ИСТИНА

During the Early Jurassic the Barents sea basin became submerged. At that time change occurred from an arid to a humid climatic regime, resulting in increased run-off and the transport of gravel, sand and mud from the mainland out onto the shelf. Many of the most important oil and gas reservoirs of the Barents sea basin were deposited during the Early and Middle Jurassic. The Jurassic stratigraphy in the Barents sea could be subdivided into five cycles caused by eustatic sea level fluctuations and sediment characteristics. The upper cycles are quite well correlated, the lower ones have a lot of internal unconformities and they are being changed by age. In the Jurassic section of the Barents sea three reservoirs J2, J1 and J0 are mapped. These collectors are represented by terrigenic section of sand stratum. In the Barents sea basin reservoir J0 has regional extension, reservoirs J2, J1 are not regional and can be mapped only on local structures. Sediments of producing reservoir J2, which was penetrated by several wells in the Eastern part of the Barents sea, were studied in most details. The profile of J2 reservoir consists of three units, different in structure, sediments composition and genetic types. Profile is characterized by transgressive-regressive structure, where sedimentary conditions are changing bottom-up from fluvial-deltaic, directly deltaic to delta fronts. Composition of sandy fractions also changes in this direction from quartz greywacke to medium-grained rocks. Several levels of ancient oil-water contact are mapped within gas-saturated part of the reservoir. This reservoir have passed through the complex generating process. Apparently, primarily small oil pools were formed in the Middle Jurassic sediments due to hydrocarbons generation by Triassic source rocks of predominantly humus composition. Then reservoir was abruptly filled by gas, which has destroyed original oil pool. Evidence of this is given by residual bitumen saturation of the rocks. Currently J2 reservoir is saturated by gas condensate which was formed by underlying and lateral flows of Upper Jurassic hydrocarbons.