Conditions of Forming Stratigraphical Traps in The Lower Cretaceous Clinoform Complex and Regularities in Their Distribution, Western Siberia

By

 Michail D. Belonin¹, Anatoliy M. Brekhuntsov², Iskander S. Jafarov³, Vladimir V. Shimansky¹, Sergey F. Khafizov³, Vladimir N. Borodkin²

(1) VNIGRI, St. Petersburg, Russia (2) SIBNAC, Tumen, Russia (3) TNK, Moskow, Russia

The Lower Cretaceous clinoform complex of Western Siberia is the most prospective oil/gas complex; it contains large and giant hydrocarbon fields in stratigraphical and structural-stratigraphical traps. However, its genesis and structure remain an engima. The paleoreconstructions of conditions of forming the clinoform deposits on the basis of interpretation of core data (petrographical, granulometric, petrochemical, paleontological, paleogeomorphological and other researches) and also geophysical data allow definite conclusions:

-         Stratigraphical traps are associated with turbidity, avandeltaic, and deltaic deposits; - The best reservoir rocks are formed by grain streams at the regressive stage of sedimentation followed by the effect of contour currents;

-         The most concentration of sorted sandy material is observed on the flanks of paleostructures in the way of grain streams;

-         A complex porous-fractured type of reservoirs has the most wide occurrence. 16 sequences of 4 order related to the stages of sedimentary basin starved filling are identified and mapped in the Frolovsky, Kaimysovsky, and Nadym-Pursky oil-gas regions.

The maps and schemes of distribution of reservoir characteristics, granulometric coefficients, and the facial environments of sedimentation are constructed. The specificity of forming the Neokomian clinoform deposits in the axial part the sedimentary basin (decrease by 75% of psammitic material entering the basin, absence of continuous areal occurrence of shelf beds; nearly the same amount of clastic material entering both from the east and the west) is shown. The depocenters of arenosity, and the ways of entering grain material are shown. The forecast of forming stratigraphical and structural-stratigraphical traps is given.