Regional synthesis of the productive Neocomian complex of West Siberia: Sequence stratigraphic framework

We have developed a regional sequence stratigraphic framework for the produ ctive Neocomian complex of central West Siberia. Sixteen depositional seque nces have been identified on the basis of analysis of regional seismic line s and well logs. A dip-oriented well-log transect that was constructed acro ss the Nizhnevartovsk and Surgut arches reveals detailed features of the st ratigraphic 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 cons ists of the clinoform. (lower part) and topset packages. Deposition of the clinoform package occurred during a period of at least 9 m.y. when clinofor ms prograded more than 550 kin from east to west. Progradation occurred thr ough lateral shelf outbuilding mostly during lowstand periods when sandston e units accumulated in shelf-edge deltas, shoreline-shelf systems at the sh elf-break zone, and submarine fans in basinal parts (Achimov Formation). Pe riods 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 w ith generally progradational. appearance. The lowstand systems tracts on th e shelf are locally present as fluvial sandstones that are incised into und erlying highstand deposits.