Abstract: 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.

Key words: sweet spot development model, diagenesis, clastic rock, tight reservoin, Yanchang Formation, Ordos Basin