Processes And Controlling Factors Of Oil Migration And Accumulation In The Tight Reservoirs Of The Lucaogou Formation In The Jimsar Sag,Junggar Basin

As the consumptions of conventional oil and gas resources increase,unconventional oil and gas resources with huge potential have gradually become new exploration targets.And tight oil has already been an important part of the unconventional oil and gas resources.The Jimsar sag,located in the eastern part of the Junggar Basin,is considered to be a typical area containing continental tight oil,especially in the Lucaogou Formation,which contains significant amount of tight oil resources and has realized commercial production.However,the frequently fluctuated depositional environments and provenance supplies are significantly responsible for the great variations in lithology and layer thickness.The reservoirs have poor physical properties and complex pore structure,which are typical tight reservoirs.To our best of knowledge,studies on the migration and accumulation processes of tight oil are still scarce.Profound understanding on the mechanism of oil migration and accumulation in the tight reservoirs and the key control factors has not been reported.In this study,the Lucaogou Formation is treated as the target,and their basic geological conditions are investigated at the beginning stage of the research,including source rock evaluation,reservoir characterization,and source rock-reservoir assemblage classification.A combination method of physical simulation,numerical simulation and comprehensive geological-geochemical analysis is utilized to illuminate the dynamic process of tight oil migration and to demonstrate the accumulation process and its controlling factors.Finally,a pattern is established to show some light on the mechanism of tight oil migration and accumulation.In the Jimsar sag,the source rocks of the Lucaogou Formation have been regarded as a“world class source rock” and spread widely throughout the whole sag.Source rocks are measured to have high abundance of organic matter with an average total organic carbon content of 2.92%.Organic matter are predominated by type I and II kerogen.Source rocks have entered the mature stage,which are considered to be good to high-quality source rock.Reservoirs are mainly distributed within the upper and lower “sweet spot”,and the reservoir rock is mainly composed of siltstone and dolomite.In general,pore structure of siltstone has been proven to be better than that of dolomite,and the tuffaceous siltstone is deemed to have the best pore structure.Source rock-reservoir assemblage has been divided into three subcategories,namely thick source rock interbedded with thin tight reservoir,thin source rock interbedded with thin tight reservoir,and thick tight reservoir interbedded with thin source rock.Elaborate oil-source correlation showed that hydrocarbon in the tight reservoirs of the “sweet spots” originated from the interbedded source rocks adjacent to these tight reservoirs.The statistical results suggested that the latter two subcategories have better oil-bearing property in the effective range of hydrocarbon expulsion.The vertical distribution of tight oil in the “sweet spots” is revealed to present great heterogeneity,which is controlled by both characteristics of source rocks and reservoirs.Authentic sandstone micro-model and micro-CT in-situ characterization of fluid injection experiment are used to visualize the process of oil migration in tight reservoir under different injection pressures,aiming to elucidate migration mechanism of tight oil.Oil migration in the porous media shows an obvious jump pattern at the condition of low pressure.The jump event presents as breaking through throat,filling pore body and occurring snap-off.Multiple and repeated jump events result in the movement of the oil-water interface.However,there is no uniform oil-water interface,and the displacing rate of water by oil varies in the direction of migration.Oil preferentially enters into the connected large pores,forming the dominant migration path.As a result,the majority of oil clusters are isolated and distribute dispersedly in the reservoir space and oil saturation of the reservoirs is very low.The divergence and convergence of these migration paths result in these isolated oil clusters converging slowly into continuous phases under high injection pressure,and the configurations of oil clusters vary as their volume increase.A large quantity of oil clusters fill in several pores with a small proportion of oil clusters distribute over the relatively small mesh network.The oil migration pattern in the porous media is dominately controlled by local capillary pressure,and meanwhile the external driving force provides sufficient energy supply for oil migration,which determines the minimum size of throat radius to breakthrough.Physical simulation and numerical simulation of accumulation process are conducted to analyze the growth pattern of oil saturation in tight reservoirs,in order to demonstrate the accumulation processes and their controlling factors.On the basis of growth rate and final oil saturation,the growth pattern of oil saturation can be divided into three types,which is dominantly influenced by the injection pressure and pore structure of tight reservoirs.The injection pressure is a prerequisite for the increase of oil saturation,and pore structure is the main factor to control the growth pattern of oil saturation.It is proved that variations of pore throat radius and coordination number play a significant controlling role in the growth pattern of oil saturation.In the Lucaogou Formatioin,pore type assemblage and pore structure of tight reservoirs with various lithology are distinctly influenced by the dissolution of minerals.Dolomite reservoirs are dominated by intercrystalline pore and residual primary pore with minor amount of dissolved pore.These reservoirs have small pore and throat radius,and the throat size is predominatly distributed in the range of nano scale.Correspondly,they have small coordination number and poor connection.As a result,the growth rate of oil saturation is very slow in the dolomite reservoirs,which have low final oil saturation.Siltstone reservoirs show obvious difference in the pore type assemblage and pore structure,which also present great variation in oil saturation.Remarkably,tuffaceous siltstone reservoirs are predominated by dissolved pore and residual primary pore,which have large pore and throat radius.It is noteworthy that throats of tuffaceous siltstones are distributed in the range from nano-scale to micro-sacle,leading to a better connection.These reservoirs have great growth rate in oil saturation,which commonly have higher final oil saturation.It is evidenced that there are two stages of oil charging in the Lucaogou Formation,and reservoirs have already been tight when the late stage of hydrocarbon charging happened at late Jurassic.Results by basin modeling indicate that source rocks reentered the oil window at early Jurassic,and since then residual pressure between source rocks and reservoirs started to accumulate.The gradually increasing residual pressure has been the primary driving force for vertical migration of tight oil.Residual pressure is proved to be capable of overcoming the displacement pressure of tight reservoirs,driving hydrocarbon into the reservoir space.Oil migration through the pore network predominantly occurred in vertically,and scattered fractures and stylolite are important path for hydrocarbon migrating laterally.The accumulation of tight oil is obviously controlled by source rock-reservoir assemblage.Tight oil mainly accumulated in the high-quality reservoirs with good physical properties in the thick source rock interbedded with thin tight reservoir assemblage.Whereas,tight oil dominantly accumulated in the high-quality reservoirs adjacent to high-quality source rocks in the rest two assemblages.The oil saturation of tight reservoir decreases with the increase of the distance from source rock.