Abstract: The composite study of five sections and 11 exploratory wells shows that Chang-6 reservoir of Yanchang formation of Upper Trias-sic in Jing'an oilfield underwent a series of complex diageneses with compaction, cementation and dissolution with various and distinctive features. The study shows that the early diagenetic compaction and multi-mineral cementation are major effective factors that lead to its poor petrophysical property and low porosity; while dissolution, in particular, dissolution of laumontite is one of significant factors controlling the quality of reservoir.

Key words: Ordos basin, diagenesis, clastic rock, reservoir, laumontite