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Giant Incised Valley Fill and Shore-Face Ravinement Traps, Urna, Ust-Teguss and Tyamskaya Field Areas, Southern West Siberian Basin, Russia

Abstract

The West Siberian Basin, in addition to being a huge geographic province, has recoverable reserves in excess of 427 BBOE, most of which is within Upper Cretaceous strata, sourced primarily from the underlying Upper Jurassic Bazhenov Shale. However, at least 28 BBOE exists within stratigraphic and structural traps in Jurassic strata, sourced from both Bazhenov shales and numerous deeper Jurassic strata. Jurassic depositional systems are strongly controlled by a pronounced unconformity overlying Triassic and older strata which was heavily folded and faulted prior and during, burial of the Jurassic sediments. Paleo-topographic relief of over 1 KM occurs locally, much of the Jurassic on-lapped around paleo-structural highs. The widespread occurrence of thin coals and carbonaceous shales interbedded with fining upward log signatures has resulted in widespread classification of much of the Jurassic as ‘non-marine’. This model has resulted in widely practiced lithostratigraphic correlations that show ‘layer cake’ correlations based on the vertical patterns of sands and coals. Detailed examination of cores, seismic and new log correlations in this study, however, show that pervasive estuarine sedimentation is common, confined in nested incised valley fill deposits. Towards the upper ‘J1’ intervals, strata become progressively more marine and shore face ravinement traps are common. Layer cake correlations and assumptions about fluvial architecture from isopachs are grossly in error and do not explain high resolution seismic facies maps or information from cores and log correlations. The fields illustrated in this study are giant stratigraphic accumulations. More will be found with more careful integration of cores with 3D seismic.