classification of petroleum accumulation. But theclassification in terms of trap origins is most favorable,where accumulations are commonly divided intostructural, stratigraphic and combined ones. However,more and more studies have indicated that the formationof many unconventional accumulations such as shale oiland gas, coalbed methane and tight oil and gas, are notcontrolled by traps. In this case, a classification based ontrap origins is not suitable for unconventionalaccumulations. Moreover, due to unaffected by traps,traditionally defining a reservoir as “petroleumaccumulation in a single trap” is also unfit forunconventional accumulation. As a result, bycomprehensive investigation and analyses of conventionaland unconventional accumulations, we put forward a newconcept of reservoir and a new classification of petroleumaccumulation.The updated concept defines the reservoir as acontinuous hydrocarbon-bearing stratum. The updatedclassification is based principally on the properties ofhydrocarbon distribution. According to such a principle,petroleum accumulations are grouped into continuous,quasi-continuous and discontinuous accumulations (Fig. 1,Table 1).
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