Seismo-Geologic Study of a Detached-Shelf Sandstone Within the Volgian-Neocomian Clinoforms of the Middle Ob Region, West Siberian Basin, Russia

O. M. Mkrtchyan

The Volgian-Neocomian succession contains most of the liquid petroleum reserves of the West Siberian Basin. These reserves occur within regressive/transgressive sandstone cycles which are part of the Volgian-Neocomian complex of prograding clinoforms infilling the West Siberian Basin. Correlation of electric-logs with regional seismic reflection profiles demonstrates the occurrence of reserves within clinoform topset (coastal plain), foreset (shelf-edge), and bottomset (basin-floor) facies. Of particular note are the shelf-edge sandstones referred to as detached transgressive sandstones.

Electric-log analysis results in correlation of some shelf-edge reservoir sandstone stratigraphically below the overlying regionally regressive-phase sandstone. This suggests that the shelf-edge sandstone correlates with shale that grades laterally into the transgressive facies of the shelf area. Local stratigraphic nomenclature refers to this relationship as a detached transgressive sandstone. In some areas, the seismic interval correlative to the reservoir sandstone pinches-out against the foreset of the preceding clinoform. This geometric relationship suggests that sandstone may have been deposited as part of a lowstand prograding complex. Along the structurally lower, more northern area of the same sandstone unit, the stratigraphic pinch-out is not as well defined. Here the sandst ne interval may correlate with thin shelf sandstones mapped within the transgressive interval along the eastern margin of the study area. This mapped pattern can also be interpreted as lowstand deposition with the correlative transgressive sandstones representing the infilling of the river valley system which brought sands to the basinal-depocenter margin. Once the sands reached the shoreline they were distributed by long-shore processes into a shore-parallel north-south oriented sandstone body.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995