Characteristics and main controlling factors of shale gas reservoirs in transitional facies: A case study of Upper Paleozoic in Ordos Basin

Micro pores and micro fractures in shales are the main reservoir space and drainage channels for shale gas and the pore/fracture growth degree is related to gas content. To learn more about shale reservoirs of the Upper Paleozoic in the Ordos Basin, we classified the reservoir space into nanometer pores, micron pores and tiny cracks. Technologies such as scanning electron microscopy, argon ion polishing and nuclear magnetic resonance were applied to quantitatively characterize the development of different reservoir space. Combined with testing data such as TOC, organic composition and maturity, X-ray diffraction of whole rock and clay minerals, the controlling factors for different pore characteristics were analyzed. Two conclusions were made as follows. (1) The diameter of nanometer pores ranged from 17.13 nm to 67.65 nm on average, and the average plane porosity ranged from 5.75% to 9.37%. For micron pores, the average diameter was 3.17-5.61 μm, and the average plane porosity was 10.2%-19.32%. Tiny cracks accounted for 0.36%-0.72%. (2) TOC, organic maturity and I/S content promoted the generation of nanometer pores, while calcite content inhibited it. For micron pores, quartz and illite contents were positively correlated with their development, while carbonate content and burial depth were negatively correlated. Quartz content, organic maturity and TOC promoted the development of tiny cracks, while carbonate content inhibited it.