Abstract: In this study, lithofacies types and characteristics were analyzed in detail through examination of the tight finegrained sedimentary rocks of Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin by methods of core observation, thin section identification, SEM (scanning electron microscope) and physical property testing. The results show that: (1) The tight finegrained sedimentary rocks have a complex mineral composition of multiple components including terrigenous detrital, pyroclastic debris and carbonate components, representing a type of transitional diamicton. (2) Based on factors such as component, sedimentary structure and organic carbon, the mixed sedimentary rock lithofacies can be divided into two types, namely the unicomponent lithofacies and multicomponents mixed lithofacies. The former is mainly massive lithofacies and the later consists of massive, strippedlumpy and laminated lithofacies. (3) There are six major lithofacies types that can be identified in the Lucaogou Formation: massive micritic dolostone, massive silty dolarenite, massive tuffaceous siltstone, stripped tuffaceous dolostone, laminated silty/argillaceous tuffites and laminated dolomitic mudstone. Different lithofacies types have different reservoir qualities, porethroat structures and reservoir spaces. And the massive tuffaceous siltstone, stripped tuffaceous dolostone and laminated silty tuffites are the favorable lithofacies.

Key words: mixed finegrained sedimentary rock, lithofacies division, reservoir characteristic, Lucaogou Formation, Jimsar Sag