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Tight Oil Reservoir Characterization of the Lower-Middle Permian Lucaogou Formation, Jimusar Sag, Junggar Basin, Northwest China


The Lower–Middle Permian Lucaogou (P2l) Formation is a “sandwich–type” combination of source rocks and reservoirs and is considered to be a potential tight oil reservoir in the Jimusar Sag (JS). However, the P2l tight oil reservoir in the JS area has not been evaluated comprehensively. This paper investigated the P2l Formation reservoir using a designated workflow on the basis of a series of analysis using core samples (including geological, petrographical, and petrophysical characterization). Results indicate that reservoir lithology comprises mainly dolomites and siltstones. The main pore types are inter- and intragranular dissolved pores, accounting for 62%, followed by residual intergranular pores and micro-fractures. The P2l Formation reservoir is characterized by medium-high porosity (1.0% - 17.3%, averaging 9.1%) and relative low permeability (0.004 mD - 3 mD, averaging 0.26 mD). The porosity is positively correlated with the permeability overall. Some of the extremely high permeability data points might be related to the presence of micro-fracture networks. The P2l Formation reservoir has extremely complex mineral compositions with dolomite, quartz, feldspar, and calcite, comprising the main mineralogical constituents of the P2l Formation reservoir, with average contents of 30.3%, 22.4%, 25.2%, and 9.2%, respectively, which are mostly brittle minerals that can thus be favorably hydraulically fractured. The clay minerals in the P2l Formation reservoir are composed of montmorillonite, illite, chlorite, and a montmorillonite mixed layer, with an average content of 11.2%. Among the clay minerals, chlorite/montmorillonite combination is the most common clay minerals with an average content of 38.1%. The samples also contain montmorillonite, illite/montmorillonite, illite, and chlorite with corresponding values of 26.1%, 27%, 3.5%, and 5.3%. This suggests that the water sensitivity of the P2l Formation reservoir is low considering the absolute low content of montmorillonite. Consequently, the P2l Formation reservoir is an ideal horizon for the exploration and development of the tight oil in the JS area. These results provide a basis for further evaluation of the tight oil potential of the P2l Formation reservoir in the wide Junggar Basin and for identifying similar plays in other areas with exploration potential. Key words: Tight Oil; Reservoir characterization; Lower-Middle Permian Lucaogou Formation; Jimusar Sag; Junggar Basin; China