Abstract:

The shale oil sweet spot reservoirs of the Permian Lucaogou formation in the Jimsar sag of Junggar Basin are characterized by complex lithology and greatly varying physical properties, and diagenesis has played an important role in the pore evolution of these reservoirs. Using rock slice, cast thin section, scanning electron microscopy (SEM) and X-ray diffraction (XRD), this paper studies the microscopic characteristics of the pores in the reservoirs to reveal pore genesis, and variations and controlling factors of reservoir physical properties. The results indicate two types of reservoirs, i.e. sandstone reservoir and dolomite reservoir are developed in the sweet spots of Lucaogou formation in the Jimsar sag. Both reservoirs are dominated by secondary pores such as intergranular dissolution pores, intragranular dissolution pores, moldic pores and dissolution fractures, and contain the macropores with diameter >50 μm accounting for more than 50%. The sandstone reservoir and dolomite reservoir have similar physical properties, belonging to medium porosity and low-ultra-low permeability reservoirs. The average porosity and permeability of the sandstone reservoir are 13.51% and 0.81mD, respectively. The dolomite reservoir exhibits an average porosity of 12.86% and an average permeability of 2.38 mD. Both reservoirs have undergone compaction, weathering/leaching dissolution, cementation, and organic acid dissolution, and are in phase A of the middle diagenetic stage. The weathering/leaching dissolution in the epidiagenetic stage accelerated the formation of large pores like capillary pores and supercapillary pores, which are the main contributors to reservoir space. The dissolution of organic acid in the middle diagenetic stage promoted the formation of nanoscale microcapillary pores, which exerted a limited improvement on reservoir physical properties.

Key words: Jimsar sag, Lucaogou formation, shale oil, pore genesis, pore type, pore structure, diagenesis, diagenetic evolution