Since the discovery of giant hydrocarbon fields in the north of Western Siberia, no unified concept regarding the mechanism and stages of their formation has been developed. This paper on the example of the Urengoy field demonstrates that the formation of HC accumulations from Jurassic to Cenomanian is related to hydrocarbon fluids, flowing upwards from the deep depth, and their subsequent transformation. In the sedimentation process, the gases of the secondary kerogen destruction form an upward fluid flow, which dissolves oil components from source rocks and carry them to shallower depths. The formation waters of the north Western Siberia are methane-saturated; so, due to changes in its solubility during the Neogene uplift, methane comes out into a free phase. The calculations were performed on the upward flow phase separation and oil and gas content changes in reservoirs with depth. The addition of 50 mole% of methane released from the water to the Neocomian reservoirs gives a good agreement on the C1-C4 components and the C5+ content in the formation gas. The calculations were based on the proposition that methane captures light fractions from oil rims, thus increasing oil density. At shallow depths, the hydrocarbons are biodegraded, which leads to formation of almost pure methane accumulations in the Cenomanian reservoirs. The main mechanism of the upward flow transformations, controlling the oil and gas accumulation, is phase transitions. The additional factors, like methane dissolution in water and its transition into a free phase, microbial converting of hydrocarbons assure consistency between the calculated formation fluid properties and the actual data in the entire sedimentary section.
The Relation between Temperature and Pressure Conditions and Oil and Gas Content in Bazhenovsky-Abalaksky (Upper Jurassic) Complex of Western Siberia