Reservoir characterization of the Upper Cretaceous shale for the Changling Sag, Southern Songliao Basin, NE China

Abstract

The Upper Cretaceous Qingshankou (K2n) and Nenjiang (K2q) shale oil from the Changling Sag (CLS) of the Southern Songliao Basin located in NE China was studied to determine shale reservoir properties and evaluate oil potential. Multiple methods, including optical microscope, scanning electron microscope and energy dispersive spectrum analysis (SEM-EDS), nitrogen adsorption/desorption analysis, and X-ray diffraction, were conducted to determine the shale reservoirs. The mineral analyses present in this shale consist of blende, albite, chlorite, pyrite, and calcite, whose textures are crystals, schistose, particle, or clusters. The mineral results show a significant percentage of brittle minerals (40.5–61.7%), and clay minerals are dominated by illite and montmorillonite. The mineralogical brittleness indexes of the Upper Cretaceous K2n and K2q shales are 0.15–0.67 and 0.18–0.42, respectively. The K2q shale has a relatively low brittle mineral content, which may not be favorable for shale oil seepage. Higher brittleness in K2n shales is more favorable for hydraulic fracturing with larger width and smaller initial stress. Although the reservoir has an ultra-low permeability, they have a relatively good porosity. The organic matter is residual in bioclastic pyritization, calcareous bioclastic grain pores, animal skeletal grain, and algous cystocarp. Moreover, the K2n and K2q shales contain a substantial number of pores, micro-fractures, and oil. The mineral intragranular pores, dissolution pores, organic pores, and micro-fractures are significantly developed. Hysteresis loop is acquired in open pores, which have mainly inkbottle-shaped pores and few parallel-plate pores or cylindrical pores. Our findings may fill significant gaps in understanding and predicting pore systems and mineralogical brittleness indexes, with respect to the lacustrine shale oil in China.