Densification of clastic reservoirs and difficulties in predicting their sweet spots constrain petroleum discovery in basins in central and western China.In order to investigate the mechanisms of reservoir densification and formation and preservation of sweet spots

a comprehensive study of sedimentology

petrology

and geochemstry were performed by using various methods

including core observation

microscopic observation of casting thin section and under fluorescence and cathode luminescence

fluid inclusion analysis

particle size analysis

mercury injection and permeability tests under overburden pressure

scanning electron microscopic analysis and so on.According to the generic types

sweet spots of the Chang 8 oil layer of Yanchang Formation in Zhenjing area

southwest Ordos Basin

are divided into a primary type and a secondary type.Early developments of chlorite films moderately inhibited later diagenesis processes

which is the main factor contributing to the formation and preservation of the primary sweet spot.The secondary type can be subdivided into a dissolution-type and a fracture-type according to their major pore space being dissolution pores or microfractures.The key factor determining the formation of the secondary sweet spot is constructive diagenesis.It can be concluded that the early charging of hydrocarbons effectively inhibited later cementation

and the multiphase diagenesis resulted the formation and preservation(or lost)of sweet spots.In vertical sections

individual sand body de-densification started from layers of relative high permeability

or areas of fracture and dissolution superimposing

and propagated outwards progressively.In planar view

sand body de-densification shows similar features

the sweet spots distribute in “bamboo-like” shape along the sand body.