Deep-water gravity flow deposits in a lacustrine rift basin and their oil and gas geological significance in eastern China

The types,evolution processes,formation mechanisms,and depositional models of deep-water gravity flow deposits in a lacustrine rift basin are studied through core observation and systematic analysis.Massive transport of slide and slump,fluid transport of debris flow and turbidity currents are driven by gravity in deep-water lacustrine environment.The transformation between debris flow and turbidity current,and the transformation of turbidity current between supercritical and subcritical conditions are the main dynamic mechanisms of gravity flow deposits in a lake basin.The erosion of supercritical turbidity current controls the formation of gravity-flow channel.Debris flow deposition gives rise to tongue shape lobe rather than channel.Deep-water gravity flow deposits are of two origins,intrabasinal and extrabasinal.Intrabasinal gravity flow deposits occur as single tongue-shape lobe or fan of stacking multiple lobes.Extrabasinal gravity-flow deposits occur as sublacustrine fan with channel or single channel sand body.However,the nearshore subaqueous fan is characterized by fan of stacking multiple tongue shape lobes without channel.The differential diagenesis caused by differentiation in the nearshore subaqueous fan facies belt results in the formation of diagenetic trap.The extrabasinal gravity flow deposits are one of the important reasons for the abundant deep-water sand bodies in a lake basin.Slide mass-transport deposits form a very important type of lithologic trap near the delta front often ignored.The fine-grained sediment caused by flow transformation is the potential"sweet spot"of shale oil and gas.

Basic principles of the whole petroleum system

This paper expounds the basic principles and structures of the whole petroleum system to reveal the pattern of conventional oil/gas-tight oil/gas-shale oil/gas sequential accumulation and the hydrocarbon accumulation models and mechanisms of the whole petroleum system.It delineates the geological model,flow model,and production mechanism of shale and tight reservoirs,and proposes future research orientations.The main structure of the whole petroleum system includes three fluid dynamic fields,three types of oil and gas reservoirs/resources,and two types of reservoir-forming processes.Conventional oil/gas,tight oil/gas,and shale oil/gas are orderly in generation time and spatial distribution,and sequentially rational in genetic mechanism,showing the pattern of sequential accumulation.The whole petroleum system involves two categories of hydrocarbon accumulation models:hydrocarbon accumulation in the detrital basin and hydrocarbon accumulation in the carbonate basin/formation.The accumulation of unconventional oil/gas is self-containment,which is microscopically driven by the intermolecular force(van der Waals force).The unconventional oil/gas production has proved that the geological model,flow model,and production mechanism of shale and tight reservoirs represent a new and complex field that needs further study.Shale oil/gas must be the most important resource replacement for oil and gas resources of China.Future research efforts include:(1)the characteristics of the whole petroleum system in carbonate basins and the source-reservoir coupling patterns in the evolution of composite basins;(2)flow mechanisms in migration,accumulation,and production of shale oil/gas and tight oil/gas;(3)geological characteristics and enrichment of deep and ultra-deep shale oil/gas,tight oil/gas and coalbed methane;(4)resource evaluation and new generation of basin simulation technology of the whole petroleum system;(5)research on earth system-earth organic rock and fossil fuel system-whole petroleum system.

Reservoir formation conditions and key technologies for exploration and development in Qingcheng large oilfield

The Qingcheng oilfield,the largest shale oil field in China,was discovered in 2019 in source rock of Chang 7 Member of Mesozoic Yanchang Formation,Changqing Oilfield in Ordos Basin,with newly increased proven geological reserves of 358 million tons,predicted geological reserves of 693 million tons,and a total of 1.051 billion tons of shale oil resources.This achieves a historic breakthrough in exploration of shale oil in Chang 7 Member.In recent years,focusing on key issues,such as whether there develop sweet spots in source rock,whether commercial oil production capacity can be formed,and whether scale-effective development can be achieved,studies about geological conditions for shale oil accumulation have been continuously carried out,key supporting technologies have been developed,and a series of theoretical innovations and technological breakthroughs have been achieved in oil exploration in source rocks.The results indicate that shale oil accumulation in Chang 7 Member is controlled by following factors.Firstly,lacustrine black shale,dark mudstone and other high-quality source rocks lay a material foundation for shale oil formation.Secondly,sandy deposits intercalated within black shale and dark mudstones are exploration sweet spots,and sandbody combination types are controlled by morphology of bottom of the lacustrine basin.Thirdly,reservoir properties are good due to development of micro-nano pore throats.Finally,reservoirs with high oil saturation and high gas-oil ratio are formed due to high-intensity oil charging in source rock.Effective matching of multiple factors is a key for shale oil accumulation in Chang 7 Member in Ordos Basin.Through innovation of key supporting technologies,exploration and development technologies for shale oil are developed,and substantial breakthrough on exploration and development of shale oil is realized.The first application of logging-seismic combination technology in loess plateau improves significantly quality of 3D seismic data,which effectively guides prediction of sandy sweet spots.Three-quality well logging technology is used for further evaluation of geological and engineering sweet spots of shale oil.With volumetric fracturing technology based on precise segmentation of long horizontal wells,initial daily oil production of individual well is increased from previously 10 tons to more than 18 tons.The exploration breakthrough in source rocks in Chang 7 Member in Qingcheng oilfield provides an important resource base for the second accelerated development of Changqing Oilfield.It is predicated that oil production from shale in Chang 7 Member in Qingcheng oilfield will reach three million tons in 2022 and five million tons in 2025.