Effect of Sequence Boundary on Sandstones Diagenesis and Reservoir Quality: 〖WTBZ〗an outcrop study from the Upper TriassicYanchang Formation, Ordos Basin, Northwest China

Key words: 
• sequence boundary /
• diagenesis /
• reservoir quality /
• Yanchang Formation /
• Yanhe River Outcrop /
• Ordos Basin /

Abstract: Diagenesis and reservoir quality in sandstones are important research scopes for petroleum exploration. Taking the delta front to delta plain facies from upper Chang 7 to middle Chang 4+5 member of Yanchang Formation, Yanhe River outcrop, eastern Ordos Basin as an example, this paper addresses the differences of authigenetic minerals, porosity types, petrophysical attributes and oilshowing in sandstones between baselevel semirising and semidowning cycles, and their origins. Both of abundant laumontite and early calcite cement in concretion prevent the formation of authigenetic minerals including chlorite, chlorite/smectite, quartzovergrowth and albiteovergrowth in baselevel semidowning cycle under the 3rd sequence stratigraphic boundary surface, whereas authigenetic chlorite, chlorite/smectite, quartzovergrowth and albiteovergrowth are relatively well developed resulting from poor development of laumontite cement owing to lack of tuffaceous deposites and lower formation temperature, and lack of early calcite cement due to little influence of lake in baselevel semirising cycle above the 3rd sequence stratigraphic boundary sequence. The 3rd sequence boundary surface together with well developed Chang16sandbodies immediately above the sequence boundary surface was the predominant fluid conduits through which acidic fluid, which dissolved laumontite and calcite cement and feldspar grains, and oil migrated originated from the mature source rocks. 