Sedimentation model of the middle Devonian clastic succession of the South Tatar Arch, Pashyian Regional stage, Volga-Ural Oil and Gas Province, Russia

Abstract

The Pashyian Regional stage (horizon) is the main productive unit of the middle Devonian clastic succession of the South Tatar arch. This article presents, for the first time, maps of the lower and upper parts of the Pashyian, including data on sand-shale ratio, number of sand layers (reservoirs) and thickness, based on the analysis of logging data from 25,000 wells. The maps were created by spatial interpolation of Natural Neighbor and ArcGIS Pro software. The model of sedimentation of the Pashyian Regional stage reflects the interpretation of the plotted maps as well as the synthesis of the results of detailed core investigations (lithological, sedimentological, ichnotextural, petrophysical, etc.) and analysis of archive and published materials. The main points of the proposed model are as follows. The Pashyian sediments were formed in a marine basin, in an environment comparable to that of the middle shelf of modern seas – in an offshore zone dominated by current activity. The basin floor was a relatively flat plateau, on which sandy, silty and clay sediments were simultaneously accumulated. Sediments of all types accumulated during sea transgression. Sea regression caused erosion and destruction of the already formed sediments. Positive landforms of seabed relief, composed predominantly of sandy well-sorted material, comprised autochthonous underwater sand bars, formed by constant currents parallel to the bathymetric contour of the seabed. Underwater sand bars formed extensive systems nearly throughout the entire territory of the modern South Tatar arch. At the same time, allochthonous, poorly sorted, less mature sediments were formed in underwater troughs produced by transversal currents (directed from the shore towards the sea). The proposed model explains the consistent thickness of the Pashyian Regional stage, the mosaic distribution of sand bodies over the area, and the lens-like shape of the sand and siltstone reservoirs. The model can be extrapolated to other stratigraphic intervals of the Devonian clastic succession with similar sedimentological features.

From Clastics to Carbonates and Black Shales: Deciphering Sedimentary Continuity and Discontinuity in the Devonian Landscape of the Volga-Ural Petroleum Province

“…At the end of the Givetian (Pashyian Regional Stage), carbonate sedimentation in the studied area completely terminated. Steady accumulation of carbonate sediments was recovered and stabilised in the studied area in the early Frasnian and continued until the global regression in the Early Visean (Silantiev et al, 2023). Transgression in the Early Frasnian for the first time extended carbonate sedimentation to shallow water environments located on tectonic arches.…”

“…In the works of many researchers, the origin of clastic sediments was associated with continental alluvial processes (Danilova, 2008;Loscheva et al, 2017). Meanwhile, recent studies (Silantiev et al, 2022, Miftakhutdinova, 2023 have confirmed the marine origin of these sediments and linked them to transgressive episodes of the Devonian marine basin.…”

“…The total thickness of the core intervals studied exceeds 600 metres. Some of the findings, particularly on the clastic interval of the Givetian, which include detailed well logs, rock descriptions, and ichnofossil illustrations have been published recently (Silantiev et al, 2022;Miftakhutdinova, 2023). In-depth analysis of the well cores was conducted directly at the core storage facility, where highly representative rock samples were selected for closer examination.…”

“…The thicknesses range from the first hundreds of metres in depressions (and basinal low-plains) to the first tens of metres in uplifted areas of the seabed (including tectonic arches) representing shallow water environments. Marine fossils are rare; meanwhile, a diverse assemblage of ichnofossils including Chondrites, Spirophyton, Scolicia, Planolites, Palaeophycus, Zoophycos and Skolithos indicates the marine origin of these sediments (Silantiev et al, 2022, Miftakhutdinova, 2023. The Paschyian Regional Stage represents the time during which clastic sedimentation completely covered the entire studied area, displacing all other types of sedimentation.…”

“…Recent studies have concentrated on examining the sedimentological and facies characteristics of Devonian sediments to develop detailed sedimentary models. These models have been developed separately for clastic sediments (Fortunatova et al, 2013;Loscheva et al, 2017;Silantiev et al, 2022), carbonate sediments (Fortunatova et al, 2016;Yousef et al, 2023), and black shales (Stupakova et al, 2015(Stupakova et al, , 2017Liang et al, 2020). In most cases, the authors examine only a certain stratigraphic interval of interest, not always using new stratigraphic schemes and data.…”