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Abstract: Sequence Stratigraphic Setting of the Priob Field Within the Neocomian Prograding Complex of the West Siberian Basin, Russia

Oleg M. Mkrtchyan, John A. Armentrout

The Neocomian strata of the West Siberian basin are a prograding complex unique in its aerial extent, structure and hydrocarbon reserves, such as Priob Field in the Ob River area. Interpretation of the basin history is based on well data and seismic reflection profiles. As many as 45 transgressive-regressive depositional sequences, called cyclites, prograde into the deep-water basin formed during Late Jurassic and Early Cretaceous. Each transgressive-regressive sequence is defined by a coarsening-upward cycle of shale, siltstone and sandstone, and is considered a chronostratigraphic subdivision of the prograding complex. Each sequence boundary is regionally correlatable on seismic reflection profiles, and is identified on well logs by sharp contacts between regressive san stones below and thin transgressive shales above.

Subordinate progradational wedges are locally developed within these sequences and contain major hydrocarbon reserves. These depositional wedges map as lens-shaped packages downlapping the outermost shelf (Priob zone) or as narrow progradational wedges downlapping the foreset reflections of the slope clinoforms immediately beyond the shelf break (Sugmut zone). These shelf-edge wedges correspond to deposition during lowest relative sea-level. Transgressive facies are identified both by thin shales deposited over the slope and shelf edge and by correlative thin sandstones on the shelf. Maps of these transgressive sandstones indicate infilled lowstand sediment transport routes subsequently buried by the shale dominated transgressive shelf facies. Regressive facies of the shelf consist of thin but wide spread sandstones that also contain major hydrocarbon reserves. Pervasive sediment starvation during the Late Neocomian resulted in deposition of thin regionally extensive shales that provide top-seal to the Neocomian hydrocarbon system.

At the Priob field, a deep erosional incision has been mapped at the AS11 shelf-edge. Sands transported through this incised valley were deposited as a prograding wedge along the shoreline, forming the reservoir facies for the Priob hydrocarbon accumulation. Stratigraphic aspects of the Priob trap include top and lateral shale seals and subtle regional structural tilt. Types of stratigraphic traps are discussed and the possibility of predicting additional such traps are analyzed.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France