Abstract:

The dynamic mechanism of petroleum accumulation in sand lens remains poorly understood.Therefore,an explanation is still needed to address the key issue concerning how hydrocarbons charged into and expulsed from the closed or semi-closed hydraulic system made of sandstones encircled by source mudstones.By comparing the characteristics of lithology and the reservoiring features of structural traps,a comprehensive study and dynamic analysis was carried out on the driving forces of the primary hydrocarbon migration and the phase transformation and buoyancy effect during the secondary migration in mudstone.The results show that the predominant charging forces include abnormally high reservoir pressure,capillary pressure difference,molecular diffusion,and osmotic pressure,and that the buoyancy and relevant additional pressure contribute greatly to hydrocarbon accumulation and water displacement in sandbodies.These dynamics vary with different evolution stages of source mudstones.The compaction and seepage water dominated the charging during low maturity stage;while in mature stage,two phase flow(hydrocarbon and water seepage),capillary pressure difference,and molecular diffusion,controlled the charging process,and hydrocarbons entered into the lens by the action of buoyancy.In the highly matured stage,the charging process continued and was further accompanied by natural gas diffusion from the top of sandbodies,resulting in a final oil accumulation in the lens.Factors that affect the sand lens reservoirs include the quality and thickness of source mudstone,and the physical properties and occurrence of the sandbodies.In summary,hydrocarbon accumulation in sand lens is much more difficult than that in structural traps.

Key words: sand lens, reservoiring dynamics, oil-gas charging, abnormally high reservoir pressure,capillary pressure difference, molecular diffusion