducing environment, which is favorable for marine organic matter deposition [8].