Sequence Stratigraphy of the Jurassic Strata and Occurrences of Potential Sandstone Reservoirs in the ST Gas Field, Northern West Siberia Basin

Jurassic strata in the ST gas field of the northern West Siberia Basin have been regarded as a potential exploration target with undiscovered hydrocarbon resources. However limited research has been performed on the sequence stratigraphy of the Jurassic strata, as well as its sandstone distribution controlled by variable sea level change and sediment input. In this paper, four third-order sequences (SQ1, SQ2, SQ3, and SQ4) and nine fourth-order sequences for the Jurassic strata are interpreted based on seismic facies analysis and the lithology stacking patterns of seven wells. SQ1 is characterized by the special Bazhenov Formation which is featured by regionally distributed deep marine shales. SQ2 (J1) is composed of a coarsening upward sequence, the base of which is an unconformable surface that can be recognizable in both seismic profiles and well logging data. SQ3 (J2-J8) is composed of a complete fining-upward and coarsening-upward sequence, showing a series of transgressive and regressive successions. A complete SQ4 has not been drilled through by all the seven wells, only showing a coarsening upward succession on its top (J9) which evolves into a fining upward succession at the base of SQ3. Combined with the seismic inversion result, which predicts sandstone distribution, a sequence evolution model was built for SQ3 showing a full unit of transgressive system tract and highstand system tract (TST-HST) which often occurs in shallow marine shelves. During sequence development, most reservoir sandstones are deposited in the shelf and tidal delta environment at the bottom of the TST and the top of HST, and mudstones are deposited as shelf mudstones, especially at maximum flooding surface. That is controlled by both accommodation and sediment input. Generally, under this sequence framework, the depositional architecture can be further analyzed with implications for source rock, reservoir sandstones, and sealing rock, which may guide future gas exploration and exploitation in this area.