Influences of feldspars on the storage and permeability conditions in tight oil reservoirs: A case study of Chang-6 group, Ordos Basin

Abstract

Based on the results from rock thin section observation, casting thin section identification, high-pressure mercury injection, rate-controlled porosimetry, X-ray diffraction, and high resolution field emission CMOS scanning election microscope observation and analysis, influences of feldspar minerals on the storage and permeability conditions in Chang-6 oil layer group in the Yanchang Formation of the Ordos Basin were studied from crystallography and mineralogy perspectives. Due to the differences of crystal structure and chemical composition, the brittleness and solubility of feldspar minerals vary greatly. The results indicate that basic plagioclase such as anorthite tend to split along the cleavage to form new flow channel in formation conditions. Alkaline feldspar rich in potassium such as orthoclase tend to corrode along the cleavage to form secondary porosity. The brittleness and solubility of acid plagioclase, such as albite, are between anorthite and orthoclase. The dominant pore type in Chang-6 oil layer group of Ordos Basin is residual inter-granular pore, the secondary pores generated by corrosion and fracture of feldspar have little contribution to porosity. In the area with higher content of anorthite, the micro-cracks resulted from breaking in cleavage effectively improve the permeability. The contribution of feldspar minerals to permeability is ranked from great to small as anorthite, orthoclase and albite. Key words: Ordos Basin, Chang-6 oil layer group, tight oil, feldspar