Abstract: The Lower Cretaceous source rocks in most Erlian Basin sags developed a set of tight rocks including tuff, tuffite, tuffaceous mud and sandstones. Here, in the first member of the Tenggeer Formation in the Lower Cretaceous of the Anan Sag, and taking tuffaceous rocks as research objects, the petrologic and diagenetic characteristics of mixed rocks were studied using core observation, microscopy, scanning electron microscopy (SEM), cathode luminescence (CL) and Xray diffraction techniques. The results show that the thin lithic tuff, developed in the semideep to deep lakes, exhibits strong devitrification with localized occurrences of microcrystalline feldspar, quartz and dolomite. In comparison, the tuffite and tuffaceous mudstone, developed in the sand sheet of fandelta front and semideep to deep lakes, are mainly in the A2 mesogenetic stage, characterized by dolomite cementation with strong devitrification in authigenic clay minerals. Its reservoir space is relatively small and composed primarily of developing micropores and structural microcracks. Lastly, the tuffaceous sandstone, developed in the sand sheet of fandelta front, is mainly in the A1 mesogenetic stage and characterized by strong compaction and calcite cementation, followed by dolomite and siliceous cementation. Its reservoir space, containing less primary developing pores, is mainly made of dissolution pores formed by dissolved feldspar fragments and carbonate cements. The semideep to deep lacustrine tuff and fandelta front tuffaceous sandstone, therefore, are the favorable reservoir zones in the Anan Sag.

Key words:  tuff, tuffaceous rock, diagenesis, first member of Tenggeer Formation, Anan Sag, Erlian Basin