Abstract: The Permian Lucaogou Formation in the Junggar Basin contains typical tight reservoir with low porosity, where natural fractures are of great importance for improving the seepage structure. Based on analysis of outcrops, cores, casting thin sections and scanning electron microscope (SEM), three types of natural fractures were identified in the Lucaogou Formation tight reservoir, including structural fractures, diagenetic fractures and abnormally high-pressure fractures. The structural fractures include shear fractures and expansion fractures, which are usually high-dipped and were developed during several stages of tectonic movements in the Junggar Basin. The diagenetic fractures consist of bedding, intragranular and intergranular fractures, of which the bedding fractures might be formed during tectonic action and hydrocarbon generation, while the intragranular and intergranular fractures are related to dissolution. The abnormally high-pressure fractures, mainly drainage fractures, are the products of extremely high fluid pressure which induced one principal stress to convert into tensile stress and produced fracture clusters in bent orientation and with different openings. Imaging logging statistics of 13 wells drilled into the upper "sweet spot" and 8 wells drilled into the lower "sweet spot" reveal less structural fractures-generally less than 0.5 fractures/m, and well-developed bedding fractures-generally more than 2 fracture/m. Core observation further confirms that the bedding fractures are primary in the Lucaogou Formaion tight reservoir, accounting for over 70%. Primary factors controlling the development of natural fractures in the Lucaogou Formation tight reservoir are sedimentary microfacies, lithology, heterogeneity, layer thickness, TOC and tectonic action.

Key words: Jimsar sag, Lucaogou Formation, natural fractures, imaging logging, development characteristics, controlling factors