Priobskoye field is located in Khanty-Mansiysk Autonomous Region of Tyumen Region 65 km east of the town of Khanty-Mansiysk. Commercial oil occurs in the Tyumen and Bazhenov Formations of Jurassic age and in Neocomian rocks of Early Cretaceous age. The main reserves are in Neocomian Strata AS - 10-12. More than 20 pools have been found in this part of the section in a stratum of Hauterivian age in the 2300-2700 m depth range. Most of these are classified as large. The Neocomian productive strata are clinoforms.
Productivity of the Neocomian sediments at Priobskoye is controlled essentially by one factor: presence in the section of permeable reservoirs. Absence of formation water during testing indicates that the pools occur in closed lenses filled completely by oil. There are no water-oil contacts.
Paleogeographic conditions and seismic data show the presence of a large zone of Neocomian clinoforms extending to the north and south of Priobskoye field. This constitutes an independent zone of oil accumulation, productivity of which is not determined by regional structure but rather by the presence of the clinoforms. This field then is part of a large north-south zone in the area of junction of the Khanty-Mansiysk depression and the Salym arch to its east.
During the course of Mesozoic history of the West Siberian platform the Khanty-Mansiysk depressioin was the deepest part of the basin of sedimentation, and it was the most argillaceous. During Volgian time at the end of the Jurassic the area of Priobskoye field was in the most subsided axial zone of the basin, which was uncompensated. This was the site of accumulation of the organic-rich cherty deposits of the Bazhenov Formation. Then in the early Berriasian of the Early Cretaceous on a background of general regression there were alternating regional and zonal regressions and transgressions. Clinoforms began to be deposited in the depression, first in the east-southeast and then filling the entire depression. Largely clays collected during the transgressions and sand-silt deposits during the regressions.
The Bazhenov Formation has a high oil-generating potential. It is generally held that these rocks are the source of most of the oil in the Lower Cretaceous fields of West Siberia. Such a source at Proobskoye, however, is questionable. Comparison of geochemical parameters of the oils of the Lower Cretaceous Neocomian strata of Priobskoye with the oils of the Jurassic Bazhenov Formation of Salym field shows that these oils differ from one another and belong to different genetic zones. The oil of Priobskoye field is undersaturated by gas, and there is an abrupt increase in formation pressure in passing from Cretaceous into Jurassic. There are thus two practically isolated stages of oil occurrence: Lower Cretaceous and Jurassic. Formation of the oil pools in the Neocomian at Priobskoye took place independently and is not related to migration from the Bazhenov.
The mechanism for formation of the Priobskoye oil pools was probably lateral up-dip migration of oil from clayey sediments of the same age into the sandy parts of the clinoforms. This model is is important to exploration.
In direct proximity to Priobskoye are similar sandstone bodies in the Khanty-Mansiysk, Frolov, and other areas. A large zone of Neocomian clinoforms extends north and south of Priobskoye field in a belt 25-50 km wide. Oil pools of similar genesis are to be found also in other areas of West Siberia. (Taken from Nemchenko (2000); digested in Petroleum Geology, vol. 34, no. 4, Autumn 2000; two cross sections - in press). Copyright 2000 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at http://geocities.com/internetgeology/ This News Letter is circulated by e-mail at no charge. To become a subscriber please send your e-mail addeess to: jamesclarke@erols.com