We have developed a regional sequence stratigraphic framework for the productive Neocomian complex of central West Siberia. Sixteen depositional sequences have been identified on the basis of analysis of regional seismic lines and well logs. A dip-oriented well-log transect that was constructed across the Nizhnevartovsk and Surgut arches reveals detailed features of the stratigraphic architecture and depositional history of the Neocomian section in two-dimensional view. Integration of ammonite biostratigraphic data led to development of a reliable chronostratigraphic framework for our sequence interpretation.
The Neocomian marine complex ranges in thickness from 350 to 700 m and consists of the clinoform (lower part) and topset packages. Deposition of the clinoform package occurred during a period of at least 9 m.y. when clinoforms prograded more than 550 km from east to west. Progradation occurred through lateral shelf outbuilding mostly during lowstand periods when sandstone units accumulated in shelf-edge deltas, shoreline-shelf systems at the shelf-break zone, and submarine fans in basinal parts (Achimov Formation). Periods of progradation were commonly interrupted by regional transgressions with significant retreat of depocenters landward over the shelf (up to 200 km). The transgressive systems tracts on the shelf are laterally extensive shale horizons that represent useful markers for correlation. The overlying highstand deposits are interpreted as thin but broad sand-prone packages with generally progradational appearance. The lowstand systems tracts on the shelf are locally present as fluvial sandstones that are incised into underlying highstand deposits.