The pay zones of Samotlor field are in the Lower Cretaceous Megion and Vartov Formations. Those of the upper Vartov are designated as Stratum A, and those of the underlying Megion and lower Vartov are Stratum B. Their age range is Berriasian, Valan- ginian, Hauterivian, Barremian.
Anomalous oils are found in both Strata A and B. These oils are heavier in comparison with the average for each stratum. This is due to higher content of tars and asphalt and to more naphthene hydrocarbons with large amounts of iso-alkanes. The oils and condensates with naphthene base do not bear signs of oxidation and are always related to gas accumulations.
Samples for microfossil analysis were taken from sectors with different composition of the oils. These occur at the crest of the field and on its flanks. Seven samples were taken from Stratum A and five from Stratum B. Two groups are recognized.
The first group consists of oils of various pay zones of Strata A and B that have similar microfossil assemblages with a pre- dominance of acritarchs. A few of these are Paleozoic varieties; the rest are Mesozoic forms that have a wide stratigraphic range.
The second group includes oils that have a high level of migrated microfossils. The migrational component of two samples from Stratum B stands at 37 percent of the total population. Paleo- zoic acritarchs (11 percent Lower Paleozoic and 17 percent Upper Paleozoic) constitute on average 28 percent of this total population. Microfossils of four samples from Stratum A are represented by 5 percent Lower Paleozoic individuals and 12 percent Upper Paleozoic.
The oils of the second group (migrated oils) are localized in several sectors on the steep east flank of Samotlor field and in a narrow belt of southeast trend on the crest of the high.
The structure of Samotlor field has proved to be more complex than had been supposed earlier. Several faults have been found, which are associated with differences in level of the water-oil contact. The displacement of the water-oil contact is taken to indicate recent tectonic movements. Sectors of anomalously heavy, naphthenic oils tend to be associated with zones of fracturing and folding of the Neocomian rocks. The microfossil data indicate active migration processes in these same zones. It can thus be concluded that the oil pools in the Neocomian rocks of Samotlor field are secondary, as indicated by presence in them of the older microfossils, and that they formed by vertical migration along zones of fracturing and faulting on both the steep flanks and crest of the structure. (Taken from Klimushina and Medvedeva, 1982; digested in Petroleum Geology, vol. 20, no. 2, p. 53-55, one cross section).
The extension of the Proterozoic and Lower Paleozoic rocks of East Siberia westward beneath the West Siberian oil-gas province is addressed in News Letter No. 4 of August 2, 1999, available at our webpage: http://geocities.com/internetgeology/
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