--> ABSTRACT: Giant Petroleum Accumulations in the Volgian-Neocomian Deposits of Western Siberia: A Complete Petroleum System Within a Second Order Depositional Sequence, by Y. N. Karogodin, J. M. Armentrout; #91020 (1995).

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Giant Petroleum Accumulations in the Volgian-Neocomian Deposits of Western Siberia: A Complete Petroleum System Within a Second Order Depositional Sequence

Y. N. Karogodin, J. M. Armentrout

In the Middle Ob Region of the West Siberian Basin, the Volgian-Neocomian succession consists of as between 35 and 45 transgressive/regressive cycles which contain 80 percent of the liquid hydrocarbons in this province. Each of these transgressive-regressive cycles progrades into the basin where the accommodation space allowed deposition of sigmoid clinoforms ranging from 200 to 600 meters in undecompacted thickness. Each clinoform consists of top set (undaform) shelfal sandstone and siltstone cyclites, foreset (clinoform) slope siltstone, and bottomset (fondaform) basinal sapropelic claystones. The composite lithofacies units of the sapropelic claystones are the Bazenhov Formation (Suite), and the composite lithofacies units of the slope siltstones and shelf sandstones i cluding such units as the Megion, Akh, Kulomzin and Cherkashin formations (Suites).

In a basin-scale context, the Volgian phase of the cycle is dominantly Bazenhov facies due to regional transgression which confined most of the terrigenous sediment supply to the basin margins. Similarly, the Neocomian phase of the cycle is dominantly Megion facies due to regression as the clinoforms rapidly prograded across the basin, infilling the southern end. The infilling of this deep water Volgian-Neocomian basin ended with deposition of the Alym Formation (Suite) shale, a regional non-marine, shallow water deposit. This large scale transgressive-regressive cycle of more than 20 million years duration is interpreted as a second-order depositional cycle. Individual regional clinoform cycles are interpreted as third-order depositional cycles of perhaps 1 to 3 million years duratio . Smaller scale clinoforms represent higher frequency cyclic deposition.

The Bazenhov sapropelic claystones average 10 percent total organic carbon, shich has been correlated using biomarkers to the oils reservoired in the Megion sandstones Migration is most probably along conduits within the clinoforms from the basin up through slope sandstones to the shelf sandstones. Within a structure, vertical migration occurs up faults and between sandstones in communication due to multiple cycles of erosion and deposition. The Alym Formation appears to be a regionally effective top-seal while the shales of the clinoforms series provide a hierarchy of more local seals.

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